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Explain in Lay Terms

Explain in Lay Terms

Explain in Lay Terms

(OP)
I have a 10' rough masonry opening supporting a wood roof trusses bearing on 4' vertical of concrete masonry over the opening.

I need to explain to the mason, the expediter of the general contractor, and the project manager, why i need a 40" concrete masonry lintel at f'm= 1.5 ksi, but the alternate concrete beam at f'c= 3ksi is only 12" deep.

I'm just frustrated that i have to give an account, ultimately to the complaining mason, even though the general has NOT had a problem with 5 or 6 course CMU lintels over the past several years.

On the other hand, it IS reasonable, to me, that the general contractor client has a right to have, at the very least, a reasonable, simple comprehensive explanation, of what i design. I often deal with a reasonable contractor, with site foremen that occasionally tell me, "I'll DO anything you want, just give me a REASONABLE answer that i can understand, that makes sense." I AM fortunate with the typical people i deal with.

SO! Considering the simple proportions of depth versus ultimate stress, how would you offer a simple, reasonable explanation, in lay terms, as to why i can't allow a 24" CMU lintel rather than the 40"?

Thank you!

RE: Explain in Lay Terms

What are the stress numbers at the center of both?
Show that one fails, the other does not.
(Don't specifically try to show him the calculations. SHOW him the results at the center:

The masonry you are installing is good for 1234.5 psig tensile. Then it breaks.
At the bottom center of the 24 inch deep section, the masonry faces 2345.6 psig. That will break it. But, if I double the depth of the masonry section to 48 inches, I quadruple the strength: so it can resist 4567.8 psig. That's too strong, because I only needed 40 inch deep section. So we have to use 40 inches deep.

RE: Explain in Lay Terms

(OP)
racookpe, thank you for your effort.

I agree about the calcs. I have rarely had to surrender calcs to the General Contractor client and i have never had to surrender to any of the trades unless, they themselves were material supplying engineers who were proposing a smaller structural element in their submittals. (I hate that when they do that!)

I've decided that i can offer my own empirical conclusion such as:

"The allowable stress for concrete (0.45f'c) is (+/-) 4x that of concrete masonry (0.25f'c)."

Thank you!

RE: Explain in Lay Terms

A non-engineer might reason, "I've built it like this 8 times before and it didn't fail, why should this one be built any differently?"

And perhaps a helpful approach to that line of thinking would be to calculate the nominal strength required to support dead load, plus the extra required for snow, wind, seismic, etc., plus the additional margin required on top of that to meet the building code.

RE: Explain in Lay Terms

I say that masonry is basically shitty concrete (no offense to masono-philes). This seems to play reasonably well with builder folk. If I need to get any fancier than that, I mention that placing flexural rebar advantageously is much more difficult in masonry because of the geometric constraints.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Explain in Lay Terms

On the other hand, why do you need to defend your design at all? If you showed the contractor a stress of 1,000,000 psi, what if he said that's OK with me? I don't like to break out calculations for people who don't understand them, because then you're treating them as peers. You can respect what they do, as long as that respect is returned. And criticizing designs is not showing respect. Tell them 40 inches is what you need and if they don't like it, do their own calculation and seal it themself.
It's not his design. They don't put contractor's names on drawings, just the engineer's. You can over design any aspect you want, until the owner gets mad.

RE: Explain in Lay Terms

Take a pine board to a rip saw and cut two "sticks", one one inch wide and another 2 " wide. Take them to the job site and ask the mason to judge the difference in strength in bending by breaking each one identically loaded. Of course try this yourself first so you know the answer. The test is with the board's section long side vertical.

RE: Explain in Lay Terms

BSVBD:
If the mason can/could build the CMU lintel as strong as the 24" deep reinforced conc. beam, then the 40" deep reinforced and grouted masonry lintel wouldn’t need to be so deep. But, since he can’t build it strong enough at only 24" deep, the 40" depth is required. Ask the mason what f’c and f’m are, and what their respective values are on this job. He should know these because they are in the Structural Notes and in the specs., has he read them? Then, ask him how they are reduced for design, don’t show him as you suggested on 14MAR16, 16:40. If he can’t answer any of these type questions, just suggest that that is why you are the engineer on the job, and signed/stamped the plans, and he isn’t. And then, double the inspection and critical eye on his work, and call him on anything that doesn’t meet code and stds., in the slightest.

If he’s worth his salt as a masonry contractor, and trying to keep current on his own education as a qualified masonry contractor, he should belong to some masonry design, building and contracting organizations. They are there, in good part, for his education and edification, he should take advantage of that opportunity if he wants an education in masonry design, that’s what he pays their dues for. Ask him to show you his copy of the codes and stds. for masonry and concrete design, and to show you the sections and/or para’s. that he thinks make your design an over design.

RE: Explain in Lay Terms

I have no idea where you are, or what code you're working to, but you make my 16" lintels (lots of snow but no seismic loads) on 10' openings sound pretty scary. Funny how they've worked for 30 years though. That aside, I agree with Jed: it's your name on the drawing, not theirs.

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