Pre-Fabricated Wood Roof Truss Long Span 1

[Stenbrook]

I have a project where the manufacturer has specified a roof truss with a span greater than 60'-0". I seem to recall reading somewhere that when a roof truss exceeds 40'-0" it requires some kind of special rigging as well as a design engineer on site during erection. Does anyone know about this or have an idea as to where to find out this information? (I'm working in Texas if that helps).

 

[Triangled]

"Requires" to me is the operative word in your post. Things very long pose special challenges during erection which a contractor may or may not be capable of properly addressing. Long trusses capable of carrying large gravity loads, in place with diaphragm and bridging/bracing fully and properly attached, but especially flimsy in the "weak direction" during their erection and unabridged/braced state, are a prime example. Your 'recollected requirements' might be municipality specific, but, nevertheless, be based upon some problem in that municipality's past. Los Angeles has historically required additional requirements based upon lengths. But these 'requirements' are at best an attempt to address the true challenge.
There are trusses which, capable of supporting large gravity loads when fully/properly laterally braced, would nevertheless twist and fail and fall to the ground during erection if not erected properly.
Texas is irrelevant, it is Earth that governs in this regard.

 

[RPMG]

I'm not familiar with this provision, but I wouldn't be because I live in snowy Michigan where there are no 60'-0" wooden trusses. However, it should be the manufacturer's engineer that's on-site, as you should not dictate how the truss should be handled.

 

[XR250]

Unless you have hands-on, practical experience with long span truss erection, I would (and your insurance company) would recommend staying away from this

 

[Canuck67]

Within the last year we became aware of a project in our area with 60' span trusses (in a snowy area with 50psf snow load). Contractor was not aware and/or ignored the requirement to have a person/engineer knowledgeable in long span wood truss erection and guess what .... after erection of about 20% of the trusses, they all collapsed (fell sideways). Luckily no one was hurt that we are aware of. It does happen and after seeing the photos, it definitely requires someone knowledgeable and experienced onsite during erection.

 

[XR250]

I have seen contractors gang 4 or 5 trusses on the ground and sheath them. Then use a crane to set the assembly. Seems like possible a safer alternative to sheathing them once they are set.
FWIW, just designing the top chord bracing on piggyback trusses is difficult. I try to use sheathing there instead of diagonal bracing.

 

[RWW0002]

For what its worth, IBC requires additional special inspection of temporary and permanent bracing of truss components for trusses spanning 60' or greater.

 

[Triangled]

I don't believe there is anything sacred about 40' or 60' thresholds, as a 39' truss or a 59' truss could also fail during erection, twisting in place and falling to the ground, if not handled properly. Of course, 40' or 60' may be municipality stipulated thresholds, but also of course, a municipality non-requirement doesn't mean it's OK. The combination of gravity and geometry will determine what happens to your trusses.

The IBC requirement pointed out by R

http://WWW0002

certainly makes good sense.

Assuming your truss has been designed properly to support required loads when placed vertically and bridged, braced and diaphragmed, during erection it should be supported in place until that bridging and bracing and diaphragm is fully installed. What is minimum support? That is to be determined professionally, but be conservative.

I think non-snow Texas is worse than heavy snow areas, simply because your trusses will be lighter; and, therefore, I'd guess, have even smaller weak direction properties, and, therefore, be even more likely to buckle during erection if not properly supported.

There are wood truss industry associations that almost certainly would provide general guidance. APA-EWS in Washington very well may have guidance. I know that SJI provides guidance for steel joists including certain requirements for rigging to remain in place until certain bridging/bracing is installed.

 

[Stenbrook]

I mean at the end of the day these trusses are being designed by a truss manufacturer. So I don't have all the information about how they were designed and braced, etc. I only need to know if I need to specify that an engineer be on site during erection or if that is something that a truss manufacturer deals with or what exactly the code says about it. I understand all the risks associated with erecting a long span truss, but at some point it goes from a means and methods issue that as a designer I'm not going to tell the contractor how to do his job to a let me make sure it goes well because it is complicated.

Also, I'm on the coast in Texas so we have high winds which may make the trusses a little bit bigger than you would expect.

 

[Triangled]

OK, I totally misunderstood where you were coming from.
Answer: I don't know.
Where I'd begin: wood truss associations and APA-EWS and local building department.

Good luck!

 

[boo1]

ANSI/TPI 1-2007 standard states in Chapter 2,
Section 2.3.1.6.1, which addresses Long Span Truss Requirements,
In all cases where a Truss clear span is 60 feet or greater, the Owner shall contract with any Registered Design Professional for the design of the Temporary Installation Restraint/Bracing and the Permanent Individual Truss Member Restraint and Diagonal Bracing.

2.3.1.6.2,
Special Inspection. In all cases where a Truss clear span is 60 ft. (18 m) or greater, the Owner shall Contract with any Registered Design Professional to provide special inspections to assure that the Temporary Installation Restraint/Bracing and the Permanent Individual Truss Member Restraint and Diagonal Bracing are installed properly.


2009 International Building Code (IBC)
2303.4.1.3 Trusses spanning 60 feet or greater. The owner shall contract with any qualified registered design professional for the design of the temporary installation restraint/bracing and the permanent individual truss member restraint/bracing for all trusses with clear spans 60 feet (18,288 mm) or greater.

Section 1704–Special Inspections:
1704.1 General. Where application is made for construction as described in this section, the owner or the registered design professional in responsible charge acting as the owner's agent shall employ one or more approved agencies to perform inspections during construction on the types of work listed under Section 1704.


See The BCSI B10 Summary Sheet, Post Frame Truss Installation, Restraint & Bracing

http://support.sbcindustry.com/images/publication_images/b10.pdf

and

http://www.sbcmag.info/sites/sbcmag...eral_movement_of_unbraced_trusses_d_16231.pdf

 

[Triangled]

Very cool boo1

 

[KootK]

Call my former employer:

Structural Building Components Association (formerly Wood Truss Council of America)
(608) 274-4849

http://www.sbcindustry.com

They will know of, or be able to quickly determine, any unique requirements that may apply in your area. They'll likely also refer you to this 100+ page document on truss erection that may or may not contain the specific information that you seek: Link



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Stenbrook]

Thank you!

 

[peal724]

over 60' requires special inspection on the temporary bracing per IBC

 

[boo1]

Lifting requires special care too
See WTCA’s BCSI 1-03, pgs. 7-9 for hoisting information.