Division IDESIGNCopyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.Section 1GENERAL PROVISIONS1.1 DESIGN ANALYSIS AND GENERALSTRUCTURAL INTEGRITY FOR BRIDGESThe intent of these Specications is to produce in-tegrity of design in bridges.1.1.1 Design AnalysisWhen these Specications provide for empirical for-mulae, alternate rational analyses, based on theories ortests and accepted by the authority having jurisdiction,will be considered as compliance with these Specica-tions.1.1.2 Structural IntegrityDesigns and details for new bridges should addressstructural integrity by considering the following:(a) The use of continuity and redundancy to provideone or more alternate load paths.(b) Structural members and bearing seat widths thatare resistant to damage or instability.(c) External protection systems to minimize the ef-fects of reasonably conceived severe loads.1.2 BRIDGE LOCATIONSThe general location of a bridge is governed by theroute of the highway it carries, which, in the case of a newhighway, could be one of several routes under considera-tion. The bridge location should be selected to suit the par-ticular obstacle being crossed. Stream crossings should belocated with regard to initial capital cost of bridgeworksand the minimization of total cost including river channeltraining works and the maintenance measures necessaryto reduce erosion. Highway and railroad crossings shouldprovide for possible future works such as road widening.1.3 WATERWAYS1.3.1 General1.3.1.1 Selecting favorable stream crossings shouldbe considered in the preliminary route determination tominimize construction, maintenance, and replacementcosts. Natural stream meanders should be studied and, ifnecessary, channel changes, river training works, andother construction that would reduce erosion problemsand prevent possible loss of the structure should be con-sidered. The foundations of bridges constructed acrosschannels that have been realigned should be designed forpossible deepening and widening of the relocated channeldue to natural causes. On wide ood plains, the loweringof approach embankments to provide overow sectionsthat would pass unusual oods over the highway is ameans of preventing loss of structures. Where reliefbridges are needed to maintain the natural ow distribu-tion and reduce backwater, caution must be exercised inproportioning the size and in locating such structures toavoid undue scour or changes in the course of the mainriver channel.1.3.1.2 Usually, bridge waterways are sized to passa design ood of a magnitude and frequency consistentwith the type or class of highway. In the selection of thewaterway opening, consideration should be given to theamount of upstream ponding, the passage of ice and de-bris and possible scour of the bridge foundations. Whereoods exceeding the design ood have occurred, or wheresuperoods would cause extensive damage to adjoiningproperty or the loss of a costly structure, a larger water-way opening may be warranted. Due consideration shouldbe given to any federal, state, and local requirements.1.3.1.3 Relief openings, spur-dikes, debris deectorsand channel training works should be used where neededto minimize the effect of adverse ood ow conditions.Where scour is likely to occur, protection against damagefrom scour should be provided in the design of bridgepiers and abutments. Embankment slopes adjacent tostructures subject to erosion should be adequately pro-3Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.tected by rip-rap, exible mattresses, retards, spur dikesor other appropriate construction. Clearing of brush andtrees along embankments in the vicinity of bridge open-ings should be avoided to prevent high ow velocities andpossible scour. Borrow pits should not be located in areaswhich would increase velocities and the possibility ofscour at bridges.1.3.2 Hydraulic StudiesHydraulic studies of bridge sites are a necessary part ofthe preliminary design of a bridge and reports of suchstudies should include applicable parts of the followingoutline:1.3.2.1 Site Data(a) Maps, stream cross sections, aerial photographs.(b) Complete data on existing bridges, including datesof construction and performance during past oods.(c) Available high water marks with dates of occur-rence.(d) Information on ice, debris, and channel stability.(e) Factors affecting water stages such as high waterfrom other streams, reservoirs, ood control projects,and tides.(f) Geomorphic changes in channel ow.1.3.2.2 Hydrologic Analysis(a) Flood data applicable to estimating oods at site,including both historical oods and maximum oodsof record.(b) Flood-frequency curve for site.(c) Distribution of ow and velocities at site for ooddischarges to be considered in design of structure.(d) Stage-discharge curve for site.1.3.2.3 Hydraulic Analysis(a) Backwater and mean velocities at bridge openingfor various trial bridge lengths and selected discharges.(b) Estimated scour depth at piers and abutments ofproposed structures.(c) Effect of natural geomorphic stream patternchanges on the proposed structure.(d) Consideration of geomorphic changes on nearbystructures in the vicinity of the proposed structure.1.4 CULVERT LOCATION, LENGTH, ANDWATERWAY OPENINGSCulvert location, length, and waterway openingsshould be in accordance with the AASHTO Guide on theHydraulic Design of Culverts in Highway DrainageGuidelines.1.5 ROADWAY DRAINAGEThe transverse drainage of the roadway should be pro-vided by a suitable crown in the roadway surface and lon-gitudinal drainage by camber or gradient. Water owingdowngrade in a gutter section should be intercepted andnot permitted to run onto the bridge. Short, continuousspan bridges, particularly overpasses, may be built with-out inlets and the water from the bridge roadway carrieddownslope by open or closed chutes near the end of thebridge structure. Longitudinal drainage on long bridgesshould be provided by scuppers or inlets which should beof sufficient size and number to drain the gutters ade-quately. Downspouts, where required, should be made ofrigid corrosion-resistant material not less than 4 inches inleast dimension and should be provided with cleanouts.The details of deck drains should be such as to prevent thedischarge of drainage water against any portion of thestructure or on moving traffic below, and to prevent ero-sion at the outlet of the downspout. Deck drains may beconnected to conduits leading to storm water outfalls atground level. Overhanging portions of concrete decksshould be provided with a drip bead or notch.1.6 RAILROAD OVERPASSES1.6.1 ClearancesStructures designed to overpass a railroad shall be inaccordance with standards established and used by the af-fected railroad in its normal practice. These overpassstructures shall comply with applicable Federal, State, andlocal laws.Regulations, codes, and standards should, as a mini-mum, meet the specications and design standards of theAmerican Railway Engineering Association, the Associa-tion of American Railroads, and AASHTO.1.6.2 Blast ProtectionOn bridges over railroads with steam locomotives,metal likely to be damaged by locomotive gases, and allconcrete surfaces less than 20 feet above the tracks, shallbe protected by blast plates. The plates shall be placed to4 HIGHWAY BRIDGES 1.3.1.3Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.take account of the direction of blast when the locomotiveis on level or superelevated tracks by centering them on aline normal to the plane of the two rails at the centerlineof the tracks. The plates shall be not less than 4 feet wideand shall be cast-iron, a corrosion and blast-resisting alloy,or asbestos-board shields, so supported that they may bereadily replaced. The thickness of plates and other parts indirect contact with locomotive blast shall be not less than34 inch for cast iron, 38 inch for alloy, 12 inch for plain as-bestos-board, and 716 inch for corrugated asbestos-board.Bolts shall be not less than 58 inch in diameter. Pocketswhich may hold locomotive gases shall be avoided as faras practical. All fastenings shall be galvanized or made ofcorrosion-resistant material.1.7 SUPERELEVATIONThe superelevation of the oor surface of a bridge ona horizontal curve shall be provided in accordance withthe standard practice of the commission for the highwayconstruction, except that the superelevation shall not ex-ceed 0.10 foot per foot width of roadway.1.8 FLOOR SURFACESAll bridge oors shall have skid-resistant characteris-tics.1.9 UTILITIESWhere required, provisions shall be made for trolleywire supports and poles, lighting pillars, electric conduits,telephone conduits, water pipes, gas pipes, sanitary sew-ers, and other utility appurtenances.1.6.2 DIVISION IDESIGN 5Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.Section 2GENERAL FEATURES OF DESIGN2.1 GENERAL2.1.1 NotationsAf area of anges (Article 2.7.4.3)b ange width (Article 2.7.4.3)C modication factor for concentrated load, P, used inthe design of rail members (Article 2.7.1.3.1)D clear unsupported distance between ange compo-nents (Article 2.7.4.3)d depth of Wor I section (Article 2.7.4.3)Fa allowable axial stress (Article 2.7.4.3)Fb allowable bending stress (Article 2.7.4.2)Fv allowable shear stress (Article 2.7.4.2)Fy minimum yield stress (Article 2.7.4.2)fa axial compression stress (Article 2.7.4.3)h height of top rail above reference surface (Figure2.7.4B)L post spacing (Figure 2.7.4B)P railing design loading 10 kips (Article 2.7.1.3and Figure 2.7.4B)P railing design loading equal to P, P/2 or P/3 (Article2.7.1.3.5)t ange or web thickness (Article 2.7.4.3)w pedestrian or bicycle loading (Articles 2.7.2.2 and2.7.3.2)2.1.2 Width of Roadway and SidewalkThe width of roadway shall be the clear width mea-sured at right angles to the longitudinal center line of thebridge between the bottoms of curbs. If brush curbs orcurbs are not used, the clear width shall be the minimumwidth measured between the nearest faces of the bridgerailing.The width of the sidewalk shall be the clear width,measured at right angles to the longitudinal center line ofthe bridge, from the extreme inside portion of the handrailto the bottom of the curb or guardtimber. If there is a truss,girder, or parapet wall adjacent to the roadway curb, thewidth shall be measured to the extreme walk side of thesemembers.2.2 STANDARD HIGHWAY CLEARANCESGENERAL2.2.1 NavigationalPermits for the construction of crossings over naviga-ble streams must be obtained from the U.S. Coast Guardand other appropriate agencies. Requests for such permitsfrom the U.S. Coast Guard should be addressed to the ap-propriate District Commander. Permit exemptions are al-lowed on nontidal waterways which are not used as ameans to transport interstate