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TPI Repetitive Stress Increase

TPI Repetitive Stress Increase

TPI Repetitive Stress Increase

(OP)
Can someone give me an interpretation as to when you can use the repetitive stress increase?  I am being told by my superior that you can take the increase if you have two trusses adjacent to each other that are spaced 24" o.c. or less.  I am pretty sure this is incorrect, but want to confirm.  Thanks.

RE: TPI Repetitive Stress Increase

Section 4.3.9 of the 2005 NDS indicates that the increase can be applied to joists, truss chords, rafters, studs planking and similair members.  

According to the NDS I would believe it would acceptable to apply the repetitive member increase to the allowable bending stress in the design of truss chord, provided the other criteria of the section are met.  Three or more members spaced  not more then 24" o.c. joined by a load distributing element.

The NDS Commentary indicates the increase is based on the increase in load carrying capacity and stiffness when multiple  members are joined together.  Consult the NDS Commentary and ASTM D245.

RE: TPI Repetitive Stress Increase

(OP)
The scenario that came up was a case where you have a small first floor bump-out on say a house.  The bump-out is adjacent to the second floor.  The bump out only requires 2 roof trusses.  You have a ledger agaist the house to catch the roof sheathing.  In this case, could you apply the repetitve stress increase?

RE: TPI Repetitive Stress Increase

Probably --

BUT does it nake THAT much of a difference or is this just a polite enginering discussion (argument).

My understanding is three members (more or less alike) spaced at 24'' oc or less.

RE: TPI Repetitive Stress Increase

(OP)
Just a polite argument.  I have found that it makes a difference on certain trusses, however.    

RE: TPI Repetitive Stress Increase

It definitely can make a difference when you are right at the limit - say a two ply girder or some long span trusses.

So - who buys the beer??

RE: TPI Repetitive Stress Increase

The reasoning behind the repetiitve stress increase is as follows:

Timber is a highly variable material and the allowable stresses are based on the worst case scenario.

If one truss happens to be at this lower threshold, then it is very unlikely that the adjacent trusses are also at this lower threshold.

Therefore at ultimate load if the weaker truss starts to yield then some of the load will transfer to the stronger trusses.

If you have three trusses then the load can be transferred to the stronger by way of a simple span or a cantilever (depending on which truss is the weak one).

If you have only two trusses then you have no redundancy and therefore if one truss starts to yield then it will continue to yield. This is the reason why they state that it must be minimum 3 trusses.

RE: TPI Repetitive Stress Increase

3 trusses minimum and they must be carrying equal loads. I have seen some truss designers say a common and an adjacent 2-ply girder that have the same profile qualify for the repetitive increase. This is incorrect. Also a 3-ply girder does in fact qualify when the 3 plies are attached to share a load.

RE: TPI Repetitive Stress Increase

(OP)
After I show him this thread, looks like he's buying the beer. :)

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