Inverted T-Beam Analysis

[BadgerPE]

Hey all!

First, my boss is out of town for the next week and I am just looking for a little guideance on this topic. When he gets back we will be reviewing this before it goes out for construction. Now that I have stated my disclaimer on to the matter at hand.

Attached are sketches and calcs.

We have a pre-engineered spec building that is already in place. The interior subgrade is compacted and ready for a slab. The lateral ties are encased in concrete and are already in place and cured. We need to underpin an existing 4' frost wall and column footing for the addition of a pit dock. I am looking at underpinning the existing column footing first. The bearing strata is natural sand. As it is sitting on sand, I am quite worried about the possibility of undermining the existing footing to the point of failure. We are trying to find a solution for this as the client would rather not drive sheet piling. If we can't find an adequate solution, sheet piling may become an option.

Therefore, I am analyzing a section of existing footing to determine the available strength in the event that some undermining does occur. The frost wall is non-loadbearing so it only has temp/shrinkage steel in it. However, our company did the original design and inspection so we are confident of the approximate location of bars in the wall/footing. For the T-beam analysis, I used the two bars in the footing and the lowest in the stem as tension steel.

When I run the calcs, I get a phi*Mn=206 k*ft, Mcr=362 k*ft and a design moment, Mu=72.8 k*ft. Since Mcr is greater than phi*Mn it raised a red flag for me. According to Section 3.7 in "Reinforced Concrete Design" by Wang and Salmon..."When steel reinforcement in a flexural member is only a small amount because of the factored moment Mu is low, the beam may perform uncracked at service load. The computation of nominal moment strength Mn, in accordance with Section 3.4, assumes the tension concrete to be cracked. Thus, the computed nominal strength Mn for a section having a small amount of reinforcement could be less than the strength Mn (called Mcr) of the same section of plain uncracked concrete. Since a ductile failure mode is desired, the lowest amount of steel permitted should be the amount that would equal the strenght of an unreinforce section.

So based upon that information, I am thinking of sistering a channel to the outside of the wall that will increase the moment capacity phi*Mn such that it is higher than Mcr thus eliminating the potential for a rupture type failre. Do any of you have any input on this that you feel would help me out any?

Some other considerations that I will be going over with my boss next week:
1) In ACI 318-08 it states that a T-Beam should be poured monolithically. This was not.
2) There is no shear reinforcing in the footing/flange.

Sorry for the long post. This is my first attempt at underpinning design, and like I alluded to earlier, I will be reviewing all of this with my boss before it goes out for construction. I would appreciate any comments/suggestions/criticisms.

 

[BadgerPE]

I forgot to save it in an older verison of Word. So for all of you that don't have 2010, here are the sketches/calcs. 3 pages.

 

[BAretired]

I guess I am getting old because I haven't the faintest idea of what you are talking about. Why don't you simply tell us what the problem is? If anyone else understands it, please explain because I do not.

BA

 

[bootlegend]

To add a little to your note 1), I think ACI means typical T-beams with slab above flange should be poured monolithically. Your situation is different and it would be hard to form AND be sure of concrete consolidation. I've always seen these poured as two pours.

I got 350 heads on a 305 engine; I get ten miles to the gallon, I ain't got no good intentions.

 

[mtu1972]

It feels like you are over thinking this. If Mn > Mu you should be OK. Also checking Vn > Vu. But I do not think you can automatically look at the wall/footing as a T-beam.

The wall by itself does not satisfy Mn > Mu. If the wall/footing interface has enough vertical steel such that VQ/I can be satisfied with a shear friction review - then maybe you can count on them acting "compositely".

Your instinct seems to be better safe than sorry and I would agree with that approach.

gjc

 

[ishvaaag]

Do not forget that older codes allowed for 4/3 of Mu for the steel to be placed at foundations; this most surely in the acknowledgement that on the thick sections of foundations to go for the whole cracking moment strength anywhere could (and may) become overkill. A fragile failure is most undesirable, and so is the developed criteria for most recent codes, buy you might be far from it.

Anyway, by the standards of reinforced concrete, minimum steel seems at lack, what is a severe shortcoming precisely for rheological solicitations, that sometimes go dismissed in calculations, minor errors in design, design changes as your case is, and some degree of ductile behaviour.