Mike,
Yes there is a straight forward way to get the required section
modulus for a cantalevered trapeze member:
weight (lb) * distance (inches) / 15000. For example, hanging 4" Sch.
10 24" off a joist (say to the right)--
15 feet of water-filled pipe plus 250 lbs = 426.7 lb, * 24" =
10,240.8, / 15000 = .68. The straight forward part comes in by looking
at the table and seeing the required section modulus is .34 for a
'normal' situation where the span is 48". (The table is only GOOD up
to normal spans of 10 feet, but straight forward also means if we have
a 12 foot span we need to double the required section modulus for a 6
foot span).
Jim and Todd make valid points-- it is best not to torque a single
beam or joist, and we can avoid it by extending the trapeze member (to
the left) so that it is fastened to two beams or joists.And just like
we techs want an FPE to review and advise on our sprinkler work, we
want a structural PE to do the same thing for any hanger thinking
outside The Redbook box.
For normal trapeeze situations the required section modulus is half
the weight in pounds times half the span in inches, divided by 15000,
the span being 4ab/a+b.
For any Forumites curious about req. sec. mod. when hanging pipe other
than s10 or s40, and/or closer than 15', #13 uses 490 lb/ft3 for steel
and 62.4 lb/ft^3 for water.
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