Masonry-to-steel-joist CONNECTION

1 The structural composition of abuilding often consists of ma-sonry walls and steel this type of system, masonrywalls perform as both bearing andnonbearing components. Whendetailing steel -joist-to-masonryconnections, designers shouldtake into account several factors,such as the adjustability of theconnection, potential movementbetween the two systems, and lat-eral bracing of the masonry and LH-series joists aremost commonly used with mason-ry. The span and depth of theseopen-web joists determine theirbearing requirements, as estab-lished by the steel Joist joists range from 8 to 30inches in depth, with a maximumspan of 60 feet. The minimumbearing requirement on masonryfor a K-series joist is 4 inches. Ifthe joist is detailed to be an-chored to a steel bearing plate,the plate must be at least 6 inch-es wide (parallel to the wall).LH-series joists range from 18to 48 inches in depth, with a maxi-mum span of 96 feet. The mini-mum bearing requirement onmasonry for an LH-series joist is 6inches.

2 If the joist is to be an-chored to a steel bearing plate,the plate must be at least 9 incheswide (parallel to the wall). masonry -to- steel -joistCONNECTIONC onsider connectionadjustability, potentialdifferential movement, and lateral bracing of the masonry wallsBy Walter LaskaFigure 1. If a welded CONNECTION is desired between a masonry bearing walland steel roof joist, you can specify a bearing plate with studs solidly em-bedded in a grouted bond 2. A continuous bulb tee embedded solidly in a grouted bond beam pro-vides a welded CONNECTION between a masonry bearing wall and a steel roof masonry unitBrickRigid insulationHorizontal joint reinforcement at 16 inches on center verticallyMetal decking and insulationBond beamConcrete masonry unitRigid insulationBrick11 2-inch minimum air space11 2-inch minimum air spaceHorizontal joint reinforcement at16 inches on center verticallySteel joistMetal decking and insulationField-weldjoist to plateReinforced bond beamBearing plate with studs solidly embedded in grouted bond beamSawcut concrete masonry unit or special unitSteel joist spaced at designated intervalsContinuous bulb tee embedded solidly in grouted bond beamSteel joistwelded to bulb teeIt is sometimes necessary, how-ever, for a steel joist to bear onmasonry less than the requireddimensions when constructingtall, slender (single-wythe)

3 Rein-forced masonry walls, for exam-ple. In such circumstances, thestructural engineer should givespecial consideration to the de-sign of the masonry and steeljoist a roof joist spans its max-imum length and carries its maxi-mum allowable load, the ends ofthe joist will rotate. This inwardmovement (joist shortening) isthe result of deflection. If this con-dition is overlooked, the masonrycan bow and way to eliminate this prob-lem is to specify joists stiff enoughto resist maximum deflection. Youalso can minimize deflection byincreasing the number of joists ,thereby reducing the space be-tween should be insulated prop-erly to prevent joist movementdue to temperature should also consider the timeof year the joists are erected andthe type of enclosed environmentthey will be subjected to. If joistsare erected in hot weather andsubjected to heavy cooling or re-frigeration upon completion, theywill contract. joists erected incold weather and later subjectedto heating will expand.

4 To accom-modate this thermal movement,you can specify that the steel fab-ricator modify the camber (up-ward bowing) of the top chord ofthe joist. A special slippage con-nection to accommodate joistlengthening may be required ifthe walls are braced laterally bysteel addition, do not design joiststo be in direct contact with theouter face shell of concrete ma-sonry units. Provide a clearanceof 3 4inch. If the two materials arein contact, any structural move-ment of the joist can stress theconcrete masonry , causing clearance is especially criti-cal in the design of single-wytheFigure 3. If a bolted CONNECTION is desired, specify a slotted channel solidlyembedded in the 4. To increase the allowable height of the wall, one option is to ex-tend the bottom chord of the joist and connect it to the wall after all roofdead loads are in capProvide continuous sealant under drip on both sides of cap and where cap sections overlapThrough-wall brick or concrete masonry unitsSlotted channel solidly embedded in grouted bond beamReinforced bond beamHorizontal joint reinforcement at 16 inches on center verticallyMetal decking and insulationConcrete masonry unitor through-wall brickSawcut concrete masonryunit or special unitReinforced bond beamMetal decking and insulationContinuous bulb tee embedded solidly in grouted bond beamClip angle anchored to concrete masonry unit with expansion anchorsWeld extended bottom chord of steel joist to clip angle after all roofing dead loads are in placeFill cells of concrete masonry solid with grout or mortar directly below bond beamSteel joist with oversized slots in top chord bolted to slotted channelSteel joist

