Wind Design Considerations for Steel Joists and Joist Girders

SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 1

Wind Design Considerations for Steel Joists and Joist Girders

Perry S. Green, PhD, Technical DirectorSteel Joist Institute TEE Center, Myrtle Beach, SC


Introduction

Commercial manufacture of open web steel joists began in 1923The Steel Joist Institute was formed in 1928 The use of steel joists has continued to

grow year after year for both floors and roofs.

Millions of steel joists and Joist Girders are put in service each year.


General Nature of Wind Loads

Typical Steel Joist and Joist Girder Buildings

WindstormsBuilding type commercial, industrialBuilding shape low rise, rectangularRoofing systems

PresenterPresentation NotesRoofs are subjected to uplift forces induced by wind blowing on and over the building. These forces vary in intensity depending on building exposure, building geometry and wind velocity. The force also varies in intensity over the roof surface. It is greater in intensity at roof edges and corners. Building codes provide minimum wind forces on buildings, but frequently these forces are intended for gross design of the lateral force resisting system.


Windstorm Damage to Roof in Texas 05 March 2004

PresenterPresentation NotesThis damage was supposedly caused by micro burst straight line winds. The storm was very localized and only damaged a few structures. There was no identified tornado in the area.


Windstorm Damage to Roof in Texas 05 March 2004


Hurricane Charley Category 4 Storm Across Florida 13-14 August 2004

PresenterPresentation NotesHurricane Charley made landfall on the west coast of Florida on the morning of August 13, 2004. The hurricane made landfall approximately 25 miles northwest of Fort Myers just before 1 PM, Eastern Daylight Time, first crossing Captiva Island, then tracking up Charlotte Bay, crossing onto the Florida mainland near Punta Gorda. Charley then turned to the northeast, with the center of circulation moving through Orlando and Daytona Beach before exiting the east coast of the Florida peninsula shortly after midnight. As a powerful and dangerous Category 4 hurricane, Hurricane Charley came ashore packing sustained winds of 145 mph. The combination of heavy rain, wind, and a 10- to 15-foot storm surge caused widespread damage and 23 deaths. Recent estimates put the total property damage from Charley at $7.5 billion.


Hurricane Charley Category 4 Storm Across Florida 13-14 August 2004


Population Trends in Hurricane-Prone Regions of the U.S.

Southeast and Gulf of Mexico: Most rapid coastal growth in recent decades and will continue to grow.

Southeast: 8 million (1960) 23 million projected (2015)

Gulf of Mexico: 8 million (1960) 22 million projected (2015)


Top 10 Deadliest Hurricanes to Strike the US: 1851-2005

372 390 400 408 7001,250 1,323 1,500

2,500

8,000

01,0002,0003,0004,0005,0006,0007,0008,0009,000

LA-G

rande

Isle

(1909

)

Audr

ey-S

W LA

,TX (1

957)

LA-La

st Isl

and (

1856

)FL

Key

s (19

35)

GA/SC

(188

1)

LA-C

henie

re (18

93)**

***

Katri

na (S

E LA,

MS)

****

SC/G

A Se

a Isla

nds (

1893

)***

SE FL

/L. O

kech

obee

(192

8)**

Galva

ston (

1900

)*

Footnotes:*Could be as high as 12,000. **Could be as high as 3,000. ***Midpoint of 1,000 2,000 range.****AP total as of Dec. 11, 2005. *****Midpoint of 1,100-1,400 range.Sources: NOAA; Insurance

Information Institute.

Hurricane Katrina was the deadliest hurricane to strike the US since 1928


Roof Design to Resist Uplift Loads

Codes and Standards2005 SJI Standard Specifications and Code of Standard PracticeProvisions from 2006 International Building CodeProvisions from ASCE/SEI 7-05

Design of Joist Bearing SeatsDesign Example - Placement of Joist BridgingSummary and Conclusions


Standards and Codes

2005 SJI Standard Specifications and Code of Standard PracticeProvisions from 2006 International Building CodeProvisions from ASCE/SEI 7-05


Roof Design to Resist Uplift Loads

The nominal loads and load combinations shall be as stipulated by the applicable code under which the structure is designed, and as shown by the Specifying Professional in the contract documents.In the absence of a specified building code such as the International Building Code (IBC 2006), the ASCE/SEI 7-05 (ASCE 2005) Minimum Design Loads for Buildings and Other Structures shall be used as the basis for the loads and load combinations.