or foreign commerce, and arenot susceptible to such use in their natural condition or byreasonable improvement.2.2.2 Roadway WidthFor recommendations on roadway widths for variousvolumes of traffic, see AASHTO A Policy on GeometricDesign of Highways and Streets, or A Policy on DesignStandardsInterstate System.2.2.3 Vertical ClearanceVertical clearance on state trunk highways and inter-state systems in rural areas shall be at least 16 feet overthe entire roadway width with an allowance for resurfac-ing. On state trunk highways and interstate routes throughurban areas, a 16-foot clearance shall be provided exceptin highly developed areas. A16-foot clearance should beprovided in both rural and urban areas where such clear-ance is not unreasonably costly and where needed for de-fense requirements. Vertical clearance on all other high-ways shall be at least 14 feet over the entire roadwaywidth with an allowance for resurfacing.2.2.4 OtherThe channel openings and clearances shall be accept-able to agencies having jurisdiction over such matters.Channel openings and clearances shall conform in width, height, and location to all federal, state, and localrequirements.7Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.2.2.5 Curbs and SidewalksThe face of the curb is dened as the vertical or slop-ing surface on the roadway side of the curb. Horizontalmeasurements of roadway curbs are from the bottom ofthe face, or, in the case of stepped back curbs, from thebottom of the lower face. Maximum width of brush curbs,if used, shall be 9 inches.Where curb and gutter sections are used on the road-way approach, at either or both ends of the bridge, thecurb height on the bridge may equal or exceed the curbheight on the roadway approach. Where no curbs are usedon the roadway approaches, the height of the bridge curbabove the roadway shall be not less than 8 inches, andpreferably not more than 10 inches.Where sidewalks are used for pedestrian traffic onurban expressways, they shall be separated from thebridge roadway by the use of a combination railing asshown in Figure 2.7.4B.In those cases where a New Jersey type parapet or acurb is constructed on a bridge, particularly in urban areasthat have curbs and gutters leading to a bridge, the samewidths between curbs on the approach roadways will bemaintained across the bridge structure. Aparapet or otherrailing installed at or near the curb line shall have its endsproperly ared, sloped, or shielded.2.3 HIGHWAY CLEARANCES FOR BRIDGES2.3.1 WidthThe horizontal clearance shall be the clear width andthe vertical clearance the clear height for the passage ofvehicular traffic as shown in Figure 2.3.1.The roadway width shall generally equal the width ofthe approach roadway section including shoulders. Wherecurbed roadway sections approach a structure, the samesection shall be carried across the structure.2.3.2 Vertical ClearanceThe provisions of Article 2.2.3 shall be used.2.4 HIGHWAY CLEARANCES FOR UNDERPASSESSee Figure 2.4A.2.4.1 WidthThe pier columns or walls for grade separation struc-tures shall generally be located a minimum of 30 feet fromthe edges of the through-traffic lanes. Where the practicallimits of structure costs, type of structure, volume and de-sign speed of through traffic, span arrangement, skew, andterrain make the 30-foot offset impractical, the pier orwall may be placed closer than 30 feet and protected bythe use of guardrail or other barrier devices. The guardrailor other device shall be independently supported with theroadway face at least 2 feet 0 inches from the face of pieror abutment.The face of the guardrail or other device shall be atleast 2 feet 0 inches outside the normal shoulder line.2.4.2 Vertical ClearanceA vertical clearance of not less than 14 feet shall beprovided between curbs, or if curbs are not used, over theentire width that is available for traffic.2.4.3 CurbsCurbs, if used, shall match those of the approach road-way section.2.5 HIGHWAY CLEARANCES FOR TUNNELSSee Figure 2.5.2.5.1 Roadway WidthThe horizontal clearance shall be the clear width andthe vertical clearance the clear height for the passage ofvehicular traffic as shown in Figure 2.5.Unless otherwise provided, the several parts of thestructures shall be constructed to secure the followinglimiting dimensions or clearances for traffic.8 HIGHWAY BRIDGES 2.2.5FIGURE 2.3.1 Clearance Diagram for BridgesCopyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.2.5.1 DIVISION IDESIGN 9FIGURE 2.4A Clearance Diagrams for Underpasses (See Article 2.4 for General Requirements.)*The barrier to face of wall or pier distance should not be less than the dynamic deection of the barrier for impact by a full-sized automobile atimpact conditions of approximately 25 degrees and 60 miles per hour. For information on dynamic deection of various barriers, see AASHTO Road-side Design Guide.FIGURE 2.5 Clearance Diagram for TunnelsTwo-Lane Highway TrafficCopyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.The clearances and width of roadway for two-lane traf-c shall be not less than those shown in Figure 2.5. Theroadway width shall be increased at least 10 feet andpreferably 12 feet for each additional traffic lane.2.5.2 Clearance between WallsThe minimum width between walls of two-lane tunnelsshall be 30 feet.2.5.3 Vertical ClearanceThe vertical clearance between curbs shall be not lessthan 14 feet.2.5.4 CurbsThe width of curbs shall be not less than 18 inches. Theheight of curbs shall be as specied for bridges.For heavy traffic roads, roadway widths greater thanthe above minima are recommended.If traffic lane widths exceed 12 feet the roadway widthmay be reduced 2 feet 0 inches from that calculated fromFigure 2.5.2.6 HIGHWAY CLEARANCES FORDEPRESSED ROADWAYS2.6.1 Roadway WidthThe clear width between curbs shall be not less thanthat specied for tunnels.2.6.2 Clearance between WallsThe minimum width between walls for depressed road-ways carrying two lanes of traffic shall be 30 feet.2.6.3 CurbsThe width of curbs shall be not less than 18 inches. Theheight of curbs shall be as specied for bridges.2.7 RAILINGSRailings shall be provided along the edges of struc-tures for protection of traffic and pedestrians. Other suit-able applications may be warranted on bridge-length cul-verts as addressed in the AASHTO Roadside DesignGuide.Except on urban expressways, a pedestrian walkwaymay be separated from an adjacent roadway by a trafficrailing or barrier with a pedestrian railing along the edgeof the structure. On urban expressways, the separationshall be made by a combination railing.2.7.1 Vehicular Railing2.7.1.1 General2.7.1.1.1 Although the primary purpose of trafficrailing is to contain the average vehicle using the struc-ture, consideration should also be given to (a) protectionof the occupants of a vehicle in collision with the railing,(b) protection of other vehicles near the collision, (c) pro-tection of vehicles or pedestrians on roadways underneaththe structure, and (d) appearance and freedom of viewfrom passing vehicles.2.7.1.1.2 Materials for traffic railings shall be con-crete, metal, timber, or a combination thereof. Metal ma-terials with less than 10-percent tested elongation shallnot be used.2.7.1.1.3 Traffic railings should provide a smooth,continuous face of rail on the traffic side with the posts setback from the face of rail. Structural continuity in the railmembers, including anchorage of ends, is essential. Therailing system shall be able to resist the applied loads atall locations.2.7.1.1.4 Protrusions or depressions at rail jointsshall be acceptable provided their thickness or depth is nogreater than the wall thickness of the rail member or 38inch, whichever is less.2.7.1.1.5 Careful attention shall be given to the treat-ment of railings at the bridge ends. Exposed rail ends,posts, and sharp changes in the geometry of the railingshall be avoided. Asmooth transition by means of a con-tinuation of the bridge barrier, guardrail anchored to thebridge end, or other effective means shall be provided toprotect the traffic from direct collision with the bridge railends.2.7.1.2 Geometry2.7.1.2.1 The heights of rails shall be measured rela-tive to the reference surface which shall be the top of theroadway, the top of the future overlay if resurfacing is an-ticipated, or the top of curb when the curb projection isgreater than 9 inches from the traffic face of the railing.2.7.1.2.2 Traffic railings and traffic portions of combination railings shall not be less than 2 feet 3 inches10 HIGHWAY BRIDGES 2.5.1Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.from the top of the reference surface. Parapets designedwith sloping traffic faces intended to allow vehicles toride up them under low angle contacts shall be at least 2feet 8 inches in height.2.7.1.2.3 The lower element of a traffic or combina-tion railing should consist of either a parapet projecting at least 18 inches above the reference surface or a rail centered between 15 and 20 inches above the referencesurface.2.7.1.2.4 For traffic railings, the maximum clearopening below the bottom rail shall not exceed 17 inchesand the maximum opening between succeeding rails shallnot exceed 15 inches. For combination railings, accom-modating pedestrian or bicycle traffic, the maximumopening between railing members shall be governed byArticles 2.7.2.2.2 and 2.7.3.2.1, respectively.2.7.1.2.5 The traffic faces of all traffic rails must bewithin 1 inch of a vertical plane through the traffic face ofthe rail closest to traffic.2.7.1.3 Loads2.7.1.3.1 When the height of the top of the top trafficrail exceeds 2 feet 9 inches, the total transverse load dis-tributed to the traffic rails and posts shall be increased bythe factor C. However, the maximum load applied to anyone element need not exceed P, the transverse design load.2.7.1.3.2 Rails whose traffic face is more than 1 inchbehind a vertical plane through the face of the traffic railclosest to traffic or centered less than 15 inches above thereference surface shall not be considered to be traffic railsfor the purpose of distributing P or CP, but may be con-sidered in determining the maximum clear vertical open-ing, provided they are designed for a transverse loadingequal to that applied to an adjacent traffic rail or P/2,whichever is less.2.7.1.3.3 Transverse loads on posts, equal to P, or CP,shall be distributed as shown in Figure 2.7.4B. A loadequal to one-half the transverse load on a post shall si-multaneously be applied longitudinally, divided amongnot more than four posts in a continuous rail length. Eachtraffic post shall also be designed to resist an indepen-dently applied inward load equal to one-fourth the out-ward transverse load.2.7.1.3.4 The attachment