Atiesadores - Extracto

7/30/2019 Atiesadores - Extracto

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3.3-20 / Column-Related DesignAt the center of the 3" bar, the bolt loads are

snpported by tension and compression forces in the1" thick web platcs above and below the bar. Theweb plates are attached to the column flange, oppositethe column web, by welds that carry this moment andshear into the column.

The shear pnd moment caused by the anchor boltforces, which are not in the plane of the weld, deter-mine the size of the vertical welds. The welds extend15" above and 3" below the 3" transverse bar.

The properties and stresses on the vertical weldsare figured on the basis of treating the welds as a line,having no width. See Figure 30.

FIGURE 30

Take area moments about the base line (y-y) :

moment of inertia about N.AM"I, = I, + I, - - A

2 w e l d r X 1 5 "Total

= 11.5" (up from base line y-y)distance of N.A. from outer fibercbotbm 11.5"

-3036

section moddus of weld

= 112 in.'( 1288)S =--9.5)

15.3

= 135.5 in."

--05.0414.0

-. I

maximum bending force on uel d

5467.5

shear force on weld

562.5

resultant force on weld

6048

required flkt weld size3000a =-113J0 WI E70 allowable

This requircs continuous fillet welds on bothsides for the full length of the 1" vertical web plate.If greater weld strength had been required, the 1" webplatc could be made thicker or taller.

For bolts of ordinary size, the upper portion ofthe plates for this detail can be cut in one piece fromcolnmn sections of 14" flanges. This insures fnll con-tinuity of the web-to-flange in tension for carrying thebolt loads. By welding across the top and bottom edgesof the liorizontal plate to the column flange, the re-quired thickness of flange plate in bending is reducedby having support in two dircctions.6. TYPICAL COLUMN BASESIn ( a ) of Figure 31, small brackets are .groove butt


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olumn Bares /

FIGURE 31

y stiffeners moy be

\voided to the oirtcr edges of thc colnmr Annges todevelop greatcr moment resistance for the attachmentto the bas? plate. This will help for moments abouteither the x-x or the y-y :tsis. A single bovel or single Vjoint is preparcd by beveling just the edge of thebrackets; no hcveling is done on the column flanges.For colnnrn flanges of nominal thickness, it mighthe easier to simply add two brackets, fillet welded tothe base of the column; see ( h ) and ( c ) . No bevelingis required, and handling and assembling time is re-duced hecat~se nly two additional pieces are requirod.In ( b ) thc bracket plates are attached to the faceof the coluin~r lange; in ( c ) the p1atr.s are> attached tothe outer edge of the column Nange. In any rolledsection used as a column, greater berrtling strengthand stiifiress is obtained about the x-x nxis. If themoment is ahont the x-x axis, it would be better toattach the additional plates to the face of the columnas in (b). This will provide a good transverse filletacross the n)lumn flange and two longitudinal filletwelds along the outer edge of the column flange withgood acct%ssihility for melding. Thc attaching platesand the welds connecting thein to the base plate arein tho most effcct~vcposition and location to transfer

this moment. The only slight drawback is that theattaclring plntcs will not stiffen the overhung portionof the base plate for the hending due to tension in thehold-down bolts, or due to the upward hearing pressureof the masonry support. Mowevrr if this is a problem,smxll hrackrxts shown in dottrd lines may be easilyadded.The plates can he fillet wrlded to the outer edgesnf thc column flange as in ( c ) , although there is notgood accessibility for the welds on the inside. Some ofthese inside fillet welds can be made before the unitis assembled to the base plate.For thick Ranges, clctail ( a ) might represent thelrast amount of \velding and additioml plate material.Short lengths of pipe have been welded to theouter edge of the cohnnn flange to develop the neces-sary moment for the hold-down bolts; see (d). Thelength and leg size of the attaching fillet welds aresufficicnt for thc moment.

