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Re: Masonry Expansion Joints

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Yes, Brad, I have also found that detail interesting.  I have frequently seen 
it at windows and at doors.  It seems that it is a misapplication of a 
commonly recognized problem.

We all know that corners of doors and windows are re-entrant corners that 
produce stress concentrations and stress concentrations produce cracks and 
since the cracks appear at window and door corners, builders put "expansion 
joints" there, never considering that the lintel "locks" the expansion 
joint.  (That is, if the lintel is properly anchored to the wall instead of 
just sitting on the masonry.)  Of course, now cracking from expansion is 
going to start at the end of the lintel and progress downward to the edge of 
the opening, reducing all bearing capacity for the lintel to whatever 
friction is developed across the crack and whatever resistance the opening 
frame can provide.  Thank God for masonry bridging capability!

Remember also that the Phoenix area believes that an earthquake cannot occur 
there and designs to seismic zone 1 forces.

In this climate (Southern Arizona), masonry expansion joints should be every 
15 - 20 feet.  To verify this, look at any long masonry wall, preferably a 
west wall, that does not have any expansion joints and measure the distance 
between cracks.  Windowless end walls of houses (24 - 30 feet long) will 
typically have a vertical crack right at the middle of the wall.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Brad Smith wrote:

. > The discussion of masonry reinforcing reminded me of expansion joint I 
. > saw in Mesa Arizona while visiting folks who retired there. It was at a 
. > tract of new single family residences built with masonry exterior walls 
. > and wood roofs. I noticed that the vertical expansion joints were located 
. > at one end of sliding glass door openings. The location made sense since 
. > this is at the weak link of thermal contraction. But how do lintels for 
. > the openings find bearing with the expansion joint at the end?  
. > 
. > Perhaps the expansion joints I saw aren't true expansion joints and that 
. > all the horizontal reinforcing continues through?
. > 
. > In my practice, vertical expansion joints are indicated on masonry wall
. > elevations (I use 2x wall height as the maximum spacing as Jim Amhrein 
. > once recommended).  However, when there are numerous door and window 
. > openings, locating the expansion joint between the openings seem futile. 
. > I should like to locate the expansion joint at the edge of an opening.  
. > 
. > Any thought?
. > 
. >