of each rail required in atraffic or combination railing shall be designed to resist avertical load equal to one-fourth of the transverse designload of the rail. The vertical load shall be applied alter-nately upward or downward. The attachment shall also bedesigned to resist an inward transverse load equal to one-fourth the transverse rail design load.2.7.1.3.5 Rail members shall be designed for a mo-ment, due to concentrated loads, at the center of the paneland at the posts of PL/6 where L is the post spacing andP is equal to P, P/2, or P/3, as modied by the factor Cwhere required. The handrail members of combinationrailings shall be designed for a moment at the center of thepanel and at the posts of 0.1wL2.2.7.1.3.6 The transverse force on concrete parapetand barrier walls shall be spread over a longitudinal lengthof 5 feet.2.7.1.3.7 Railings other than those shown in Figure2.7.4B are permissible provided they meet the require-ments of this Article. Railing congurations that havebeen successfully tested by full-scale impact tests are ex-empt from the provisions of this Article.2.7.2 Bicycle Railing2.7.2.1 General2.7.2.1.1 Bicycle railing shall be used on bridgesspecically designed to carry bicycle traffic, and onbridges where specic protection of bicyclists is deemednecessary.2.7.2.1.2 Railing components shall be designed with consideration to safety, appearance, and when thebridge carries mixed traffic freedom of view from passingvehicles.2.7.2.2 Geometry and Loads2.7.2.2.1 The minimum height of a railing used toprotect a bicyclist shall be 54 inches, measured from thetop of the surface on which the bicycle rides to the top ofthe top rail.2.7.2.2.2 Within a band bordered by the bikewaysurface and a line 27 inches above it, all elements of therailing assembly shall be spaced such that a 6-inch spherewill not pass through any opening. Within a band bor-dered by lines 27 and 54 inches, elements shall be spacedsuch that an 8-inch sphere will not pass through anyopening. If a railing assembly employs both horizontaland vertical elements, the spacing requirements shallapply to one or the other, but not to both. Chain link fence2.7.1.2.2 DIVISION IDESIGN 11Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.is exempt from the rail spacing requirements listedabove. In general, rails should project beyond the face ofposts and/or pickets.2.7.2.2.3 The minimum design loadings for bicyclerailing shall be w 50 pounds per linear foot transverselyand vertically, acting simultaneously on each rail.2.7.2.2.4 Design loads for rails located more than 54inches above the riding surface shall be determined by thedesigner.2.7.2.2.5 Posts shall be designed for a transverse load of wL(where Lis the post spacing) acting at the cen-ter of gravity of the upper rail, but at a height not greaterthan 54 inches.2.7.2.2.6 Refer to Figures 2.7.4A and 2.7.4B formore information concerning the application of loads.2.7.3 Pedestrian Railing2.7.3.1 General2.7.3.1.1 Railing components shall be proportionedcommensurate with the type and volume of anticipated12 HIGHWAY BRIDGES 2.7.2.2.2FIGURE 2.7.4A Pedestrian Railing, Bicycle RailingCopyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.pedestrian traffic. Consideration should be given to ap-pearance, safety and freedom of view from passing vehi-cles.2.7.3.1.2 Materials for pedestrian railing may beconcrete, metal, timber, or a combination thereof.2.7.3.2 Geometry and Loads2.7.3.2.1 The minimum height of a pedestrian railingshall be 42 inches measured from the top of the walkwayto the top of the upper rail member. Within a band bor-dered by the walkway surface and a line 27 inches aboveit, all elements of the railing assembly shall be spacedsuch that a 6-inch sphere will not pass through any open-ing. For elements between 27 and 42 inches above thewalking surface, elements shall be spaced such that aneight-inch sphere will not pass through any opening.2.7.3.2.2 The minimum design loading for pedestrianrailing shall be w 50 pounds per linear foot, transverselyand vertically, acting simultaneously on each longitudinalmember. Rail members located more than 5 feet 0 inchesabove the walkway are excluded from these requirements.2.7.3.2.3 Posts shall be designed for a transverse loadof wL (where L is the post spacing) acting at the center ofgravity of the upper rail or, for high rails, at 5 feet 0 inchesmaximum above the walkway.2.7.3.2.4 Refer to Figures 2.7.4A and 2.7.4B formore information concerning the application of loads.2.7.4 Structural Specications and Guidelines2.7.4.1 Railings shall be designed by the elastic meth-od to the allowable stresses for the appropriate material.2.7.3.1.1 DIVISION IDESIGN 13FIGURE 2.7.4B Traffic RailingTRAFFIC RAILINGCopyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.For aluminum alloys the design stresses given in theSpecications for Aluminum Structures Fifth Edition, De-cember 1986, for Bridge and Similar Type Structures pub-lished by the Aluminum Association, Inc. for alloys 6061-T6 (Table A.6), 6351-T5 (Table A.6) and 6063-T6 (TableA.6) shall apply, and for cast aluminum alloys the designstresses given for alloys A444.0-T4 (Table A.9), A356.0-T61 (Table A.9) and A356.0-T6 (Table A.9) shall apply.For fabrication and welding of aluminum railing, seeArticle 11.5.2.7.4.2 The allowable unit stresses for steel shall beas given in Article 10.32, except as modied below.For steels not generally covered by these Specica-tions, but having a guaranteed yield strength, Fy, the al-lowable unit stress, shall be derived by applying the gen-eral formulas as given in these Specications under UnitStresses except as indicated below.The allowable unit stress for shear shall be Fv 0.33Fy.Round or oval steel tubes may be proportioned usingan allowable bending stress, Fb 0.66Fy, provided the R/tratio (radius/thickness) is less than or equal to 40.Square and rectangular steel tubes and steel W and I sections in bending with tension and compression on extreme bers of laterally supported compact sec-tions having an axis of symmetry in the plane of loading may be designed for an allowable stress Fb 0.60Fy.2.7.4.3 The requirements for a compact section areas follows:(a) The width to thickness ratio of projecting elementsof the compression ange of Wand I sections shall notexceed(b) The width to thickness ratio of the compressionange of square or rectangular tubes shall not exceedbt Fy60002 2 ( ) -bt Fy16002 1 ( ) -14 HIGHWAY BRIDGES 2.7.4.1FIGURE 2.7.4B (Continued)Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.(c) The D/t ratio of webs shall not exceed(d) If subject to combined axial force and bending, theD/t ratio of webs shall not exceedbut need not be less than(e) the distance between lateral supports in inches ofWor I sections shall not exceedor20 000 0002, ,( )AdFfy- 72 4002 6,( )bFy-Dt Fy1 60010 50 320301050,( ) (( )( ) for s inches - 30)M for s inches (3- 31)yR Ps for s inchesRPss for inchesyy >6 1 000 50 3220 50 3 29/ , () ( )- 28)or, ( s -3.25.1.4 DIVISION IDESIGN 39*This shear transfer may be accomplished using mechanical fasteners,splines, or dowels along the panel joint or spreader beams located at in-tervals along the panels or other suitable means.Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.3.25.3.2 ShearWhen calculating the end shears and end reactions foreach panel, no longitudinal distribution of the wheel loads shall be assumed. The lateral distribution of thewheel load at the supports shall be that determined by theequation:Wheel Load Fraction per PanelFor wheel loads in other positions on the span, the lateraldistribution for shear shall be determined by the methodprescribed for moment.3.25.3.3 DeectionsThe maximum deection may be calculated by apply-ing to the panel the wheel load fraction determined by themethod prescribed for moment.3.25.3.4 Stiffener ArrangementThe transverse stiffeners shall be adequately attachedto each panel, at points near the panel edges, with eithersteel plates, thru-bolts, C-clips or aluminum brackets. Thestiffener spacing required will depend upon the spacingneeded in order to prevent differential panel movement;however, a stiffener shall be placed at mid-span with ad-ditional stiffeners placed at intervals not to exceed 10 feet.The stiffness factor EI of the stiffener shall not be less than80,000 kip-in2.3.25.4 Continuous FlooringIf the ooring is continuous over more than two spans,the maximum bending moment shall be assumed as being80% of that obtained for a simple span.3.26 DISTRIBUTION OF WHEEL LOADS ANDDESIGN OF COMPOSITE WOOD-CONCRETE MEMBERS3.26.1 Distribution of Concentrated Loads forBending Moment and Shear3.26.1.1 For freely supported or continuous slabspans of composite wood-concrete construction, as de-scribed in Article 16.3.14, Division II, the wheel loadsshall be distributed over a transverse width of 5 feet forbending moment and a width of 4 feet for shear.3.26.1.2 For composite T-beams of wood and con-crete, as described in Article 16.3.14, Division II, the ef-fective ange width shall not exceed that given in Article10.38.3. Shear connectors shall be capable of resistingboth vertical and horizontal movement.3.26.2 Distribution of Bending Moments inContinuous Spans3.26.2.1 Both positive and negative moments shallbe distributed in accordance with the following table:3.26.2.2 Impact should be considered in computingstresses for concrete and steel, but neglected for wood.3.26.3 DesignThe analysis and design of composite wood-concretemembers shall be based on assumptions that account forthe different mechanical properties of the components. Asuitable procedure may be based on the elastic propertiesof the materials as follows:1 for slab in which the net concrete thickness isless than half the overall depth of the compos-ite section2 for slab in which the net concrete thickness isat least half the overall depth of the compositesection18.75 (for Douglas r and Southern pine)in which,Ec modulus of elasticity of concrete;Ew modulus of elasticity of wood;Es modulus of elasticity of steel.EsEwEcEwEcEwWbut not less thanp4 001..40 HIGHWAY BRIDGES 3.25.3.2Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.3.27 DISTRIBUTION OF WHEEL LOADS ONSTEEL GRID FLOORS*3.27.1 General3.27.1.1 The grid oor shall be designed as continu-ous, but simple span moments may be used and reducedas provided in Article 3.24.3.27.1.2 The following rules for distribution of loadsassume that the grid oor is composed of main elementsthat span between girders, stringers, or cross beams, andsecondary elements that are capable of transferring loadbetween the main elements.3.27.1.3 Reinforcement for secondary elements shallconsist of bars or shapes welded to the main steel.3.27.2 Floors