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# concrete, rubble walls, CMU

• To: <seaint(--nospam--at)seaint.org>
• Subject: concrete, rubble walls, CMU
• From: "Andrew D. Kester" <andrew(--nospam--at)baeonline.com>
• Date: Wed, 12 Mar 2003 15:35:49 -0500

```GAIL:

Since you and Mark, and others, seem to be the PT concrete gurus, any other
light you could shed on the Miami condo would be quite interesting. Is it
normal practice to include regular reinforcement along with the post
tensioned cables? According to Mark's example, there wasn't much holding the
concrete slab up after the cables failed.

I laughed a little at your post that the stone wall was out of your
expertise! If you understand PT I am sure you could engineer the heck out of
a stone wall. I would think that you could design the wall like you said, as
an unreinforced structure. Include the vertical weight of the stone to
resist bending and overturning. Compute the tensile stress in the inside
face of the wall, compare with the allowable tensile stress of the mortar. I
would fully mortar all of the pieces of stone together, especially on the
inside face where all of the tension would be. Then I think you could
appropriately model it based on max. tensile stress. If not, consider a
backing wall out of reinforced concrete that is integral with the stone. You
could build the wall with mortar, then pour a small 5" concrete wall behind
it, that is reinforced. It will  be hidden, so noone would know the
difference. You could include wire tires from the mortar bed back into the
concrete wall. Shoot, you are only going up 3 feet so what is the worry? If
it is your own house, "in theory", my FS would be small. What is the risk?
You will know if isn't working when it starts leaning :) Now that is real
engineering. Now overturning is another issue, if you cannot achieve
stability thru weight of the stone, and leaning of the all back into the
slope, then I guess you would need a foundation and a reinforced wall behind
it. I am sure some of the "older sages" on the list have some better advice
:)

CMU flange spacing:
About whether to use 48" for effective comp. flange or larger, I would say
go with the ACI code for regular cell spacing. Like someone else said, what
is the difference between a pilaster and a filled cell? Another issue to
consider is how far can you assume the CMU can span horizontally between
filled cells or pilasters before that stress (if it is unreinforced horiz)
becomes an issue. I always thought that was one of the other reasons for the
48" oc limit, but I may be wrong..

Andrew Kester, EI
Longwood, FL

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