42nd Edition SJI Catalog 2005

K-Series Standard Specifications K-Series Load Tables KCS Joists

LH- and DLH-Series Standard Specifications LH- and DLH-Series Load Tables

Joist Girders Standard Specifications Joist Girder Weight Tables


2005 SJI Standard Specification for Open Web Steel Joists, K-Series

5.11 UPLIFTWhere uplift forces due to wind are a design requirement, theseforces must be indicated on the contract drawings in terms ofNET uplift in pounds per square foot (Pascals). The contractdocuments shall indicate if the net uplift is based upon LRFD orASD. When these forces are specified, they must be consideredin the design of joists and/or bridging. A single line of bottomchord bridging must be provided near the first bottom chordpanel points whenever uplift due to wind forces is a designconsideration.*

PresenterPresentation Notes* For further reference, refer to Steel Joist Institute Technical Digest #6, Structural Design of Steel Joist Roofs to Resist Uplift Loads.



2005 SJI Standard Specification for Longspan Steel Joists, LH-SeriesDeep Longspan Steel Joists, DLH-Series



2005 SJI Standard Specification for Joist Girders

1004.9 UPLIFTWhere uplift forces due to wind are a design requirement, theseforces must be indicated on the contract drawings in terms ofNET uplift in pounds per square foot (Pascals). The contractdrawings must indicate if the net uplift is based on ASD orLRFD. When these forces are specified, they must beconsidered in the design of Joist Girders and/or bracing. If theends of the bottom chord are not strutted, bracing must beprovided near the first bottom chord panel points wheneveruplift due to wind forces is a design consideration.*



2005 SJI Code of Standard Practice

1.4 DESIGNIn the absence of ordinances or specifications to the contrary,all designs prepared by the specifying professional shall be inaccordance with the Steel Joist Institute Standard SpecificationsLoad Tables & Weight Tables of latest adoption.



6.1 PLANS FURNISHED BY BUYER(a) LoadsThe Steel Joist Institute does not presume to establish theloading requirements for which structures are designed.

The Steel Joist Institute Load Tables are based on uniformloading conditions and are valid for use in selecting joist sizesfor gravity loads that can be expressed in terms of "pounds perlinear foot" (kiloNewtons per Meter) of joist. The Steel JoistInstitute Joist Girder Weight Tables are based on uniformlyspaced panel point loading conditions and are valid for use inselecting Joist Girder sizes for gravity conditions that can beexpressed in kips (kiloNewtons) per panel point on the JoistGirder.



6.1 PLANS FURNISHED BY BUYER(a) Loads (contd)The specifying professional shall provide the nominal loads andload combinations as stipulated by the applicable code underwhich the structure is designed and shall provide the designbasis (ASD or LRFD).

The specifying professional shall calculate and provide themagnitude and location of ALL JOIST and JOIST GIRDERLOADS. This includes all special loads (drift loads, mechanicalunits, net uplift, axial loads, moments, structural bracing loads,or other applied loads) which are to be incorporated into thejoist or Joist Girder design. For Joist Girders, reactions fromsupported members shall be clearly denoted as point loads onthe Joist Girder. When necessary to clearly convey theinformation, a Load Diagram or Load Schedule shall beprovided.



6.1 PLANS FURNISHED BY BUYER(a) Loads (contd)The specifying professional shall give due consideration to thefollowing loads and load effects:

1. Ponded rain water.2. Accumulation of snow in the vicinity of obstructions such

as penthouses, signs, parapets, adjacent buildings, etc.3. Wind.4. Type and magnitude of end moments and/or axial forces

at the joist and Joist Girder end supports shall be shownon the structural drawings. For moment resisting joists orJoist Girders framing near the end of a column, dueconsideration shall be given to extend the column lengthto allow a plate type connection between the top of thejoist or Joist Girder top chord and the column.


