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Re: Sawtooth Roof

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An old rule of thumb is to use sag rods for roof slopes greater than 3 on 12. I
prefer to locate the sag rods in the upper third of the beam since I believe
they are more effective at that location. This helps provide lateral restraint
as well as takes out the lateral force.

If the deck is not positively attached to the beams to resist lateral forces,
than you cannot count on it to provide lateral bracing or resist the lateral
force component. I do not recommend counting on friction to provide lateral
support!

Many buildings, even those 60 years old, may not have ever seen their full
design loads. That may be the case here, especially since you have a sloped
roof. Signs of distress aren't always evident or even occur since these
structures have to be loaded to approx. 1.7 or more times their design load
before a failure may occur. Since there is a precast roof, the overload has to
be even higher to cause a failure since you have to also overcome the DL safety
factor. Just because there is no distress or failure does not mean there is an
adequate safety factor provided.

The best you can do is advise the owner about this situation and recommend what
you consider to be the best construction practices to provide an adequate safety
factor.

Jim Kestner, P.E.
Green Bay, Wi.

Ed Marshall wrote:

> 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
>