In ( e ) two channels with additional stiffeners arewcldd to the cohnnrr flanges for the required momentfrom the hold-down bolts. By setting this channelassenibly back slightly from the milled end of thecolumn, it does not have to be designed for any bear-


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3.3-22 / Column-Related Design

ing, but just the tension from the hold-down bolts. Ifthis assembly is set flush with the end of the columnand milled to bear, then this additional bearing loadmust be considered in its design. Any vertical tensileload on the assembly from the holddown bolts, orvertical bearing load from the base plate (i f iu con-tact), will produce a horizontal force at the top whichwill be applied transverse to thc column flange. If thecolumn flange is too thin, then horizontal plate stiffenersmust be added between the column flanges to eflec-tively transfer this force. These stiffeners are shown in( e ) by dotted lines.

In ( f ) built-up, hold-down bolt supports arewelded to the column flanges. These may be designedto any size for any value of moment.

In (g), the attaching plates have been extendedout farther for very high moments. This particulardetail uses a pair of channels with a top plate for thehold-down bolts to transfer this tensile force back tothe main attaching plates, and in turn back to thecolumn.

One of the many possible details for the base ofa built-up crane runway girder column in a steel millis shown in Figure 32. Two large attaching plates arefillet welded to the flanges of the rolled sections of thecolumn. This is welded to a thick basc plate. Two longnarrow plates are next welded into the assembly, withspacers or small diaphragms separating them from thebase plate. This provides additional strength and stiff-ness of the base plate through beam action for theforces from the hold-down bolts. Short sections of Ibeam can also be welded across the ends between theattaching plates.7. HIGH-RISE REQUIREMENTS

A 14" WF 426# column of A36 steel is to carry a com-pressive load of 2,000 kips. Using a bearing load of730 psi, this would require a 30" X 60" base plate.Use E70 welds.

For simplicity, each set of lxackets together witha portion of the base plate formed by a diagonal linefrom the outer comer of tlir plate hack to the coh~snnflange, will be assrsmcd to resist the bearing pressureof tho masonry snpport; see Figure 34. This is a con-servative analysis because the base plate is not cutalong these lines and thcse portions do not act inde-pendently of each other.

Columns for high-rise buildings may use brackets ontheir base plates to help distribute the column loadout over the larger area of the base plate to themasonry wpport.


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This portion of the assembly occupies a trapezoidalarea; Figure 35.

/ + h i = 167"tL = 50' -

FIGURE 35

P = A w= (690 in." (750 psi)= 516 kips

Determining thickness of base plateTo get an idea of the thickness of the base plate ( t ) ,consider a 1" wide strip as a uniformly loaded, con-tinuous beam supported at two points (the brackets)and overhanging at each end. See Fignre 36.From beam formula #6Bh in Section 8.1:

-w a2M, (a t support) = 2

Since:M = a S

or:where:

t = I a = 7 5 a, (ALSC L.J.1.4.8)

= 5.51" or use 6"-thick plateCheck bending stresses & shear stresses inbase plate bracket sectionStart with lYzf'-thick brackets ( 2 x 1M" = 3" flangethickness) at right angles to face of column flange. Findmoment of inertia of the vertical section throughbrackets and base plate, Figure 37, using the methodof adding areas:

moment of inertia about N.A.

FIGURE 36


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3.3-24 / Column-Related

distance of N.A. to outer fibercb = 9.27"

bending stressesM CbVb = I

= 4370 psi

= 9770 psi OK-n~aximum hear forcc at neutral axis

Bendtng stress [a) Shear force (f)PI (4

FIGURE 37corresponding shear stress iu brackets

= 8400 psi OKshear force at face of 6" base plate(to be transferred through fillet welds)

