Section modulus is an engineering formula which provides a property
(measurement) of a cross sectional shape and reflects both the shape and
orientation of the object. It can be determined through the equation S
= I/c, where I = moment of inertia about an axis through the centroid,
and c = distance from the centroid to the extreme edge of the section.
This is according to the "Manual for Steel Construction, 8th Edition"
sitting on my bookshelf. This volume also has page after page of
various S values for dozens of shapes in dozens of sizes, including both
slotted and non-slotted channel, S members, I, members, W, members,
C-channel, etc.
And NFPA-13 ('02) would certainly allow you to use trap members other
than those shown in the standard, as long as they had the appropriate
section modulus. Section 9.1.1.6.2 advises "Any other sizes or shapes
giving equal or greater section modulus shall be acceptable" which
seems pretty final to me.
And I don't know about Unistrut not working for trapeze members. Base
don the values shown as a minimum in NFPA-13, I could use a 0.202
section modulus uni-strut to create a 3'-0" span trapeze to support
2-1/2" schedule 40.
PARSLEY CONSULTING
Ken Wagoner, SET
760.745.6181 voice
760.745.0537 fax
[EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]> e-mail
www.ParsleyConsulting.com <http://www.ParsleyConsulting.com> website
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