Filled with Concrete3.27.2.1 The distribution and bending moment shallbe as specied for concrete slabs, Article 3.24. The fol-lowing items specied in that article shall also apply toconcrete lled steel grid oors:Longitudinal edge beamsUnsupported transverse edgesSpan lengths3.27.2.2 The strength of the composite steel and con-crete slab shall be determined by means of the trans-formed area method. The allowable stresses shall be asset forth in Articles 8.15.2, 8.16.1, and 10.32.3.27.3 Open Floors3.27.3.1 Awheel load shall be distributed, normal tothe main elements, over a width equal to 11 4 inches per ton of axle load plus twice the distance center to center ofmain elements. The portion of the load assigned to eachmain element shall be applied uniformly over a lengthequal to the rear tire width (20 inches for H 20, 15 inchesfor H 15).3.27.3.2 The strength of the section shall be deter-mined by the moment of inertia method. The allowablestresses shall be as set forth in Article 10.32.3.27.3.3 Edges of open grid steel oors shall be sup-ported by suitable means as required. These supports maybe longitudinal or transverse, or both, as may be requiredto support all edges properly.3.27.3.4 When investigating for fatigue, the mini-mum cycles of maximum stress shall be used.3.28 DISTRIBUTION OF LOADS FOR BENDINGMOMENT IN SPREAD BOX GIRDERS**3.28.1 Interior BeamsThe live load bending moment for each interior beam ina spread box beam superstructure shall be determined byapplying to the beam the fraction (D.F.) of the wheel load(both front and rear) determined by the following equation:where,NL number of design traffic lanes (Article 3.6);NB number of beams (4 NB 10);S beam spacing in feet (6.57 S 11.00);L span length in feet;k 0.07 WNL(0.10NL 0.26) 0.20NB 0.12;(3-34)W numeric value of the roadway width betweencurbs expressed in feet (32 W66).3.28.2 Exterior BeamsThe live load bending moment in the exterior beamsshall be determined by applying to the beams the reactionof the wheel loads obtained by assuming the ooring toact as a simple span (of length S) between beams, but shallnot be less than 2NL/NB.3.29 MOMENTS, SHEARS, AND REACTIONSMaximum moments, shears, and reactions are givenin tables, Appendix A, for H 15, H 20, HS 15, and HS 20loadings. They are calculated for the standard truck orthe lane loading applied to a single lane on freely sup-ported spans. It is indicated in the table whether thestandard truck or the lane loadings produces the maxi-mum stress.D FNNkSLLB. . ( +23- 33)3.27 DIVISION IDESIGN 41*Provisions in this article shall not apply to orthotropic bridge super-structures.**The provisions of Article 3.12, Reduction in Load Intensity, werenot applied in the development of the provisions presented in Articles3.28.1 and 3.28.2.Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.3.30 TIRE CONTACT AREAThe tire contact area for the Alternate Military Load-ing or HS 20-44 shall be assumed as a rectangle with alength in the direction of traffic of 10 inches, and a widthof tire of 20 inches. For other design vehicles, the tire con-tact should be determined by the engineer.42 HIGHWAY BRIDGES 3.30Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.Section 4FOUNDATIONSPart AGENERAL REQUIREMENTS AND MATERIALS4.1 GENERALFoundations shall be designed to support all live anddead loads, and earth and water pressure loadings in ac-cordance with the general principles specied in this sec-tion. The design shall be made either with reference to ser-vice loads and allowable stresses as provided in SERVICELOAD DESIGN or, alternatively, with reference to loadfactors, and factored strength as provided in STRENGTHDESIGN.4.2 FOUNDATION TYPE AND CAPACITY4.2.1 Selection of Foundation TypeSelection of foundation type shall be based on an assessment of the magnitude and direction of loading,depth to suitable bearing materials, evidence of previousooding, potential for liquefaction, undermining or scour, swelling potential, frost depth and ease and cost ofconstruction.4.2.2 Foundation CapacityFoundations shall be designed to provide adequatestructural capacity, adequate foundation bearing capacitywith acceptable settlements, and acceptable overall sta-bility of slopes adjacent to the foundations. The tolerablelevel of structural deformation is controlled by the typeand span of the superstructure.4.2.2.1 Bearing CapacityThe bearing capacity of foundations may be estimatedusing procedures described in Articles 4.4, 4.5, or 4.6 forservice load design and Articles 4.11, 4.12, or 4.13 forstrength design, or other generally accepted theories. Suchtheories are based on soil and rock parameters measuredby in situ and/or laboratory tests. The bearing capacitymay also be determined using load tests.4.2.2.2 SettlementThe settlement of foundations may be determinedusing procedures described in Articles 4.4, 4.5, or 4.6 forservice load design and Articles 4.11, 4.12, or 4.13 forstrength design, or other generally accepted methodolo-gies. Such methods are based on soil and rock parametersmeasured directly or inferred from the results of in situand/or laboratory tests.4.2.2.3 Overall StabilityThe overall stability of slopes in the vicinity of foundations shall be considered as part of the design offoundations.4.2.3 Soil, Rock, and Other Problem ConditionsGeologic and environmental conditions can inuencethe performance of foundations and may require specialconsideration during design. To the extent possible, thepresence and inuence of such conditions shall be evalu-ated as part of the subsurface exploration program. Arep-resentative, but not exclusive, listing of problem condi-tions requiring special consideration is presented in Table4.2.3Afor general guidance.4.3 SUBSURFACE EXPLORATION ANDTESTING PROGRAMSThe elements of the subsurface exploration and testingprograms shall be the responsibility of the designer basedon the specic requirements of the project and his or herexperience with local geologic conditions.4.3.1 General RequirementsAs a minimum, the subsurface exploration and testingprograms shall dene the following, where applicable: Soil strataDepth, thickness, and variability43Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.Identication and classicationRelevant engineering properties (i.e., shearstrength, compressibility, stiffness, permeability,expansion or collapse potential, and frost suscep-tibility) Rock strataDepth to rockIdentication and classicationQuality (i.e., soundness, hardness, jointing andpresence of joint lling, resistance to weathering,if exposed, and solutioning)Compressive strength (e.g., uniaxial compres-sion, point load index)Expansion potential Ground water elevation Ground surface elevation Local conditions requiring special considerationExploration logs shall include soil and rock strata de-scriptions, penetration resistance for soils (e.g., SPT orqc), and sample recovery and RQD for rock strata. Thedrilling equipment and method, use of drilling mud, typeof SPThammer (i.e. safety, donut, hydraulic) or cone pen-etrometer (i.e., mechanical or electrical), and any unusualsubsurface conditions such as artesian pressures, bouldersor other obstructions, or voids shall also be noted on theexploration logs.4.3.2 Minimum DepthWhere substructure units will be supported on spreadfootings, the minimum depth of the subsurface explo-ration shall extend below the anticipated bearing level aminimum of two footing widths for isolated, individualfootings where L 2B, and four footing widths for foot-ings where L 5B. For intermediate footing lengths, theminimum depth of exploration may be estimated by lin-ear interpolation as a function of L between depths of 2Band 5B below the bearing level. Greater depths may be re-quired where warranted by local conditions.44 HIGHWAY BRIDGES 4.3.1TABLE 4.2.3A Problem Conditions Requiring Special ConsiderationProblemType Description CommentsOrganic soil; highly plastic clay Low strength and high compressibilitySensitive clay Potentially large strength loss upon large strainingMicaceous soil Potentially high compressibility (often saprolitic)Soil Expansive clay/silt; expansive slag Potentially large expansion upon wettingLiqueable soil Complete strength loss and high deformations due to earthquake loadingCollapsible soil Potentially large deformations upon wetting (Caliche; Loess)Pyritic soil Potentially large expansion upon oxidationLaminated rock Low strength when loaded parallel to beddingExpansive shale Potentially large expansion upon wetting; degrades readily upon exposure to air/waterPyritic shale Expands upon exposure to air/waterRock Soluble rock Soluble in owing and standing water (Limestone, Limerock,Gypsum)Cretaceous shale Indicator of potentially corrosive ground waterWeak claystone (Red Beds) Low strength and readily degradable upon exposure to air/waterGneissic and Schistose Rock Highly distorted with irregular weathering proles and steep discontinuitiesSubsidence Typical in areas of underground mining or high ground water extractionSinkholes/solutioning Karst topography; typical of areas underlain by carbonate rock strataCondition Negative skin friction/ Additional compressive/uplift load on deep foundations due toexpansion loading settlement/uplift of soilCorrosive environments Acid mine drainage; degradation of certain soil/rock typesPermafrost/frost Typical in northern climatesCapillary water Rise of water level in silts and ne sands leading to strength lossCopyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.Where substructure units will be supported on deepfoundations, the depth of the subsurface exploration shallextend a minimum of 20 feet below the anticipated pile orshaft tip elevation. Where pile or shaft groups will beused, the subsurface exploration shall extend at least twotimes the maximum pile group dimension below the an-ticipated tip elevation, unless the foundations will be endbearing on or in rock. For piles bearing on rock, a mini-mum of 10 feet of rock core shall be obtained at each ex-ploration location to insure the exploration has not beenterminated on a boulder. For shafts supported on or ex-tending into rock, a minimum of 10 feet of rock core, or alength of rock core equal to at least three times the shaftdiameter for isolated shafts or two times the maximumshaft group dimension for a shaft group, whichever isgreater, shall be obtained to insure the exploration has notterminated in a boulder and to determine the physicalcharacteristics of rock within the zone of foundation in-uence for design.4.3.3 Minimum CoverageA minimum of one soil boring shall be made for eachsubstructure unit. (See Article 7.1.1 for denition of sub-structure unit.) For substructure units over 100 feet inwidth, a minimum of