From: Neil Moore <nmoore(--nospam--at)innercite.com>
Date: Fri, 12 Jan 2001 14:21:09 -0800
We usually receive a computer output sheet with all of the loads. The
one's I've seen require you to identify which member is in compression and
will provide the compression force. Their sketch indicates which members
have to be braced. You should require the truss company to provide you
with these analysis's.
If there are compression chord forces, then you have to insure that there
is a diaphragm to prevent the chord buckling or possibly have to design
some sort of lateral sysytem to accomodate this.
Neil Moore, S.E.
At 04:23 PM 1/12/2001 -0500, you wrote:
>George and Neil -
>When you receive truss shop drawings from the fabricator, do the
>drawings give the forces in the webs and each segment of the chords? In
>my experience when we review MPC wood truss shop drawings the web and
>chord forces are not provided to us 99% of the time.
>Albert J. Meyer, Jr., P.E.
>From: George Richards, P.E. [mailto:george(--nospam--at)BORM.com]
>Sent: Friday, January 12, 2001 4:02 PM
>Subject: RE: Question on wood Roof Trusses
>I second Mr. Moore's opinion. Our plans (for houses, not even
>require that we review the roof truss drawings for conformance with our
>design intent. Over 90% of our builders do send us the drawings for
>prior to fabrication. The truss drawings are then forwarded to the
>Official so that he can approve them. This is in accordance with the
>section on deferred submittals. The Clients know up front that this is
>process and they pay for the service. What the get is a safer home more
>likely to be free of defects in design and construction.
>George Richards, P. E.
>From: Neil Moore [mailto:nmoore(--nospam--at)innercite.com]
>Sent: Friday, January 12, 2001 10:27 AM
>Subject: Re: Question on wood Roof Trusses
>WEB BRACING ANCHORAGE TO WHAT?
>The problem is that there are two phases to the design of any of the
>commercial truss systems. The truss designer indicates in his
>which web members must be braced and his small sketches that come to the
>job site usually will indicate a 1x4 in the proper locations.
>The second step is for the engineer of record for the project to review
>truss design, note where the web members have to be braced and extract
>compression force in these web members from the truss designers
>calculations. Once the structural engineer has this value, he then will
>determine the force required to keep the web member from buckling.
>this is about 2% of the compressive load. Then he counts the number of
>trusses, multiply this times the 2% compression force and has an
>approximate value for what the 1x4 web bracing members have to transfer
>an anchor point.
>Our local county structural committee has discussed this problem. In
>past, truss calculations and layouts were not being properly reviewed by
>many of the local engineers. The county started enforcing this a few
>ago. At our last meeting, one of our prominent structural engineers
>he was amazed at the mistakes and problems that he has found in the
>submittals by the truss companys. (he didn't specify which one's, as
>are a number of them here). Problems such as the final bracing points
>forces were easy; problems such as the trusses coming down on bearing
>that weren't bearing walls was something else. Problems with the proper
>loadings, ect, were also found.
>Section 2321.1 of the 97 UBC specifies that the design of trusses will
>per ANSI/TPI 1-1995, which many people don't have. In the TPI
>Plate Institute) is the language about who's responsible for what, as I
>stated in my earlier email.
>The dilemna here is that in the design stage, the E.O.R. won't have this
>information. This may have to be performed after the actual contract is
>awarded. A similar problem exists in sprinkler systems when the
>subcontractor submits his design, which may be nothing like what was
>envisioned during the design phase.
>Make sure your client knows that there may be additional fees to perform
>Neil Moore, S.E.
>At 12:29 PM 1/12/2001 -0500, you wrote:
>>Until the plated truss industry will provide me with the forces to be
>>resisted and locations of the braces while the project is still in the
>>stage, I am going to require that the truss designer be responsible for
>>design and detailing of all bracing that is required as a condition of
>>of the truss.
>>While the plated truss industry is reluctant to admit it, there *are*
>>self-equilibrating methods of bracing compression members that do not
>>to be carried down into the basic structure.
>>I would be very hesitant to modify anything that the truss designer
>>since they could then point to the modifications and say that that was
>>cause of a problem.
>>I have in my files (somewhere) a TPI addendum/letter, ca. late '60's,
>>'70's, that says that the building designer is *not* responsible for
>>that is a condition of the design; that the truss designer is
>>A. Roger Turk, P.E.(Structural)
>>Neil Moore wrote:
>>>>It appears that you have discovered one of the problems with the
>>industry. These member are supposed to be braced and this usually
>>the truss company's drawings that come to the site. Apparently these
>>installed. What many engineers that specify prefabricated trusses
>>is that THEY are responsible for the bracing of the web members. The
>>company design will indicate what web members are to be braced, but the
>>ENGINEER (if there is one) has to complete the bracing design. That
>>possibly being braced off to the gable walls or strutted up or down to
>>diaphragm or to walls below.
>>>From an older commentary in the TPI-85 publication: "The need for
>>location of lateral bracing that may be required to reduce the buckling
>>individual truss members is determined as part of the wood truss design
>>the only requirement for bracing that will be shown on the truss
>>Lateral bracing details, including method of connection and transfer of
>>buckling forces to the structure, are to be determined by the building
>>designer." ...... there's more.
>>In your case, you can probably add another 2x4 to the compression webs
>>a tee section. The installation should be done with care to prevent
>>Neil Moore, S.E.<<