SELECTING

Modern Steel Construction • April 2002

Since the Steel Joist Instituteadopted the first standardspecification and first loadtable in 1928 and 1929, respec-tively, the building designer

has been able to specify standard joistdesignations rather than design eachstructural component of each steeltruss. The current Steel Joist Institute’sStandard Specification Load Tables andWeight Tables for Steel Joists and SteelGirders contains three standard specifi-cations for three distinct series.

K-SERIESOpen Web Steel Joists or K-series are

defined as simply supported uniformlyloaded trusses that can support a flooror roof deck (see Figure 1). The topchord of the joist is assumably bracedagainst lateral buckling by the deck.The K-series is distinguished by thedepth range of 8” to 30” with a maxi-mum span of up to 60’ and standardseat depth of 21/2”. Maximum uniformload for K-series joists is 550 plf. Thestandard load table found in the SteelJoist Institute Specification uses stan-dard designations which define thejoist depth, a series designation, thetotal load capacity, live load capacity

based on L/360 allowable deflection,erection stability bridging require-ments and approximate joist weight. Italso includes the K-series economytable so the lightest joist for a givenload can easily be selected. The ends ofK-series joists must extend at least 2 ½”over steel supports (see Figure 2).

The KCS joist is a K-series joist de-veloped to allow the building designerto easily specify a standard joist to sup-port not only uniform loads but alsoconcentrated loads or other non-uni-form loads. KCS joist chords are de-signed for constant moment capacity atall interior panels. All webs are de-signed for a vertical shear equal to theshear capacity. In addition, all webs(except the first tension web, which re-mains in tension under all simple spangravity loads) will be designed for 100% stress reversal. The building de-signer will calculate the maximum mo-ment and maximum shear and selectthe appropriate KCS joist from the KCSjoist load tables. If a KCS joist cannot beselected, or if any uniform load exceeds550plf or any concentrated load ex-ceeds the shear capacity of the joist,

double KCS joists can be used, or anLH-series joist can be selected.

LH AND DLH JOISTSThe second series, the Long Span

and Deep Long Span steel joist or theLH and DLH series, is defined as sim-ply supported uniformly loadedtrusses (see Figure 3). LH series maysupport a floor or a roof deck. DLH se-ries may support a roof deck. Both se-ries are designed assuming the topchord is braced against lateral bucklingby the deck. Its depth of 18” to 48” dis-tinguishes the LH series joist. It has amaximum span of 96’ and a maximumuniform loading up to 1000 plf. Itsdepth of 52” to 72” and a maximumspan of 144’ distinguish the DLH se-ries. It has a maximum loading of 700 plf.

The standard seat depth is 5”, al-though a 71/2” seat depth is preferredfor the larger joist designations. Theends of LH and DLH series joists mustextend a distance of no less than 4”over a steel support (see Figure 4). Thestandard load tables found in the SteelJoist Institute’s Specification use stan-dard designations that define the joistdepth, a series designation, the totalload capacity, live load capacity basedon L/360 allowable deflection, erection

SPECIFYINGand

Wesley B. Myers, P.E.

An insider’s guide to selecting and specifying K-series, LH, DLH-series joists and joist girders

Figure 1.

Figure 3.

Figure 2.

stability bridging and approximatejoist weight.

JOIST GIRDERSThe third series is Joist Girders de-

signed as simply supported, primaryload carrying members. Loads will beapplied through steel joists and typi-cally will be equal in magnitude andevenly spaced along the joist girder topchord. The ends of joist girders mustextend a distance of no less than 6”over a steel support. Joist girder tablesfound within the Steel Joist Institute’sSpecifications include member depth,number of joist spacings, loading ateach joist location and an approximateweight of the joist girder. The Steel JoistInstitute’s Weight Table for Joist Gird-ers includes approximate weights forjoist girders with depths from 20” up to72” and spans up to 60’. Standard seatdepth is 71/2”. The ends of Joist Girderseries joists must extend a distance ofnot less than 6” over a steel support.

When Joist Girders support equal,uniformly spaced concentrated loads,the joist girder designation provides anadequate specification of the member.For example, the joist girder designa-tion 60G10N12K indicates the joistgirder is 60” deep. The G indicates thatit is the joist girder series, the 10N indi-cates the number of joist spaces, andthe 12K indicates the magnitude of theconcentrated load in kips. The buildingdesigner should include the self-weight of the joist girder in the panelpoint load. The joist manufacturer willdesign the joist girder using the mosteconomical web configuration, typi-cally where the diagonals are locatedunder the concentrated loads (see Fig-ure 5).

