Control Joints in Special Reinforced Masonry Shear Walls 2

[volney68]

In the book Concrete Construction Engineering Handbook (edited by Edward G. Nawy) I found in Section 28.9.2.4 the statement:

"Due to the large quantities of prescriptive horizontal reinforcement, control joints are typically not necessary for special reinforced masonry shear walls"

Being relatively new to masonry design in high-seismic areas, I have no personal experience to draw upon. I've searched and found no similar statements made anywhere else. Does anyone have insight to offer?

 

[MacGruber22]

The rationale in that statement certainly makes sense. The IBC, ASCE 7, or Masonry Code doesn't address as far as I can tell; it is up to us to get into those details. Is it going to be a pain to have expansion joints, or are you just trying to take advantage of this concept? Often times the expansion joints proposed by the architects make lateral analysis a pain in the butt, but it isn't often practical to have no expansion joints. However, if you really need to remove expansion joints, why don't you apply a self-straining masonry force and combine in your load combinations at service and strength? If you aren't performing the calcs by hand, it may not be a ton of extra effort. Check out ASCE 7-10 C2.3.5 "Load Combinations Including" - load factors for self-straining loads have to be determined by the engineer, hence the commentary discussion.

Also try:
“Control Joints for Concrete Masonry Walls,”
NCMA TEK 10-2A, National Concrete Masonry
Association, Herndon, VA, 1998, 6 pp.

“Control Joints for Concrete Masonry Walls Empirical
Method,” NCMA TEK 10-2B, National Concrete
Masonry Association, Herndon, VA, 2005, 4 pp.

Although, I think it would be tough to find prescriptive measures to answer your question.

Hope that helps (a bit).

 

[JLNJ]

There are guidelines for the equivalent of shrinkage reinforcement which should control the shrinkage cracking. I am not at my desk right now, but there have been several papers on the subject. The prescriptive reinforcement will exceed the shrinkage, so control joints probably won't be necessary. Normally this is good news, as longer shear wall panels are generally better than short ones.

I suppose there are practical limits to consider. It might be helpful to the structure to have control joints at a reasonable spacing if your structural syetem can tolerate them.

 

[hokie66]

Not having reviewed any of the references cited above, I would just point out that reinforcement doesn't prevent shrinkage cracking, but rather controls it. If thin cracks are acceptable, then I would agree with minimising control joints.