= 24,630 llx/in. ( to be carried by four filletwelds at 1%" thick brackets)leg size of mch fillet wdd joining base plate to brackets

l/g (24,630)W =----(11,200) - E70 allowable= ,545"or use %/,Br'[l---

(The minimum fillet wcld leg size for 6" plateis WB .)Determining vertical weld requirementsIn determining fitlet weld sizes on the usual beam seatbracket, it is often assumed that the shear reaction isuniformly distributed along the vertical length of thobracket. The hvo unit forces resulting from shear andbending are then resolved together (vectorially added) ,and the resultant force is then divided by the allow-able force for the fillet weld to give the weld size. Thisis of course conservative, because the maximum unitbending force does not occur on the fillet weld at the


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Column Bases / 3.3-25

same region as does the maximum unit shear force.However the analysis docs not take long:bending force on weld

f, = u= (9770 psi) (1%")= 14,660 lbs/in. (o ne bracket a nd two filletwelds )

or = 7330 lbs/in. (o ne fillet we ld)vertical shear force on weld(assuming unifolm distribution)

resultant force o n weld

required leg size of cer tical fillet we ldactual force

0 = allowable force

FIGURE 38

Alternate method. In cases whe re the forces arehigh, and the requirement for welding is greater, itwould be wcll to look further into the analysis in orderto reduce the amount of welding.In Figure 37, it is seen that the maximum unitforce on the vertical wt:ld due to bending momentoccurs at the top of the bracket mnnection ( b ) in arcgion of very low shear t~msfcr.Likewise the maxi-mum unit she ar force occurs in a region of low bend ingmoment ( c ) . In the following analysis, the weld sizeis determined both for bending and for shear, and thelarger of these two values are used:ccrtical shear requirement(maximum condit ion at N.A.)

f l = 25,200 lbs/in.to be carried by four fillet welds

actual force0 = allowable force

= ,562" or %,/,,"bending requirement(maximum condit ion at top of bracket)

actual force= -.allowable force

Hence use the larger of the two, or 3/4" fillet w elds..4lthough this altrrmate method required a slightlysma ller fillet weld (.654") as agains t (.75 8"), theyboth endod u p at % wheu they were rounded off. So,in this particular example, there was no saving inrising this method.Column stiffenersA rather high eompr~~ssiveorce in the top portion ofthese brackets is applied horizontally to the columnRange. It would hs wcll to add stiifenors behveen thecolumn flanges to transfer this force from one bracketthrough the column to thc opposite column flange;Figure 38.It might he argncd that , if the brackets are milledto br ar against the column flanges, th e bearin g areamay then be considered to carry the compressive hori-zontal force bctwecn the bracket an d the column flange.Also, the connecting welds may then be considered to


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/ Column-Related

FIGURE 39Slight tensile prestressUnit sheor ' between bracket in weld before load isforce on weld ond column flange applied

carry only the vertical shear forces. See Figure 39, left.If the designer questions whether the weld would

load up in compression along with the bearing areaof the bracket, it should be remembered that weldshrinkage will slightly prcstrrss the weld in tension and,the end of the bracket within the weld region in com-pression. See Figure 39, right. As the horizontal com-pression is applied, the weld must first unload intension before it would be loaded in compression. Inthe meantime, the bracket bearing area continues toload up in compression.

This is very similar to standard practice in weldedplate girder design. Even though the web is not milledalong its edge, it is fittpd tight to the flange and simplefillet welds join the hvo. In almost all cases, these weldsare designed just for the shear transfer (parallel to theweld) between the web and the flange; any distributedfloor load is assnmed to transfer down through theflange (transvrrse to the weld) into the cdge of theweb which is in contact with the flange. Designersbelieve that even if this transverse force is transferredthrough the weld, it does not lower the capacity ofthe fillet weld to transfer the shear forces.

Refer to Figure 37 (b ) and notice that the bendingaction provides a horizontal compressive force on thevertical co nn edn g wclds along almost their entirelength. Only a vcry small lcngth of the welds nearthe base plate is subjected to horizontal tension, andthese forces are very small. The maximum tensileforces occur within the base plate, which has no con-necting welds.shear force on certical weld(assuming uniform distribution)

516.5kf -.-------- 4 x 30"= 4310 lbs/in. (o ne weld)

t;crtical weld size(assuming it to transfer shear force only)

bnt 3" thick column flange would require a minimumlhr' h (Table 2, Sect. 7.4).

