Footing with cold joint 1

[ottles]

Let's say you have concrete placement done one half of it (see the red lines) and the other top half (or rest of portion) of it placed next day. Can it affect the punching shear strength? How about one way shear strength?

 

[KootK]

You need to be able to deal with the horizontal shear demand on the cold joint. If that's satisfied then I believe that one way and punching shear should be ok.

If you can penetrate the first pour with an internal vibrator then the concrete should be able to bond as though the two pours were monolithic. If not, you may need to consider bonding agents or rebar dowels across the joint. Depending on the size of the footing etc, it may be cheapest to just rip out the first pour.

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.

 

[ottles]

You need to be able to deal with the horizontal shear demand on the cold joint. If that's satisfied then I believe that one way and punching shear should be ok.

If you can penetrate the first pour with an internal vibrator then the concrete should be able to bond as though the two pours were monolithic. If not, you may need to consider bonding agents or rebar dowels across the joint. Depending on the size of the footing etc, it may be cheapest to just rip out the first pour.

1. Should footing and lower part of column be casted monolithic? Or can footing have concrete placement first and then say 5 days later the column would be casted?

2. Let's say you have the footing casted first with dowels or even column bars sticking out already. And say the footing depth is 20 inches. What would happen if the contractor pour the extra concrete they didn't use in other part of building (because the concrete truck has certain volume and sometimes it arrives with more than the requirements and there is extra) into the footing. Instead of the designed 20 inches footing depth.. say it becomes 25 inches.. then after it the column is casted the following week.. would this unintentionally make the 2 way punching shear (and even one day shear) stronger? This happens in construction when they have extra concrete and they can't dump it elsewhere so they add it to the footing or foundation.

Thank you.

 

[hokie66]

You seem to be trying to solve the contractor's problem of ordering the wrong amount of concrete.

For your first question, KootK is correct. For one isolated footing, rip it out and do it again. The horizontal shear issue for a pad footing can be quite severe, and is not easily solved after the first lift hardens.

It is normal to cast footings first, then columns after some time has passed. Casting them together is not only difficult, it can lead to quality issues. And column concrete is often a higher strength grade than the footing.

There is not a problem with casting a footing thicker than designed. But then wouldn't the column starter bars be too short?

 

[KootK]

1) Columns can, and should, be cast after footings.

2) The footing over pour may well improve shear capacity but it will shorten the lap between your footing dowels and column vertical bars.

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.

 

[ottles]

For your first question, KootK is correct. For one isolated footing, rip it out and do it again. The horizontal shear issue for a pad footing can be quite severe, and is not easily solved after the first lift hardens.

Hokie66. Books I read like Design of Concrete Structure 14t Edition doesn't mention this horizontal shear thing in the Footing and Foundation part but only punching shear and 1 way shear. May I know why it's not mentioned and what books mentioned them? Checking 2 other books.. it's not mentioned either. Thank you.

 

[hokie66]

I don't know specifically about that book. Maybe the authors just thought that no one would do such a stupid thing in a footing. But I imagine that your book does address horizontal shear in different contexts. Horizontal shear stress in beams and slabs is typically of lesser magnitude than in a pad footing, because the contact area of the pad joint is less, and the applied pressure is greater.

 

[ottles]

It is normal to cast footings first, then columns after some time has passed. Casting them together is not only difficult, it can lead to quality issues. And column concrete is often a higher strength grade than the footing.

The contractor usually casted the footing and lower part of column at same time. The president reasoned it must be monolithic. Also the foundation and column happen to be both 4000 psi so they can be casted monolith (at least the lower part of the column and footing). How can there be quality issues for this?

 

[hokie66]

Who is the "president"? Why would you want to cast the column in two pieces when one will do? The less operations, the better. Or is this again due to concrete ordering issues?

 

[ottles]

Who is the "president"? Why would you want to cast the column in two pieces when one will do? The less operations, the better. Or is this again due to concrete ordering issues?

concrete ordering issues.. and the reason for the partial column up to that point is to add a ground beam to it.. so they order a concrete truck that can fill them at same time.. because it's difficult and expensive to order separate truck just for that one meter column height.. the purpose of the ground beam is to make the column more stiff.

the president is owner of the contractor company who follows the structural plans by any methodology he sees fit (that meets the structural company plan criteria like 4000 psi).

