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IS TRUSS DEFLECTION A PROBLEM

IS TRUSS DEFLECTION A PROBLEM

IS TRUSS DEFLECTION A PROBLEM

(OP)
When a run of roof trusses deflect different amounts, does this present a problem? For example. Let's say you have run of 5 standard commons, then the run of trusses changes to 5 attic trusses boxed out for an FAU. Say the deflection (in the center panel) for the attic trusses are twice that of the standard commons--in the same panel. What are the negative ramifications of this? How should you handle this? References to any articles that address this will be appreciated. Thanks.

RE: IS TRUSS DEFLECTION A PROBLEM

I think you have to look at the magnitude of the difference instead of percentage.  And Figuring trusses at 24" oc, i would consider limiting the relative deflection to 4'x12"/240 = .2" (The equivalent of a 4' span of drywall limited to L/240 deflection within that span.)  

I don't think there is any "Code" about deflection from truss to truss, specifically.

RE: IS TRUSS DEFLECTION A PROBLEM

Differential deflections with adjacent trusses is a major callback item for truss suppliers. Structurally sound trusses give the impression of inadequacy when sheetrock ceilng wave appearances give homeowners a concern. Differing cambers and strongbacks can help.

RE: IS TRUSS DEFLECTION A PROBLEM

I have seen cracks appear at this condition.....

CTruax is correct --- try to get cambers set correct or strong back.  I have even used 2 or 3 "extra" trusses until they reached a natural "break" or wall!!

This can also be a real problem in floors where joists of varying spans meet up.  One is short and one is very long although both are acceptable concerning loads and deflections....

RE: IS TRUSS DEFLECTION A PROBLEM

(OP)
Thanks to everyone. All of your comments have been very helpful.

RE: IS TRUSS DEFLECTION A PROBLEM

Differential deflection is a problem. Most issues stem from interior bearing walls that do not run the whole truss span. A good truss manufacturer will keep all designs within 1/4" diffirence. There are even some instances where we have 2-ply trusses just for this purpose.

A 7' setback hip girder with a bearing post is a common problem. The 9' setback truss will deflect a lot more.

RE: IS TRUSS DEFLECTION A PROBLEM

You mentioned roof trusses - I have a real problem with differential deflection there as the local working of the roof diaphragm can cause the over-working of, and a premature breakdown of the roof membrane and, heaven forbid, the nemesis of us all and the income of a lot of lawyers, water leaks.  Try for uniformity wherever poossible, and don't be afraid to double up trusses locally to minimize any necessary differential deflection - spread out the difference over a greater area.

At the floors, I am always trying to avoid intermedoiate hard points where possible.  These can give unsightly reads in the floor molding.  Doubling or trippling trusses in the area for two or three joist bays helps to spread out the deflection difference here too, as in 3,2,1,1,1, etc.

Mike McCann
McCann Engineering

RE: IS TRUSS DEFLECTION A PROBLEM

(OP)
Thanks Mike. That helps a lot.

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