Estimeting footing loads of a tower

[pelelo]

Hello,

I have a couple of questions regarding the design load of a deep foundation project (drilled shafts) I am working on at the moment. I currently work for a contractor firm and recently became interested in how engineers obtain the design load for the drilled shafts.

This deep foundations project is in order to support a 20 story tower (only for apartments, no offices). I understand the idea of using deep foundations as the underlaying soil is mostly soft clay and soft sandy clay.

Somehow, it seems this project did not have a structural engineer to provide information about the loads therefore I heard the geotechnical engineer had to come up with an estimation of the footing loads and at the same time the project was 5 months delayed. Geotechnical engineers assumed some load per story (I think for apartments it is around 200 psf, please let me know if I'm wrong).

So, 200 psf/story x 20 stories = 4000 psf.

Using the total footing area which i don't remember and multiplying 4000 psf by the total footing area I would get the total estimated weight of the tower.

Using this weight, I would distribute it proportionally to each footing as there are different footing sizes (from 6 x 6 to 10 x 10).

Having that weight per footing, that would be my total pile group load per footing, and then i would play with the diameter and how many piles would be drilled in each footing.

Please let me know If I am way too off of reality,

Thanks again.

 

[cntw1953]

"Using the total footing area which i don't remember and multiplying 4000 psf by the total footing area I would get the total estimated weight of the tower."

1. For gravity load alone, pil/pile group load = tributary area/column x 4000 psf.
2. How about lateral load effect - wind, seismic?

The rough estimate and looking into similar cases in nearby areas can provide a feel of the efforts required in the planning stage, but shouldn't take the place of knowledgeable engineers (geotech & structural).

 

[ishvaaag]

I find extremely unusual and if not, inadequate, that a 20 stories high building has no engineer to provide the foundation point solicitations. Even if the author of the project was sacked it would be unprofessional to retain such data if required ... except of course if envisaging some kind of litigation. But even in such case another engineer should be in charge and entirely aware of the design items, designed by another or by him, and then correctly lead the works to proper ends, this including of course coordination with any foundation works team involved with the project.

However, I also have experiences where, without the project author retaining any information, foundation companies choose to go alone with their concept for the loadings in order to get THE practical setup to be used in the works. This also involves risks and could go very wrong were not for that the rules of thumb they use are conservative, not to say the safety factors usually employed for deep foundations.

 

[pelelo]

Guys,

Thanks a lot for your reply.

cntw1953,

why would you use the trib area of each colum instead of the footing area??, Is there any reason?. By the way, is 200 psf / story acceptable for apartment buildings?, do you know where could i find the weight per area if I am working on a hotel building and on an office building?, I know it is higher than 200 psf, but I have no idea what would be the value.

The geotechnical engineer told me they considered only axial load, as lateral load are not critical in this case.

ishvaaag,

Due to several problems and time contraints we had in order to start this project, our firm hired a geotechnical engineering firm in order to come up with the pile dimensions. I know a strutural engineer should be present in order to provide that kind of information but in this case the geotechnical engineer based his calculations according to his experience.

Actually that is what i want to know: how did he come up with his numbers, but, since i don;t see him very frequently, it's hard for me to figure it out.

Thanks again for the input!

 

[cntw1953]

pelelo:

I don't think his (geotech consultant) method to com up the pile load is applaudable, so I wouldn't elaborate/comment further. You would need to track him done, understand and somehow validate the load he presented. In general, the rough estimate came from one's accumulated experiences, however, it would be greatly affected by the proposed use, type of building, building material, location/region it situates. Good luck, my friend.

 

[Ron]

pelelo...first of all, the distribution of load is not equal to all footings. Take an example of a simple building with 9 columns. The columns on the corners have the lower load of the column. Next the interior, perimeter columns take twice the load of a corner column. The fully interior column takes 4 times the load of the corner column.

Secondly, if a geotech came up with the loads, they are likely to be conservative.

Finally, the load depends on the type of construction (reinforced concrete, post-tensioned concrete, steel frame with concrete floors, architectural pre-cast exterior, etc.) as well as the use. A residential use generally has lower floor loads than an office use.

Lateral load should be considered as even a moderate wind zone produces lateral loading on a 20-story tower. Uplift might also need to be considered, depending on the net overturning moment.

 

[pelelo]

Thank a lot again guys,

Ron,

You are right, a geotech usually overstimate the loads.

 

[BigH]

