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Top Chord member selection...

Top Chord member selection...

Top Chord member selection...

(OP)
Working on a truss and viewing the different methods for accurate analysis, there is no shear and moment diagrams for them. Why? When digging into the AISC construction manual to find an appropriate member, its simply a compression property that we look into? For example, I have a truss that spans 127 ft. with a tributary width of 47 ft. A simple DL of 25 and LL of 20. I can not reduce the Live Load because its an assembly structure and in doing a traditional moment and shear diagram then my results are huge. Finding a member that can hold a 1,771 kip ft. moment is crazy. If I look for a compression property(Table4-7), bases on my method of joint and section analysis, well then my selections are very reasonable such as a WT12x52 could do the trick.

How do you guys go about selecting the members.

Thank you,

Hemi79

RE: Top Chord member selection...

Trusses are typically designed based mainly on gross tension and compression. Depending on the connections at the joints, which may be continuous for some members, moments can be present at some member ends that have to be considered too.

The development of computer programs such as RISA 3D computes what these moments are too, as well as moments and shears along set points of the member's length if needed.

If you are not well versed in the design of trusses, you really should engage a structural engineer to help you. They can be pretty specialized, especially at the span you are talking about.

Mike McCann
MMC Engineering

RE: Top Chord member selection...

(OP)
Thank you very much for the reply. It really helps. The structural engineering firm i do work for hasnt put me to work on something like this before. However, im attending summer school for a Masters in Civil with a structural concentration and everytime we look at something like a joist or truss there seems to be so much uncertainty amongst our group and ide really like to put something together to present. Maybe something for a 30ft span and considering different truss types. I will do it for joists as well.

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