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top chords bowing from stress

top chords bowing from stress

top chords bowing from stress

(OP)
I am hanging 60'- 3" gable trusses(4/12)and my top chords are bowing(and racking)in opposite directions(uniformly) on all my installed trusses. They are literally trying to lay down on the headers. One side toward the front of the building, the other toward the rear. When I try to plumb and lathe the top chords on one side, it aggravates the bow in the other side in an extreme(and scary)way. The builder I worked for prior to starting my own company inspected my build to try and help me dtermine the cause of this problem. The only thing he could come up with was that this truss behavior is due to the rafter lengths being too long and causing compression stress. He was right, they are too long. We measured them this afternoon. The bottom chord is built to the precise measurements shown on the spec sheet, but the top chords are not. Most are a combined total of 3/4"(or more)longer than the design drawing calls for. Could this be causing enough compression stress to force the trusses to behave like this? This is the second time I have attempted to hang them. This is a dangerous situation for my crew. Any help would be appreciated.

RE: top chords bowing from stress

Sorry but I am having a hard time trying to get what you are saying. Are the trusses deflecting different from the gable? are the top chords bowing out away from the "face" of the gable? Are these trusses 2x4 or 2x6 top chords? Are they clearspan or do they have interior bearing? What are they loaded for , tile , shingle or ??

 Let me say this , If you are setting a 60-0 + span truss, you will have to go to extra efforts in order to keep them plumb. Can you say extra bracing???

JP
Truss Designer

RE: top chords bowing from stress

(OP)
I should have been more descriptive in my original post. the top and bottom chords are 2 x 8. They are triple fink type trusses, loaded for 2 x 4 purlins (on 2'-0" centers)and metal roofing. They are clearspan. The building (100' x 60') is pole construction(6 x 6)with a double header(2 x 8). I'm not sure I have the vocabulary to properly describe the truss behavior, but, here goes: The top chord on the west side, from the heel to the peak, is bowing toward the south end. the top chord on the east side is bowing toward the north end. We call it an S-bow, I have no idea if ther is a proper term? We braced the trusses adequately, I believe, with advice from a P.E. friend of mine(he is mechanical, however, and would not claim to be an expert). Several other builders  we brought to the site, to help us identify our errors, agreed that the trusses were adeqately braced and the building construction was not improper.  the bowing seemed to worsen the more the trusses were braced, which led to the theory of improper rafter lengths causing stress when they were forced into our building width dimension. According to the cad drawings furnished with the trusses, the rafters are 1 1/4" too long, on average. which leads me to my question. Could a combined rafter length overrage of 2 1/2" cause this type of behavior when the trusses are lathed together? Would this be bowing due to stress? I apologize if my vocabulary is limited and my explanation difficult to understand, and appreciate the offered help greatly.

RE: top chords bowing from stress

I'm not a structural either but it sounds like typical lack of bracing.  Did the truss manufacturer provide bracing details?  Were they perhaps designed for continuous bracing such as sheathing?

RE: top chords bowing from stress

The behavior is typical of long-span trusses.
The truss supplier should have supplied you with literature on the correct method of installation (or where to obtain it).
Goto WoodTruss.com and read the free technical sheets:
-Installation of Long-Span trusses
-Temporary (during construction) bracing
-Permanent Bracing

The building designer should make sure the top chords are adequately braced with the purlins and metal roof diaphragm.

Too many of these trusses end up on the ground.
I'm sure the fabricator will provide the literature when he sells you the second set (replacement) trusses.

RE: top chords bowing from stress

I don't think the extra 2"-2.5" really makes a difference with your buckling in this case.  Probably what is going on is the standard bracing details do not work for trusses this long.  The information provided by CTruax is some good information and contains a good suggestion:  to be safe you may want to hire a professional to design the bracing you need if the extra bracing required is not already supplied by the manufacturer.  I definitely think more bracing is required.

RE: top chords bowing from stress

If you bought these trusses from a legit manufacturer, they will have the stamped drawings on file, get a copy and make sure that you have all the additional bracing required.  IN my experience, even "experienced" installers seriously underbrace their trusses.  
Of course if you bought the trusses from a legit company you should complain that they arent built to spec if the top cords are too long.

RE: top chords bowing from stress

Ok Rafter , I think is understand now. CTruax is spot on is saying this TYPICAL of long span trusses. you are asking alot from this component!! This is also complicated by them being used in a purlin style roof. Either way you are lacking the proper bracing.
 My initial guess is going to be that the only bracing is the purlins themselves and they are all perpendicular(sp?)to the trusses themselves. Is this true? If so youll need to apply some "X" bracing or "45 degree" style bracing.
 Check this TPC info out
http://www.woodtruss.com/projects/woodtruss/images/publication_images/b2.pdf?PHPSESSID=68ad12bdaf92a406145032ac30117ef9
 One other question I have is were the trusses plumb when they were istalled? Im sure if you asked the framer he will say he did. What kind of bracing is on the bottom chords?
 One other thing is what is the truss spacing and loading? I would make a check on that also.
 What style bracing is being used at the gable ends? Is there a diagonal brace from the bottom chord going back into the truss system?

Best of luck... JP

JP
Truss Designer

RE: top chords bowing from stress

Correction to my previous post , the link is from the WTCA / TPI web site.

 Not the tpc.... thats a golf club.

JP
Truss Designer

RE: top chords bowing from stress

I think jptruss just seconded my thoughts, you should take a look at the stamped engineering drawings for the truss and look for any additional bracing it requires.  When I was designing trusses, our drawings would show the locations of any required additional bracing.  When you look at those drawings you should make sure that they were designed for a purlin style roof system, many designers rely on the passive diagonal bracing of a solid plywood roof.

RE: top chords bowing from stress

I would strongly suggest that you hire a qualified structural engineer who deals with wood buildings (not mechanical)to go out to the site to help you figure out what is going on. He should be able to tell you whether you have adequate temporary and permenent bracing installed and whether the metal deck diaphragm is adequate to transfer lateral loads.

RE: top chords bowing from stress

You had better be real careful or you will be buying a whole new set of trusses. These long span wood trusses are notorious for collapsing due to improper permanent and temporary bracing.

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