Custom Steel Truss Design
Custom Steel Truss Design
(OP)
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.






RE: Custom Steel Truss Design
RE: Custom Steel Truss Design
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.
RE: Custom Steel Truss Design
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.
RE: Custom Steel Truss Design
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.
RE: Custom Steel Truss Design
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.
RE: Custom Steel Truss Design
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.
RE: Custom Steel Truss Design
RE: Custom Steel Truss Design
Yes. It is prudent approach. Add bridgings/braces may help in member sizing, make the chord more economical.
RE: Custom Steel Truss Design
RE: Custom Steel Truss Design
Don't they typically get welded after adjustment?
RE: Custom Steel Truss Design
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.
RE: Custom Steel Truss Design
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.