Standards and Codes



2006 International Building CodeSECTION 2206 STEEL JOISTS

2206.1 General2206.2 Design

The registered design professional shall indicate onthe construction documents the steel joist and/or steeljoist girder designations from the specifications listedin Section 2206.1 and shall indicate the requirementsfor joist and joist girder design, layout, end supports,anchorage, non-SJI standard bridging, bridgingtermination connections and bearing connectiondesign to resist uplift and lateral loads.

2206.3 Calculations2206.4 Steel joist drawings2206.5 Certification


2006 International Building CodeSECTION 1605 LOAD COMBINATIONS

1605.2 Load combinations using strength design or load and resistance factor design

1605.2.1 Basic load combinations1.2D + 1.6(Lr or S or R) + (f1L or 0.8W)1.2D + 1.6W + f1L + 0.5(Lr or S or R)0.9D + 1.6W0.9D + 1.0E

NOTE: F and/or H loads have been left out of the above equations

PresenterPresentation NotesF = Load due to fluids with well-defined pressures and maximum heights.H = Load due to lateral earth pressures, ground water pressure or pressure of bulk materials


2006 International Building CodeSECTION 1605 LOAD COMBINATIONS

1605.3 Load combinations using allowable stress design

1605.3.1 Basic load combinationsD + (W or 0.7E)D +0.75 (W or 0.7E) + 0.75L + 0.75(Lr or S or R)0.6D + W0.6D + 0.7E

NOTE: F and/or H loads have been left out of the above equations

PresenterPresentation NotesF = Load due to fluids with well-defined pressures and maximum heights.H = Load due to lateral earth pressures, ground water pressure or pressure of bulk materials


Standards and Codes



Basic parametersWind speed, importance, exposureSignificance / importance of exposure category

Exposure C is default, while charts are based on BThe difference is often 30 to 40 percent

ASCE 7-05 Specified Wind Loads


ASCE 7-05 Basic Wind Speed Map

PresenterPresentation NotesThis figure is the same as FIGURE 1609 BASIC WIND SPEED (3-SECOND GUST) from the 2006 International Building Code.


It all looks simple when the building structure appears to be a simple rectangle made up of large monolithic elements as described in Figure 6-3.

The reality is when the building shape is more complex comprised of numerous elements then it is not as easy to determine the loadings on joists in corners and Joist Girders that pass through both edge and corner zones.


PresenterPresentation NotesShow a key plan or two of complex building shapes, then zoom in to a corner with joist, Joist Girder and edge and corner zones. It all looks simple when the building appears to be totally rectangular, made of large monolithic slabs. Reality is more complexshow sample corner framing.



PresenterPresentation NotesShow a key plan or two of complex building shapes, then zoom in to a corner with joist, Joist Girder and edge and corner zones


What Constitutes Net Uplift?For ASD, the uplift load combination is 0.6D + WFor LRFD, the uplift load combination is 0.9D + 1.6W

The EOR may need to differentiate between minimum and maximum dead load.

(Note: 0.6D is NOT an allowance for collateral loads)



ASCE 7-05 Specified Wind LoadsWhat constitutes Net Uplift?

Amplified DL resistance by 1.65 for uplift is not desirable!So,

( ) ( )( )

gy

gy

gy

AFWL65.1DL65.1AFWLDL65.1

AF6.0WLDL

=+

=+

=+

( ) ( )gy

gy

AF6.0WLDL6.0AFWL65.1DL

=+

=+


Maximum Dead Load (for gravity loading)Minimum Dead Load (for wind uplift)Collateral Load (also for wind uplift)Collateral loads represent a category of dead loads which are not part of the building structure but are required for the buildings function. These include: Mechanical equipment, piping, electrical equipment, conduit, sprinkler piping fire proofing, ceilings, etc.



