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Perpendicular tension loading in truss connections

Perpendicular tension loading in truss connections

Perpendicular tension loading in truss connections

(OP)
Hi everyone.  I've used this forum often and have found lots of useful information.  This is my first posting so please bear with me.

A colleague of mine and I were discussing the issue of perpendicular to grain tension loading in a wood sill plate.  We're all taught to avoid this type of loading condition, but what happens in a truss when this occurs.

Take a simple "A" frame truss with an elevated bottom chord that is vertically loaded.  The bottom chord is bolted to the top chord at its ends.  Once the truss is loaded and the bottom chord goes in to tension and pulls on the top chord, doesn't this produce a perpendicular to grain loading on the top chord.  It may only be a small portion of the load in the bottom chord but how does the code (IBC, NDS,…) resolve this?  Are my colleague and I missing something?

Thanks in advance for any advice.

RE: Perpendicular tension loading in truss connections

This condition occurs frequently when, for instance, a girder truss has a 2x10 bottom chord, but only a 2x6 hanger is used to support framing from it.  Both Simpson and USP have tables that tell you how much to reduce the hanger's rated capacity to account for tension perp to grain.

RE: Perpendicular tension loading in truss connections

This occurs quite often in metal plate connected roof trusses.  The connection should be designed such that tension perpendicular to grain is avoided.  If tension perpendicular to the grain can not be avoided, mechanical anchorage should be provided to resist those stresses.   

RE: Perpendicular tension loading in truss connections

And don't forget that when loads get high - the plate often spans the width of member - or close to it.

RE: Perpendicular tension loading in truss connections

If you are loading the bottom chord below its centroid you will have cross-grain tension perp to grain.  Simpson and USP do have reduced allowable values for some truss to truss hangers to account for this.  It should be avoided wherever possible.

woodengineer

RE: Perpendicular tension loading in truss connections

Are there any published values for allowable stress perpendicular to grain?

RE: Perpendicular tension loading in truss connections

(OP)
Hi guys, thanks for the comments.

ctcray: Good comment.  I know this condition occurs in wood floor systems all the time.  Say for instance, you have a large GLB (5.125x16.5) with another floor girder framed into it (4x12).  Unless a top flange hanger is used to support the 4x12 it would seem a face mounted hanger would produce tension perpendicular to grain.  I don't remember seeing the reduced loads in the Simpson catalogue, but I'll look again.

CJSchwartz:  I agree.  Something should be added to the connection or the connection should be modified to avoid the tension stresses.  Although, I can't remember ever seeing a truss with a connection that had something unique or unusual.  Everything I have seen looks like a standard plate connection.  I would think in order to eliminated the tension loading the plate connection would need a backing plate similar to a Simpson "GLS" hanger.

Woodengineer:  I agree.  When ever possible the beam or truss should be loaded above the its centroid.  

Maybe this is not such a big issue with a "built-up" truss or the "A" frame I described in my earlier message.  What I mean by "built-up" truss, is a truss built with a 4x top chord and a 4x bottom chord all connected with steel plates.  In this scenario, all the connections should have the bolt groups centered around the top and bottom chord centerlines so as not to produce eccentric loading in the joints.  

The Timber Manual, 4th edition, shows an example of a wood truss, but only the connection with the compression loading is shown.  Later, in Chapter 7, there is the usual comment about tension loading perpendicular to grain should be avoided.  

I hope I'm not over thinking this, but it would seem there would be some type of documentation on this somewhere.

Thanks again for the advice.

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