Perhaps I am a bit confused - but I believe that I see apples and oranges. The OP indicated that he was working on drilled shaft design and was wondering how the geotechnical engineer came up with the estimated loads. He then indicated the total load of the "tower" and looked to how much each 'footing' would hold - but this seems to contradict his working on drilled shaft foundations.
Perhaps the story is like this: A site investigation was carried out and, due to the soft clay, deep foundations would be appropriate. The geotechnical engineer then gave, based on his profile and assessment, allowable working loads for deep shafts - perhaps end bearing permissible for enlarged base shafts. Alternatively he could have indicated allowable support values for other types of deep foundations. The geotechnical engineer "may" have indicated that typical vertical loads for such 20-story structures are "XXX" based on, perhaps, similar projects. Of course, the wind loads, seismic loads and the like probably weren't mentioned.
The assessment of allowable loadings for a given deep foundation type founded at different depths, etc. are based on the capacity of the ground to support such foundations. It is the structural engineer, then, to determine how many such deep foundations, the layout of such, etc. would be needed to support the design loads (dead and live loadings including moments, etc.). If the structural engineer throws his hands up in the air and says he can't make it work, then the two will need to sit down and find a way - for there is a way forward. What I am alluding too is that the geotechnical engineer doesn't actually need the loadings to determine the allowable capacities for preliminary design - unless there is concern of settlement issues being "red". The scenario would be for the geotech, in absense of loads, to give his best advice on what can be used - what restrictions (e.g. settlement) would be expected and then work with the structural engineer to develop a system to support the structure once the loads are finalized and other problems that surface can be addressed.
That's my take during the celebration of Boxing Day!!

 

[beton1]

I think BigH is making sense here. The value arrived at may be a method that belongs in the rule of thumb thread, for preliminary/ballpark sizing.

 

[pelelo]

Thanks again, I appreciate your thread. I just became courious how the geotech engineer came up with the loads on top of the pile groups.

 

[cntw1953]

Similar projects from his experience.

 

[kelowna]

Pelelo

This is not the way to work. There are obviously live and dead loads, but they have to be factored properly (not 100% of the building will be loaded at the same time). There are also lateral loads (Ron put it very nicely) due to wind and maybe earthquake loading. Those could be very significant and cannot be ignored.

It is not possible for a geotechnical engineer to 'guess' the loads and produce the design of the foundations that way. You really do need the calculated loads from the structural engineer. Trying to go around it will only create bigger problems in the future.

 

[cntw1953]

For your curiousity only, he can work out the number (psf) this way:

1. Estimate building weight by type of construction - concrete, steel with concrete floor and masonry partitions...etc.
2. Add live load for proposed building use - residential, hotel...etc., per code.
3. Estimate lateral load effects based on building configuration and region.
4. Sum above to get maximum/minimum loads, then apply some safety factor to it. (I think uplift is not likely a problem for this case, but you need to raise this concern to see if it was investigated).
5. Divide the maximum load above by total building floor area to get average uniform floor load.

He might have done this so many times in the past. For certain type of construction, type of building usage, and certain area/region, it could be just a pick from his memory bank with minor adjustment for deviation, if any. You need to find out either by the tedious way - do your own cal, or simply talk to him. But, engage a structural guy should be the best approach as pointed out by manys.

 

[pelelo]

kelowna,

What you say makes sense. Probably I will not be able to see the structural engineer of this tower. And even more due to other problems (money), I don;t think that person will show up in this project.

cntw1953,

I will try to get in touch with the geotech engineer to get into details as for sure there are thing he made based on his experience and either don't want to share or he assumes it is something I know.

Thanks again.

 

[paddingtongreen]

This sounds like a stage in preparing a turnkey proposal. Even so, I always supplied the estimates to the Geotech Eng.

Sometimes, in preparing a turnkey, because of time restraints, we perform some items of work in parallel that would normally be done in series, but only for the estimate.

pelelo, you don't say where this is to be built, I cannot conceive of the process you describe getting accepted by the Directors of Code Enforcement, here in the USA.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[cntw1953]

It could be a duplication of same kind. Check it out.

 

[BigH]

kelowna - never suggested that the geotechnical engineer produce the "design" of the foundations - the geotech comes up with the permissible loads for a particular type of deep foundation unit, say allowable vertical (and estimate of settlement under such loading) and allowable horizontal (and estimate of lateral movement). So, for instance, the allowable vertical load on a deep foundation is 1000 kN. The structural engineer would determine that his vertical loading and apply the geotechnical engineers permitted loadings per deep foundation unit. Obviously there would need to be discussions during the foundation design (structural engineer's lead).

 

[kelowna]

BigH - I agree with you. I probably should have been more clear on my post.

I get very worried about some posts like this in which somehting that is clearly wrong is discussed. I know Pelelo question was different (how does the Geotech estimate the loads), but the fact remains that his project has a much more serious problem than how to estimate loads.

 

[TDAA]

I agreee with Big H's take on the question. It is likely that there was a "we have assumed" load written in the report. This is typically there, and should be there, to show you have a certain expectation of the building when loads have not been provided. This statement should be replaced with "the structural engineer has provided (estimated?)loads of. . . ). These statements should fall in line with your understanding of the anticipated construction (CIP vs steel vs masonry vs wood . . .), which may alter tolerances for movements and the overall loads.

This helps address why you have chosen deep foundations vs shallow, why you have chosen a particular bearing stratum, etc..

By getting a ballpark of the load, you can correctly identify if a particular system is suitable or not. As far as the loads, look at IBC (in the US) for live loads and dead loads of different building items and uses. Estimate a column spacing, and figure the estimated loads. Document your assumed foundation loads in the report, so discrepancies can bee addressed when the real loads from the structural are known (or after the architect changed the design, making the structural redesign, forcing loads way above what you had expected, all so they could have a nice open area in the lobby).

Pelelo, take the Geotech's numbers very lightly, as they are just intended to be a working tool to show he is on the same page with the rest of the design team (even if they have not been assembled yet).