 

[hokie66]

I see. Of your practices, the worst one would be casting a footing in two lifts, one one day and the other the next. That needs to be avoided.

 

[ottles]

I don't know specifically about that book. Maybe the authors just thought that no one would do such a stupid thing in a footing. But I imagine that your book does address horizontal shear in different contexts. Horizontal shear stress in beams and slabs is typically of lesser magnitude than in a pad footing, because the contact area of the pad joint is less, and the applied pressure is greater

Does this horizontal shear affect more on 2 way punching shear or 1 way shear? How about for mat and bigger footing? If someone can find any reference about this in foundation section of books and whether it is called by another name, please let me know.. thanks

The contractor told me they did this in other buildings when they ran out of concrete (estimate error of volume) or when the truck broke down. We just ended in arguments because of his stubbornness.

 

[hokie66]

Shear is shear, but horizontal shear is different from either one way or two way shear as you know it. Horizontal shear is on the horizontal plane, and in your case the concern is the along the joint between the two placements. I imagine you know that shear in short, heavily loaded flexural elements typically controls, and a pad footing is a good example of such.

 

[KootK]

I would say that it does affect two way shear more than one way shear because:

1) Vertical and horizontal shear are complementary.
2) Two-way shear stresses will be much higher than one way.

That said, I'll usually design for the horizontal shear associated with one-way shear only and leave it at that. I do so because I believe that the high localized horizontal shears associated with two-way action can be redistributed laterally.

I believe that the same procedure is applicable to rafts as well.

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.

 

[ottles]

Kootk.. Thanks. I'd like to ask something unrelated. How does building with rocks surrounding the basements perform. Would the rock push against the walls?

 

[KootK]

would the shorter pink building foundation moves along with the rock underneath it.. or would it slide and crush the wall of the tall building?

It should be designed not to slide along the rock so it shouldn't crush the wall of the tall building.

Won't the rock surrounding the basement of the tall building crush into the tall building wall during seismic activity? Or would the wall and rock move together?

The rock will likely impart its seismic load to the building primarily by pushing against the diaphragm below grade that is closest to the surface. It shouldn't impact the walls themselves much as the walls will be flexible relative to the rock mass. If this were soil instead of rock, however, then you would have to worry about the soil getting seismically thrown against the walls.

Lastly. If you will say wall and rock would move along together. Can the pink short building foundation also move along with the rock due to coefficient of friction making it not able to slide (let's say no key or doweled are installed between the pink building foundation and rock)? What do you think?

Yes. That's just how it ought to be designed in my opinion.

Here's an article that discusses this issue a bit from Los Angeles. And another from the Kiwis.

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.

 

[ottles]

The rock will likely impart its seismic load to the building primarily by pushing against the diaphragm below grade that is closest to the surface. It shouldn't impact the walls themselves much as the walls will be flexible relative to the rock mass. If this were soil instead of rock, however, then you would have to worry about the soil getting seismically thrown against the walls.

You mention the walls will be flexible relative to the rock mass. But isn't it the retaining wall is made up of column and beam grid touching the rock. Let's say the rock impart its seismic load to the column-beam joint portion connecting to the diagraphm.. won't the rock crack the column-beam joint?

When you mention walls. Did you mean the concrete that is not part of the column and beams that is external and touching the rock? But wall should also include the external column, beams and column-beam joints. If it's this you were saying.. you mean even column, beams, column-beam joints are flexible relative to the rock mass and rock will not affect/crack these external column,beams?

If you mean the diaphragm as composing of the column,beam, column-beam joint, what would happen to them as the rock push against them? Would the rock crack them? the rock mass is so huge.. could any diaphragm even resist them? Imagine you have a rock in your hands and you hit a wooden toothpick with it.

