excavated deeper than calculated depth 1

[mattdukes]

The actual digged or excavated depth is 2 meters to reach certain tuff rock layer, calculated depth of design is 1.5 meters.. what do you normally do in case like this?

1. Increase column size to control deflection
2. Put more bars to control deflection
3. Or nothing at all taking into account that the extra column length is below grade and the soil covering it can just hold it?

What do you usually do in your experience?

 

[mattdukes]

What we intend to do is to put the 150kpa designed footing putting right on the rock and just let the designers change the column parameter for the extra 0.5 meter length. We don't want to change the design of the footing anymore. So there is absolutely no problem in putting the 150 kpa footing - as is- right on the say 1000kpa rock, correct?

 

[msquared48]

Taper the footings to lower the volume.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[hokie66]

mattdukes,
I agree with your last statement. One other (pedantic) thing. Mr Pascal's name should alway be respected by capitalizing...kPa.

 

[BAretired]

What we intend to do is to put the 150kpa designed footing putting right on the rock and just let the designers change the column parameter for the extra 0.5 meter length. We don't want to change the design of the footing anymore. So there is absolutely no problem in putting the 150 kpa footing - as is- right on the say 1000kpa rock, correct?

There is no problem doing that, but you are wasting material. Why don't you want to change the design of the footing anymore? Why not stop and think about it and come up with a sensible design? Why do you insist on making decisions without reviewing all possibilities?

I suggest you sit down with all concerned and see if you can come up with something a little more intelligent than what you have proposed.

BA

 

[mattdukes]

Because the designers said they no longer have time to redesign the whole footing just to accomodate the increased bearing capacity.. and they also said 0.5 to 1 meter length increase of the column can be made without changing any of the design... because they have large margin of safety already. So we'll use as is. And base on my own calculations, the bonus of having increase bearing capacity than designed is one more floor can be added in the future... which of course will be coordinatated with the designers. Thanks to your assistance anyway.

 

[hokie66]

mattdukes,
By chance, would you be related or work with Pattontom?

 

[BAretired]

So we'll use as is. And base on my own calculations, the bonus of having increase bearing capacity than designed is one more floor can be added in the future... which of course will be coordinatated with the designers.

If load is assumed to be spread evenly over the footing area, you will have no additional capacity to carry another floor. Footing capacity will be controlled by the reinforcement which has presumably been designed to carry the present design load.

If you want to obtain additional capacity, use a square concrete pedestal 0.5m high on top of each footing. If the side dimension of the pedestal is 1/3 the dimension of the footing, you will increase the footing capacity substantially because you will reduce the cantilever length of the footing to L/3.



BA

 

[mattdukes]

Thanks BAretired for the statement. Bending moments of footings are indeed controlled by reinforcement in the form of qu= factored column load / bearing area. So even if the soil capacity increases 5 times fold (from say 150 kpa to 750 kpa). If the shear, moment reinforcement is the same, increasing the column load would only strain the moment and shear bars even though actual soil bearing is higher. So from the beginning the reinforcements must be designed already. This is what you are emphasizing, agree?

Also from the formula. Increasing footing area would make designed qu or net upward soil pressure less requiring lesser bars. I'm wondering. Do you in practice use larger footing and less bars but more concrete for more overturning stability (instead of smaller footing and strong bars to obey the qu=factored column load/bearing area formula?

 

[BAretired]

Consider a footing 2.7m x 2.7m with column 0.4m x 0.4m bearing directly on it. Assume that the factored uniform soil pressure is q_u. Cantilever length = (2.7-0.4)/2 = 1.15m. M_u= 2.7*q_u*1.15²/2 = 1.785q_u.

Now, suppose we add a pedestal 0.9 x 0.9 x 0.5 high to the top of footing. Factored soil pressure q_udoes not change but the cantilever length becomes (2.7-0.9)/2 = 0.9m and M_u = 2.7q_u*0.9²/2 = 1.0935q_u, a mere 61% of the moment without the pedestal. If the soil bearing is sufficient, this means a potential increase of 63% in permissible column load using the original footing reinforcement. The cost of the additional 0.4 cubic meters of concrete is trivial compared to the added benefit in additional load carrying capability of the footing.

Do you in practice use larger footing and less bars but more concrete for more overturning stability (instead of smaller footing and strong bars to obey the qu=factored column load/bearing area formula?

In the above, I was considering a centrally loaded square footing with no consideration for overturning. In most buildings, overturning is not carried by individual pad footings but rather by stiff elements such as elevator shafts.

BA

 

[dhengr]

It seems to me that if you have increased the soil bearing capacity by a factor of 5 (150 to 750kPa), by going .5m lower to bear on solid rock; you might be able to reduce the bearing area (size) of the of the footings enough, so that the original 60 cubic meters of concrete would allow the original .625m depths to be extended by about .5m to the rock surface. Thus the top elevation of the footings and the column designs would not have to change. Another possible solution would be just to lower the whole building by .5m, then neither the columns or the footing would have to change.

BA and Hokie.... I wonder if this OP’er. isn’t from the same neck of the woods, or at least the same school of engineering thought, knowledge and logic, as the guy we dealt with a few weeks ago on the eccentric footings, etc. etc. They don’t have the time to redesign footings for rock, but they can find the time to redesign the frames and columns, and now are even planning on adding another floor?

What the heck kind of engineering really goes on in their neck of the woods? He claims there was some GeoTech involvement in the foundation design, but then they didn’t know there was solid rock .5m below the intended bottom of footing elevation during their original design? I can’t believe this. This shouldn’t even be called engineering, because it certainly isn’t by any stretch of the imagination. One wonders if some of the good advice being given here might not actually be twisted around, and be mis-applied or mis-used. This kind of so called engineering is down right dangerous, and probably shouldn’t be aided and abetted.

 

[hokie66]

dhengr,
I was wondering the same thing. See my latest post. I am not expecting an answer, but if not, there are other ways of finding out.

 

[BAretired]

I'm not sure why the soil report did not reveal the location of a "tuff rock layer" at 2.0m depth. I wonder whether it remains the same 2.0m depth across the site or if its depth is variable.

I'm not sure whether the footings at design depth (1.5m) can safely sustain the loads using design pressures provided by the geotechnical engineer without extending them down by an additional 0.5m.

I'm not sure why it would be a huge cost to increase the thickness of the footings by 0.5m but there would be no financial consideration for reducing material quantities where possible to achieve a better design.

I'm not sure why the Structural Engineer of Record does not have time to get properly involved in the construction phase of the project.

In the absence of more information, I don't see how I can contribute to this thread in a meaningful way.

BA

 

[mattdukes]

Thank you.