As the depth to span ratios increase,it will be more economical to load boththe diagonal panel points and the verti-cal panel points (see Figure 6). Whilethe designation VG is not shown in theSteel Joist Institute’s Specifications,joist manufacturers will recognize thisdesignation as an indicator to locate thevertical panel points underneath theconcentrated load (see Figure 7). Be-cause the joists align with the web ver-ticals and do not block the open panelsabove the bottom chord, this configu-ration has the largest amount of unob-structed openings to accommodatemechanical ductwork.

BRIDGINGThe final component, joist bridging,

is required to:� Align the joist during erection;� Provide stability for the joist during

erection;� Control the slenderness ratio of the

bottom chord;� Assist in stabilizing the web systems.

The size, type and number of rowsof bridging depend on the span of thejoist spacing between the joist and thestandard joist designation. Joist bridg-ing may also be required to brace thebottom chord for wind uplift and axialloads. It may also provide lateral stabil-ity for the joist under gravity load withstanding seam roofs.

There are two types of bridging:horizontal and diagonal. Horizontalbridging consists of continuous anglesconnected to the top or bottom chords(see Figure 8). Diagonal bridging con-sists of two angles that cross diagonallyfrom the top chord to the bottom chordbetween each joist and are connected attheir point of intersection (see Figure 9).

For typical situations, the number ofrows of bridging required is given intabular form in the Steel Joist Institute’sStandard Specifications. These specifi-

cations also indicate when bolted diag-onal erection stability bridging is re-quired during construction. Bridgingfor all joists requires the end of eachbridging load have positive anchorage.When a joist is at the end of a bridgingline, such as at expansion joints or joistsat end walls, x-bridging should be usedbetween the last two joists. Standardbridging is required to laterally stabi-lize the joist against torsional bucklinguntil the permanent deck is attached.Construction loads must not be appliedto the joist until the bridging is attachedto the joist and anchored at its ends.

Diagonal bridging between the lastjoist and a rigid end wall may cause thediagonal bridging to act like a verticalsupport and therefore will attempt tocarry the joist vertical load. When thejoist deflects and the end wall does not,damage may occur in the bridging, andit will no longer be effective. Substitut-ing horizontal bridging for the diago-nal bridging after erection in this lastspace will allow a joist near the endwall to deflect independently.

Joists and joist girders in roof sys-tems will be subject to net uplift loadswhen the wind suction forces on theroof exceed the permanent dead load.Uplift loads could affect the design ofthe joists’ components as well as thebridging. Under gravity loads, the topchord of the joist is in compression, andthe bottom chord is in tension. Under anet uplift loading, the bottom chord ofthe joist will be in compression. Due tothis load reversal, the bridging designmay need to be adjusted to properlybrace the bottom chord.

April 2002 • Modern Steel Construction

Figure 4.

Figure 5.

Figure 6.

Figure 7.

Figure 8.

Figure 9.

The Steel Joist Institute’s Specifica-tions also requires joists subject to a netuplift to have a line of bridging neareach of the first bottom chord panelpoints. Depending on the actual mag-nitude of the net uplift, additionalbridging may also be required. Upliftalso causes a stress reversal in the joistwebs, and the manufacturer will be re-quired to design them accordingly.Steel Joist Specifications require the netuplift on joist and joist girders be spec-ified to the joist manufacturer. A noteon the drawings, such as “Design joistand bridging for net uplift of 20 psf,”can be utilized. When the building re-quires increased wind loadings at thecorners and edges, a diagram showingthe net uplift may be the best methodfor providing the joist manufacturerwith the required loading information(see Figure 10). Joist girders should beconsidered primary members whendetermining uplift loads, and thereforethe building designer may elect tospecify a lower net uplift on the joistgirders. The manufacturer will designthe bottom chord and the bottom chordbraces for the girder as required by thenet uplift loadings.

