lightweight cmu design 1

[msucog]

i've got a question geared more toward the design aspects of cmu walls. i have a project where 2800 psi lightweight block was specified with a f'm=2000psi. we're near the end of the project and had a few recent prisms breaks lower than 2000psi. upon review it turns out that all the cmu does not meet the 2800psi since it's lightweight. the block supplier says that they don't even make a lightweight cmu to that strength. the contractor is faced with having to grout the entire wall at their expense since they supposedly didn't bother to provide the supplier with all the information. the initial cmu testing showed some failing cmu unit strength results during the foundation block installation. the work was okayed since all the cells would be grouted and apparently the supplier was sending the subpar block to be used below grade. once they sent out "above grade" block, the cmu unit test results came up above 2800psi and we (testing firm) were directed to test using masonry prisms. all the results have been fine until very recent. so here is my question:
the questionable area is located in shear walls (apparently significant ones since these things have a substantial amount of reinforcement). i'm thinking that one approach to help my client (owner) might be to see if we can't get a better seismic site class (which will in turn improve the SDC to help the structural engineer make his design numbers work better when he reevaluates the design). we've already performed the site specific hazard analysis to get the reduction in the parameters as allowed by code, so that's out--only thing left is trying to get a better site class. the owner simply wants the thing open so the engineer is reevaluating how the lower strength block effects the design. if we did some additional testing to evaluate the Vs30, then i'm certain the seismic parameters will improve. so in turn i see this potentially helping the design engineer feel better about what is in the field (in turn helping the project move forward). i'm not familiar with all the steps in cmu wall design so do my assumptions about getting improved seismic parameters helping the structural engineer get his numbers to work make sense? (and yes, i know no good deed goes unpunished--i'm not even sure if i'm going to discuss this possibility with the architect/engineer or not since the contractor has been nothing but a complete arse the entire project. it would almost be sweet revenge to let the SOB flounder out there and pay out of his own pocket.) i'd appreciate any opinions. thanks.

 

[msquared48]

I don't mean to pry into your contractual relationship here, but as the testing firm, it appears to me you are trying to do the job of the EOR. Why take the responsibility? Are you the EOR too?

Mike McCann
McCann Engineering

 

[concretemasonry]

msucog -

How can you say all the block do not meet the specifications since they are "lightweight"? - Apparently, the recent prism tests have been satisfactory, but the early tests are now being reviewed and found to be low. The supplier was stretching things to cover his or the contractor's error when he said they do not make that strength lightweight block. - They may not be carried in inventory, but there no reason they cannot be made. - What did the submittal test reports look like and what is the history of the suppliers lightweight prism testing. Those strengths specified are not unreasonably high if the correct units are ordered.

I don't know what "sub-par" means in the specification. Apparently they met the ASTM C90 specifications for loadbearing CMUs, but not the project specifications for those walls. I assume the engineer approved the grouting based on the wall loads, and when the supplier was informed of the specifications, the correct strength units finally got to the job (too late).

The whistle should have been blown when the first block did not meet the project specifications. That is the only professional way to insure a correction, rather than reverting to a poor substitute of grouting.

Grouting cores full is a poor engineering solution (maybe the only one available at the time) since all it does is provide a higher grouted f'm that can be very misleading in many applications. As an example, in a wall subjected to flexure, the outer fibers are subjected to the highest compressive stresses, but are lower than the "avewrage" grouted prism f'm. - Very misleading, especially when high strength grout is used.

The lab should just provide the information on the units used as soon a possible so any corections can be made as soon as possible. If things slide too long, the responsiblity is his and not the testing lab. - The EOR knows the required loads on the walls and must make the calls, especiaaly with shear that is heavily reliant on reinforcement if the structure is reinforced.

 

[msucog]

we provide all results immediately (within a day or so). the initial individual unit test failed. retests of the individual units eventually passed. the architect/engineered approved going forward becuase they were still below grade at that time and the tests began passing. the many sets of prisms since then passed but the problem now is that the last few prisms have failed to meet f'm. that is when the supplier and contractor came out and said that the lightweight blocks were not designed/intended to meet 2800psi. as far as the submittals, i asked the contractor, architect/engineer and owner on several occassions prior to the beginning of masonry work. i have yet to see the submittals so i have no idea what they show. in emails between the architect/engineer and contractor, they noted some results that did not meet the specifications.

as far as "sub par" apparently (this is my interpretation of what was said by the contrator), the below grade cmu's were "rejects" for lack of a better word. i think the supplier knew they were sending out block that would not meet particular project requirements but since the below block were fully grouted, they knew that no one would require them to be removed. once we notified everyone of the initial below grade results, the architect/engineer demanded that those block be immediately taken offsite so that they didn't inadvertently end up in the walls. those blocks failed multiple tests.

i am not the EOR and do not intend to play such. i did provide the seismic site class parameters and do have the capability to provide a better site class if we're instructed to perform additional field work. the client looks to me as the independent guy and often asks for input to situations. i have the capability to help the project if my assumptions are correct...that is why i ask for opinions. i have no input as to whether fully grouted is better than tearing out the block...and frankly, i don't care if they raze the entire building. as much as i would like to see the contractor suffer, my primary interest is the interest of my client...his interest is to get the facility open. if a better site class will not help the engineer or their numbers, then that is the answer i'm looking for.