Wind Loads: Net Uplift Zone Diagram

24 psf

15 psf

11 psf

120'

80'

8'

8'

1460 lbs 1228 lbs

60 plf96 plf

PresenterPresentation NotesDefine the zones in the Diagram: Edge Zone, Corner Zone, distance aThe joist and deck manufacturer is not responsible for interpreting building codes, thus the designer must specify all loads on the joists. Modern codes specify varying wind uplift loading depending on location, thus the designer should provide a diagram describing the loads and zones of loading. Do not specify uplift as gross uplift such as the wind loads for the MWFRS or Components and Cladding from ASCE 7-05. The joist manufacturer will not be able to determine the value for D or you may not want to use 0.6D in Equation 16-11. Also, if you have chosen to use the load cases in 1605.3.2, the joist manufacturer will not be able to determine the value for D and they would not know if you have included (w) in your calculation for W in Equation 16-14.Again, specify wind loads in psf (net uplift or provide the load combination). Do not specify wind in mph.


Properly Applying Wind Loads to Steel Joists and Joist Girders

What are the qualifications to use the simplified method?Is there an advantage to Method 2 even if simplified Method 1 is allowed?How often does or does not a typical joist low-rise building qualify for the simplified method?Net pressure vs. net uplift

30netztnet pIKp =

PresenterPresentation NotesPoints to make:Can ASCE simplified method be used? Review qualificationsASCE7-05 Section 6.4.2.2 Components and Cladding Equation 6-2


The chart on the following slide is a typical components and cladding roof wind pressures chart provided on the contract documents.

Roof pressure needs to be converted to NET uplift, or more correctly the result of the appropriate load combination for wind forces acting upward.



ROOF SURFACES

EFFECTIVE WIND AREA

POSITIVE PRESSURES (PSF)

NEGATIVE PRESSURES (PSF)

ZONE

1 2 3 1 2 3

10 SF 5.3 5.3 5.3 -13.0 -21.8 -32.8

20 SF 5.0 5.0 5.0 -12.7 -19.5 -27.2

50 SF 4.5 4.5 4.5 -12.2 -16.4 -19.7

100 SF 4.2 4.2 4.2 -11.9 -14.1 -14.1


PresenterPresentation NotesThere are some minimum pressures that apply.


Per ASCE definition of Effective Width, take span times an effective width that is not less than one third the span.Note: This is specifically referenced for the ASCE Method 2 charts, but it should also apply to ASCE Method 1 (simplified).



So for steel joists, a simple rule is that for all joist spans of 18 foot or greater, use the 100 square foot values, i.e. 18 x 6 = 106 > 100 ft.2

So if a project does not have any spans less than 18 feet, there is no need for a detailed chart with values by square foot.

The light weight of joists under 18 foot spans often allows for a conservative uplift value to be used rather than a detailed interpolation for the exact square footage.



For spans of at least 13 feet (13*13/3 = 56.33 ft.2), just use the 50 square foot value, or if no values are listed for 50 sq. ft., use the average of 10 and 100 sq. ft. values.

For joist spans less than 13 feet, the 10 sq. ft. value could conservatively be used.



Clarifications and Interpretations:ASCE simplified method described in Section 6.4.2.2 provides a formula for net design wind pressure. This is NOT the same as SJI section 5.11 NET uplift.

ASCE net is the sum of internal and external pressures.SJI net, is the final resultant pressure, less appropriate dead load result of the load combination



Steel joists are considered components and cladding (C&C).Joist Girders are considered Main Wind Force Resisting System (MWFRS).

Most often, separate MWFRS pressure values are not provided for the Joist Girders, and the joist supplier applies the end reaction (net) uplift forces from the component and cladding joists to the girders.

Is this conservative?



Other considerationsOverhangs have significant uplift

TCXs automatically have same capacity as downward gravity.But uplift on overhangs can easily exceed gravity, particularly in coastal areas or hurricane prone regions.

Kickers that carry horizontal wind forces need to have both components defined.



The first consideration relative to the design of the structure is to determine if rigid frame action is required.For single story structures the optimum framing system generally consists of braced frames in both directions, and the use of a roof diaphragm system to transfer wind and seismic loads to the vertical bracing elements. The specifying professional must specify the necessary loading and stiffness data to the joist manufacturer.

Properly Applying Lateral Loads to Steel Joists and Joist Girders

PresenterPresentation NotesThe specifying professional and the joist manufacturer must communicate design data and information to each other.


