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Re: z factor for crack control

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Granted, strain rates for GR40 are _lower_ than for GR60, but does that
mean that the investigation of crack control should be neglected? I think
not. A slab reinforced with #5s @ 18" will have larger crackes than a slab
with #4s @ 12". I believe service performance should never be ignored even
though USD and LRFD are in vogue.

Thanks for the reference to the "crack-u-lator". Although this helps in
measuring crack width, it is still an open debate about what is an
acceptable crack width before needing repair.

Bill Allen

> From: TVDEsq(--nospam--at)
> To: seaoc(--nospam--at)
> Subject: Re: z factor for crack control
> Date: Monday, August 11, 1997 8:15 PM
> 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
>  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
> much less strain and crack potential in the surrounding concrete.  
> Interestingly, hidden in the 'z' factor is the allowable crack size per
> Gurgely-Lutz (sp?) equation.  I recall that interior exposure corresponds
> crack width of 0.016 in. and exterior exposure to a crack with of 0.013
>  (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
> rather than the corresponding crack-widths were incorporated into the
>  Imagine the liability. 
> The intent of the code is to get better performance by forcing designers
> use several smaller bars rather than only a few large ones.
> Tom VanDorpe, P.E.
> VCA Engineers