 

[KootK]

You mention the walls will be flexible relative to the rock mass. But isn't it the retaining wall is made up of column and beam grid touching the rock. Let's say the rock impart its seismic load to the column-beam joint portion connecting to the diagraphm.. won't the rock crack the column-beam joint?

In my region, it would be walls and slabs below grade at the perimeter rather than beams and columns. No matter though. Columns will also be flexible compared to the rock and your beams should be braced by, and deliver their load to, the diaphragm (slab).

If it's this you were saying.. you mean even column, beams, column-beam joints are flexible relative to the rock mass and rock will not affect/crack these external column,beams?

Yes.

If you mean the diaphragm as composing of the column,beam, column-beam joint, what would happen to them as the rock push against them? Would the rock crack them? the rock mass is so huge.. could any diaphragm even resist them? Imagine you have a rock in your hands and you hit a wooden toothpick with it.

By diaphragm, I'm mean the stuff in the structural floor. Slabs and beams. Not columns. And yes, the diaphragms can take the load. I think that you're getting off track thinking of the basement as a stationary thing that sits still while the rock pounds on it. Instead, imagine your building floating in a bowl of jello. As you shake the jello, two things happen:

1) The jello imparts forces to the basement and;
2) The basement moves around along with the jello.

For the most part, the basement does move along with the earth mass. Never perfectly so however. If it were perfectly so, there would be no seismic force imparted to the building from the earth anywhere other than at the footing to rock interface. And sometimes, we do design buildings assuming that to be the case. But then we also assume that those buildings' height is the the height from the footings to the roof for seismic purposes.

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.

 

[ottles]

In my region, it would be walls and slabs below grade at the perimeter rather than beams and columns. No matter though. Columns will also be flexible compared to the rock and your beams should be braced by, and deliver their load to, the diaphragm (slab).

You mean solid concrete wall and slabs in the perimeter? Many buildings have 2 floors of basement parking and I think some use column-beams and hollow blocks (or masonry blocks) at perimeter to make up the walls just like above grade. Are you saying you haven't encountered this in practice and this is not allowed??

Back to the pink short building. If it slides, would it inertia be greater than the rock and break the wall or equal inertia and force? Or would the wall even cushion the sliding of the pink short building foundation (a short period short building)? What is the usual case in practice?

Would a building with pad footings slide more? If one is build right next to a building with 2 basement parkings.. are there guidelines how the foundation should be distance from the adjacent building.. but lets' say it's one meter distance.. it's very big and so much space would be used up and I don't think it's the norm in practice. There are thousands of buildings with pad footings that is touching the adjacent building basement parking wall. Had there reports where the pad footings (or combined footings) shear into the basement parking wall? Thank you.

 

[KootK]

Are you saying you haven't encountered this in practice and this is not allowed??

No. I'm saying that perimeter beams and columns in the basement wall is not common where I practice. I have seen the system and it is allowed to my knowledge.

Back to the pink short building. If it slides, would it inertia be greater than the rock and break the wall or equal inertia and force? Or would the wall even cushion the sliding of the pink short building foundation (a short period short building)? What is the usual case in practice?

One of two things needs to happen:

1) The short building foundation needs to be designed so as not to slide. This is the usual practice.

2) The basement wall of the main building needs to be designed to restrain the leftwards movement of the short building foundation. I've never heard of this being done and can imagine that it would be difficult to make it work.

Would a building with pad footings slide more?

More than what? I'm not sure what we're comparing pad footings to.

If one is build right next to a building with 2 basement parkings.. are there guidelines how the foundation should be distance from the adjacent building

Sometimes there are legal requirements for spacing based on ownership. It's a weird thing structurally. Even if two basement buildings have some separation, they would still push on one another below grade through the interstitial soil. The soil can, of course, transfer axial loads across the gap. It's much less of an issue with rock of course.

.. but lets' say it's one meter distance.. it's very big and so much space would be used up and I don't think it's the norm in practice.

You're right. It is very common to have adjacent building foundations pretty much in contact with one another.

Had there reports where the pad footings (or combined footings) shear into the basement parking wall?

I've never heard of such a report. Again though, this should be a moot point as the shorter building should be designed not to slide.







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.