The specifying professional must indicate the type of joist to column connections so that the joist manufacturer can provide the joists with the geometry that meets the design intent.

The joist manufacturer must design the joists in conformance with the SJI Specifications and other contract requirements specified by the specifying professional.



Specification of Required Forces and MomentsMinimum thickness of bottom chord (weld requirements).Chord splices must conform to the requirements of the 2005 AISC Seismic Provisions, Section 7.3a.Use IBC Load Combinations


PresenterPresentation NotesTalk about wind moment frames and their special requirements.



All top chord axial loads and end moments are transmitted directly into the columns via the tie plates. No horizontal forces are transferred through the girder seats.

e

F

F

M

PresenterPresentation NotesOne of the major difficulties with edge joists or edge joist girder is transfer of the chord force through the seat, a couple is produced (Fe) which creates a moment in the top chord. Joists can resist only very small chord moments, thus some other connection type must usually be designed. The connection shown here can only be used for small forces. The designer must make sure that the moment and force can be carried by the edge member.


Design of Bearing Seats to Resist Uplift Loads

Research2005 SJI Standard SpecificationsRecommended Design Procedure


Typical Roof Framing using K-Series Open Web Steel Joists


End Bearing Seat Connections


Profile of SJI Standard K-Series Open Web Steel Joists


Components of Uplift Resistance for Test Program

Anchorage Weld Strength Ductility

Seat Angle Strength Ductility


Joist Seat Test Program ParametersVary seat angle size (leg and thickness) S1 L 1 x 1 x 7/64 S2 L 1-1/2 x 1-1/2 x 1/8 S3 L 2 x 2 x 3/16 S4 L 2 x 2 x 1/4

Vary seat length 4, 6, 8 nominal

Vary anchorage weld length 1, 3, 5 nominalSpecimen Nomenclature SAS-SL-FWS-WL


TOP CHORD ANGLES

JOIST SEAT WELD

3/4" BASEPLATE

9/16" DIA. HOLEFOR A325N BOLT( 4 PLACES)

JOIST SEAT ANGLES

PULL PLATE

BUTT WELDPROVIDED BETWEEN ANGLES

Typical Test Specimen Configuration

FILLET WELDS PROVIDED BETWEEN TOP CHORD TOE AND SEAT ANGLE AND SEAT ANGLE TOE AND TOP CHORD


Experimental Test Setup

REACTION PLATE

INSTRUMENTATION

TEST SPECIMEN


0.30 in. Vertical Displacement at 6.5 kips Applied Load

End View During and After TestTest Specimen S3-4-1/8-3

Failure Mechanism


0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0 0.1 0.2 0.3 0.4 0.5

Displacement (in.)

Load

(kip

s)

Avg P1 & P2

Avg P4 & P7

Typical Load-Deformation BehaviorTest Specimen S3-4-1/8-3


Yield Line Perimeter

Profile and End View After TestTest Specimen S1-6-1/8-1


Anchorage Weld (typ.)

Yield Line Formation (typ.)

Anchorage Weld (typ.)

Yield Line Formation (typ.)

Yield Line PatternsShort and Long Anchorage Welds


a

Plastic Hinge

Yield Line Analysis Model for Prediction of Uplift Capacity

Pu/2

Pu/2 Yield Line

a

LwLs

a

a


We = (Pu / 2) We = External WorkPu = Predicted ultimate uplift load = Distance which the load moves thru

Wi = Mp (Lyl)Wi = Internal WorkMp = Plastic moment capacity of plate,

per unit length = Fy Z = Angle through which YL rotatesLyl = Length of yield line, the lesser of

Lw + a and Ls

Yield Line Analysis Using Virtual Work


Wi + We = 0

(Pu / 2) - Mp (Lyl) = 0

But since tan = for small angles, = / a

Solving for Pu gives:

Pu = 2 Mp Lyl / a

Assumption of a = 2.3 t provides reasonablygood prediction of ultimate uplift strength of joist bearing seat

Yield Line Analysis Using Virtual Work


The flexural resistance of K-Series joist bearing seats can be predicted using a yield line approach.The yield line model is based on principles of basic mechanics, not on empirical curve fitting.A 5/32 fillet weld is adequate to develop the flexural strength of the yield line.