two borings shall be required.4.3.4 Laboratory TestingLaboratory testing shall be performed as necessary todetermine engineering properties including unit weight,shear strength, compressive strength and compressibility.In the absence of laboratory testing, engineering proper-ties may be estimated based on published test results orlocal experience.4.3.5 ScourThe probable depth of scour shall be determined bysubsurface exploration and hydraulic studies. Refer to Article 1.3.2 and FHWA (1988) for general guidance regarding hydraulic studies and design.Part BSERVICE LOAD DESIGN METHODALLOWABLE STRESS DESIGN4.4 SPREAD FOOTINGS4.4.1 General4.4.1.1 ApplicabilityProvisions of this Article shall apply for design of iso-lated footings, and to combined footings and mats (foot-ings supporting more than one column, pier, or wall).4.4.1.2 Footings Supporting Non-RectangularColumns or PiersFootings supporting circular or regular polygon-shaped concrete columns or piers may be designed as-suming that the columns or piers act as square memberswith the same area for location of critical sections for mo-ment, shear, and development of reinforcement.4.4.1.3 Footings in FillFootings located in ll are subject to the same bearingcapacity, settlement, and dynamic ground stability con-siderations as footings in natural soil in accordance withArticles 4.4.7.1 through 4.4.7.3. The behavior of both thell and underlying natural soil shall be considered.4.4.1.4 Footings in Sloped Portions ofEmbankmentsThe earth pressure against the back of footings andcolumns within the sloped portion of an embankmentshall be equal to the at-rest earth pressure in accordancewith Article 5.5.2. The resistance due to the passive earthpressure of the embankment in front of the footing shallbe neglected to a depth equal to a minimum depth of 3 feet, the depth of anticipated scour, freeze thaw action,and/or trench excavation in front of the footing,whichever is greater.4.4.1.5 Distribution of Bearing PressureFootings shall be designed to keep the maximum soiland rock pressures within safe bearing values. To preventunequal settlement, footings shall be designed to keep thebearing pressure as nearly uniform as practical. For foot-ings supported on piles or drilled shafts, the spacing be-tween piles and drilled shafts shall be designed to ensurenearly equal loads on deep foundation elements as may bepractical.When footings support more than one column, pier, orwall, distribution of soil pressure shall be consistent withproperties of the foundation materials and the structure,and with the principles of geotechnical engineering.4.4.2 NotationsThe following notations shall apply for the design ofspread footings on soil and rock:A Contact area of footing (ft2)A Effective footing area for computation ofbearing capacity of a footing subjected toeccentric load (ft2); (See Article 4.4.7.1.1.1)4.3.2 DIVISION IDESIGN 45Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.bc, b, bq Base inclination factors (dim); (See Article4.4.7.1.1.8)B Width of footing (ft); (Minimum plan di-mension of footing unless otherwise noted)B Effective width for load eccentric in direc-tion of short side, L unchanged (ft)c Soil cohesion (ksf)c Effective stress soil cohesion (ksf)c* Reduced effective stress soil cohesion forpunching shear (ksf); (See Article 4.4.7.1)ca Adhesion between footing and foundationsoil or rock (ksf); (See Article 4.4.7.1.1.3)cv Coefficient of consolidation (ft2/yr); (SeeArticle 4.4.7.2.3)c1 Shear strength of upper cohesive soil layer below footing (ksf); (See Article4.4.7.1.1.7)c2 Shear strength of lower cohesive soil layer below footing (ksf); (See Article4.4.7.1.1.7)Cc Compression index (dim); (See Article4.4.7.2.3)Ccr Recompression index (dim); (See Article4.4.7.2.3)Cc Compression ratio (dim); (See Article4.4.7.2.3)Co Uniaxial compressive strength of intactrock (ksf)Cr Recompression ratio (dim); (See Article4.4.7.2.3)C Coefficient of secondary compression de-ned as change in height per log cycle oftime (dim); (See Article 4.4.7.2.4)D Inuence depth for water below footing(ft); (See Article 4.4.7.1.1.6)Df Depth to base of footing (ft)e Void ratio (dim); (See Article 4.4.7.2.3)ef Void ratio at nal vertical effective stress(dim); (See Article 4.4.7.2.3)eo Void ratio at initial vertical effective stress(dim); (See Article 4.4.7.2.3)ep Void ratio at maximum past vertical effec-tive stress (dim); (See Article 4.4.7.2.3)eB Eccentricity of load in the B direction mea-sured from centroid of footing (ft); (See Ar-ticle 4.4.7.1.1.1)eL Eccentricity of load in the L direction mea-sured from centroid of footing (ft); (See Article 4.4.7.1.1.1)Eo Modulus of intact rock (ksf)Em Rock mass modulus (ksf); (See Article4.4.8.2.2)Es Soil modulus (ksf)F Total force on footing subjected to an in-clined load (k); (See Article 4.4.7.1.1.1)fc Unconned compressive strength of con-crete (ksf)FS Factor of safety against bearing capacity,overturning or sliding shear failure (dim)H Depth from footing base to top of secondcohesive soil layer for two-layer cohesivesoil prole below footing (ft); (See Article4.4.7.1.1.7)Hc Height of compressible soil layer (ft)Hcrit Critical thickness of the upper layer of atwo-layer system beyond which the under-lying layer will have little effect on the bear-ing capacity of footings bearing in the upperlayer (ft); (See Article 4.4.7.1.1.7)Hd Height of longest drainage path in com-pressible soil layer (ft)Hs Height of slope (ft); (See Article 4.4.7.1.1.4)i Slope angle from horizontal of ground sur-face below footing (deg)ic, i, iq Load inclination factors (dim); (See Article4.4.7.1.1.3)I Inuence coefficient to account for rigidityand dimensions of footing (dim); (See Arti-cle 4.4.8.2.2) Center-to-center spacing between adjacentfootings (ft)L Length of footing (ft)L Effective footing length for load eccentricin direction of long side, B unchanged (ft)L1 Length (or width) of footing having positivecontact pressure (compression) for footingloaded eccentrically about one axis (ft)n Exponential factor relating B/L or L/B ra-tios for inclined loading (dim); (See Article4.4.7.1.1.3)N Standard penetration resistance (blows/ft)N1 Standard penetration resistance correctedfor effects of overburden pressure (blows/ft); (See Article 4.4.7.2.2)Nc, N, Nq Bearing capacity factors based on the valueof internal friction of the foundation soil(dim); (See Article 4.4.7.1)Nm Modied bearing capacity factor to accountfor layered cohesive soils below footing(dim); (See Article 4.4.7.1.1.7)Nms Coefficient factor to estimate qultfor rock(dim); (See Article 4.4.8.1.2)Ns Stability number (dim); (See Article4.4.7.1.1.4)46 HIGHWAY BRIDGES 4.4.2Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.Ncq, Nq Modied bearing capacity factors for ef-fects of footing on or adjacent slopingground (dim); (See Article 4.4.7.1.1.4)P Tangential component of force on footing(k)Pmax Maximum resisting force between footingbase and foundation soil or rock for slidingfailure (k)q Effective overburden pressure at base offooting (ksf)Q Normal component of force on footing (k)qaii Allowable uniform bearing pressure or con-tact stress (ksf)qc Cone penetration resistance (ksf)qmax Maximum footing contact pressure (ksf)Qmax Maximum normal component of load sup-ported by foundation soil or rock at ultimatebearing capacity (k)qmin Minimum magnitude of footing contactpressure (ksf)qo Vertical stress at base of loaded area (ksf);(See Article 4.4.7.2.1)qult Ultimate bearing capacity for uniform bear-ing pressure (ksf)q1 Ultimate bearing capacity of footing sup-ported in the upper layer of a two-layer sys-tem assuming the upper layer is innitelythick (ksf); (See Article 4.4.7.1.1.7)q2 Ultimate bearing capacity of a ctitiousfooting of the same size and shape as the ac-tual footing, but supported on surface of thesecond (lower) layer of a two-layer system(ksf); (See Article 4.4.7.1.1.7)R Resultant of pressure on base of footing (k)r Radius of circular footing or B/2 for squarefooting (ft); (See Article 4.4.8.2.2)RQD Rock Quality Designation (dim)sc, s, sq Footing shape factors (dim); (See Article4.4.7.1.1.2)su Undrained shear strength of soil (ksf)Sc Consolidation settlement (ft); (See Article4.4.7.2.3)Se Elastic or immediate settlement (ft); (SeeArticle 4.4.7.2.2)Ss Secondary settlement (ft); (See Article4.4.7.2.4)St Total settlement (ft); (See Article 4.4.7.2)t Time to reach specified average degree of consolidation (yr); (See Article 4.4.7.2.3)t1, t2 Arbitrary time intervals for determinationof Ss(yr); (See Article 4.4.7.2.4)T Time factor (dim); (See Article 4.4.7.2.3)zw Depth from footing base down to the high-est anticipated ground water level (ft); (SeeArticle 4.4.7.1.1.6) Angle of inclination of the footing basefrom the horizontal (radian) Reduction factor (dim); (See Article4.4.8.2.2) Length to width ratio of footing (dim)m Punching index BL/[2(B L)H] (dim);(See Article 4.4.7.1.1.7)z Factor to account for footing shape andrigidity (dim); (See Article 4.4.7.2.2) Total unit weight of soil or rock (kcf) Buoyant unit weight of soil or rock (kcf)m Moist unit weight of soil (kcf) Angle of friction between footing and foun-dation soil or rock (deg); (See Article4.4.7.1.1.3) Differential settlement between adjacentfootings (ft); (See Article 4.4.7.2.5)v Vertical strain (dim); (See Article 4.4.7.2.3)vf Vertical strain at nal vertical effectivestress (dim); (See Article 4.4.7.2.3)vo Initial vertical strain (dim); (See Article4.4.7.2.3)vp Vertical strain at maximum past verticaleffective stress (dim); (See Article4.4.7.2.3) Angle of load eccentricity (deg) Shear strength ratio (c2/c1) for two layeredcohesive soil system below footing (dim);(See Article 4.4.7.1.1.7)c Reduction factor to account for three-di-mensional effects in settlement analysis(dim); (See Article 4.4.7.2.3) Poissons ratio (dim)f Final vertical effective stress in soil at depthinterval below footing (ksf); (See Article4.4.7.2.3)o Initial vertical effective stress in soil atdepth interval below footing (ksf); (See Ar-ticle 4.4.7.2.3)p Maximum past vertical effective stress insoil at depth interval below footing (ksf);(See Article 4.4.7.2.3) Angle of internal friction (deg) Effective stress angle of internal friction(deg)* Reduced effective stress soil friction anglefor punching shear (ksf); (See Article4.4.7.1)4.4.2 DIVISION IDESIGN 47Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.The notations for dimension units include the follow-ing: dim Dimensionless; deg degree; ft foot; k kip; k/ft kip/ft; ksf kip/ft2; kcf kip/ft3; lb pound;in. inch; and psi pound per square inch. The dimen-sional units provided with each notation are presented forillustration only to demonstrate a dimensionally correctcombination of