8" masonry back-up to 4" brick wall 2

[PostFrameSE]

I have a couple of questions. When calculating the value "d" for rebar location in a grouted masonry cell, is it common to use 1/2 the block depth? I've normally used .45 x block depth. Is that too conservative?

I've done some split-face block design before, but never played around with brick veneer. The contractor deviated from my 10" CMU design and used an 8" block with 4" brick. He is contending that the brick adds to the strength of the wall (there is an air space between.) To me, it seems that the ties only transfer the load to the block, which does all of the work. Is there a way to increase the capacity of the 8" block wall with the 4" brick?

Thanks

 

[concretemasonry]

The brick is strictly a non-structural veneer, since there is no means to transfer the load between the different wythes.

You just got a very fine 8" bearing wall with great weather resistance (no more insulation though) if the reinforcement spacing is the same. The wall built is not as strong (for flexure) as the 10" wall you designed. There is no practical way to increase the flexural strength of the wall unless you want to be exotic and apply sheets of Kevlar to one side of the wall. You probably have more compressive strength that you need since the ASTM C90 strength requirement are so low that manufacturing methods/material handling usually produce block well beyond ASTM minimums. I know of block producers that cannot afford to make minimum stregth and typically are 40-60% over ASTM requirements. Obviously, that has all but eliminated the concept/availability of non-loadbearing CMUs except for specialized uses.

Many engineers take the easy conservative route and use 1/2 of the block thickness for d, but you can specify a greater one by requiring the reinforcement placed close to one face or the other. For an 8" block wall you can get a d of just under 6" if the location is specified and controlled.

You must make sure grout (8" to 11" slump) is used in the filled cores and not mortar or concrete.

Dick

 

[civilperson]

Ivany block have slots for positive location of reinforcement adjacent to both faces prior to grouting. This is the only type of block that I would use a d greater than 0.5 depth. The brick adds nothing flexural strength wise unless bonded to the block.

 

[PostFrameSE]

Thanks to each of you for your feedback. That is very helpful. I have a hard enough time in getting masons to even look at the drawing, much less follow a specification to place the rebar a certain place in the hole so I think I'll stick with 1/2d or .45d to be safe.

 

[concretemasonry]

PostFrameSE -

I understand your quandry. That is why many engineers take the route of putting the bar near the middle and use extra rebar and grout.

Many areas do not have good mason contractors that bid jobs in a price-oriented market situation or project. The good masonry contractors make money by charging a reasonable (not cheap) price and getting the job done on schedule and according to specs. Usually, they have a long term relationship with the GC, although in some markets GCs do their own masonry because of the importance of scheduling. Some masonry subcontractors are just blocklayers.

The key is good job site inspection or enforcement of the design specified, after you have a reliable mason contractor. Often, a good strong position at a pre-construction meeting will go a long way to sort out what is really critical. I saw a project of 20 - 15 to 22 story, 6" loadbearing partially reinforced block (4 different strengths) buildings built with very few clean-outs or job site prisms because of good, detailed quality control measures taken in advance. - It was cheaper to design it correctly nad build it right than to assume a lower standard and add extra material and labor to the cost.

There are many methods and/or products that insure rebar placement IF a specific location is requird. CIVILPERSON's suggestion about the "Ivany" block is just one. George Ivany was a structural engineer that was involved in many multi-story (3 to 7 story) loadbearing masonry buildings. There are also similar uints or rebar spacers that can be used to insure the location and eliminate the detailed inspection of each grout core. The use of specilized block is more consistant and eliminates the ordering, scheduling and placement problems common to using hardware.

Good luck!!

Dick

 

[JKW05]

It would seem to me that if there were adequate joint reinforcing tying the veneer to the block backup and the brick assembly were of adequate strength, then the load could be estimated to be distributed to the block and brick based upon their individual stiffnesses.

 

[civilperson]

Comment to JKW05 post:
Unless the brick veneer was on both sides of the block, then the flexural strength would differ for each type of moment. The joint reinforcement would function similar to the web steel of joists and the compressive block would exist in the brick, (exterior fiber), making the block a interior element only used to transfer shear. It would be an interesting problem to model brick as a composite structural element in flexural/axial loading case.

 

[concretemasonry]

JKW05 -

The key word is "IF". The tie between the two wythes must be able to handle the vertical shear required for the 2 wythes (block & brick) to act as a composite section - very difficult. It appears flexural equivalency is the concern.

Dick

 

[JKW05]

I'm not thinking of it as a composite wall. I'm thinking of it as an 8" wall carrying a percentage of the load, and a 4" wall carrying a percentage of the load. The percentages would be based on the relative stiffness of each individual wall. The joint reinforcing links the two walls together, forcing the two walls into consistent deflections. Stresses for each indvidual wall would need to be within allowable values.

 

[miecz]

JKW05

I believe you are correct, that proper ties allow load distribution. I have a publication by the Portland Cement Association that states that "each wythe in a cavity wall hepls to resist the wind acting as a separate wall." It has graphs representing this reprinted form the "Masonry Structural Design of Buildings, TMS-809-3, Department of the Army, or AFM 88-3, Chapter 3, Department of the Air Force. These should be available on line.

 

[civilperson]

Put the minimum required eccentricity on the axial loaded wall or apply horizontal loads from wind or seismic, and the flexural strength becomes relevant. Thus the model you describe is still required to be within the P-M diagram.

 

[jike]

Common designb practice of cavity walls by the engineered method is to only count on the backup block to carry the load and assume that the veneer simply transfers the load it sees to the backup block. Even if you evaluate the stiffness of the venner and the block, one would find the large majority of the load being carried by the backup wythe.

Surprisingly, the ACI 530 allows you to add the nominal thicknesses of the backup and venner wythes together to determine a maximum height-to-thickness ratio by the empirical method.
Ref ACI 530-99, Table 5.5.1. I do not have the latest version handy. Has this changed or is it still the same?

 

[JKW05]

For an axially loaded wall, I would consider the eccentricity on the 8" wall alone, with the veneer only contibuting to the resistance of the normal wind load. If the axial load is very small, or if this is a non-load bearing wall, the flexural stress from those loads is obviuosly less influential.

I agree that common practice is to only count on the back-up. The only time I resort to trying to count on some contribution from the veneer is in evaulating existing construction, or perhaps in a condition stated in the O.P. I also agree that the back-up carries the large majority of the load. But if the design stress in the backup is just slightly above the allowable, considering the slight contibution from the veneer may be enough.