Research Programs Recommendations


2005 SJI Standard Specification for Open Web Steel Joists, K-Series5.6 END ANCHORAGE(b) Steel

Ends of K-Series Joists resting on steel supports shall beattached thereto with a minimum of two 1/8 inch (3 millmeters)fillet welds 1 inch (25 millmeters) long, or with two 1/2 inch (13millimeters) ASTM A307 bolts, or the equivalent. When K-Series Joists are used to provide lateral stability to thesupporting member, the final connection shall be made bywelding or as designated by the specifying professional.

(c) UpliftWhere uplift forces are a design consideration, roof joistsshall be anchored to resist such forces (Refer to Section 5.11Uplift).



104.7 END ANCHORAGE(b) Steel

Ends of LH- and DLH-Series Joists resting on steel supportsshall be attached thereto with a minimum of two 1/4 inch (6millmeters) fillet welds 2 inches (51 millmeters) long, or with two3/4 inch (19 millimeters) ASTM A307 bolts, or the equivalent.When LH/DLH-Series Joists are used to provide lateral stabilityto the supporting member, the final connection shall be made bywelding or as designated by the specifying professional.

(c) UpliftWhere uplift forces are a design consideration, roof joists shallbe anchored to resist such forces (Refer to Section 104.12Uplift).


2005 SJI Standard Specification for Joist Girders1004.6 END ANCHORAGE(b) Steel

Ends of Joist Girders resting on steel supports shall beattached thereto with a minimum of two 1/4 inch (6 millmeters)fillet welds 2 inches (51 millmeters) long, or with two 3/4 inch(19 millimeters) ASTM A307 bolts, or the equivalent. In steelframes, bearing seats for Joist Girders shall be fabricated toallow for field bolting.

(c) UpliftWhere uplift forces are a design consideration, roof JoistGirders shall be anchored to resist such forces (Refer toSection 1004.9 Uplift).


Pn = 2 Mp Lyl / aWhere:

Pn = Nominal uplift capacityMp = Plastic moment capacity of plate per

unit length= Fy Z

Z = t2 / 4Lyl = Length of yield linea = 2.3 t = 1.67 (AISC-ASD safety factor for

bending)Pn/ = Allowable uplift strength

ASD Design Procedure


Pn = 2 Mp Lyl / aWhere:

Pn = Nominal uplift capacityMp = Plastic moment capacity of plate per

unit length= Fy Z

Z = t2 / 4Lyl = Length of yield linea = 2.3 t = 0.90 (AISC-LRFD resistance factor for

bending)Pn = Design uplift strength

LRFD Design Procedure


Recommended Bearing Seat Design to Resist Uplift Loads

Length Thickness Fy LW Mp a LYL Pn / PweldLs (in.) t (in.) (ksi) (in.) (in.-k/in.) (in.) (in.) (kips) (kips)

4 0.125 50 1 0.195 0.288 1.903 1.55 3.714 0.125 50 1.5 0.195 0.288 2.403 1.96 5.574 0.125 50 2 0.195 0.288 2.903 2.36 7.424 0.125 50 2.5 0.195 0.288 3.403 2.77 9.284 0.125 50 3 0.195 0.288 3.903 3.18 11.146 0.125 50 4 0.195 0.288 4.903 3.99 14.856 0.125 50 5 0.195 0.288 5.903 4.80 18.56

4 0.156 50 1 0.304 0.359 2.127 2.16 4.634 0.156 50 1.5 0.304 0.359 2.627 2.67 6.954 0.156 50 2 0.304 0.359 3.127 3.18 9.274 0.156 50 2.5 0.304 0.359 3.627 3.68 11.584 0.156 50 3 0.304 0.359 4.127 4.06 13.906 0.156 50 4 0.304 0.359 5.127 5.21 18.536 0.156 50 5 0.304 0.359 6.127 6.09 23.16



Length Thickness Fy LW Mp a LYL Pn / PweldLs (in.) t (in.) (ksi) (in.) (in.-k/in.) (in.) (in.) (kips) (kips)