units for the footing capacity procedurespresented herein. If other units are used, the dimensionalcorrectness of the equations shall be conrmed.4.4.3 Design TerminologyRefer to Figure 4.4.3Afor terminology used in the de-sign of spread footing foundations.4.4.4 Soil and Rock Property SelectionSoil and rock properties dening the strength and com-pressibility characteristics of the foundation materials arerequired for footing design. Foundation stability and set-tlement analyses for design shall be conducted using soiland rock properties based on the results of eld and/orlaboratory testing.4.4.5 Depth4.4.5.1 Minimum Embedment and Bench WidthFootings not otherwise founded on sound, non-de-gradeable rock surfaces shall be embedded a sufficient48 HIGHWAY BRIDGES 4.4.2FIGURE 4.4.3A Design Terminology for Spread Footing FoundationsCopyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.depth to provide adequate bearing, scour and frost heaveprotection, or 2 feet to the bottom of footing, whichever isgreatest. For footings constructed on slopes, a minimumhorizontal distance of 4 feet, measured at the top of foot-ing, shall be provided between the near face of the foot-ing and the face of the nished slope.4.4.5.2 Scour ProtectionFootings supported on soil or degradable rock stratashall be embedded below the maximum computed scourdepth or protected with a scour countermeasure. Footingssupported on massive, competent rock formations whichare highly resistant to scour shall be placed directly on thecleaned rock surface. Where required, additional lateralresistance should be provided by drilling and groutingsteel dowels into the rock surface rather than blasting toembed the footing below the rock surface.Footings on piles may be located above the lowest an-ticipated scour level provided the piles are designed forthis condition. Assume that only one-half of the maximumanticipated scour has occurred when designing for earth-quake loading. Where footings on piles are subject todamage by boulders or debris during ood scour, ade-quate protection shall be provided. Footings shall be con-structed so as to neither pose an obstacle to water trafficnor be exposed to view during low ow.4.4.5.3 Footing ExcavationsFooting excavations below the ground water table, par-ticularly in granular soils having relatively high perme-ability, shall be made such that the hydraulic gradient inthe excavation bottom is not increased to a magnitude thatwould cause the foundation soils to loosen or soften dueto the upward ow of water. Further, footing excavationsshall be made such that hydraulic gradients and materialremoval do not adversely affect adjacent structures. Seep-age forces and gradients may be evaluated by ow net procedures or other appropriate methods. Dewatering orcutoff methods to control seepage shall be used wherenecessary.Footing excavations in nonresistant, easily weatheredmoisture sensitive rocks shall be protected from weather-ing immediately after excavation with a lean mix concreteor other approved materials.4.4.5.4 PipingPiping failures of ne materials through rip-rap orthrough drainage backlls behind abutments shall be pre-vented by properly designed, graded soil lters or geotex-tile drainage systems.4.4.6 AnchorageFootings founded on inclined, smooth rock surfacesand which are not restrained by an overburden of resistantmaterial shall be effectively anchored by means of rockanchors, rock bolts, dowels, keys, benching or other suit-able means. Shallow keying or benching of large footingareas shall be avoided where blasting is required for rockremoval.4.4.7 Geotechnical Design on SoilSpread footings on soil shall be designed to support thedesign loads with adequate bearing and structural capac-ity, and with tolerable settlements in conformance withArticles 4.4.7 and 4.4.11. In addition, the capacity of footings subjected to seismic and dynamic loads, shall be evaluated in conformance with Articles 4.4.7.3 and4.4.10.The location of the resultant of pressure (R) on the baseof the footings shall be maintained within B/6 of the cen-ter of the footing.4.4.7.1 Bearing CapacityThe ultimate bearing capacity (for general shear fail-ure) may be estimated using the following relationship forcontinuous footings (i.e., L 5B):qult cNc 0.5BN qNq(4.4.7.1-1)The allowable bearing capacity shall be determined as:qall qult/FS (4.4.7.1-2)Refer to Table 4.4.7.1Afor values of Nc, N, and Nq.If local or punching shear failure is possible, the valueof qultmay be estimated using reduced shear strength pa-rameters c* and * in Equation (4.4.7.1-1) as follows:c* 0.67c (4.4.7.1-3)* tan1(0.67tan ) (4.4.7.1-4)Effective stress methods of analysis and drained shearstrength parameters shall be used to determine bearing capacity factors for drained loading conditions in all soils.Additionally, the bearing capacity of cohesive soils shall4.4.5.1 DIVISION IDESIGN 49Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.be checked for undrained loading conditions using bear-ing capacity factors based on undrained shear strength parameters.4.4.7.1.1 Factors Affecting Bearing CapacityAmodied form of the general bearing capacity equa-tion may be used to account for the effects of footingshape, ground surface slope, base inclination, and inclinedloading as follows:qult cNcscbcic 0.5BNsbi qNqsqbqiq(4.4.7.1.1-1)Reduced footing dimensions shall be used to accountfor the effects of eccentric loading.4.4.7.1.1.1 Eccentric LoadingFor loads eccentric relative to the centroid of the foot-ing, reduced footing dimensions (B and L) shall be usedto determine bearing capacity factors and modiers (i.e.,slope, footing shape, and load inclination factors), and tocalculate the ultimate load capacity of the footing. The re-duced footing dimensions shall be determined as follows:B B 2eB(4.4.7.1.1.1-1)L L 2eL(4.4.7.1.1.1-2)The effective footing area shall be determined as follows:A BL (4.4.7.1.1.1-3)Refer to Figure 4.4.7.1.1.1Afor loading denitions andfooting dimensions.The value of qultobtained using the reduced footing di-mensions represents an equivalent uniform bearing pres-sure and not the actual contact pressure distribution be-neath the footing. This equivalent pressure may bemultiplied by the reduced area to determine the ultimateload capacity of the footing from the standpoint of bear-ing capacity. The actual contact pressure distribution (i.e.,trapezoidal for the conventional assumption of a rigid50 HIGHWAY BRIDGES 4.4.7.1TABLE 4.4.7.1A Bearing Capacity FactorsCopyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.footing and a positive pressure along each footing edge)shall be used for structural design of the footing.The actual distribution of contact pressure for a rigidfooting with eccentric loading about one axis is shown in Figure 4.4.7.1.1.1B. For an eccentricity (eL) in the Ldirection, the actual maximum and minimum contactpressures may be determined as follows:for eL L/6:qmax Q[1 (6eL/L)]/BL (4.4.7.1.1.1-4)qmin Q[1 (6eL/L)]/BL (4.4.7.1.1.1-5)for L/6 eL L/2:qmax 2Q/(3B[L/2) eL]) (4.4.7.1.1.1-6)qmin 0 (4.4.7.1.1.1-7)L1 3[(L/2) eL] (4.4.7.1.1.1-8)For an eccentricity (e) in the B direction, the maxi-mum and minimum contact pressures may be determinedusing Equations 4.4.7.1.1.1-4 through 4.4.7.1.1.1-8 by re-placing terms labeled L by B, and terms labeled B by L.Footings on soil shall be designed so that the eccen-tricity of loading is less than 16 of the footing dimensionin any direction.4.4.7.1.1.2 Footing ShapeFor footing shapes other than continuous footings (i.e.,L 5B), the following shape factors shall be applied toEquation 4.4.7.1.1-1:sc 1 (B/L) (Nq/Nc) (4.4.7.1.1.2-1)sq 1 (B/L) tan (4.4.7.1.1.2-2)s 1 0.4 (B/L) (4.4.7.1.1.2-3)For circular footings, B equals L. For cases in whichthe loading is eccentric, the terms L and B shall be re-placed by L and B, respectively, in the above equations.4.4.7.1.1.3 Inclined LoadingFor inclined loads, the following inclination factorsshall be applied in Equation 4.4.7.1.1-1:ic iq [(1 iq)/Nctan ] (for 0)(4.4.7.1.1.3-1)ic 1 (nP/BLcNc) (for 0) (4.4.7.1.1.3-2)iq [1 P/(Q BLc cot)]n(4.4.7.1.1.3-3)i [1 P/(Q BLc cot)](n 1)(4.4.7.1.1.3-4)n [(2 L/B)/(1 L/B)]cos2[(2 B/L)/(1 B/L)]sin2 (4.4.7.1.1.3-5)Refer to Figure 4.4.7.1.1.1Afor loading denitions andfooting dimensions. For cases in which the loading is ec-centric, the terms L and B shall be replaced by L and B,respectively, in the above equations.Failure by sliding shall be considered by comparingthe tangential component of force on the footing (P) to themaximum resisting force (Pmax) by the following:Pmax Qtan BLca(4.4.7.1.1.3-6)FS Pmax/P 1.5 (4.4.7.1.1.3-7)In determining Pmax, the effect of passive resistanceprovided by footing embedment shall be ignored, and BLshall represent the actual footing area in compression asshown in Figure 4.4.7.1.1.1B or Figure 4.4.7.1.1.1C.4.4.7.1.1.4 Ground Surface SlopeFor footings located on slopes or within 3B of a slopecrest, qultmay be determined using the following revisedversion of Equation 4.4.7.1.1-1:qult cNcqscbcic 0.5BNqsbi(4.4.7.1.1.4-1)Refer to Figure 4.4.7.1.1.4Afor values of Ncqand Nqfor footings on slopes and Figures 4.4.7.1.1.4B for valuesof Ncqand Nqfor footings at the top of slopes. For foot-ings in or above cohesive soil slopes, the stability numberin the gures, Ns, is dened as follows:Ns Hs/c (4.4.7.1.1.4-2)Overall stability shall be evaluated for footings on oradjacent to sloping ground surfaces as described in Arti-cle 4.4.9.4.4.7.1.1.5 Embedment DepthThe shear strength of soil above the base of footings isneglected in determining qultusing Equation 4.4.7.1.1-1.If other procedures are used, the effect of embedmentshall be consistent with the requirements of the procedurefollowed.4.4.7.1.1.1 DIVISION IDESIGN 51Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.52 HIGHWAY BRIDGES 4.4.7.1.1.5FIGURE 4.4.7.1.1.1B Contact Pressure for Footing Loaded Eccentrically About One AxisFIGURE 4.4.7.1.1.1A Denition Sketch for Loading and Dimensions for FootingsSubjected to Eccentric or Inclined LoadsModied after EPRI (1983)Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.4.4.7.1.1.5 DIVISION IDESIGN 53FIGURE 4.4.7.1.1.1C Contact Pressure for Footing Loaded Eccentrically About Two AxesModied after AREA (1980)Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.54 HIGHWAY BRIDGES 4.4.7.1.1.5FIGURE 4.4.7.1.1.4A Modied Bearing Capacity Factors for Footing on Sloping GroundModied after Meyerhof (1957)FIGURE 4.4.7.1.1.4B Modied Bearing Capacity Factors for Footing Adjacent Sloping GroundModied after Meyerhof (1957)Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.4.4.7.1.1.6 Ground WaterUltimate bearing capacity shall be determined usingthe highest anticipated ground water level at the footinglocation. The