Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

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

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Jason,

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)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


__________________________________________________
Do You Yahoo!?
Tired of spam?  Yahoo! Mail has the best spam protection around 
http://mail.yahoo.com 

******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org 
******* ****** ****** ****** ******* ****** ****** ********