×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Truss Failure

Truss Failure

Truss Failure

(OP)
I recently looked at a truss failure where the vertical truss member from the bottom chord to the ridge had pulled out vertically from the metal press plate at the ridge. There is a high humidity condition within the attic and there was record snowfalls in the area where it occured. Any thoughts on causation of the failure? I think that the high humidity condition allowed the wood to swell leading the plates to slightly separate from the member. Your thoughts are appreciated.

RE: Truss Failure

Is this a cold attic - insulated at the ceiling level instead of at the roof structure level?

If so, and there was a roof leak in the area of the failure wetting the connection, it is possible that swelling due to freezing could have contributed to the plate loosening grip too...  Heck, this could have happened too since there was a lot of moisture in the wood as you describe - just a different scenario I suppose.

Then again, maybe this was the critical connection to fail under the high load condition.  Was there drifting at the time of failure?  Possibly an unbalanced snow condition?

Mike McCann
MMC Engineering

RE: Truss Failure

That is usually not a highly loaded member and is usually only in compression - depending on design.  Was this a scissors truss - both bottom and top chords pitched??

See if you can contact the mfg or the truss designer who might be able to shed some light on what is going on...

Truss plate might have been too small or placed incorrectly.

RE: Truss Failure

I tend to disagree with you Mike, the centerline vertical member from bottom chord to ridge is usually under tension loads and sometimes pretty high. But you are right, it depends on the design.  

RE: Truss Failure

We just need a whole lot more info to hazard an educated guess.  I have seen kingposts separate from chords - esp on scissors trusses - but not too common on standard trusses unless the plates were too small or misplaced or truss was way overloaded.

Design, grade and size of lumber, pitch, span, spacing, loading, etc...  all need to be reviewed

RE: Truss Failure

(OP)
Thank you for all of the feedback. I will look into all of the possibilities that were presented here.

RE: Truss Failure

Agree with swsengineer.  The kingpost is usually in tension.   

RE: Truss Failure

I mis-spoke - usually in tension - but a lot more info required to hazard a guess

RE: Truss Failure

question on roof truss,es in general.Take a 40 foot span and design a truss thats able to stand this span with a 60 lb load factor.Then lets just say after I have bought the truss,es I decide to add a bearing wall down the middle.
Will this bearing wall added after the fact have any bearing what so ever regarding future truss failure of this building???

RE: Truss Failure

Yes - it actaully could - because now some of the web members may (will) be reverse loaded.  But bracing them might solve the problem.  Also check chords.

Also the internal bearing might be overloaded?? - maybe

RE: Truss Failure

Thanks Mike
I am not actually doing this.I suggested this theory to a builder friend of mine whom says I am 100%  wrong and that adding a bearing wall down the middle to cut the free span in 1/2 would in NO WAY add to less possible truss failure do to overloading.I said to me that defies logic?
He says the truss always fails in the outter perimeter
In other words towards the outter bearing walls so therefore putting a bearing wall down the middle would in no way enhance the overall structural integrity of the roof truss in this situation??whos right?
Thanks

RE: Truss Failure

Well - again - I am worried about reverse loading in the webs and chords.  While he is basically right about trusses generally failing at the ends (perimeter) - a longer span truss could fail due to reverse loading because no one looked at that possibilty.  A long tension web or chord is all of the sudden now in compression - failure might loom.

The internal bearing will now carry about 5/8's of the total load.  This could cause a problem with perpendicualr to grain loading (allowable about 500 psi depending on grade) or might over load the bearing wall or beam below??

Simply - all loading conditons need to be checked.

RE: Truss Failure

A few years ago, I inspected a failure similar to the one mentioned in this thread.  The king post pulled away from the top chord for all but the end trusses in the building.  The building had been a pig barn, so the humidity was very high and the snow was pretty heavy that year.

Underneath all of the rubble, we found a feeding trough which had been suspended from the bottom of the trusses.  The trusses had not been designed to support a suspended load and it is probable that this is what caused the collapse.

Best regards,

BA

RE: Truss Failure

BAretired --

Funny you should mention that because it reminds of a failure I ran into many years ago.

Seems like a farmer bought some trusses off the yard - pre-made 24' trusses for residential use.

He put them like 8' oc and hung chicken cages from them.

All went well until the chickens fattened up. You can guess the "rest of the story"

A lot of dead chickens and one mad farmer..

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources