Typical Failure Point in Metal Plate Connected Truss

[AdamJSR]

Some of you may have read my previous threads on truss design and analysis...

I am still a bit stumped on actually deterring the strength of the plates on a metal plate connected truss, but can any of you shed some light on what is typically the failure point? I ran an analysis on the lumber (chords and webs) and it looks like I'm getting values of around 0.5 CSI (Using the analysis method outlined in ANSI TPI 1 2002).

I understand that the plates are usually the weak point, but i'm still figuring out where and how weak! Any experience with what ones typically fail first under vertical load (snow)

Thanks all

 

[rb1957]

a couple reasons i can think of ...

1) intentionally, the plates are being designed as a structural fuse, something that'll give way to protect the structure from more extensive (and expensive) damage, or
2) unintentionally, the plate loads were under-estimated (or the plates under-designed) and hd premature failure.

Quando Omni Flunkus Moritati

 

[TXStructural]

In my experience, these trusses fail:
1) plates begin to peel away once the member held by the plate exceeds the tension capacity of the portion of plate embedded into the wood. The appearance is that of a piece of tape coming loose from an edge and lifting away. This is incipient failure indicating overload in tension.
2) failure to properly brace long members and top/compression chords result in our-of-plane buckling of the truss or the member(s)

The computations I have done on "bad" trusses tell me that the wood is seldom the problem. The wood is selected for its convenience in limited sizes, and for its ability to resist deflection/buckling, but the plates can be provided in a host of sizes and shapes. The shape is determined by the geometry and demand, not the strength of the connected members. In other words, the plates are the place where money can be saved and will almost always be the weak link.

 

[woodman88]

It depends on the manufacturer inventory.
Some will design for the minimum lumber and plates. Than use the sizes and grades that meet or are greater than the minimum required.
Some have the inventory in the program to design them from. So you will see very low stresses in the members.
In any case, due to their inventory, you will never know what plates are the most stressed out without checking them all.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.