Stack Bond Masonry

[ctstally]

What are the requirements for vertical reinforcement in stack bond masonry? I am familiar with the additional seismic requirements for stack bond as it relates to horizontal joint reinforcement and/or bond beams, but what about the vertical reinforcement spacing for wind load? I can't find anywhere in the code that this is addressed. Thanks

 

[cedent]

Both vertical and horizontal minimum reinforcing requirements are listed under section 1.14 of MSJC 2005. I think that is the section you were refering to in regards to seismic design requirements. Other minimums may exist elsewhere.

As far as the reinforcing required by design, when you design the reinforcing... make sure that you use the appropriate effective width per bar. (See Section 2.3.3.3.2 of MSJC for specific limitations for stack bond.) Depending on your bond beam spacing, this may or may not be critical to you.

 

[concretemasonry]

Are you are designing engineered masonry as stack bond (technically other then running bond according to ACI 530) or are you reviewing anothers design?

Is it for architectural appearance?

In 45 years of in experience in the U.S. and 35 countries, I found it extremely rare to justify stack bond unless for aesthetic puposes, since it is more costly unless you are in a very backaward part of the world or parts of the eastern U.S.

Vertical reinforcement is essentially the same IF you have enough horizontal reinforcement to make the costly stack bond behave like running bond and possible use costly intermediate bond beams. You could also run into problems with stack bond in arch action. No difference in the f'm of the units and mortar.

 

[ctstally]

I am the structural engineer for the job and this is the first time that stack bond has come up on a job for me. The architect has specified stack bond for aesthetics.

I have some pretty high walls and I am concered about the behavior of the stack bond under wind load. I can't justify in my head that if I have vertical bars at say, 48"o.c., that the lateral load on the un-reinforced portions of the wall will transfer to the reinforced cells across the continuous head joints.

Everything I read says that you should think of stack bond as 16" vertical pilasters with a vertical joint on each side of the blocks. If this is the case, how does the lateral load get to the grouted, reinforced cells? It seems to me that you would have to have a vertical bar in each block. If you count on the joint reinforcement to transfer the load then I would need to specify truss type joint reinforcement, right? Because the ladder type would be unstable for load normal to the wall.

Also, I was under the impression that the vertical bars required on each side of a normal vertical control joint in cmu walls was required for the very reason I mentioned above. If this is the case and you assume a control joint on each side of each block in stack bond, then wouldn't the same requirement hold true?

 

[JAE]

ctstally,

I think you can successfully reason that the load from wind gets to your reinforced cells via the horizontal joint reinforcing.

Partially reinforced block walls in flexure (with running bond) assume that the reinforced cell and the face shell of the block form a type of "T" beam in flexure, similar to a concrete beam-slab system.

What you cannot do is design your block using an effective flange width more than the 16". There is no mechanism for transferring horizontal shear to adjacent face shells to help with your "T" beam design of a block wall in flexure. So your effective "b" for your flexural calculations is 16" maximum (assuming you are using 16" long blocks).

 

[doka1]

what about using a bond beam every third or fourth course?

 

[concretemasonry]

ctstally -

In addition to the ACI 530 and the commentaries, a very good source to explain how masonry works go to the Natioanl concrete Masonry Association (NCMA) site. I think it is ncma.org.

Look at the uper right quadrant of the home page for the TEK notes. Click on that, then on a state (any state) and a producer (any producer)to gain access to the 140 TEK notes on many different subjects. The TEK note are written by professional engineers that hold prominent positions on most national code and standards groups (ASTM, ACI, TMS, IBC, etc.).

If you look at section 14 for structural notes, TEK Notes 14-6 (Bond patterns) and 14-7 (working stress?) will give you good information and explain how intermediate bond beams can make a stack bond panel act like structural element just as a running bond panel does normally. This could get you past the microscopic method of looking at the individual pieces and look at the wall section as a strucural element.

There are a lot of other bits of good information in this partially hidden treasue of technical and practical information.

Dick