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RE: Components & Cladding for Wood Roof Trusses

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I've just been reading the recent CASE/ACEC publication "A Guideline
Addressing Coordination and Completeness of Structural Construction
Documents". For engineered items such as wood trusses, it refers to
responsibilities of the SER (Structural Engineer of Record) and of the SSE
(Specialty Structural Engineer). It suggests that performance criteria be
developed by the SER and be specified in the contract documents.
Furthermore, it recommends that the SER review the submittals for the
engineered item. 

I agree with these recommendations. Too often the SER just specifies the
truss materials and generic code requirements but does not clearly define
the performance criteria. While I agree that it is not practical to fully
detail all wind pressures at all surfaces, certainly basic wind design
criteria for the trusses should be provided. Even though the ASCE Commentary
does state to consider C&C wind loads at trusses, it probably would be best
to restate that as part of the design criteria. Also, too often the SER only
performs a cursory check of the submitted truss design and often relies on
the SSE to perform the design correctly. However it is my opinion that the
SER should review the truss design in sufficient detail to verify  that it
conforms to the specified requirements. That doesn't mean that every
component needs to be checked in detail, but the basic design criteria and
approach should be understood well enough to approve the design as
conforming to the specified requirements. 

A recent article in Structure magazine (April 2004), "Truss Bracing, Design
Guidelines for Bracing of Cold-Formed Steel Trusses", states "Although it is
the responsibility of the specialty engineer to ensure that the truss
elements will not fail, ultimately it is still the building designer's
responsibility to review the design and ensure it is compatible with the
other elements of the structure." 

This should apply to all engineered items specified to be designed by others
and that become part of the permanent structure. The SER is acting as the
Owner's agent in ensuring that submitted designs reasonably conform to
requirements. The SER should perform adequate reviews to ensure that the
supplied design conforms to project design requirements. The SSE is still
responsible for the detailed design, but the SER should have confidence that
it was done correctly based on review of submittals. In performing such
reviews, I have frequently found submitted designs to be deficient. 


William C. Sherman, PE 
(Bill Sherman) 
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com

> -----Original Message-----
> From: sea [mailto:sea(--nospam--at)builderspost.com] 
> Sent: Wednesday, April 14, 2004 10:05 AM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: Components & Cladding for Wood Roof Trusses
> 
> The typical truss designer does not design C&C.
> The typical EOR doesn't specify or have knowledge of it.
> The typical building department plancheck doesn't require and 
> check for it. Typically, the job is built and finaled without 
> the correct design. That 'Factor of Safety' really comes into 
> play here...
> 
> Most truss software will do it, automatically.
> All truss software will do it manually.
> 
> EOR's should specify C&C design, System reactions (& 
> uplifts), and min/max member forces for use in system bracing design.
> 
> Charlie Truax
> 
> ----- Original Message ----- 
> From: "Kestner, James W." <jkestner(--nospam--at)somervilleinc.com>
> To: "SEAINT" <seaint(--nospam--at)seaint.org>
> Sent: Wednesday, April 14, 2004 7:33 AM
> Subject: Components & Cladding for Wood Roof Trusses
> 
> 
> ASCE 7-98 requires that wood roof trusses be designed for C&C 
> loading with Zones 1, 2 and 3 and overhangs. It seems 
> impractical for the EOR to list all the loads for the truss 
> designers because of the various effective areas for 
> different trusses. About the best that we can do is to 
> reference that the truss designer needs to comply with the 
> C&C requirements of ASCE 7-98. Additionally, our drawings 
> must list: Basic Wind Speed, Importance Factor, Exposure 
> Factor and Internal Pressure Coefficient and minimum dead 
> load (to determine net uplift).
> 
> Does anyone else specify C&C loads differently?
> 
> Does the typical truss designer know how to handle this 
> loading or is it done automatically for him? Does the typical 
> truss program take these C&C loads into account?
> 
> Jim K.

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