Metal Deck to Brace Compression Member
Metal Deck to Brace Compression Member
(OP)
Hi, I am designing a long span structure with a curved roof. The have deep trusses with wide flange chords oriented sideways. Our unbraced length for the weak axis of the chords is equivalent to our panel points. We have provided beams framing into each panel point with 1 infill beam between panel points. These beams span 35-40' between trusses. We have assumed the beams framing into panel points will act as bracing to the top chord member. We have no additional framing/bracing for the infill framing between the trusses. My chord forces are approx. 2000k, therefore, the force required to brace these is around 40k. My question comes in regard to the compression strength and weak axis unbraced length of the members bracing the truss. Since my deck will be rigidly attached to the top flange of the beam, I think all will agree that the beam top of flange is braced. However, the beam bottom flange is free for 35-40 feet. What is the overall unbraced length assumption for the weak axis of this beam? I have already heard multiple opinions from yes, it is braced against weak axis buckling, to no, I must provide additional infill framing as weak axis bracing to the infill beams. I have looked and not really found any papers or solid research from AISC regarding the matter. Any help or educated opinion is welcome.






RE: Metal Deck to Brace Compression Member
The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.
RE: Metal Deck to Brace Compression Member
RE: Metal Deck to Brace Compression Member
Additionally, since the beams are at 11' o/c, how could you possibly get 40K per beam, period?
The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.
RE: Metal Deck to Brace Compression Member
Our chord load is 2000k. We assume only beams framing into panel points provide bracing for compression. To provide this bracing we are designing the beams providing the bracing for 2% of the chord force. This equates to 40k load at each panel point.
RE: Metal Deck to Brace Compression Member
The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.
RE: Metal Deck to Brace Compression Member
A number of issues spring to mind here.
RE: Metal Deck to Brace Compression Member
The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.
RE: Metal Deck to Brace Compression Member
Thanks for the thoughtful response. Regarding your suggestions;
1. We have not discussed with an erector or contractor yet, but I agree with your point.
2. I have thought about this but may be difficult to control deck attachment in the field to have such a pattern... We may insert beams to brace weak axis at every other and attach this framing to deck but this also adds quite a number of small pieces to be erected very high above the floor.
3. There is no net uplift on the long span truss due to minimal wind loading. However, we do have a full layer of framing at the bottom of truss elevation and beams framing at every panel point. There is an entire rigging system and catwalk system, so we created a diaphragm of horizontal framing/trusses at this level - this was omitted in my quick sketch.
4. We do also have a horizontal truss at the curved roof level following the contour of the truss. This is around the entire perimeter of the building. The idea was to have infill beams at the interior of the roof act as collectors to transfer all lateral loads to these perimeter horizontal trusses which then connect to vertical braced frames and all four sides of the building.
RE: Metal Deck to Brace Compression Member
The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.
RE: Metal Deck to Brace Compression Member
RE: Metal Deck to Brace Compression Member
RE: Metal Deck to Brace Compression Member
http://www.aisc.org/store/p-2304-torsional-and-con...
RE: Metal Deck to Brace Compression Member
RE: Metal Deck to Brace Compression Member
In regards to the design of the bracing members, whether they are designed for 1% or 2% of the truss top chord force, another paper that provides an in-depth treatment of columns braced on one flange is "Torsional Bracing of Columns" by Helwig and Yura (ASCE Journal of Structural Engineering, May 1999).
RE: Metal Deck to Brace Compression Member
Hokie, whether the load is 1% or 2% our problem is the same as this is a high KL/r issue. We have decided to provide kicker bracing to provide lateral restraint at center span to the roof beams. This will provide us adequate capacity to support the chord bracing loads.
Also, as was not shown in the sketch, the bottom chords are also braced at every panel point.