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

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I'm not yet convinced of the step function nature. 
Here's what I was thinking when I proposed a non-step
function transition:

At s slightly less than d/2, a 45 deg crack will cross
two stirrups, but one of them will have insignificant
development length.  So there will be no difference in
strength when s is slightly more than d/2, where that
insignificant development length drops to zero.

Then, as s gradually increases from d/2, the
development length of the remaining stirrup gradually
decreases, until it is zero at s = d.

Shear steel strength may not be linear in this region,
but I doubt it is a step-function, either.  Make
sense? Your thoughts?

D a v e

Jason Kilgore wrote...

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)] 
> Sent: Wednesday, October 27, 2004 7:13 PM
> To: seaint(--nospam--at) 
> 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
> this FEMA requirement?
> Also, why not say the shear steel is 100% effective
> d/2, dropping linearly to 0% effective at d?
> Thanks,
> d a v e  e v a n s

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