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RE: CMU Wall Strengthening

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oops... EI of strongback to equal or exceed EI of WALL.

From: "Chris Roper" <christophermroper(--nospam--at)hotmail.com>
To: seaint(--nospam--at)seaint.org
Subject: RE: CMU Wall Strengthening
Date: Tue, 09 Sep 2003 22:44:04 +0000

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 seaint(--nospam--at)seaint.org at 15:47:51 on 9 Sep 2003.
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Good points. Beware though that if the strongback is designed as non-composite and only for stresses from out-of-plane loads, it may not end up being stiffer than the wall. Make sure the EI of the strongback equals or exceeds the EI of the strongback. I have always found that making the strongback stiff enough is what drives it's size, not stresses.


From: "Mike O'Brien" <mike.obrien(--nospam--at)wdpartners.com>
Reply-To: <seaint(--nospam--at)seaint.org>
To: <seaint(--nospam--at)seaint.org>
Subject: RE: CMU Wall Strengthening
Date: Tue, 9 Sep 2003 17:24:30 -0400


I would design the strong back for the full out of plane load or you can check it as a composite section. I would design it with a single strongback per pier which also reduces the likelihood of interfering with the roll-up doors.

Since this strongbacked element is going to be alot stiffer than the adjacent wall, be sure that you anchor the top of it adequately to the roof for wall anchorage forces.

Since the wall bows out and the strongback would presumably be straight, that means that there will be a gap between the strongback and the wall at the midheight. I have heard two schools of thought on this, one is to leave the wall bowing and add steel shims between the strongback and wall. The other is to use through bolts or strong adhesive (tension) anchors and to crank the wall and strongback together. This second way makes the wall look plumber but preloads the strongback and anchors, so the remaining capacity is reduced.

Now all that you have to do is hope someone wiser than me speaks up.

Mike O'Brien, SE




-----Original Message-----
From: richard lewis [mailto:rlewistx(--nospam--at)juno.com]
Sent: Tuesday, September 09, 2003 1:28 PM
To: seaint(--nospam--at)seaint.org
Subject: CMU Wall Strengthening


I have a shipping warehouse that has some problems with its masonry
bearing wall.  The front of the building has 3 overhead door openings.
The wall is 12 inches thick, 8" CMU and 4" brick.  The doors are 12 feet
wide.  Two doors have a loading dock bumper height of about 3 feet high.
A third door opens on grade and is 14 feet high.  All door heads are same
height.  The masonry pier between doors is 5 feet wide.  The remaining
front of the building is a full height masonry wall for about 100 feet in
length.  The wall height above the doors is about 9'-8".

It appears that a truck has caught the side of the tall opening.  The
masonry piers are out of plumb.  I dropped a plumb bob from the roof to
the door head and the top of door head is about 3/4" out of plumb to the
exterior face.  I dropped the plumb bob from the door head down to the
ground and the wall went back in about 3/4".  The adjacent door was
similar, slightly less.  The pier between the last 2 doors was about 1/2"
out of plumb, same curvature as the first one.  As I progress away from
the 14' high opening the out of plumb decrease, so it appears to radiate
from this one door.  That's why I think a truck must have pulled out one
door jamb.  The top of this jamb has severely cracked brick.

My fix first is to repair the cracked brick.  The steel lintel above the
door bears 8" on the jamb, the first 4" being on the cracked brick.  Then
I think I should stabilize the masonry piers.  My thought is to use a
steel 'strongback' on the inside face.  I envision a steel wide flange
column bolted to the CMU wall.  I expect the top course to be a bond beam
since steel bar joist bear on it.  I can attach the bottom to the
concrete floor or a concrete wall at the base.  I would guess I could
connect to the masonry wall at about 2' o.c. vert. with adhesive anchors.

I want to get some feedback on what to design this 'strongback' for.
Perhaps some of you have done this before.  The strongback will not be as
stiff as the CMU wall so I don't believe it will take the lateral
loading.  I guess it would just be for buckling stabilization so the wall
does not get worse.  What kind of forces would this be?  Would one
strongback per 5' wide pier be enough?

Thanks for your help.  I really appreciate all the wisdom from your
input!

Rich

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