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RE: CMU Wall Reinforcing

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Andre,

 

You make a very good point: cost to detach a wall element from the lateral load path may exceed the cost to reinforce that element to behave resist in-plane shear loads. My thought was merely that we sometimes make shear walls of things we should not. Drew appears to be evaluating a pier.

 

That said, I am inclined to agree with you that ignoring this “shear wall” provision for a “shear pier” may be non-conservative. Missing from my ’05 copy of the code are alternatives to ordinary, intermediate, and special shear walls. It seems to me that a pier within a wall walls under the reinforcing requirements unless somebody can find an exception to the rule.

 

Drew,

 

Assuming there are no seismic criteria to follow for piers within a masonry wall, I think Andre’s got the right idea – and it’s what I’ve done in the past: at least put all the shear (horizontal) reinforcing you might need into that wall so you aren’t in a pickle when it shatters during an earthquake. Even if you were to say this was a beam – not a special reinforced shear wall – you would need shear reinforcing spaced not more than half the beam depth (d), which in this case may be a little less than pier length, so I think you’re back where you started, generally speaking.

 

Of course, if you’re talking longitudinal (vertical) reinforcing, you may be able to justify a 4-bar solution, but you’re still going to have to address that bar diameter when setting your horizontal bar spacing. So, if you’re using #8 vertical rebar, you’re still facing 16” maximum spacing. No, that’s not under the pier design criteria, but it seems more than reasonable to get the pier reinforced similar to a either a column or a beam or a wall – but you’ve got to get close to something or Andre is right, you’ll have a problem under lateral load from earthquake forces.

 

Keith Erick Fix

Principal Engineer

Red Pepper Consulting, Inc.

http://www.redpepperconsulting.com/

 

10201 W. Markham Street

Suite 215

Little Rock, AR 72205

 

+1 (501) 227 7183 office

+1 (501) 350 3629 mobile

+1 (501) 319 7319 fax

 

http://www.redpepperconsulting.com/

 

From: Andre Sidler [mailto:asidler(--nospam--at)hotmail.com]
Sent: Thursday, April 28, 2011 12:25 PM
To: SEI Listserve
Subject: CMU Wall Reinforcing

 

Drew -
 
I respectfully disagree with Mr. Fix's statement.  If you are designing a special reinforced masonry shear wall then this reinforcing spacing requirement must be met.  The reason this requirement is in the standard is to ensure that the pier does not lose stability and that it will be able to support whatever gravity loads are placed on it.  Yes the lateral shear may be small, but if the wall looses axial capacity after a few cycles and collapses then life safety is not met.  The work-around is to detach the pier from the lateral system and that is not an easy thing to do.  Depending on the case it may be more cost effective (both design fee wise and construction cast wise) to add in the additional reinforcing than to detach the pier from the lateral system.
 
Andre J. Sidler, S.E., P.E.
Sr. Structural Engineer
Port of Seattle
Seattle, WA
 
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From: "Keith Erick Fix" <keith.fix(--nospam--at)redpepperconsulting.com>
To: <seaint(--nospam--at)seaint.org>
Subject: RE: CMU wall reinforcing

Drew,

If the in-plane lateral loads are really low, is this really a shear =
wall? In other words, can the lateral force resistance be provided =
elsewhere? If the lateral force is resisted elsewhere, then the wall you =
described isn't a shear wall, and the rules change.

Keith Erick Fix
Principal Engineer
Red Pepper Consulting, Inc.
http://www.redpepperconsulting.com/=20

10201 W. Markham Street
Suite 215
Little Rock, AR 72205

+1 (501) 227 7183 office
+1 (501) 350 3629 mobile
+1 (501) 319 7319 fax

http://www.redpepperconsulting.com/