4 0.188 50 1 0.442 0.432 2.358 2.89 5.584 0.188 50 1.5 0.442 0.432 2.858 3.50 8.374 0.188 50 2 0.442 0.432 3.358 4.11 11.174 0.188 50 2.5 0.442 0.432 3.858 4.72 13.964 0.188 50 3 0.442 0.432 4.358 4.89 16.756 0.188 50 4 0.442 0.432 5.358 6.56 22.336 0.188 50 5 0.442 0.432 6.358 7.34 27.918 0.188 50 6 0.442 0.432 7.358 9.00 33.50

4 0.250 50 1 0.781 0.575 2.806 4.57 7.424 0.250 50 1.5 0.781 0.575 3.306 5.38 11.144 0.250 50 2 0.781 0.575 3.806 6.19 14.854 0.250 50 2.5 0.781 0.575 4.306 6.51 18.564 0.250 50 3 0.781 0.575 4.806 6.51 22.276 0.250 50 4 0.781 0.575 5.806 9.45 29.706 0.250 50 5 0.781 0.575 6.806 9.76 37.128 0.250 50 6 0.781 0.575 7.806 12.70 44.54


The Pweld strength given in the preceeding tables does not account for the transverse loading of the weld due to uplift and thus could be multiplied by 1.5.Where a joist seat has been detailed for a bolted connection, and for any reason the bolt is not utilized, the empty slot in the bearing seat leg severely diminishes uplift capacity. In such a condition, if a weld and no bolt is to be used on a slotted bearing seat, then the weld should be applied within the empty slot.



Seat Angles L 1-1/2 x 1-1/2 x 1/8Ls =4 Lw = 2-1/2 Fy = 50 ksi

Allowable and Design Uplift StrengthsZ = 0.125 2 / 4 = 0.00391 in.3 / in.a = 2.3 (0.125) = 0.28750 in.Lyl = 2.50 + (0.2875) = 3.403 in. < LsMp = 50 (0.00391) = 0.1953 in.-kip / in.Pn = 2 (0.1953)(3.403) / 0.2875 = 4.62 kipsPn/ = 4.62 / 1.67 = 2.77 kipsPn = 0.9 (4.62) = 4.16 kips

ASD and LRFD Design Example

PresenterPresentation NotesThe maximum factored load Pu from the applicable LRFD load combinations is to be compared to the Design uplift strength, phi Pn.The maximum load P from the applicable ASD load combinations is to be compared to the Allowable uplift strength, Pn / omega.


2005 SJI Standard Specification for Open Web Steel Joists, K-Series




2005 SJI Standard Specification for Joist Girders

1004.9 UPLIFTWhere uplift forces due to wind are a design requirement, theseforces must be indicated on the contract drawings in terms ofNET uplift in pounds per square foot (Pascals). The contractdrawings must indicate if the net uplift is based on ASD orLRFD. When these forces are specified, they must beconsidered in the design of Joist Girders and/or bracing. If theends of the bottom chord are not strutted, bracing must beprovided near the first bottom chord panel points wheneveruplift due to wind forces is a design consideration.*



Design ExampleBuilding Location:

Near Orlando, FL in open terrain minimum slope / ft.Topography: HomogenousExposure: Category C (Sections 6.5.6.2 and 6.5.6.3)

Building Framing and Layout:Flat roof system consisting of steel joists, Joist Girders, and structural roof deck. CMU walls on all four sides with debris-resistant windows and door infill. Building has a parapet height of less than 3-0 and is considered a closed building.

Building Classification: Building Category IIImportance Factor = 1.0 (Table 6-1)


Design Example

Dimensions:Length, l = 121-4Width, w = 80-0Height, h = 20-0 above the ground

Roof slope is less than or equal to 5 degreesRoof live load deflection is based on L/240

Design Roof Loads:Dead Load, D = 15.0 psfRoof Live Load, Lr = 20.0 psf

Total Load = 35.0 psf


ASCE 7-05 Basic Wind Speed Map

ORLANDO

PresenterPresentation NotesThis figure is the same as FIGURE 1609 BASIC WIND SPEED (3-SECOND GUST) EASTERN GULF OF MEXICO AND SOUTHEASTERN U.S. HURRICANE COASTLINE from the 2006 International Building Code.


