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Precast Concrete C-shaped Joists

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Hi, Everyone –


I’m trying to determine the load capacity of the floor of a building designed in 1970.  This is a ground floor structural slab with a crawl space below it.  The drawings indicate that the floor is constructed of precast “C”-shaped joists, sitting on pier-supported grade beams, with a 2”-3” topping slab.  The “C”-shaped joists are like double-T’s without any wings, and are laid next to each other, then topped.  It is unknown whether they’re prestressed.  From the details, it is clear that they are two different depths – 9” and 12”.  On plan, they’re called out as “Precast reinforced concrete joist X.XXX 2’-0” wide fillers cast as required” (plus additional information about the thickness and reinforcement of the topping slab).  The “X.XXX” in the callout is one of four numbers: 0.060, 9.070, 12.070, or 12.080.  Obviously, the 9 or 12 is the depth.  I’m having a hard time figuring out the “.060”, “.070” or “.080”, though.  Might this indicate the capacity in some way?  There aren’t any general notes or plan notes that would provide any other direction to the precast designer regarding loading, so it seems that these numbers must indicate design capacity somehow.


If anybody has experience with this type of a floor system, I’d really appreciate any help you could provide.




-- Joel


Joel Adair, P.E.

SHW Group

Plano, Texas

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