The design of steel joists assumesthe floor and roof decks have adequatestiffness to provide lateral stability tothe top chord of the joist. A standingseam roof should be assumed to nothave adequate diaphragm capacity,and bridging should be provided to lat-erally brace the joists under gravityloads. The top chord, the bridging sizeand spacing must be adjusted to pro-vide sufficient lateral bracing. Topchord horizontal bridging must be de-signed for a compressive axial force of0.0025nP where n is the number ofjoists between joist anchors, and P isthe joist’s top chord design force. Thegreater the quantity of joists betweenrigid bridging supports, the greater theforces due to the additive secondaryforces developed at the joist locations.The magnitude of these bridging forcescan be managed by the addition ofrigid bridging supports such as diago-nal bridging and/or horizontal trussesconstructed using the joists’ top chords.

END MOMENTSWhen joists are used as part of a

rigid frame or moment frame or otherbracing system, axial loads or end mo-ments may be induced into the joist or

joist girders. A load diagram (Figure11) or a schedule of loads may be usedto inform the joist manufacturer of themagnitude and the direction of the mo-ments and forces and the required loadcombinations that should be consid-ered for each load (Figure 12). The joistmanufacturer will check the affect ofthe moment and chord forces and ad-just the design of the chords and thequantity and spacing of the bottomchord braces accordingly. Suitable con-nections will also be required for thetransfer of these moments and chordforces from the bracing system orcolumns into the joists. If no other loadpath is provided, the top chord mustalso be designed to accommodate theaxial force and bending moment devel-oped due to the force coming inthrough the bottom of the joist seat (seeFigure 13). A better force path would becreated if a top plate (Figure 14), tie an-gles or knife plates (Figure 15) wereadded to connect either the adjacentjoist or the adjacent column if applica-ble. When the deck is used in the brac-ing system as a diaphragm, then theload must be transferred from the deckinto the beam or joist girder. The lateralstiffness of the joist seat may not be ad-equate to transfer this force. When thisoccurs, a tube steel or a C channel sec-tion can be used to transfer the lateralload from the deck to the supportingmember (see Figure 16). In a hybrid

roof system (Figure 17), a wood decksupported by a wood subpurlin sys-tem, and a steel joist with a wood naileris used as a diaphragm in the lateralbracing system. Nails provide the shearconnection to the subpurlins and woodnailer. Wood screws transfer the shearfrom the wood nailer to the top chordof the steel joist. Again, a top plate orknife plate should be used to transferthe developed axial load to the adjacentjoist or column.

SPRINKLER LOADSThe building designer can account

for loads from small ducts, cable traysand sprinkler systems by including auniform collateral load of a sufficientmagnitude to cover all these loads.Then a joist can be selected to resist thiscollateral load in addition to all theother uniform loads. Although theloads are delivered to the joists at dis-tinct locations, this method has provedto be reasonable and economical. Thebuilding designer should specify thatpipe hanger loads are to be located atjoist panel points so that local bendingwill not be induced in the joist chords.Large ducts and sprinkler mainsshould be considered as concentratedloads, and the magnitude and locationshould be specified on the design doc-uments.

ROOF TOP UNITS It is common practice for mechani-

cal units to be placed on the roofs ofbuildings. Rooftop units can vary inweight from 100 lbs. to more than25,000 lbs. They can cover an area of acouple of sq. ft. to hundreds of sq. ft.The building designer’s decision onhow best to provide capacity for theserooftop units will depend on the size,quantity, required location and area ofthe units. The building designer maydesignate zones on the roof where theunits may be placed and specify KCS

Figure 10.

Modern Steel Construction • April 2002

Figure 11.

joists to provide the additional capac-ity required to support these units.These zones should be located to pro-vide the maximum area while affectingthe fewest number of joists and joistgirders. Locating the zones nearcolumns will minimize the amount offlexural resistance required in the sys-tem and still allow a flexible system inwhich to locate the roof top units. Theunit’s reaction to the top chord of thejoist must be transferred to a panelpoint to avoid localized bending in thetop chord of the joist. If the location ofthe unit can be controlled, the reactionsshould be located over panel points. Ifthis is not possible, a special web mem-ber must be added in the field to sup-port this concentrated load.

When the magnitude and locationof a concentrated load can be accu-rately specified, the joist manufacturerhas the option of designing the topchord for localized bending or addingan additional web member in theirshop (see Figure 18).

