Foundation Wall Concrete Lintel/Beam

[YoungGunner]

My state has a modified building code that states that so long as an opening in a basement foundation wall is a maximum of 6' and the concrete beam has a depth of 12", then you only need (2) #4 bottom bars located 2" from the opening. There will naturally be a bar near the top of the wall but with 3" top cover per the other codes which makes it impossible to use as a compression bar. The top bar area is less than the (2) #4 bottom bars, and the equations for fixed-fixed tend to make the negative moment worse than the positive, leading me to think the state assumes pinned-pinned boundaries. I'm working on determining the maximum point loads or distributed loads such a beam can support and determining the boundary conditions will impact what the worst load effects are.

My question is that if the bars are only placed in the bottom of the beam, is the state assuming the boundary conditions are only pinned-pinned for these beams? This leads to the follow-up; is this a bad assumption for the state to assume pinned-pinned versus fixed-fixed for a beam poured integrally with the foundation wall? Or is it OK to assume pinned-pinned for this mode of construction?

I may be young in the industry, but I'm not willing to accept the standard as the standard without solid argument.

 

[dik]

Generally your top bars are larger than your bottom bars. Can you increase the top bars to #5?, else add another #4 bar? For that type of opening, I generally treat the lintel as being fixed at the ends since the stiffness of the wall in flexure is much greater than the beam.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[KootK]

1) I would say that you are correct about folks commonly designing such a beam as pinned pinned.

2) The danger in the pin-pin assumption, in my opinion, is that your "d" for shear drops to nothing at the ends if you have hogging moments which, of course, you do.

3) The traditional solution to #2 has been to provide top steel in the amount of 25% - 33% of the bottom steel. You'll see this reflected in the CRSI manuals in places.

 

[phamENG]

YoungGunner - are we talking about the IRC? (Or Utah's edited version, anyway.) I'm guessing so. The IRC is made for non-engineers and non-architects to design and build simple residential buildings. It's based on a mix of engineering and long standing practice/empirical methods.

I agree with you that it doesn't hold up to sound engineering practice - most things about basement walls in the IRC are pretty sketchy. Reinforcement quantities are typically a lot less than a wall designed by an engineering using ACI. (I don't deal with basements very often, but that is my recollection from the last time.) But for a simple house, they're allowed. So you have an up hill battle ahead if you want to enforce a higher standard.

 

[YoungGunner]

phamEng - we are talking about the amendments to the IBC actually, not IRC. Every engineer out here uses that provision for lintels and the company I work for as well - but I would like to establish some firm limitations on how we feel about it. But that implies determining load effects, which goes back to the pinned-pinned versus fixed-fixed boundary conditions.

KootK - so it's possible then, that the bar at the top of the foundation may be sufficient in this case, since it 50% of the area of the bottom (2) bars?

I may be young in the industry, but I'm not willing to accept the standard as the standard without solid argument.