tie beam not on soil

[ticas]

The usual procedure is to first put compacted soil and put the tie beam on top of the compacted soil so the weight would bear on the soil and not the columns. But in one of the tie beams in the constuction site, the tie beam was poured with concrete using just wooden support (and not soil because trying to reach schedule). The tie beam right side also served as the stair support. Now I don't think it's possible to insert tight compacted soil under the tie beam, what do you guys usually do in this case?

 

[JAE]

I'd have them place flowable fill or a lean concrete mix under the tie beam. You are correct that compacting soil beneath the tie beam is difficult if not impossible.

 

[ticas]

But the tie beam is about two meter above mat foundation. If I put lean concrete mix, the mat foundation can no longer have bending moments or the foundation can't bend at its center. Flowable fill stil not compacted enough.

 

[Larryhd2]

Tie beam above the mat foundation? Is there to be 2m of soil over the mat? If so then I can see how lean concrete would make the mat behave differenty than compacted soils.... maybe a bond breaker would solve that issue?

Or maybe have them place concrete columns under the tie beam down to the mat before importing the soil?

 

[ticas]

I think the solution is to put 1 square foot pure concrete column every 3 meter of the tie beam. Since it's a tie beam, it's designed only to distribute the main column moments and not to bend from weight. But the problem is putting the 1 square foot columns can prevent the mat foundation portion from bending due to its being constrained by the tie beam. How do you guys usually handle this situation?

 

[TLHS]

What's the problem with flowable fill? You should be able to get a compressive capacity high enough, and if you have high air volumes you apparently can get reasonably small subsidance. If you're really concerned that it won't bear you could likely flowable fill the void to a short distance below, wait for it to set and then grout the void between the fill and the beam.

 

[ticas]

TLHS, won't the flowable fill prevent the mat foundation below from bending? I haven't used such before. Has anyone actually used this between mat foundation and tie beams?

 

[TLHS]

How are you picturing it stopping your mat bending any more than regular fill would? I'm not saying you're wrong, I'm just not sure where you're coming from. It's not concrete really. I can't see you getting a structural bond or any reasonable composite action where the fill acts as part of your mat's compression block. You're talking about something that, if specified, can have a strength low enough that it can be removed with a normal excavator.

As a side note, how would a stiffer mat hurt anything? The only way it would become stiffer is if more material were activated in resistance which, it seems, would increase the moment arm of your reinforced section and presumably make the whole thing stronger?

 

[Larryhd2]

How about compacted fill to withing a few inches of the bottom of the tie beam... lean concrete under the tie beam to the compacted fill.... then complete the soil import.

Gets you the support of the tie beam and allows everything to move the way you want it to.

 

[ticas]

I assume a flowable fill is more heavy than compacted fill? I haven't used flowable fill before. And I don't want to use concrete lean mix as it would be much heavier than compacted fill and can affect settlement of the foundation.

Also I think flowable fill would be more compacted than trying to compact the soil a few inches under the tie beam? Any others with experiences on this?

What do you commonly use between the mat foundation and tie beam anyway?

 

[amec2004]

High pressure jet grout underneath the tie beam ?

anchor bolt design per ACI 318-11 crane beam design

http://www.civilbay.com

 

[hokie66]

I have never used "tie beams" in conjunction with a mat foundation, so don't have a clear picture of what you are trying to achieve. Maybe a section through your building would help.

 

[ticas]

the tie beams are just to distribute the moments in the columns of the mat foundation, why is this concept foreign to you? tie beams have two purposes:

1. to distribute moments of the columns
2. to avoid differential settlement

since the foundation is mat, there is not differential settlement, so the purpose is to distribute moments of the columns

 

[Larryhd2]

Ticas,

I'm also not familiar with this type of system you are describing. Do you have a mat with soil over it that then has tie beam between the columns?... If so, why isn't the mat up higher?....

I am very familiar with mat foundations and your statement that there is no differential settlement is not true. It is far less with a mat, but not entirely gone. I would have integrated tie beams or stiffeners in the mat to account for the column moment distribution.

I agree with hokie, you need to provide a section.

 

[ticas]

It's very simple, the building has 5 columns, 4 at corners and one at center. The rock is 3 meters below. The mat foundation is on top of the rock and the ground slab (which is at level with the tie beam) is about 2 meters above the foundation. Hence tie beams on ground slab level is needed. If you would integrate the tie beam on the foundation, the columns would become longer.

 

[BAretired]

If the tie beams are not capable of spanning between columns under their own dead load, how were they expected to distribute column moments? Maybe you should place a steel column under the tie beam at midspan and support it on the rock below. The mat foundation could be poured around it.

Why are you using a mat foundation on rock? Is it really rock?

BA

 

[ticas]

not really rock, but very dense silty sand, above it is clay that is why mat foundation is put on the dense silty sand. the tie beam can support its own dead weight, but it's supposed to be carried by the soil, not the columns which can only stress the columns, the size of the tie beam is 400mm depth, 300mm width, 6 meters, 3 top bars at support, 2 top bars at midspan, 2 bottom bars at support and midspan. the foundation is already casted in concrete.. only two tie beams at front don't have the soil between it and the mat foundation because of schedule constrain.

Maybe I can put a steel column between the tie beam and foundation at midspan? but if the foundation bends, the tie beam maybe bent upward and get damaged.

 

[hokie66]

If you are just worried about the dead load of the tie beams themselves on the column, stop worrying. That is an insignificant gravity load.

 

[BAretired]

Whether you have a column or compacted soil, the tie beam will behave about the same.

BA

 

[ticas]

but the stairs footing is right at the tie beam itself so the right side of the footing is holding half the stairs going to the next floor. I think I'll put compacted soil and lean mix a few inches above to touch the tie beam, but the problem is, in compacted soil, it is not 100% compacted so any few mm difference can make the tie beam not touching it. When you compact soil, are you sure it's 100% compacted?