f'm on a CMU building designed in 1979

[zurch1818]

I'm looking to check into the capacity left of a CMU apartment building designed in 1979. In the area in question, the building only has bearing walls and no columns and the load is being applied on the roof. Rather than the building calling out an f'm of 1500psi, it calls out a different f'm based upon what floor I am on. The problem is this capacity is much less than 1500psi. In the attached picture, I have a type C wall. I've got two questions:

1. Does it makes sense to use the f'm that is on the plans, or should I use an f'm of 1500psi?
2. What is the difference between "53% Solid f'm=xxx psi based on net area" vs "52% solid f'm=xxx psi based on gross area"? I have never seen something like this spec'd out on any newer building.

Thanks for your help,

 

[msquared48]

Are you sure it is not the actual stress in the CMU seen at each floor level?

Mike McCann, PE, SE (WA)

 

[CELinOttawa]

I vote actual stress as well. Plans examiners have required this from time to time in some jurisdictions. Where are you? 1979 is nearly/roughly old enough to toss some non-US areas under the Imperial system bus...

 

[oldrunner]

Are all of the cells filled? Try referring to the old NAVFAC P55(?) manual which had some very good design criteria in the 70's.

 

[zurch1818]

Thanks for the responses. The site is in La Crosse, Wisconsin. From what I can tell, I believe all the blocks are hollow and only have their face shells grouted. It does not call out any reinforcement. Because it is in an interior wall, I can believe that they weren't designed too carry much bending.

After thinking about this last night, I agree it could be the design f'm to satisfy the design loads. It very well could be more, but I don't know if I am willing to claim that with the huge proposed loads (approximately 10 times the original design). There is also a line item that I cut out about when I posted that screenshot of the plans. It says if there is no special inspection (which Wisconsin doesn't have today), that all the f'ms must be increased by 50%.

I also found a version of the NAVFAC online and it does look like it has some good information in it.

 

[CELinOttawa]

Filling unfilled cells can give you a great increase in capacity, but for occupied spaces or high architectural quality of finishes it can be a near impossible sell.

 

[JAE]

According to an old masonry table I have - an 8" nominal CMU wall with no grouted cells and 2" thick (non-standard) face shell thickness will
give you 48 sq. inches per foot of Net area assuming mortar bedding on the face only (i.e. no webs mortared).

This 48 sq. inches is 48/(7.625 x 12) = 0.534 or 53.4% of the gross area.

If you have block that has different face shell thickness (1 1/4" standard or also non-standard 1 3/8") then this 53% net area would only be achieved by having some cells grouted.

I would agree with the others here that the f'm the drawings refer to is NOT f'm but an actual axial or axial+bending stress - fa or fa+fb.



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[concretemasonry]

That looks like it was designed by someone that tried to be fancy a give the opportunity to use the weakest blocks possible at specific floors. Unfortunately, there many flaws in the concept.

1)In reality, the cost of the block are not cheaper and complicate construction. Extra cement is usually not necessary, since adding water to the mix is far more effective, but may affect the appearance or color slightly. The manufacturing process from the 1940s has outstripped the code requirements updating for materials. All of the f'm strengths in this project are in the very low end and much lower than the strengths of the normal block in most areas.

2)Contractors would not save any money since there is little or not cost difference and the different strengths really complicate the logistics and make it impractical to use different strength grades.

If the required strengths were much higher, it might be prudent to use 2 different grades of block for a similar building of that height. Block can be available at f'm=5000 psi, but it is hard to see why the upper strengths are necessary.

The governing factor of the prism strength of a standard 2 block prism is the strength of the masonry units and not the mortar strength. Mortar is tested in cubes and is used in a very thin bed,so the failure could be referred to as "confined compression", while the CMUs in a prism fail in a diagonal shear failure the goes through the face shells and webs. Using a full mortar bed does not increase the f'm appreciably and can require specialty units to maintain perfect alignment of the webs.

ACI 530 code, specifications and commentary provides an excellent resource on engineered masonry. For concrete masonry prior to 1990, the resource that the concrete masonry portion of the design was an older National Concrete Masonry Association (NCMA) manual called something like the "Design of Engineered Concrete Masonry Structures" that is casually referred to as the "green book". That was the mostly likely standard the building was designed to since it was issued in the early 1960's.

The use of too many strength levels of materials is counter-productive and saves no money for a owner. I was on a project of about 20 loadbearing masonry building that ranged from about 12 to 22 stories and they the contractor and engineer agreed that it was only necessary to have 3 different strengths on the entire project. There was no field testing of the units, since they were tested and marked/identified at the plant prior to delivery.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[concretemasonry]

LaCrosse, WI is on the boarder with MN, that used the ASTM and other national standards for materials. The engineer MAY have put the stresses on the drawings to indicate that and the specification may have gone into deeper material requirements. Without knowing the dimension of the structure, the stresses do appear to be the maximum actual stresses of a specific wall somewhere in the building. Normally, that would be the maximum stress required on all walls at that level in the building.

In 1979, Wisconsin was literally in a different world as far as codes and specifications. DILHR (Department of Labor, Industry and Human Relations or similar) was coming out of the woods and really did not recognize the model codes or ASTM when it came to many materials and building requirements. Consequently, the designs and specs of engineers lagged behind because of the differences.

I drove that 270 miles to Madison many times on code and material requirements, even though we supplied only 10% of our total sales into masonry construction in WI. We even had to make a special line of lightweight block to comply with the local requirements that required us to produce an inferior block to fit into the older rigid categories for load bearing block that did not agree with ASTM.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[zurch1818]

Thanks for the replies everyone. If it helps, the building is roughly ~160' x ~53'. There are two ~23' bearing walls that span in the 53' direction with a ~7' corridor in the middle. I have tracked the original design loads down to the first floor. The 1' strip of bearing wall was designed to carry the following ASD loads:

6th: 2.1k
5th: 3.92k
4th: 5.7k
3rd: 7.47k
2nd: 9.24k
1st: 11.02k

At this point the proposed load is roughly 10k being to the roof. Pending on the brick size, I could possibly spread the load out far enough to bring the design under unity. I agree that the engineer MAY have called out axial stresses, but I don't know if I want to claim that. The magnitude of the proposed load makes me leery of signing off on it. Although grouting it solid is an option, I don't think it is practical for this building.

I had no idea that the building code in 1979 was so far behind in regards to CMU. I have attached the design crit page for your reference. Does anything look out of the ordinary on this page? I see that it calls out a NCMA reference from February 1975. A prism test was also required for all cmu blocks and was required to be verified with the architect prior to construction. The schedule I already included are details 12 and 13 on S-5 (which are referenced in this attachment). I don't have access to any of the references that are mentioned. Thanks,

Kevin