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RE: Sawtooth Roof

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Jim,

Thanks for the comments.  A detail I didn't include in my original
description is the presence of L2x1 1/2 angle iron "ledgers" which are
attached to the top of 6" wide flange purlins (which run up and down the
slope at 6' o.c) after every 4th precast panel (8' clear between these
angles).  Their purpose appears to have been an erection aid in that they
keep the precast panels from sliding down the slope.  12" wide flange beams
spanning 30' running parallel to the ridge (set with their webs
perpendicular to the roof surface) support the purlins.  These 12" beams are
located on 10' centers.  So, in addition to friction and the butterfly
clips, the angle iron ledgers tie the purlins to each other, and the purlins
tie the 12" beams to each other.  However, to keep everything square (and
not sagging downhill) there is only the weak axis stiffness of the 12" beams
and, more significantly the diaphragm stiffness of the 2' x 6' precast
panels along with many layers of adhered roofing felts and shingles.

Ed 

> -----Original Message-----
> From:	Jim Kestner [SMTP:jkestner(--nospam--at)somervilleinc.com]
> Sent:	Wednesday, December 09, 1998 3:29 PM
> To:	seaint(--nospam--at)seaint.org
> Subject:	Re: Sawtooth Roof
> 
> 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
>