as far as the lightweight block, the supplier said that they do not have lightweight block to meet the specifications. they said that they would have to completely design the block...in other words, it's a special order block and that's not what the contrator paid for. i personally think the contractor screwed up but then again, it's not my decision. my understanding is that one of the options on the table is to fully grout at the contractor expense since they didn't order to appropriate block. i don't think the block supplier is necessarily at fault since they sent what was ordered by the contractor. the contractor is on the hook for everything. they can't even blame me for this one!!! (insider joke for those that have read some of my other ranting threads)

 

[msquared48]

"i did provide the seismic site class parameters and do have the capability to provide a better site class if we're instructed to perform additional field work"

Does your firm provide Geotech services too? If it is possible, and you are making the call, do not put the liabliity of your firm out there on a borderline call.

I assume that seismic controls here and not wind laterally?

You know, this is the problem with CMU, it is really hard to correct problems like this without screwing up the architecture of the structure, and the integrity of the structure itself.

That being said, from the stand point of a structural engineer, an obvious solution here would be the addition of pilasters on the wall, external, internal, or both as needed, and as often to take the lateral normal to the wall, ands possibly the vertical if the top bond beam could span between them. (Excedrin Migraine and Nitroglicerin to be made available to all Architects after they reading this comment).

Additionally, would it be possible to modify the "R" value to a different lateral system? This too would modify the seismic force. Probably not, but just a thought.

Mike McCann
McCann Engineering

 

[msucog]

i am a geotech and provided the geotechnical exploration services for this project. we weren't asked to perform the work to evaluate the shear wave analysis of the subsurface conditions. the site class was determined from the average n-value. in my part of the world, the shear wave will typically bump from a D to a C as long as the ave n-value is 25+. i'm 99.9% certain the site class will change if we do the shear wave field work. i might have mispoken in my first reply about the SDC. the reduction that we got through the site specific hazard analysis might have bumped up the SDC as much as it will go (i.e. a better site class might not change the SDC)...i simply don't remember off the top of my head. this is where i'm fuzzy since i'm not 100% sure how the seismic parameters translate in to the building design. my understanding is that even if the SDC does not change, a lower SDs and SD1 will help once the structural engineer transforms those parameters to an equivalent lateral force (i think that's the correct term but i'm not completely sure). again, this is the reason for my question.

will a lower SDs and SD1 potentially help the structural engineer avoid having to require additional work? again, i'm simply looking at a "legit" way to keep the project moving forward (if the client directs us to perform the extra work to justify such a condition).

 

[msquared48]

Anything that lowers the Cs factor will help to include decreasing the overall weight.

This is one problem with solid grouting the cells - it increases the overall weight you have to include in the seismic equation.

It is possible that the lowering of the site class from a C to a D will not work if you still have to solid grout the wall. I doubt the EOR figured solid grouted walls into his original equation. With lightweight block, this could change the weight of the walls from 49 psf at 48" OC for lightweight to somewhere around 75 psf for solid grouted lightweight - about a 70% increase in the wall weight.

Bottom line is I think you will have to do something different when all this falls out.

Mike McCann
McCann Engineering

 

[civilperson]

Site class is a function of what soil is below. A too conservative guess of soil class can be corrected by an extensive investigation, but it is my experience that the site class is usually correctly identified initially by knowing what is across the street and down the road.

 

[dik]

There are two issues: contractual and engineering.

From an engineering vantage, I would have the EOR review his work to determine if the supplied construction is acceptable and what has to be done to remedy it. The contractor should bear costs for re-engineering and may have to advise his insurance company (costs for remediation could be high).

Dik

 

[msucog]

i would never provide a site class based on something across the street here in the piedmont. i'll perform borings or shear wave analysis at each structure location to get the site class. if it's a big warehouse, i'll do multiple borings to refusal just for the site class (that's if it look like a class c is possible). if it's a definite class d based on the n-value, i won't do a lot of drilling just for seismic site class since there's no advantage and no need to charge the client for pointless borings. most sites will have borings to refusal anyway, so there's usually not a huge extra charge for the borings anyway. i do always push shear wave testing since i have yet to find a site that won't get 1200fps for the Vs average over 100'.

as far as my question, i think i'm going to just sit tight and keep my mouth shut for now. we cut a few samples from the walls in question last week and they broke at or above 2800 psi. the latest round of prisms came up above 2000psi. if i open my mouth now, i'll just be sticking my neck out for no good reason. if by some chance there's a huge amount of rework required, maybe i'll reconsider bringing it up. just knowing the architect, i feel that he'll make them do some additional grouting at critical locations to beef up the shear walls and make the contractor pay at least a little out of his pocket...that's if the architect happens to be in a good mood. it wouldn't surprise me if he made them pay for a complete redesign or even rebuild. we'll see. thanks for the input.

 

[JLNJ]

It seems problematic to spec both the block unit strength and the prism compressive strength. Isn't it better to spec f'm and permit the mason to design his own assemblage?