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RE: Reinforced conc bms with widely-spaced transverse reinf ... FEMA 356

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I don't think this is a linear relationship as you indicated.  I believe the
strength reductions are more in line with how many stirrups are engaged with
a shear crack.

At S < d/2, a crack will engage at least 2 stirrups, which is the minimum
basis for design.

At d/2 < S < d, a crack might only engage 1 stirrup, so you reduce the
strength from the equation by 50%.

At d < S, a crack can form that completely bypasses all shear stirrups.  If
the crack bypasses the stirrups your steel design strength is zero, even if
you did provide steel.

So yes, there *is* a sudden drop off in strength when the spacing is
slightly more than d/2.  A crack either can or cannot form that misses the
steel, so there is no gradual change.

Also, this is based on memory and a 45 deg. angle shear crack.  I may be off
on some of the specifics, including the actual angle of the crack, but the
core premise is what I learned back in graduate concrete design classes.

---
Jason Kilgore
Leigh & O'Kane, LLC
Kansas City, Missouri

> -----Original Message-----
> From: Gale45man [mailto:gale45man(--nospam--at)yahoo.com]
> Sent: Wednesday, October 27, 2004 7:13 PM
> To: seaint(--nospam--at)seaint.org
> Subject: Reinforced conc bms with widely-spaced transverse reinf ... FEMA
> 356
> 
> In reinf conc bms with transverse reinf spacing s >
> d/2, FEMA 356 6.4.4 says  Vs  shall be assumed not
> more than 50% effective, and it goes on to say that
> for s >= d, Vs = 0.
> 
> Obviously the transverse reinforcement becomes less
> and less effective as the spacing increases beyond
> d/2, and zero at s = d, but I can't believe it
> instantly drops 50% if the spacing is just a little
> bit beyond d/2.
> 
> Does anyone know if there a research/testing basis for
> this FEMA requirement?
> 
> Also, why not say the shear steel is 100% effective at
> d/2, dropping linearly to 0% effective at d?
> 
> Thanks,
> d a v e  e v a n s
> 
> 
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