Three options are available to thebuilding designer for specifying joiststo support uniform loads plus concen-trated or other non-uniform loads. Thefirst option would be to select a stan-dard K-series or LH-series designationusing an equivalent uniform loadingmethod. This method should not beused for large loadings that cause thelocation of zero shear to be outside thecenter 2% of the span due to possiblestress reversal in the joists webs. Thesecond option would be to calculatemaximum moment and shear and se-lect a KCS joist. A double joist canbe specified with both of theseoptions as long as the load istransferred equally to each joist.The third option is to include aload diagram on the contractdrawings for the joist manufac-turer. None of the three optionsconsider the effects of load bend-ing in the top chord, and there-fore a method must be providedto transfer the load to a panelpoint.

JOISTS SPAN DIRECTIONAND JOIST SPACING

For floor systems, it is usuallymore economical to span thejoists in the long direction. Sincethe joists sit on top of the joistgirder, they can be made deeper

than the joist girder by the amount ofthe seat depth without infringing onthe clear height requirements. Widejoist spacings provide a very economi-cal floor system. The widest spacing fora given deck profile and slab thicknessshould be used. Erection costs are typi-cally less, and the floor will usuallyhave better vibration characteristics.Deeper joists also allow larger penetra-tions through their webs. While widerspacing would also be preferred forroofs, the most economical framingwill vary based on design require-ments and layout, as well as interactionwith other building expenses such asthe wall systems. The optimum joistgirder depth in inches is approximatelyequal to the span of the girder in feet.Joist depth should be selected based onthe economy table found in the SteelJoist Institute’s Specification. The de-signer should also look at the bridgingrequirements for the selected joists. Itmay be more economical to pick aslightly heavier joist if a line of bridg-ing can be eliminated, particularly ifthe heavier joist eliminates the need forany diagonal bridging.

DEFLECTIONThe Steel Joist Institute load tables

provide deflection information as auniform load that will produce anL/360 deflection, for K, LH or DLHjoists. Other deflection criteria can bedirectly proportioned to determine theallowable capacity to produce this de-flection.

Figure 12.

Figure 13.

Figure 14.

Figure 15.

April 2002 • Modern Steel Construction

Figure 16.

Figure 17.

Figure 18.

CAMBERThe Steel Joist Institute’s Specifica-

tion designates that camber be sup-plied for the LH and DLH series.Camber is optional with the manufac-turer for K-series, so if camber isneeded on K-series joists, it should bespecified. Long LH and DLH seriesjoists will have a significant amount ofcamber and can cause problems in con-

necting the deck to the end walls if thecamber has not been considered in thisconnection. If a joist is placed next toan end wall, and the deck is to be con-nected to the shear wall, then some al-lowance must be made either in thecamber or in the location of the endwall supports so the deck will attach tothe wall system. If the edge joist iseliminated and the camber is not solarge as to cause the deck to buckle, thedeck can often be forced into contactwith the end wall support so that theconnection can be made. If specialcamber is required to make this con-nection, it must be indicated on thedrawings.

SLOPING JOISTSThe span of a parallel chord sloped

joist is the length along the slope. Min-imum depth, load carrying capacityand bridging requirements are deter-mined using this sloped definition ofspan. The load capacities from the SteelJoist Institute’s Standard Load Tableswill be the component normal to thejoist. The building designer shouldspecify the load component, parallel tothe joist (see Figure 19).

Sloping Joist ExampleRoof Slope = 6:12Dead Load (DL) = 20 psfSnow Load (SL) = 30 psfPlan dimension of Bay L = 40’-0”

Joist spacing = 4’-0” o.c.

θ = tan-1(6/12) = 26.6°

SL × cos2θ × joist space = (30 psf)(cos226.60°)(4’-0” o.c.) = 96plf

DL × cosθ × joist space= (20 psf)(cos 26.60) (4’-0” o.c.) = 72 plf

Total load (normal component) = 168 plf

Parallel load component = [(SL × cosθ) + D] × sinθ × joist space

= [(30 × cos 26.6°)+20] × sin 26.6° x 4’= 84 plf

Select joist from Economy Table: RL 168 plf/LL 96 plf

24K6 at 45’-0” + Total load = 179/Liveload (L/360) = 93 OK

Add note on drawings: Design slop-ing joists for parallel load componentof 84 plf.

Wesley B. Myers, P.E., has worked for15 years in steel and steel joist design andis a member of the Engineering PracticeCommittee of the Steel Joist Institute. Aprofessional engineer licensed in sevenstates, he is currently the vice president ofengineering of SMI Joist and the generalmanager of SMI Joist-Nevada. Visit theSteel Joist Institute’s web site at:

www.steeljoist.org

Figure 19.

Modern Steel Construction • April 2002

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