Custom Steel Truss Design

[ARLORD]

I am designing a custom steel roof truss supporting bar joists. The span is about 70ft, I am using WT's for the chords, and double angles for the webs. I am doing this for the first time. I have found design criteria in the 2005 AISC 13ed, and in the design guide #7 for the design of the truss members connections, etc. However, I have not seen anything on the out of plane bracing requirements. Does anyone know where to find or what design criteria I should use to specify the top and botton chord out-of-plane bracing. I assume the bar joists will be bracing the top chord, so it more for the bottom chord, especially at the panel points.

 

[ARLORD]

Another question that I just thought of, if I have to design for lateral loads, wind/seismic, would I just model the truss with the lateral and gravity loads and design the members and connections based on the member loads from the model results. Am I missing anything?

 

[JedClampett]

Technically, the bottom chord is in tension unless there is a uplift load that overcomes the dead load. But I would still brace it such that L/r<300.
If I understand your second question, yes, you could model it for the reactions from your members plus its own dead load. But for wind this would be conservative as you can use smaller wind loads for larger tributary areas.
The lateral load portion of your question throws me off. The lateral loads are usually taken out with a diaphragm or seperate horizontal bracing.

 

[Lion06]

The bottom chord bracing seems to be an issue that hasn't been resolved on this forum yet. It's been discussed, but I don't think there's been any consensus yet. While I have gone through some examples using energy methods for stability and (somewhat) convinced myself that bottom chord bracing isn't required (unless it's subject to uplift forces) just for gravity to brace compression diagonals I still don't feel comfortable with that.

You can use the bracing provisions in App. 6 to check the bottom chord for strength and stiffness to brace compression diagonals.

As far as lateral loads, you seem to have it right. I would suggest modeling the columns (the actual height and size) and the actual connection conditions because this will influence the behavior of the truss.

 

[ARLORD]

Yes, the bottom chord is mostly in tension, except for uplift. But if it is just in tension, how are the panel points supported laterally, by weak axis bending of the bottom chord.

As far as the lateral load question, I don't have all of the info for this job yet. I do have a roof plan of the building. It currently has joist girders and they would like to fabricate the trusses themselves. I see x-bracing perpendicular to the trusses, but none in the direction of the trusses. So if the truss girders shown on the drawings were designed to take moment, I would then have to design the custom trusses to take the moment also. I wonder if the truss can take the moment with the member connection still being pinned connected, tension/compression members.

 

[kslee1000]

arlord:

If the bottom chord is fully in concentric axial tension, there is no weak axis (side sway) bending from itself, rather, the diagonal (web member) in compression may cause local instability.

 

[ARLORD]

Yes kslee1000,

I was thinking that any local instability caused by the compression member could be resisted by weak axis bending of the bottom chord member. Design the bottom chord for tension and bending.

 

[jike]

You might consider using a joist girder in lieu of a fabricated truss. It should lower your overall construction cost. The joist girder supplier can pretty much handle anything that you can throw at them: non-uniform loads, end moments, uplift, etc. as long as the chords are not larger than 6 x 6 angles, using grade 50 steel.

 

[kslee1000]

arlord:

Yes. It is prudent approach. Add bridgings/braces may help in member sizing, make the chord more economical.

 

[Bobber1]

Just for thought, many bar joists have slotted holes to allow for girder placement tolerances. Do the use of slotted holes preclude use of the joist as a brace for the girder?

 

[Lion06]

Bobber-

Don't they typically get welded after adjustment?

 

[Ron]

ARLORD...you mentioned one thing that caught my attention...."they would like to fabricate the trusses themselves". A 70-ft span, heavily loaded truss is not something that "amateurs" should take on just to save a few bucks(I'm assuming from your post that your client does not fabricate trusses on a routine basis). You will have complete joint penetration splices for the top and bottom chords, not to mention a bunch of out of position welding. Make sure in your design you call for such things as welding in accordance with accepted standards (for the US...AWS D1.l), that all welding be inspected by a Certified Welding Inspector or qualified Special Inspector, and that all welders be certified to the particular standard, process, and positions to be encountered.

Bobber1...as for slotted holes in the joist, they are often just for erection, but even so, if they are tightened to any reasonable tension (snug tight plus 1/2 turn or similar) they are not likely to slip under the relatively small lateral bracing loads required. If for diaphram, I would weld them or require tensioning in accordance with AISC requirements for a slip critical connection.

As for bottom chord bracing, not necessary except for uplift as many have noted. For uplift, consider using the guidelines of the Steel Joist Institute for deep, long span joist girders.

 

[ARLORD]

Thanks Ron,

Good idea for the welding notes, but I think they want to do it themselves because they are steel fabricators, I think. I am not in contact with the client directly. They at least have the materials in house. Thay are trying to reduce costs, it is their idea to do this. like I said they have design drawings with joist girders.