Control Joints In CMU Shear Walls

[STR04]

Help!!!

I have a 48'x72', three story masonry structure that needs control joints. The wall openings set by the architect determined my shear walls locations. Can you place a control joint in a shear wall, say at the neutral axis or to heck with it and not place any joints letting the building crack?

TIA

 

[UcfSE]

I would say that if you put a control joint in a shear wall, then you just made two shear walls from one. I would design and detail them as two since they are seperated by the joint. The joint will allow slip between the walls causing them to act as two rather than one.

 

[PEStructural]

UcfSE -

The way I've always approached it is if the horizontal reinforcing is running continuously through the joint, then I treat it as a continuation of the adjacent wall. However, if the reinforcing is stopped, then I treat it as you have stated.

 

[14159]

"The way I've always approached it is if the horizontal reinforcing is running continuously through the joint, then I treat it as a continuation of the adjacent wall. However, if the reinforcing is stopped, then I treat it as you have stated."

If the reinforcement runs continuously through the joint, then how do you have a joint?

I think that UcfSE is right and you have two walls beside each other. It takes a very strong vertical shear connection to force the two walls to act compositely. By the way, the neutral axis is the location with the most shear -- think VQ/I...

Good luck.
DBD

 

[STR04]

However, your bond beam carries across the CJ to resist the chord forces. What happens to this element as the building begins to expand and contract?

 

[bjb]

I have always designed shear walls the way that UcfSE has mentioned. The CJ will prevent the transfer of shear across the joint. The top bond beam helps to distribute the horizontal in-plane load to the various wall panells. Regarding bond beams, if the bond beam is required for strength, as in the case of a diaphragm chord, it must be continuous across a CJ. The amount of steel in the beam compared to the cross sectional area of the wall is not enough to prevent the CJ from working. One chord will be in tension with the rebar carrying the tension, so the bond bema will probably have some cracking any way. If the building is really large, you would need a joint through the entire structure to relieve the expansion/contraction forces.

Bond beams for crack control should not be continuous across the CJ. CJ's are recommended to encourage cracking in contoleed locations when CMU shrinks, so you can prevent water from penetrating the wall. With brick, the you need joints to allow for expansion of the brick due to moisture absorbtion by the brick. If you don't have them in brick, you will probably see distress, especially at corners and any offsets as the brick grows. A brick parapet without joints can suffer a lot of distress. Joints in a brick veneer and a CMU backup should line up.

 

[MotorCity]

It really depends whether the joint is vertical or horizontal. If it is a true shear wall, there are no out-of-plane forces. If the joint is horizontal you may see slipping between two block courses. If the joint is vertical it most likely will not slip.

 

[STR04]

MotorCity,

Please explain "most likely"?

STR04

 

[MotorCity]

For the two shearwalls separated by a vertical joint, I can see them slipping out of plane in two ways.

First, if the load is large enough, it could cause the two walls to act like a hinge and fold at the construction joint (or fail from the flexure whichever comes first).

Also, if there is a significant seismic load, it could cause the walls to heave out of plane.

Not being familiar with the design parameters, I made an educated guess that neither of those events is likely to occur. But of course it depends on your seismic and wind loads. Hope that clarified my previous response.