effect of ground water level on the ultimatebearing capacity shall be considered by using a weightedaverage soil unit weight in Equation 4.4.7.1.1-1. If 37, the following equations may be used to determine theweighted average unit weight:for zw B: use m(no effect) (4.4.7.1.1.6-1)for zw B: use (zw/B)(m )(4.4.7.1.1.6-2)for zw 0: use (4.4.7.1.1.6-3)Refer to Figure 4.4.7.1.1.6A for denition of termsused in these equations. If 37, the following equa-tions may be used to determine the weighted average unitweight: (2D zw)(zwm/D2) (/D2)(D zw)2(4.4.7.1.1.6-4)D 0.5Btan(45 /2)(4.4.7.1.1.6-5)4.4.7.1.1.7 Layered SoilsIf the soil prole is layered, the general bearing capac-ity equation shall be modied to account for differencesin failure modes between the layered case and the homo-geneous soil case assumed in Equation 4.4.7.1.1-1.Undrained LoadingFor undrained loading of a footing supported on theupper layer of a two-layer cohesive soil system, qultmaybe determined by the following:qult c1Nm q (4.4.7.1.1.7-1)4.4.7.1.1.6 DIVISION IDESIGN 55FIGURE 4.4.7.1.1.6A Denition Sketch for Inuence of Ground Water Table on Bearing CapacityCopyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.Refer to Figure 4.4.7.1.1.7Afor the denition of c1. Forundrained loading, c1equals the undrained soil shearstrength sul, and 1 0.If the bearing stratum is a cohesive soil which overliesa stiffer cohesive soil, refer to Figure 4.4.7.1.1.7B to de-termine Nm. If the bearing stratum overlies a softer layer,punching shear should be assumed and Nmmay be calcu-lated by the following:Nm (1/m scNc) scNc(4.4.7.1.1.7-2)Drained LoadingFor drained loading of a footing supported on a stronglayer overlying a weak layer in a two-layer system, qultmay be determined using the following:qult [q2 (1/K)c1cot1] exp{2[1 (B/L)]Ktan1(H/B)} (1/K)c1 cot1(4.4.7.1.1.7-3)The subscripts 1 and 2 refer to the upper and lower layers, respectively. K (1 sin21)/(1 sin21) and q2equals qultof a ctitious footing of the same size andshape as the actual footing but supported on the second (or lower) layer. Reduced shear strength values shallbe used to determine q2in accordance with Article 4.4.7.1.If the upper layer is a cohesionless soil and equals25 to 50, Equation 4.4.7.1.1.7-3 reduces toqult q2exp{0.67[1 (B/L)]H/B} (4.4.7.1.1.7-4)The critical depth of the upper layer beyond which thebearing capacity will generally be unaffected by the pres-ence of the lower layer is given by the following:Hcrit [3B1n(q1/q2)]/[2(1 B/L)] (4.4.7.1.1.7-5)In the equation, q1equals the bearing capacity of theupper layer assuming the upper layer is of innite extent.56 HIGHWAY BRIDGES 4.4.7.1.1.7FIGURE 4.4.7.1.1.7ATypical Two-Layer Soil ProlesFIGURE 4.4.7.1.1.7B Modied Bearing Capacity Factor for Two-Layer Cohesive Soil with Softer Soil Overlying Stiffer Soil EPRI (1983)Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.4.4.7.1.1.8 Inclined BaseFootings with inclined bases are generally not recom-mended. Where footings with inclined bases are neces-sary, the following factors shall be applied in Equation4.4.7.1.1-1:bq b (1 tan)2(4.4.7.1.1.8-1)bc b (1 b)/(Nctan) (for 0)(4.4.7.1.1.8-2)bc 1 [2/( 2)] (for 0)(4.4.7.1.1.8-3)Refer to Figure 4.4.7.1.1.8Afor denition sketch.Where footings must be placed on sloping surfaces,refer to Article 4.4.6 for anchorage requirements.4.4.7.1.2 Factors of SafetySpread footings on soil shall be designed for Group 1loadings using a minimum factor of safety (FS) of 3.0against a bearing capacity failure.4.4.7.2 SettlementThe total settlement includes elastic, consolidation,and secondary components and may be determined usingthe following:St Se Sc Ss(4.4.7.2-1)Elastic settlement shall be determined using the unfac-tored dead load, plus the unfactored component of liveand impact loads assumed to extend to the footing level.Consolidation and secondary settlement may be deter-mined using the full unfactored dead load only.Other factors which can affect settlement (e.g., em-bankment loading, lateral and/or eccentric loading, andfor footings on granular soils, vibration loading from dy-namic live loads or earthquake loads) should also be con-sidered, where appropriate. Refer to Gifford, et al., (1987)for general guidance regarding static loading conditionsand Lam and Martin (1986) for guidance regarding dy-namic/seismic loading conditions.4.4.7.2.1 Stress DistributionFigure 4.4.7.2.1A may be used to estimate the distri-bution of vertical stress increase below circular (orsquare) and long rectangular footings (i.e., where L 5B). For other footing geometries, refer to Poulos andDavis (1974).Some methods used for estimating settlement of foot-ings on sand include an integral method to account for theeffects of vertical stress increase variations. Refer to Gif-ford, et al., (1987) for guidance regarding application ofthese procedures.4.4.7.1.1.8 DIVISION IDESIGN 57FIGURE 4.4.7.1.1.8A Denition Sketch for Footing Base InclinationCopyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.4.4.7.2.2 Elastic SettlementThe elastic settlement of footings on cohesionless soils and stiff cohesive soils may be estimated using thefollowing:Se [qo(1 2)A ]/Esz(4.4.7.2.2-1)Refer to Table 4.4.7.2.2Afor approximate values of Esand for various soil types, and Table 4.4.7.2.2B for val-ues of zfor various shapes of exible and rigid footings.Unless Esvaries signicantly with depth, Esshould be de-termined at a depth of about 1 2 to 2 3 of B below the foot-ing. If the soil modulus varies signicantly with depth, aweighted average value of Esmay be used.Refer to Gifford, et al., (1987) for general guidance re-garding the estimation of elastic settlement of footings onsand.4.4.7.2.3 Consolidation SettlementThe consolidation settlement of footings on saturatedor nearly saturated cohesive soils may be estimated using58 HIGHWAY BRIDGES 4.4.7.2.2FIGURE 4.4.7.2.1A Boussinesg Vertical Stress Contours for Continuous and Square FootingsModied after Sowers (1979)Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.4.4.7.2.3 DIVISION IDESIGN 59TABLE 4.4.7.2.2A Elastic Constants of Various SoilsModied after U.S. Department of the Navy (1982) and Bowles (1982)TABLE 4.4.7.2.2B Elastic Shape and RigidityFactors EPRI (1983)Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.the following when laboratory test results are expressed interms of void ratio (e): For initial overconsolidated soils (i.e., p 0):Sc [Hc/(1 eo)][(Ccrlog{p/o} Cclog{f/p})] (4.4.7.2.3-1) For initial normally consolidated soils (i.e., p o):Sc [Hc/(1 eo)][Cclog(f/p)] (4.4.7.2.3-2)If laboratory test results are expressed in terms of ver-tical strain (v), consolidation settlement may be estimatedusing the following: For initial overconsolidated soils (i.e., p o):Sc Hc[Crelog(p/o) Ccelog(f/p)](4.4.7.2.3-3) For initial normally consolidated soils (i.e., p o):Sc HcCcelog(f/p) (4.4.7.2.3-4)Refer to Figures 4.4.7.2.3Aand 4.4.7.2.3B for the de-nition of terms used in the equations.To account for the decreasing stress with increaseddepth below a footing, and variations in soil compress-ibility with depth, the compressible layer should be di-vided into vertical increments (i.e., typically 5 to 10 feetfor most normal width footings for highway applications),and the consolidation settlement of each increment ana-lyzed separately. The total value of Scis the summation ofScfor each increment.If the footing width is small relative to the thickness of the compressible soil, the effect of three-dimensional(3-D) loading may be considered using the following:Sc(3-D) cSc(1-D)(4.4.7.2.3-5)Refer to Figure 4.4.7.2.3C for values of c.The time (t) to achieve a given percentage of the totalestimated 1-D consolidation settlement may be estimatedusing the following:t THd2/cv(4.4.7.2.3-6)Refer to Figure 4.4.7.2.3D for values of T for constantand linearly varying excess pressure distributions. SeeWinterkorn and Fang (1975) for values of T for other ex-60 HIGHWAY BRIDGES 4.4.7.2.3FIGURE 4.4.7.2.3A Typical ConsolidationCompression Curve for Overconsolidated SoilVoid Ratio Versus Vertical Effective StressEPRI (1983)FIGURE 4.4.7.2.3B Typical ConsolidationCompression Curve for Overconsolidated SoilVoid Strain Versus Vertical Effective StressFIGURE 4.4.7.2.3C Reduction Factor to Account forEffects of Three-Dimensional Consolidation SettlementEPRI (1983)Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.cess pressure distributions. Values of cvmay be estimatedfrom the results of laboratory consolidation testing ofundisturbed soil samples or from in-situ measurementsusing devices such as a piezoprobe or piezocone.4.4.7.2.4 Secondary SettlementSecondary settlement of footings on cohesive soil maybe estimated using the following:Ss CHclog(t2/t1) (4.4.7.2.4-1)t1is the time when secondary settlement begins (typi-cally at a time equivalent to 90-percent average degree ofconsolidation), and t2is an arbitrary time which could rep-resent the service life of the structure. Values of Cmaybe estimated from the results of consolidation testing ofundisturbed soil samples in the laboratory.4.4.7.2.5 Tolerable MovementTolerable movement criteria (vertical and horizontal)for footings shall be developed consistent with the func-tion and type of structure, anticipated service life, andconsequences of unacceptable movements on structureperformance. Foundation displacement analyses shall bebased on the results of in-situ and/or laboratory testing tocharacterize the load-deformation behavior of the foun-dation soils. Displacement analyses should be conductedto determine the relationship between estimated settle-ment and footing bearing pressure to optimize footing sizewith respect to supported loads.Tolerable movement criteria for foundation settlementshall be developed considering the angular distortion(/) between adjacent footings. / shall be limited to0.005 for simple span bridges and 0.004 for continuousspan bridges (Moulton, et al., 1985). These / limits arenot applicable to rigid frame structures. Rigid frames shallbe designed for anticipated differential settlements basedon the results of special analysis.Tolerable movement criteria for horizontal foundationsdisplacement shall be developed considering the potentialeffects of combined vertical and horizontal movement.Where combined horizontal and vertical displacementsare possible, horizontal movements should be limited to 1inch or less. Where vertical displacements are small, hor-izontal displacements should be limited to 11 2 inch or less(Moulton, et al. 1985). If estimated or actual movementsexceed these levels, special analysis and/or measures tolimit movements should be considered.4.4.7.3 Dynamic Ground StabilityRefer to Division I-ASeismic Design and Lam andMartin (1986a; 1986b) for guidance