September 2004 Modern Steel Construction

You don’t have to know howto dance in order to sidestepsome of the problems that cansurface when designing withsteel joists and joist girders.

Here are a few ways to avoid commonmissteps—and make your project (andyour joists) more efficient.

1Situation: Setting the joist seat depthtoo shallow for sloped ends. Thetypical K-series joist seat depth of 

2½ inches is not adequate once the roof slope is ½:12 or greater. At the high endof the joist, it becomes difficult to placethe end web member to deliver the endreaction over the support. As shown inFigure 1, the chord angle may not clearthe support and would need to becoped.

Solution: Increase the seat depth any-where that more than a nominal roof slope is used. The joist manufacturer canreadily provide a variety of seat depthsother than the typical 2½-inch and 5-inchdepths.

2Situation: Including a web configu-ration in load diagrams or joist pro-files, when there are not any

specific geometric requirements. Drafterswill often “fill-in” a joist profile with ran-dom web members just to make it lookpresentable. The joist manufacturer thenloses the option of quoting the most effi-cient geometry.

Solution: Leave the web area blank oradd a note stating that the “web mayvary.” If there is a specific constraint forthe placement of the web members, suchas a large duct passing through the joist,show this on the joist profile.

3Situation: Limiting the use of joistgirders to only those designationsfound in the Weight Tables. The

purpose of the Weight Tables is to pro-vide the approximate self-weight of the

 joist girder to be included in the struc-tural design. The Weight Table is not acomprehensive listing of all possibilities.

Solution: Use joist girders for any rea-sonable combination of depth, length,kip loading, and number of panels. Ingeneral, most manufacturers can provide

 joist girders with chord angles of up to6×6×¾. Several manufacturers have pub-lished more extensive Weight Tables.(Steel Joist Institute is planning an expan-sion of its current Weight Tables to in-clude limits beyond 72 inches deep, 20kips per panel point, and 60 feet long.)

4Situation: Showing a weld at a joist

 bottom-chord extension when oneis not required, as shown in Fig-

ure 2. Atypical joist is designed as simplysupported with an underslung end.Welding the bottom chord extensions de-velop a fixed-end moment, which shouldnot be done unless it is considered in thedesign of both the joist and the overallframe.

Solution: Unless required, don’t call fora weld on a bottom-chord extension. Thecolumn stabilizer plate in the gap be-tween the chord angles provides thesame resistance to the joist overturningwithout being welded, as if it werewelded. If a weld is required, show theappropriate design requirement—eitheran end moment or an axial bracing forceto the bottom-chord extension.

5Situation: Don’t call for welds to

 joists that are bigger than the typi-cal chord-angle sizes. As shown in

Figure 2, a 1/4 inch fillet weld is specifiedfrom the bottom chord to the column sta-

 bilizer plate. For a typical K-series joist,this is thicker than the bottom chordangle sizes that would ordinarily beused.

Solution: Use thinner, longer welds.For smaller K-series joists, assume a max-imum angle thickness and weld size of 1/8 inch. For larger K-series, and smallerLH-series joists, assume a maximumangle thickness and weld size of 3/16 inch.Call only for a 1/4 inch weld at heavy long-span joists, or joist girders.

6Situation: Making the joist manufac-turer responsible for tracking downloading information that is not

available yet. Structural contract draw-ings that refer the joist manufacturer tothe mechanical contractor or equipmentsupplier for the joist design loads canoften delay joist fabrication.

Solution: When mechanical informa-tion is not available, consider designing aroof top unit “zone” with extra capacity.This is far cheaper than reinforcing joistslater on in the field.

7Situation: Using joists that are tooshort. Short joists that are less than8-feet long are not practical to fabri-

cate. If the drafter is not given other in-structions, a minimum joist size may beused even for very short spans, such asthe 8K1 joists in the corner of the framingplan of Figure 3.

Solution: Use joist substitutes. The joistmanufacturer can build a joist substitutemore efficiently than a very short joist.The joist substitute may consist of twoangles welded together to make a chan-nel section. It may be made of tube steelor another shape. The Steel Joist Instituterecently published a straight-forwardLoad Table for three standardized joistsubstitutes with the designations 2.5K1,2.5K2, and 2.5K3.

10 Tips for Designing withSteel Joists

by Timothy J. Holtermann, P.E.

 

steel joistsfocus on

   2 

   1   /   2   "

1 1/2"12

 

1/4" 2"TYP.

Figure 1.

Figure 2.

 

8Situation: Corner joists or substi-tutes that are not simply supportedat both ends. Notice in Figure 3 that

while the typical joist has two supportsand a cantilevered top chord extension,the shortest 8K1 joists in the corner aresimply cantilevers with no “base” span.This is a problem, whether these joists aredesignated 8K1 or if they are joist substi-tutes as discussed above.

Solution: Make sure all the framingmembers have support at each end.Where necessary, design the edge angleor other edge steel to provide supportspanning from the corner to the first non-cantilevered framing member. The size of the edge-framing member, as well as theresultant point loads where that memberis supported, should be shown as inFigure 3.

9Situation: “Over-designed” top-chord extensions on small joists. Ashort 8K1 or 10K1 will have a Load

Table capacity of 550 plf total and liveload. This capacity may be more than isneeded but comes at no great cost for themain span. As with a roof overhang (seeFigure 3) or a top chord extension of anylength, providing a capacity of 550/550 isdifficult for both strength and deflection.

Solution: Use the Steel Joist Institute’sExtended End Load Tables and specify atop chord extension type such as “S7”or “R3.”

10Situation: Using too many joistdepths in a skewed bay. Whilethe economy table might sug-

gest a different joist depth for every spanin a skewed bay, the joist manufacturer

 builds, bundles, and ships by depth.Solution: Refer to Figure 3. Instead of 

changing the joist depth several timesgoing into the corner, make only one ortwo steps down in depth, combining sev-eral span lengths in each depth. Then use

 joist subs for the smallest spans.For more information about steel

 joists, to order any of the above men-tioned guides or manuals, or to contactan individual manufacturer, please visitthe Steel Joist Institute web site atwww.steeljoist.org.

Timothy J. Holtermann, P.E., is engineeringmanager for Canam Steel Corporation, Wash-ington, MO and chairman of the Steel Joist In-stitute Engineering Practice Committee.

Figure 3.

September 2004 • Modern Steel Construction