5 Welded to bulb teewalls, where any cracks that de-velop would be on the exteriorand exposed; in double-wythewalls, in contrast, cracks woulddevelop on the interior andwouldn t be wall connectionsA CONNECTION should be de-signed so that it can be installedeasily. It needs to be adjustablesince distances between bearingpoints can vary by an inch ormore due to construction toler-ances. In addition, the anchorbolts set in the masonry duringconstruction can be a CONNECTION that allowsadjustment of the joist in two di-rections north-south and east-west. If a welded CONNECTION is de-sired, then specify a bearing platewith studs (Figure 1) or a continu-ous bulb tee (Figure 2), which willprovide adequate plates should be fabricat-ed and delivered to the jobsiteprior to masonry construction toenable masons to embed theplates before the joists are a bolted CONNECTION is de-sired, then specify a slotted chan-nel solidly embedded in the ma-sonry.

6 Also, specify the steel joistto be fabricated with oversizedslots in the top chord. The chan-nel enables the bolt to be adjust-ed from left to right, while theslotted joist allows for up-downadjustment (Figure 3).Nonbearing wall connectionsWhen connecting steel joists tononbearing masonry walls, con-nection adjustability is not as crit-ical, but you should still give spe-cial consideration to thisconnection. Two similar types ofconnection plates provide thebest the most adjustability, spec-ify a clip angle bolted to the ma-sonry with either an expansionanchor or an anchor bolt. Alterna-tively, you can specify fabricatedmetal inserts, which are installedduring construction. Clip anglesand metal inserts should be fabri-cated and delivered to the jobsiteprior to the beginning of masonryconstruction. Lateral bracing of wallsMasonry walls must be bracedlaterally to resist wind loads(which govern in nonseismic ar-eas). This bracing is commonlyprovided by a roof system con-structed of steel joists .

7 The steeljoists (and metal decking) act as adiaphragm that continuouslybraces the wall against windloads at the bearing , piers, pilasters, orcrosswalls are not required forlateral bracing if the wall is builtwithin height limitations allowedby walls are braced at themasonry and steel joist connec-tion. To increase the allowableheight of a wall without increasingthe wall thickness, adding morerebar, or constructing columns,piers, or pilasters, you can extendthe bottom chord of the joist andconnect it to the wall but onlyafter all roof dead loads are inplace (Figure 4). The joist must bestiff enough to minimize deflectionafter live loads are applied. If ex-Figure 5. To brace the wall laterally, you can extend the horizontal bridgingangles and weld them to clip angles embedded in the masonry wall. Theextended section of each angle should align with the horizontal bridging.(Nonalignment could cause joist chords to crimp from wind pressure.)

8 Metal decking and insulationConcrete masonry unit or through-wall brick unitExtend bridging angle and weld to clip angleFill cells of concrete masonryunit solid with grout or mortarHorizontal joint reinforcement at 16 inches on center verticallyClip angle bolted to masonry with expansion anchorExtend angle and weld to clip angleReinforced bond beamFigure 6. Another option for bracing a wall laterally is to extend the horizon-tal bridging angles and weld them to metal inserts embedded in the decking andinsulationSteel joistsExtend angle and weld to fabricated insertConcrete masonry unitHorizontal joint reinforcement spaced at 16 inches on center verticallyExtend bridging angle and weld to fabricated metal insertsFabricated metal inserts solidly embedded in concrete masonry with mortarcessive deflection occurs, a hori-zontal crack can develop in themortar joint immediately aboveor below the bottom chord walls also must bebraced laterally by the steel brace these walls, you can ex-tend the horizontal bridging an-gles and weld them to the metalplate or angle, which is connect-ed to the wall (Figures 5 and 6).

9 Align the extended angle with thehorizontal bridging to preventcrimping in the joist chords dueto wind pressure. However, ifspacing between the bridging an-gles is in excess of 6 feet, bridgingalone may not adequately transferwind loads. In this case, you mustincrease the number of diagonal bridging is used forsteel joists , specify a horizontalbridging CONNECTION at the endjoists to brace the walls these joists to themasonry is not recommendedand may cause the walls to crack;when the joist deflects, loadswould be induced into the cross-bracing, pulling the top connec-tion inward while pushing thebottom CONNECTION outward, re-sulting in additional design of masonryand steel requires a thorough un-derstanding of the two problems can beavoided and successful con-struction can be assured byproviding well-developed andworkable CONNECTION details. Walter Laska is the senior consultant andpresident of masonry Technologies Inc., inDowners Grove, Ill.

10 , and is a technical con-sultant for masonry #M960472 Copyright 1996, The Aberdeen GroupAll rights reserv