Need a book? Engineering books recommendations...

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

RE: Cracked cmu basement wall

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Will,
Codes change all of the time.  And unreinforced masonry basement walls 
crack.  We can't rehabilitate structures every time there is a change those 
 of us sitting in some hotel conference room.  You could not build the 
cathedrals of Europe by modern standards, but they work through various 
mechanisms (another topic).  That is unless there is an earthquake like 
happens in Italy from time to time.

Go to the NCMA Tek Notes to look for standards for basement walls, but you 
will have to go back to old residential codes to determine the standard of 
care employed at the time of construction.

I take a different approach.  A crack as you indicated in the photos does 
not look like a flexural crack.  It appears to be more of a settlement 
crack because it is diagonal.  Clays settle in time.  It is just what they 
do.  A CMU wall on top of a strip footing will settle and will crack unless 
 there is significant steel and bond beams to control the cracks.  Most 
residential codes do not require that volume of reinforcing steel even in 
modern residential codes.

I would suggest that you measure the out of plane movement of the wall.  If 
 it is still within normal construction tollerance, I would suggest just 
sealing it up to avoid excessive water leakage.  If you have out of plane 
movement of the wall, then there are a number of remedies.  I have used 
2x6's @ 12" o.c. strong backs anchored to the joists above and to the floor 
 below.  You should use treated lumber and you have to provide a means to 
shim between the wall and the wood strong back.  Keep the strong back wall 
vertical and shim as required to take load from the CMU wall into the 
strong back wall.  Then hang peg board on the strong back wall and organize 
 the tools.

Another alternative is to cut a slot in the face of the CMU and reinforce 
the cells.  That is a lot of work, but you don't loose basement space.

There are other propritary strong back techniques using anchors and plates, 
 but I am a little dubious.  They warrant their work for about a year which 
 is only one cycle of soil expansion and contraction.  So unless it is 
cheap, I don't recommend it.

If you want to drive yourself nuts, check the ledger connection bolts to 
the rim joist and joists and see if the ledger plate has enough bolts to 
resist the out of plane movement of the wall.  Generally they do not by 
quite a margin.  You have to count on the friction and the dead load to 
even have a chance.  The added strong back wall solves that issue too.

When you look at the older codes for residential construction, you will 
find that if they required reinforcing steel at all, it is not enough to be 
 categorized as reinforced concrete or masonry, by the definitions in the 
code for reinforeced CMU or concrete.

After 50 years a little settlement in a CMU wall is understandable.  Ask 
the owner when he first noticed the crack and if it has gotten worse.  I 
would suspect that the crack is fairly stable and probably cracked 20 or 30 
 years ago.  I just would not get too aggressive unless there is some 
significant out of plane movement.

The old residential codes provide the standard of care if you are looking 
for legal cover.  If you are going to try to use modern soil pressures on 
basement walls, you will probably have to strengthen the wall.

Regards, Harold Sprague

> Date: Wed, 17 Oct 2012 18:05:25 -0400
> From: gtg740p(--nospam--at)gmail.com
> Subject: [SEAINT-SEAOSC] Cracked cmu basement wall
> To: seaint-seaosc(--nospam--at)mail-list.com
> 
> I am looking at a house that was built in 1961 and in the past year or 
> two has developed a crack in the 8" block basement wall. There were a 
> couple of tree roots pushing on the wall that have been removed. The 
> crack was caught during an inspection for the house to be sold.
> 
> I have told the owner to get a pachometer and see if there is any rebar 
> in the wall. If there is, then the wall may not move further after the 
> dirt is put back against it on the outside. If there isn't any rebar in 
> the wall, what would you suggest be done? I told him it should have rebar 
> in it if it was built today even by residential code. The wall is a total 
> of 12" thick with some kind of concrete on the exterior face of the 8" 
> cmu you can see in the last picture. So I couldn't see the damage to the 
> cmu on the exterior face. The brick is resting on this ledge with the 8" 
> block extending up in the last picture.
> 

Truncated 1054 characters in the previous message to save energy.

-------------------------------------------------------------------------
Post your message to the list by sending it to: 
   SEAINT-SEAOSC(--nospam--at)mail-list.com.

The email messages sent to the list will be saved in an archive on the World Wide Web.
These archives are located at:
    http://archive.mail-list.com/SEAINT-SEAOSC

To contact the list owner, send your message to:
   SEAINT-SEAOSC-list-owner(--nospam--at)mail-list.com.

5700 Ralston Street, Suite 300, Ventura, CA 93003

To unsubscribe, switch to/from digest, get on/off vacation, or change your email address, click here.
http://cgi.mail-list.com/u?ln=seaint-seaosc&nm=seaintma%40euken.net