If partial-penetration groove welds are used (as-suming a tight fit) the following applies:allowables (E70 welds)

compression: same as plateshear: 7 = 15,800 psi

shear jorce on one weldf. = 4310 lbs/in.

required effective throat

i j using bevel ioint Y6,ik

t = t, + '/a= ,273'' 4- W"= ,398"raot face (land) = ll/z" - ( .39Wr )

= ,704'' or use -'


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if using 1 joint k-%"4

1 %"t = t, "= ,273"root face (land) = 1Yz" - (.273")= ,954" or use '/8"

A portion of the shear transfer represented by theshear force di~ tribut ion n Figure 37 ( c ) lies below aline through the top surface of the base plate. I t mightbe reasoned that this portion u a ~ l d e carried by thebase plate and not the vertical connecting welds be-tween tire bracket and the colnmn flange. If so, thistriangular arcs would approximately represent a shearforce of

?5. (24,63O#/in.) 6" = 73.9"to be deducted:

516.5&- 3.9' = 4426&

However, in this example, the column flange thicknessof 3" would require a %" fillet weld to be used.Brackets to column flange edgesThe base section consisting of the brackets attachedto the edge of the column flanges, Fignre 40, is nowconsidered in a similar manner. From Illis similaranalysis, thc brackets will be made of 1W-thick plate.

Figure 41 shows the resulting column base detail.

FIGURE 40 FIGURE 41


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COLUMN BASE PLATE DIMENSIONS (AISC, 1963)- COLUMN BASE PLATES / Forc Dimensions fo r max imumII olumn l oadsL?Q Base nlaien, ASTM 1116. h - 27 ir iCuacil ic , - 30UO or, 'Or / COLUMN BASE PLATESDimensions for maximumcolumn loads mT1.-Bare OaiPs, hSTM A16. F,, 27 kr ,coacrsts. l , 3OM nri 1 . J.- -

Wt .P"Fi.-b .1w1611331201069992857972655853504540I1210089777266605 44945393367584840353128242017

~-

tn.

61: 6W

4 X 14%W

1X 12W1 X 10W:

1 x 8W

~.Note' iSI lO

Wh t--This and following toblei prenenisd here by cauttery of American Institute ofSteel Conltruction.


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Column Bases / 3.3-2


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2.3-30 / Column-Related

E 5! I S r , s 8. 2 a a a F Y . L D F F Fxk X x x r x s x z x m x X S x x X- X- X- X-- - - a " L D - - - a m F " ?"


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ases / 3.3-31

Column base plates for the 32-story Commerce Towers, Kansas City,Mo., were shop-fabr icated and shipped separately. At the site theywere positioned and bolted to the concrete. The heovy columns werethen erected ond field welded to base plates. This was facilitated byuse of semi-automatic arc welding with self-shielding cored electrodewire. Process quadrupled the speed of manual welding and producedsounder welds.

Ten-ton weldments were required for tower bases on lift bridges alongthe St. Lawrence Seaway. Edges of attaching members were double-beveled to permit fuil penetration. Iron powder electrodes were speci-fied for higher welding speeds and lower costs. Because of highrestraint, LH-70 (low hydrogen) E7018 electrodes were used on rootposses to avoid cracking, while E6027 was used on subsequent passesto fill the ioint.


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3.3-32 / Column-Related Design

In designing a scenic highway bridge with 700' archspan, near Santa Barbora, Col., engineers called fortower columns to be anchored to the concrete skew-backs by means of 1% " prestressing rods. The bot-tom of the column is slotted to accommodate thebase, an "eggbox" gr ill made up of vertical plateswelded together and to the box column. The towerssuppoFt heavy vertical girder loads but also safelytransmit horizontal wind and seismic loads from thedeck system to the foundation.

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