Structural soundness of combining lintels !

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We have a dilemma. We will soon be placing precast concrete floor slabs on the first floor of our new build. The manufacturers mandate 9 inch deep concrete lintels for any opening under 1800 mm. Builder has already placed 6 inch lintels there. There is now a further 6-7 inches of small bricks between these 6 inch lintels and the ceilikng.
To correct is it sufficient to add another lintel on top of the existing one? ie. will a 6 inch plus a 6 inch lintel, one on top of the other be as good as a SINGLE 9 or 12 inch lintel?
Much obliged Chad.
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Chad wrote:

It would be grand if things worked that way, but they don't. Particularly in precast concrete. I doubt ( < weasel word) that you'd have a catastrophic collapse, but the lower lintel would most likely crack.
New building, right? Why start off on the wrong foot?
The only people that can definitely answer your question are the people that designed the lintels.
R
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In a previous post RicodJour wrote...

I agree with Rico on this. I would assume that there is an architect or engineer of record for a building that uses precast concrete floor panels.
Ask that that person the question and see what kind of answer you get.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
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Bob Morrison wrote:

Aww, c'mon, Bob. We agree on everything. ;)
R
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I agree about doing it right. Just getting the story straight before asking builder to correct it.
RicodJour wrote:

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OK I agree about doing it right first time. So only option is take off the old lintels. Thanks
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Hmmm
Now seems that the people who manufacture the floor slabs are instructing the builder to simply add a 3 inch lintel on top of the 6 inch one. Starting to get worried now.....
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Chad wrote:

And I'm starting to get confused. Who signed off on the drawings that were submitted to the building department for approval? They should be your first stop in the line of questioning as they are the engineer/ architect of record and responsible for the building, not a material supplier who is feeding a builder.

responsible for both and it's less of a worry, but I would not automatically assume that what they're suggesting is the best for _you_ as opposed to the builder who is buying their products.
If you don't have someone watching out for your interests (sounds like you don't), and you are not qualified to determine the correct sizing of the lintels (you're not), then you are in a bit of a position.
All of construction, all of it, is simply a risk vs. reward calculation. I could build you a building that would last forever, but the price would kill you, and forever wouldn't last very long for you. You have not paid for perfection, but your building has an expected life expectancy that far exceeds the builder's warranty period.
If the builder wants to make a substitution from the contract documents, require them to sign an indemnification clause (you do have a lawyer, I hope) that is good for, oh, say twenty years, that will require the builder to correct any deficiencies in the lintels after the normal warranty period has run out. The indemnification should include materials, labor and some number to reflect lost productivity to the building's occupants during the repair work, which would presumably affect you or your tenants (and they'd withhold money because of it). The indemnification agreement would be worded that it is the builder's wish to sign the agreement and is voluntarily waiving arbitration and litigation rights in their own interest.
I would never in a million years personally sign such an agreement, but it would be the only way that I'd let a builder sell me on something that is against the original contract documents and the manufacturer's own literature just to correct the builder's oversight. Essentially what you're doing is giving the builder an option to not correct the undersized lintels. Either sign the waiver or fix the lintels - his choice.
You have provided very limited information, and I have no idea of what the real situation is. My suggestion may work, or it may be ridiculous. This is why you need someone who is knowledgeable in construction to protect your interests. You can hire such help at any time.
R
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My question is simple:
We currently have a 6 inch concrete lintel over the opening. The builder plans to add on top of this a 3 inch PRE-STESSED lintel. Do the two combine in strength? According to the builder they do, "6+3=9" as he said himself.
If anybody on this message board can answer this THEORETICAL question I would greatly appreciate it. I can clarify any TECHNICAL details.
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I haven't read the answers above but in answer to your question the answer is no.
Not the formula but an example would be 6 squared = 36 and 3 squared = 9 or a total of 45. BUT, 9 squared = 81.
Maybe that will make it easer for you to see. The 6 will hold it's weight and so will the 3 but they would have to be bonded together to hold what the 9 would carry.
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Chad wrote:

Your question is simple because you don't know what's involved. To the people that understand what's involved it's a question that can't be answered simply.

That is as dangerous of a comment as he could make. It's one step _down_ from, "Don't worry about it."

I already answered that.
NO. 6+3 DOES NOT EQUAL 9
Is that any clearer?
You can ask the question any way you want, it doesn't matter the answer will be the same. I could go into shear planes and all sorts of crap, but it'd fly right over your head, so there's no point.
How differently constructed lintels would react in layers is a question that can only be decisively answered by the engineer who designed the thing. Since you're asking here, after already being told that you need to ask the designer of record, you probably don't have one. If that's the case you are on thin ice and asking the guy on shore if the ice looks thick enough from where he is.
It sounds to me like you're being sold a bill of goods, or being sold the goods by someone who can't read the bill, and you're looking here for approval. No one here can conclusively determine that the proffered solution would fly without a _lot_ more information, and even then, what? You're going to risk your building on an answer from a newsgroup? Professionals are inherently conservative and with good reason. I'm being conservative with good reason. I'm _not_ being very conservative in warning you.
Some random guy on the internet (you) asking for a FREE engineered solution to his problem is more than a little presumptuous. It takes away the monetary reward that the engineer deserves - you do believe that people that do work for you deserve to get paid, don't you? - and creates a liability for the engineer. So let's see, that sums to added risk with no reward. Does that sound like a good deal to you? Would you take that deal? If you're lucky Bob Morrison may explain to you why a professional doesn't engage in that sort of activity in a newsgroup.
Good luck with your project.
R
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In a previous post RicodJour wrote...

