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Re: z factor for crack control

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In a message dated 97-08-11 20:42:34 EDT, you write:

<< Why don't concrete sections using Gr 40 steel need to be checked for crack
 control? >>

Grade 40, having a lower Fy will yield at less strain than Gr 60.
(e=40/2900=0.014in/in versus e=60/2900=.0207in/in). Therefore, there is also
much less strain and crack potential in the surrounding concrete.  

Interestingly, hidden in the 'z' factor is the allowable crack size per the
Gurgely-Lutz (sp?) equation.  I recall that interior exposure corresponds a
crack width of 0.016 in. and exterior exposure to a crack with of 0.013 in.
 (Call CTL at 1-800-522-2CTL for a credit-card sized crack comperator for
your wallet!) Due to good forsight by the authors of ACI 318, the z factor,
rather than the corresponding crack-widths were incorporated into the code.
 Imagine the liability. 

The intent of the code is to get better performance by forcing designers to
use several smaller bars rather than only a few large ones.

Tom VanDorpe, P.E.
VCA Engineers