regarding the devel-opment of ground and seismic parameters and methodsused for evaluation of dynamic ground stability.4.4.8 Geotechnical Design on RockSpread footings supported on rock shall be designed tosupport the design loads with adequate bearing and struc-tural capacity and with tolerable settlements in confor-mance with Articles 4.4.8 and 4.4.11. In addition, the re-sponse of footings subjected to seismic and dynamicloading shall be evaluated in conformance with Article4.4.10. For footings on rock, the location of the resultant4.4.7.2.3 DIVISION IDESIGN 61FIGURE 4.4.7.2.3D Percentage of Consolidation as a Function of Time Factor, TEPRI (1983)Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.of pressure (R) on the base of footings shall be maintainedwithin B/4 of the center of the footing.The bearing capacity and settlement of footings onrock is inuenced by the presence, orientation and condi-tion of discontinuities, weathering proles, and other sim-ilar features. The methods used for design of footings onrock should consider these factors as they apply at a par-ticular site, and the degree to which they should be incor-porated in the design.For footings on competent rock, reliance on simple anddirect analyses based on uniaxial compressive rockstrengths and RQD may be applicable. Competent rock isdened as a rock mass with discontinuities that are tightor open not wider than 1 8 inch. For footings on less com-petent rock, more detailed investigations and analysesshould be used to account for the effects of weathering,the presence and condition of discontinuities, and othergeologic factors.4.4.8.1 Bearing Capacity4.4.8.1.1 Footings on Competent RockThe allowable contact stress for footings supported onlevel surfaces in competent rock may be determined usingFigure 4.4.8.1.1A(Peck, et al. 1974). In no instance shallthe maximum allowable contact stress exceed the allow-able bearing stress in the concrete. The RQD used in Fig-ure 4.4.8.1.1A shall be the average RQD for the rockwithin a depth of B below the base of the footing, wherethe RQD values are relatively uniform within that inter-val. If rock within a depth of 0.5B below the base of thefooting is of poorer quality, the RQD of the poorer rockshall be used to determine qall.4.4.8.1.2 Footings on Broken or Jointed RockThe design of footings on broken or jointed rock mustaccount for the condition and spacing of joints and otherdiscontinuities. The ultimate bearing capacity of footingson broken or jointed rock may be estimated using the fol-lowing relationship:qult NmsCo(4.4.8.1.2-1)Refer to Table 4.4.8.1.2A for values of Nms. Values ofCoshould preferably be determined from the results oflaboratory testing of rock cores obtained within 2B of thebase of the footing. Where rock strata within this intervalare variable in strength, the rock with the lowest capacity62 HIGHWAY BRIDGES 4.4.8FIGURE 4.4.8.1.1A Allowable Contact Stress for Footings on Rock with Tight DiscontinuitiesPeck, et al. (1974)Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.should be used to determine qult. Alternatively, Table4.4.8.1.2B may be used as a guide to estimate Co. Forrocks dened by very poor quality, the value of qultshouldbe determined as the value of qultfor an equivalent soilmass.4.4.8.1.3 Factors of SafetySpread footings on rock shall be designed for Group 1loadings using a minimum factor of safety (FS) of 3.0against a bearing capacity failure.4.4.8.2 Settlement4.4.8.2.1 Footings on Competent RockFor footings on competent rock, elastic settlements willgenerally be less than 1 2 inch when footings are designedin accordance with Article 4.4.8.1.1. When elastic settle-ments of this magnitude are unacceptable or when the rockis not competent, an analysis of settlement based on rockmass characteristics must be made. For rock masses whichhave time-dependent settlement characteristics, the proce-dure in Article 4.4.7.2.3 may be followed to determine thetime-dependent component of settlement.4.4.8.2.2 Footings on Broken or Jointed RockWhere the criteria for competent rock are not met, theinuence of rock type, condition of discontinuities and de-gree of weathering shall be considered in the settlementanalysis.The elastic settlement of footings on broken or jointedrock may be determined using the following: For circular (or square) footings; qo(1 2)rI/Em, with I ( )/z(4.4.8.2.2-1) For rectangular footings;4.4.8.1.2 DIVISION IDESIGN 63TABLE 4.4.8.1.2A Values of Coefficient Nmsfor Estimation of the Ultimate Bearing Capacity of Footings on Broken or Jointed Rock (Modied after Hoek, (1983))Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law. qo(1 2)BI/Em, with I (L/B)1/2/z(4.4.8.2.2-2)Values of Ipmay be computed using the zvalues pre-sented in Table 4.4.7.2.2B from Article 4.4.7.2.2 for rigidfootings. Values of Poissons ratio () for typical rocktypes are presented in Table 4.4.8.2.2A. Determination ofthe rock mass modulus (Em) should be based on the resultsof in-situ and laboratory tests. Alternatively, values of Emmay be estimated by multiplying the intact rock modulus(Eo) obtained from uniaxial compression tests by a reduc-tion factor (E) which accounts for frequency of disconti-nuities by the rock quality designation (RQD), using thefollowing relationships (Gardner, 1987):Em EEo(4.4.8.2.2-3)E 0.0231(RQD) 1.32 0.15 (4.4.8.2.2-4)For preliminary design or when site-specic test data can-not be obtained, guidelines for estimating values of Eo(such as presented in Table 4.4.8.2.2B or Figure4.4.8.2.2A) may be used. For preliminary analyses or fornal design when in-situ test results are not available, avalue of E 0.15 should be used to estimate Em.4.4.8.2.3 Tolerable MovementRefer to Article 4.4.7.2.3.4.4.9 Overall StabilityThe overall stability of footings, slopes, and founda-tion soil or rock shall be evaluated for footings located on64 HIGHWAY BRIDGES 4.4.8.2.2TABLE 4.4.8.1.2B Typical Range of Uniaxial Compressive Strength (Co) as a Function ofRock Category and Rock TypeCopyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.4.4.9 DIVISION IDESIGN 65TABLE 4.4.8.2.2A Summary of Poissons Ratio for Intact RockModied after Kulhawy (1978)TABLE 4.4.8.2.2B Summary of Elastic Moduli for Intact RockModied after Kulhawy (1978)Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.or near a slope by limiting equilibrium methods of analy-sis which employ the Modied Bishop, simplied Janbu,Spenser or other generally accepted methods of slope sta-bility analysis. Where soil and rock parameters andground water levels are based on in-situ and/or laboratorytests, the minimum factor of safety shall be 1.3 (or 1.5where abutments are supported above a slope). Otherwise,the minimum factor of safety shall be 1.5 (or 1.8 whereabutments are supported above a retaining wall).4.4.10 Dynamic/Seismic DesignRefer to Division I-A and Lam and Martin (1986a;1986b) for guidance regarding the design of footings sub-jected to dynamic and seismic loads.4.4.11 Structural Design4.4.11.1 Loads and Reactions4.4.11.1.1 Action of Loads and ReactionsFootings shall be considered as under the action ofdownward forces, due to the superimposed loads, resistedby an upward pressure exerted by the foundation materi-als and distributed over the area of the footings as deter-mined by the eccentricity of the resultant of the downwardforces. Where piles are used under footings, the upwardreaction of the foundation shall be considered as a seriesof concentrated loads applied at the pile centers, each pilebeing assumed to carry the computed portion of the totalfooting load.66 HIGHWAY BRIDGES 4.4.9FIGURE 4.4.8.2.2A Relationship Between Elastic Modulus and Uniaxial Compressive Strength for Intact RockModied after Deere (1968)Copyright 2002 AASHTO. All rights reserved. Duplication is a violation of applicable law.4.4.11.1.2 Isolated and Multiple Footing ReactionsWhen a single isolated footing supports a column, pieror wall, the footing shall be assumed to act as a cantilever.When footings support more than one column, pier, orwall, the footing slab shall be designed for the actual con-ditions of continuity and restraint.4.4.11.2 Moments4.4.11.2.1 Critical SectionExternal moment on any section of a footing shall bedetermined by passing a vertical plane through the foot-ing, and computing the moment of the forces acting overthe entire area of the footing on one side of that verticalplane. The critical section for bending shall be taken at theface of the column, pier, or wall. In the case of columnsthat are not square or rectangular, the section shall betaken at the side of the concentric square of equivalentarea. For footings under masonry walls, the critical sec-tion shall be taken halfway between the middle and edgeof the wall. For footings under metallic column bases, thecritical section shall be taken halfway between the columnface and the edge of the metallic base.4.4.11.2.2 Distribution of ReinforcementReinforcement of one-way and two-way square foot-ings shall be distributed uniformly across the entire widthof footing.Reinforcement of two-way rectangular footings shallbe distributed uniformly across the entire width of footingin the long direction. In the short direction, the portion ofthe total reinforcement given by Equation 4.4.11.2.2-1shall be distributed uniformly over a band width (centeredon center line of column or pier) equal to the length of theshort side of the footing. The remainder of reinforcementrequired in the short direction shall be distributed uni-formly outside the center band width of footing. is the ratio of the footing length to width.4.4.11.3 Shear4.4.11.3.1 Critical SectionComputation of shear in footings, and location of crit-ical section, shall be in accordance with Articles 8.15.5.6or 8.16.6.6. Location of critical section shall be measuredfrom the face of column, pier or wall, for footings sup-porting a column, pier, or wall. For footings supporting a column or pier with metallic base plates, the critical section shall be measured from the location dened in Article 4.4.11.2.4.4.11.3.2 Footings on Piles or Drilled ShaftsShear on the critical section shall be in accordance withthe following: Entire reaction from any pile or drilled shaft whosecenter is located dp/2 or more outside the criticalsection shall be considered as producing shear onthat section. Reaction from any pile or drilled shaft whose centeris located dp/2 or more inside the critical sectionshall be considered as producing no shear on thatsection. For the intermediate position of pile or drilled shaftcenters, the portion of the pile or shaft reaction to beconsidered as producing shear on the critical sectionshall be based on linear interpolation between fullvalue at dp/2 outside the sectio