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[SEAOC] Re: [SEAOC] Re: [SEAOC] Roof Truss shop drawings[Subject Prev][Subject Next][Thread Prev][Thread Next]
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- Subject: [SEAOC] Re: [SEAOC] Re: [SEAOC] Roof Truss shop drawings
- From: Frank Woeste <fwoeste(--nospam--at)vt.edu>
- Date: Sun, 04 Aug 1996 16:53:30 +0600
At 09:39 PM 8/3/96 -0700, you wrote: > >Question: Have you ever been provided loads of the lateral bracing so that >you could provide connections and resisting members by your local truss >supplier? Tom and Neil: This is a good question and it has been fixed at least "on paper." If you will refer to WTCA 1-1995 (Standard Responsibilities in the Design Process Involving Metal Plate Connected Wood Trusses), Section 6.0 Truss Designer Responsibilities, paragraph 6.2 states "For each Truss Design Drawing, set forth as a minimum the following:....................................... 6.2.11 Maximum axial compression forces in the Truss members to enable the Building Designer to design the size, connections and anchorage of permanent continous lateral bracing. Forces may be shown on the Truss Design Drawing or on supplemental Documents; and 6.2.12 Required permenent truss member bracing location. So to calculate the lateral bracing loads, you need the information from 6.2.11 from the Truss Designer and a factor to convert from truss member force to a required lateral restraining force. TPI literature has given a 2% rule for many years; ie, "The force required to restrain the top chord against buckling in the lateral directiom is assumed to be two percent of the maximum axial force, per lateral brace, per truss braced."(Paragraph H-220.127.116.11, HIB-91 Handling, Installing & Bracing Metal Plate Connected Wood Trusses, Truss Plate Institute) The TPI telephone number is 608-833-5900 if you would like to purchse HIB-91, a 109 page pocket size manual. The word "Maximum" in 6.2.11 is very important because trusses are now analyzed under 3 or 4 load cases. So--to design the lateral bracing, you need the maximum compression in each member for ALL load cases used in the truss design. Sincerely, Frank Woeste >> I would value other opinions on the review of residential roof truss >>shop drawings by the engineer of record. > >Tom: > >Your question opens up many a can of worms concerning the prefabricated >truss industry and maybe this is the place to air things out. The ICBO >people informed me a couple of years ago that there were some problems >and our local building official tightened up the submittal requirements >here after some problems were brought to his attention. > >The engineer of record may require on his drawings that the truss company >provide him with calculations and drawings, but unless he follows this up, >he may never get them. > >There was a comment about trusses being supported by walls which are not >bearing walls; this has happened. > >But there are some other subtle things going on. The design for "lateral >bracing that may be required to reduce the length of individual truss is >determined as part of the wood truss design and is the only requirement for >bracing that will be shown on the truss design. Lateral bracing details, >including method of connection and transfer of member buckling forces to >the structure are to be determined by the BUILDING DESIGNER. Lateral >bracing is to be supplied in the size specified and installed at the location >specified by the BUILDER or ERECTION CONTRACTOR. The BUILDING DESIGNER must >ascertain that the specified lateral bracing is properly installed and that >this bracing is sufficiently anchored or restrained by diagonal bracing to >prevent its movement." - This is from Appendix B, Commentary on Bracing from >TPI-85, recommened design practice the Truss Plate Insititue. This is not >in the UBC. > >Question: Have you ever been provided loads of the lateral bracing so that >you could provide connections and resisting members by your local truss >supplier? > >Going on. Do you provide details of how non-bearing walls are to be framed >under prefabricated trusses? The only detail that I know of is in the >Simpson catalog - and this is not usually a problem - unless you are in >snow country with 100 psf+ snow loads. Then this can be a problem if your >contractor connects the top of a wall in the between web points of a bottom >chord and doesn't provide a method to allow the truss to deflect. > >Further. I think there may have been a discussion about this topic here >before, I know it was well discuss the the twdwelling group on Compuserve, >but the problem is seasonal truss uplift due to the difference in relative >humidity between chords. A real problem when the truss starts to bow upward >and starts pulling apart the taped joints at the interior walls. You client >gets real skittish from the noises as well as watching the cracks grow at >the ceiling-wall joints. Sometimes this is misinterpreted as settlement of >the interior footings! There is more information in a paper put out by >the Small Homes Council-Building Research Council, University of Illinois at >Urbana-Champaign, Techical Note 18 by Donald Percival - title: "Ceiling- >Floor Partition Separation" What Is It and Why Is Is Occuring?" > >This problem can be made worse by the contractor not providing adequate >ventilation. I usually see a note on drawings specifying so much area of >ventilation per the roof area of the building; but maybe it's time to start >actually specifying the various openings and making sure that it's done. > >If you don't get to review the final truss design from the fabricator, then >how do you know if he provided the correct loads. There is a case pending >presently where the designer missed three foot eaves with snow loads over >200 psf! > >It might be a good idea to inspect the truss's installation. We found one >project where the electrician had run his wiring through the bottom chord of >a truss. > >A final troubling thought concerning the design of heel joints; the code >(Sec. 2343.3) refers you to Table 23-III-PP for connector capacity >reductions for spans up to 35 feet. What is done when the span is greater >than 35 feet? Supposively, the various truss designers have this taken >care of in their computer programs. I've only reviewed and compared one >program and this required manual input to model. The problem is the heel >joint eccentricity and I have not seen an ICBO metal plate report that >indicates that their plates can resist a moment connection. > >Again, most engineers will not run into there problems unless the trusses >are used in a highly loaded situation and maybe you won't encounter these >questions until you get into court. > > >Neil Moore, S.E. > > >... > > Frank E. Woeste, Ph. D., P. E. Biological Systems Engineering Virginia Tech Blacksburg, VA 24061-0303 Home:(540)951-0469 Office:(540)231-6093 FAX:(540)231-3199 ...
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