Rico has done a damn good job of explaining why I recommended you contact the architect or engineer of record. If don't have one, then get one.
If this is a THEORETICAL problem, then THEORETICALLY you should get a REAL engineer to solve it. As a REAL engineer, I wouldn't touch this problem with a 10-foot pole without being able to physically inspect the situation first hand -- and maybe even then I would turn down the job.
Definitely one of those situations where 6+3 does not equal 9. If you think it does then you might try putting 3 bullets in a 6-gun, putting the gun to your head and pulling the trigger 9 times. That's about how much 6+3=9 in this situation.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
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Common sense tells you that 6+3 does not equal 9 in this case. Imagine trying to replace a 2x8 floor joist with 2 2x4's on top of each other...Wouldn't work, would it? As a carpenter of some experience, if I were in your shoes, I would get an engineer involved.
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To belabor it some more. The illustration of stacking 2 x 4's while true is better than stacking 2 concrete lintels. Each would carry what a 2 x 4 would carry or twice what one would carry. Stacking the lintels would be different in that the top one would crack first and you would have only the bottom one carrying... Kinda similar to the World Trade Center. Each one would pancake down.

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Thanks Glenn,
I'm now intrigued. Would you say that the addition of the three inch lintel does not improve the situation? Is it adding strength at all? One would imagine that there should be some benefit to adding the lintel. I'm trying to get an idea of the perentage improvement. The figures you have given suggest that two lintels are only about half the strength of the one 9 inch lintel.
Don't be afraid to tease out the idea on the newsgroup. Thats what it is for.
Thanks, Chad -------------------------------- Glenn wrote:
haven't read the answers above but in answer to your question the answer is no.
Not the formula but an example would be 6 squared = 36 and 3 squared = 9 or a total of 45. BUT, 9 squared = 81.
Maybe that will make it easer for you to see. The 6 will hold it's weight and so will the 3 but they would have to be bonded together to hold what the 9 would carry.
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Chad wrote:

Well, gee, Sparky, what percentage of failure is acceptable to you?

Don't take this the wrong way, but are you fucking insane? The "figures" Glenn gave...?! They weren't figures, they were arbitrarily chosen numbers. He was making a complex situation easier by giving an analogy that was so simple that even an idiot could understand the concept. Unfortunately he erred in guessing the density he was dealing with.
It was a good try, Glenn. You summed it up nicely. I was sure you had it. Better luck next time!
Glenn's analogy was illustrating that beam strength is not linear - that is, it does not ADD. You can't add a 3" member to a 6" member and have it equal the load capacity of a 9" member. The load capacity is proportional to the SQUARE of the depth. Remember when you dropped out of 9th grade as they were hitting those funny math problems where they'd multiply a number by itself? That's a number squared. EACH member is dealt with separately.
If things were so simple, why do you think people use deep pieces of wood, steel or concrete as beams instead of a bunch of shallower ones? It'd be a hell of a lot easier to handle the smaller pieces, transport them, install them and they'd be cheaper. If it could be done, it would be done EVERYWHERE. Hint, hint. It's not.
If this is sinking in at all, please stop me. It's an effort to talk down to someone this much.

Hey, here's a thought. Why not tease the idea out of an engineer? Make sure you print out this thread and show it to him. Engineers like a good laugh, too.
Good luck with your project.
R
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Contact the source of the lintels.
I am not an engineer, but I think the bearing capacity of the 6 could be added to the bearing capacity of the 3. I started out thinking that they would never add up to the bearing capacity of a 9.
If I am reading these charts correctly, a huge if, it would appear that perhaps you're OK. <http://www.castcrete.com/catalogs/SAFE%20LOAD%20TABLES.pdf
Verify with a structural engineer or the/a lintel manufacturer.
--
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Keep the whole world singing . . . .
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DanG wrote:

The lintel manufacturer is a party to the oversight. It is unclear whether the contractor or the lintel manufacturer/supplier is responsible for the oversight. In this instance it is unclear what relationship there is between the builder and the lintel supplier/ manufacturer, and it sounds like the builder is relaying the information.
It plays out like this:
Of course there could be no possible reason for a builder to withhold information. Oh, wait - greed. So there's one reason. So that's it, just the one reason - greed...and not understanding the physics involved (ignorance). Okay, two reasons. That's it. There can be no other reasons. Just those two. Greed, and ignorance...and sloth. The contractor could be too lazy to investigate the situation. So that's three reasons. There are ONLY three reasons...
You get the idea?

You're not. You misread the tables and don't understand the physics. Sorry, Dan.
Think of it this way. One of the lintels is weaker than the other. It will yield (deflect) first under load. At that point the other lintel would be required to support the weaker lintel. Take it from there.
Glenn's analogy of two 2x4's stacked not equaling a 2x8 is good.

The only advice that can be given.
A solution can almost always be designed, but a solution can not be guessed at. Not unless you're an idiot, that is. A designed solution requires someone that understands the design conditions, materials and structure to be corrected. Neither the OP nor the builder is up to the task.
R
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RicodJour wrote:

I'm sorry, Marson. My error - it was your analogy, not Glenn's. Glenn had the other good one.
R
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I presume a 2 x 4 is 2 inches deep and a 2 x 8 is also 2 inches deep? If so then a more relevant analogy is whether two 2x4 are as strong as a 4x4.
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