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Re: Torsion in Wood Members

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Title: Re: Torsion in Wood Members

Before I answer that, please read what the LRFD Commentary says regarding torsion in wood members:

C5.5 Resistance of Members in Torsion
Timber members are sometimes, but not often,
designed to resist torsional loadings. In general, it
is advised that the structure be designed to avoid
wood member torsional stiffness and strength being
the principal or only path for the carrying of

A wood member loaded to failure in torsion displays
a longitudinal, parallel to grain splitting. A
rectangular wood member has a fairly large torsion
constant, J, but low relevant material strength and
stiffness. Thus, wood is not very efficient as a torsion
member and the design principles for its use
as such have not been well defined. The contents
of Sec. 5.5 are those in the Timber Construction
Manual (AITC, 1994), including the torsional
shear strength definition for glued laminated members.
The torsional shear strength limit for solid sawn
members is from the Wood Handbook (Forest
Products Laboratory, 1987).

Based on the Wood Handbook, I think you still have to use b as the smaller dimension and d as the larger dimension regardless of beam orientation. Take a look at the Wood Handbook page 8-4 for background on the torsion equation.



From: "Joseph Grill" <jrg40581(--nospam--at)>
To: seaint(--nospam--at)
Subject: Re: Torsion in Wood Members

What if the depth is the smaller side dimension?

On 2/7/07, AWC Info <AWCInfo(--nospam--at)> wrote:
> AF&PA/ASCE 16-95 Standard for Load and Resistance Factor Design
> (LRFD) for Engineered Wood Construction Section 5.5:
> 5.5 Resistance of Members in Torsion
> The adjusted torsion resistance, Mt', of a solid
> rectangular beam shall be computed as:
> Mt' = (b^2 d^2 Ftv')/(3d + 1.8b)
> where b is section width, the smaller side dimension;
> d is section depth, the larger side dimension;
> and Ftv' is adjusted torsional shear strength.
> For section shapes other than rectangular, the
> ajusted member torsion resistance shall be based on
> linear elastic torsion analysis with Ftv' used as the
> maximum torsional shearing strength.
> For solid sawn lumber, Ftv' shall be taken as
> two-thirds of the adjusted horizontal shear
> strength, Fv'. For glued laminated members, Ftv'
> shall be limited to Frt', the adjusted radial tension strength.
> The torsional resistance of structural composite
> lumber members is not within the scope of this
> standard and requires special investigation.
> ********
> Buddy
> John "Buddy" Showalter, P.E.
> Director, Technical Media
> AF&PA/American Wood Council
> 1111 19th Street, NW, Suite 800
> Washington, DC 20036
> P: 202-463-2769
> F: 202-463-2791
> The American Wood Council (AWC) is the wood products division of the
> American Forest & Paper Association (AF&PA). AWC develops
> internationally recognized standards for wood design and construction.
> Its efforts with building codes and standards, engineering and
> research, and technology transfer ensure proper application for
> engineered and traditional wood products.
> *********************
> The guidance provided herein is not a formal interpretation of any
> AF&PA standard. Interpretations of AF&PA standards are only available
> through a formal process outlined in AF&PA's standards development
> procedures.
> *********************
> From: "John J. Treff" <jjtreff(--nospam--at)>
> To: SEAInt <seaint(--nospam--at)>
> Subject: Torsion in Wood Members
> Hello Colleagues,I've been looking for information about design for
> torsion= in wood members (timber and glulam). The only decent
> reference I've found = is the AITC Timber Construction Manual (5th
> edition). In section 4.3 gives = an excellent approach to design for
> torsion, but using ASD. I am looking fo= r references with the LRFD
> approach. Any ideas?JJ