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Sawtooth Roof

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We are working on a project that, among other things, requires that we
evaluate the safe load carrying capacity of an existing sawtooth roof of an
industrial building.  The roof was designed for 30 psf live load.  The
current building code (SBC) requires between 12 and 16 psf depending on the
tributary area for the particular supporting member.  There may now be as
much as 20 psf of equipment, utilities, or catwalks suspended from some roof
purlins (in some cases the load has been present for many years).  The
question is how much load can safely be suspended from this steel.

The building was built about 1940.  The slope on each sawtooth is 5 on 12.
The building bays are 24' parallel to the ridges and 30' parallel to the
slope.  The building is approximately 1000' long (with expansion joints) by
270' wide.   

The roof cladding is 2' x 6' x 1 7/8" reinforced precast concrete panels
(135 pcf).  Each panel is attached to purlins with a pair of 14 gage
butterfly clips at diagonally opposite corners (specified to have a driving
fit on the beam flange).  The allowable stress for the supporting structural
steel is noted as 18 ksi (the AISC code permitted 20 ksi for A7 steel at
that time). The supporting steel was originally sized assuming that the
individual beams were laterally supported, but there are NO SAG RODS. For a
roof system of this type it has been our long standing practice to require
sag rods between the purlins.  On a smaller building we might simply
recommend to the owner that he add sag rods but on a building of this size
the cost would be very substantial.  Moreover this roof has successfully
stood for almost 60 years with no signs of distress.

The original roofing is composition.  Above that at least one layer of
asphalt shingles was eventually added.  Above these layers a sprayed-on
membrane has been added.  We assume that the joints between the precast
panels were pointed with a mastic when originally installed.  We are
inclined to accept that the precast panels with the butterfly clips, mastic
pointing, and adhered roofing acts as a diaphragm within individual bays of
the building providing lateral support to the supporting steel up to at
least the original 18 ksi stress level.

Comments would be appreciated.

Ed Marshall, PE
Atlanta