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Re: Residential construction-hip beams

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Domenic:

An interesting test of a hip roof system was reported in 1980 in the Forest
Products Journal (Load distribution on a full-scale nailed-glued hip-roof
system; by D. H. Percival and Q. B. Comus; FPJ Vol 30(11)).  The title
reference to nailed-glued pertains to the girder supporting the hip rafters
- the girder was a plywood gussetted wood truss spanning 32 ft.  The hip
system was a terminal hip system, where the girder truss supports only one
2x6 end rafter and two 2x10 hip rafters (each a single 2x10), all of which
intersect the girder truss at its midspan.  Each hip rafter supported jack
rafters at 2 ft o.c. (all 2x6 except one each side closest to girder which
was 2x8) and at a 4:12 pitch.  Ceiling joists were 2x8s at 2 ft o.c. and
spanned 16 ft from the end wall to the girder truss.  All framing was
either SS or #1 Douglas Fir.  The hipped area was 16 ft x 32 ft and sat on
short wood-frame walls on 3 sides, sheathed at the corners.  The system had
plywood roof sheathing applied prior to loading and included 1 standard
truss at 2 ft o.c. to the far side of the girder.  Actual dead weight of
the material used was 4.66 psf.  Imposed loads through use of bundles of
roof shingles were applied to result in 4 psf load on the ceiling joists
(for drywall & insulation) and 42.65 psf on the roof sheathing (2.65 psf
for shingles & felt, rest for live load).  This resulted in a total applied
load of 27,913 lb.  Actual reactions at each end of the girder truss were
measured with load cells.  The measured total reactions of the girder was
only 8,600 lb.  The authors make comparisons of this with the typical
approach, where the framing is designed as being only structural members
and loads are calculated on a tributary area approach, and found the design
total load on the girder is calculated as 14,100 lb, which is 64% greater
than measured in the test.  As most of this calculated load is imposed on
the girder by the hip rafter, this result correlates with the good
performance of seemingly under-designed hip rafters you cite.

Hope this is of interest.

Stu Lewis



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