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RE: Precast Concrete C-shaped Joists

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The product you are referring to is most commonly called a channel joist or slab.


The numbering sequence you referred to sounds to me like you are looking at shop drawings instead of contract drawings.


If you scan the drawings and email the PDF’s I will take a look.


Otherwise, if there is no load capacity or reinforcement called out on the drawings your choices are:


1.      Find out what the building use was originally and research the Code required live load at the time it was constructed to determine the likely minimum live load capacity (the cheap but not very versatile/flexible solution).


2.      X-ray some a typical stem of the 12” and 9” channel slab (the expensive but all is revealed solution).


D. Matthew Stuart, P.E., S.E., F.ASCE, SECB
Structural Division Manager

Pennoni Associates Inc.
One Drexel Plaza
3001 Market Street, 2nd Floor
Philadelphia, PA 19104
Office 215-222-3000 x7895 | Direct 215-254-7895
Fax 215-222-0789 | Mobile 908-309-8657 | mstuart(--nospam--at)


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From: Adair, Joel [mailto:jadair(--nospam--at)]
Sent: Wednesday, May 26, 2010 10:24 AM
To: seaint(--nospam--at)
Subject: Precast Concrete C-shaped Joists


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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