Design Example

Basic wind speed, from Figure 6-1b for Orlando, Florida area V = 110 mph.

Design approach is based on the Simplified Procedure (Method 1) for both Components and Cladding and Main Wind Force System since the following conditions exist:

Simple diaphragm building (Section C6.2).Building shape is basis and has a symmetrical cross section in both directions and a flat roof.There is no expansion joints in the building.Its a low-rise building with a mean roof height, h less than 60 ft. and does not exceed the least horizontal dimension (Section 6.2).


Design Example

Since the building has debris-resistant glazing and has no dominant opening in any wall it can be classified as a closed building. (Section 6.5.9.3). Building has a regular shape.Rigid building, where height/width,

w = 20 ft./80 ft. = 0.25 < 4 (Section C6.2).The building is not subjected to the topographic effects of Section 6.5.7No torsional effects meets Note 5 of Figure 6-10.


Steel Joist and Joist Girder Layout


Wind Zone Definitions


Steel Joist Design

20K6 Rod Web @ 40-0Considering no uplift -

2-0 18 @ 2-0 2-0

3-0 17 @ 2-0 3-0

Bottom Chord = 2 angles 1.5 x 1.5 x 0.137, A = 0.784 in.2End Web = 5/8 in. dia. round bar , A = 0.307 in.2


Steel Joist Design

20K6 Rod Web @ 40-0With (net) uplift -

8-0 32-0

Bottom Chord = 2 angles 1.5 x 1.5 x 0.155, A = 0.882 in.2End Web = 7/8 in. dia. round bar , A = 0.601 in.2

84 plf108 plf


Design DataEnd Web, left end l = 37.49 in.

( )

( )

( ) OKksi90.5601.055.3

APksi21.702.89.0

ksi02.8F

ksi35.13137.1

E877.0F877.0F

137.121875.037.490.8

rKL

c

cr

2

2

ecr

==>=

=

===

==

Steel Joist Design

Reduce to 90% for eccentricity at bearing seat


Design DataBottom Chord, Pc = 10.62 kips

OKksi04.12882.062.10ksi16.12F

ksi26.20F

4.81295.024

r

controls3.111r

.in96

cr

cr

z

y y

b

b

=>=

=

==

=

=

l

l

l

Steel Joist Design

4 rows (40)(12)/(4+1) = 96 in.


Placement of Bridging to Resist Uplift Loads20K6 Bridging Configuration: Option 1

5 @ 8-0

Erection Stability BridgingUplift Bridging

A Common Alternative (not for this case)4 Rows Equally Spaced

4 Rows Equally Spaced Between Uplift Bridging

PresenterPresentation NotesIn the common alternative, the bottom chord bridging rows are equally spaced between the uplift bridging at the first bottom chord panel points. However, this is applicable only if there is no required erection stability bridging or if the number of top chord bridging rows is an odd number (the cross bridging is at the midspan of the joist). So, it does not work here.



7-0

2 @ 8-9 3 @ 7-6

7-6 7-07-65-0

Design DataBottom Chord, 2 angles 1.5 x 1.5 x 0.137, A = 0.784 in.2

Pc = 10.62 kips


Placement of Bridging to Resist Uplift LoadsAt midspan of the joist:

OKksi44.13784.054.10ksi43.13F

85.104r

ksi55.13784.062.10ksi47.18Fksi84.30F

controls4.81295.024

r9.69

r

cr

y y

b

crcr

zy y

b

=>=

=

=>=

=

===

l

ll

For compression, 7-6 space controls; Pc = 10.54 kips


Placement of Bridging to Resist Uplift LoadsWith revised bridging locations at the TC, check spacing

OK"9'8'2.1014'51

OK145110956.0

105r

.in105"9'8

y y

b

b

>=+



5 Equal Spaces Between First BC Panel Points

5 @ 6.8 ft.

9.8 ft. 3 @ 6.8 ft. 9.8 ft.

End TC space = 9.8 ft.

OK'2.10'8.9

OK145123ry y

b

Documents

Wind Design Considerations for Steel Joists and Joist Girders