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Unit weight of precast roof plank from 1

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I wasn't able to find anything specific about your situation, but I did find 
a table in Architectural Graphic Standards, 6th Edition, 1970, page 498, that 
does show precast concrete planks as a roof system, but they start out at 4" 
and list 4", 6", 8" and 10" as standard thicknesses, the weight as 40 to 75 
psf, live load capacity as, "up to 40 psf" (but I wouldn't hang my hat on 
it), and a span range of 15' to 50'.

Normal weight concrete weighs 12 - 12.5 psf/inch thickness (144 to 150 pcf), 
so if the plank was 3" thick, it would weigh 37.5 psf assuming normal weight 

A lot of the precast roof systems such as you described may be proprietary, 
and may even be proprietary of the joist manufacturer.

Maybe this gives you some additional ideas.  Good Luck.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Mark Swingle wrote:

>>We are investigating an existing building in Massachusetts, built in 1959
and 1964, to which an addition is currently being attached.  It is a
one-story building which framing consists of open-web joists at 4' oc
supported by steel beams and columns on spread and strip footings. 
Exterior walls are CMU infill.

The existing drawings call out <3" PRECAST ROOF PLANK> spanning the 4 ft
between joists.  Does anyone know the unit weight of this material? 
Breyer's 3rd edition lists <concrete plank> at 6.5 psf per inch, which is
78 pcf.  Regular-weight concrete is 145 pcf.

Is Breyer correct?  Is this the same thing?  In other words, was
light-weight concrete in use for roof planks in 1959 and 1964?

By the way, if I use 145 pcf, some of the existing joists and beams are
overstressed, which seems unlikely.

Any history lessons from you old-timers will be greatly appreciated.


Mark Swingle, PE, SE<<