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Unrestrained truss top chord analysis

Unrestrained truss top chord analysis

Unrestrained truss top chord analysis

(OP)
Hi everyone,

I have a client who wishes to use a cold-formed steel truss flooring system, with minimal (~84mm) concrete floor and timber framed upper floor walls with metal deck roofing, in two-storey residential housing construction. These systems are well established at my company but this client has been introducing more and more ideas into the system and they are very hungry to save a buck or 5 minutes of construction time, and I have come across a problem I can't find a solution to. The client wants to use these trusses in such a fashion that they are supporting flooring for only a fraction of their length (~0.5 of the length) and extending outwards to lower floor supports. This leaves a substantial length of the truss top chord susceptible to lateral buckling failure. I have analyzed many of these trusses and I am not sure how to determine the required amount of lateral restraint due to the changing compression in the top chord. The top chord experiences maximum compression under the floor which is fully restrained, but then each successive bay of the top chord away from the restraint has less and less compression. I have only found literature on using the conservative approach of assuming the top chord has the maximum level of compression throughout it's entire length and restraining it to suit that. Does anyone have any or know of any documentation of a more accurate and refined approach to determining the actual lateral restraint requirements? It would be greatly appreciated by my client. Thanks everyone.

RE: Unrestrained truss top chord analysis

Dear Pertheng

you have the following two solutions that will yield the same answer:

1-use the equation given in the paper "Elastic Buckling of a Column Under Varying Axial Force"from the AISC

2- perform an elastic buckling analysis using any FEM, in my case I use SAP2000, get the multiplier factor. this factor should be multiplied by the LARGEST NORMAL FORCE VALUE IN THE NORMAL FORCE DIAGRAM, in the length that is buckling.
by this you get the Pcr load (elastic buckling analysis). from Pcr you can get the K factor that you will use in the design formula.


RE: Unrestrained truss top chord analysis

It can be performed by hand using Newmark's Numerical Method.

BA

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