Multi-storey Building Design - Pile Foundations

[EireChch]

Hi All,

My company is providing Civil Engineering services (i.e. drainage design etc) for a large multi storey development. The project is at its early stages and there are some preliminary designs beginning to take place. I am working as a Civil Engineer but looking to get into geotechnical engineering which I have done for approximately 3 years prior to my current job.

In my brief time as a geotech I have not had that much involvement in the design of large multistorey buildings. I am essentially trying to do some preliminary design myself and compare it to what is being done.

I am attempting pile design for the building. This will be undertaken using hand calcs and spreadsheets only as I dont have access to any software. (Plus its better to do it by hand first right?!)

The building is rectangular in shape with one half 16 floors high and the other 12 storeys. I understand that it will be constructed using a mixture of post tensioned and in situ slabs with precast and cast in place walls/columns. I understand that wall panels will generally be precast panels. I am using loads from the structural mark ups I have seen. Beneath the taller section the loads are between 10 - 21,000kN and the smaller section the loads are up to 10,000kN.

The grids are 8 x 8m and at the first grid along the breath of the building i.e. Gridline A, the column loads range from 2,980 up to 8,600kN. The total load along the grideline is approx 40,000kN. I have made a spreadsheet calculating capacity of a single 800mm dia pile extending 20m through clay. Assuming a FoS of 2.5 the allowable pile capacity is 890kN. Spreadsheet attached for those interested. It includes a pile group capacity sheet, its pretty basic but feel free to critisise. Use at your own peril.

I did a check and for a pile group comprising 800mm at 2.4m c/c, 6 piles long x 2 piles wide, I get an allowable capacity of 30,000kN. However according to Bowels and others, the total pile group capacity cant be more than the sum of the capacity of the individuals piles. Therefore the capacity of the group is 12 * 890 = 10,680kN.

Dividing the avaerage column load of 6000/890 I need approximately 6.7 piles so say 7 piles. Repeating the exercise for all the column loads and Dotting pile caps of say 6 - 24 piles beneath columns and keeping them symmetrical is more or less resulting in piles all over the floor plan and not just isolated beneath the columns. I suppose this is required given the large loads. Taking the first gridline A-A I am considering designing it as a long thin pile cap which will be about 45m long.

A couple of questions on the above:

1) Does any one have any other good references for pile group design (I have Bowels and Tomlinson).

2) Is it correct that the capacity of the pile group is cant be more than the sum of the individual pile capacities? I know it states this in Bowels and not in Tomlinsons?

3) Is there ever a case when designing pile groups that the capacity of the group is less than the sum of the individual piles?

4) If i do design one long thin pile cap beneath the grideline A-A can I assume that the pile cap is sufficiently stiff and that it will distribute the loads evenly onto all the piles or is that too much of a simplification? Can i repeat this along all gridlines?

5) One thing I thought about is that I am working from some mark ups and that I havent seen structural drawings yet. I am wondering will structural recommend a thick slab i.e. 1.5m deep that will distribute loads and therefore I will just have piles every 3D spacing beneath the slab.

6) Comments on the attached spreadsheet please

All help appreciated.

Thanks

 

[oldestguy]

Is the cart before the horse? Are there any cases nearby with experience as to what loads can be carried on what kind of pile? Does the budget allow for load testing of individual as well as groups of piles?

 

[EireChch]

Possibley OG, do you mean doing pile design without full structural details?

I cant answer those other questions as of yet. I don't have any geotechnical experience in the area since I am new.

 

[SlideRuleEra]

ErieChCh - The following are my responses to your questions:

2) Pile group efficiency can not exceed 100% of the total of all individual piles in the group.

3) For friction piles, group efficiency is always < 100%.
For point bearing piles, group efficiency really does not apply. If spacing is adequate (3D should be), the pile group could equal the sum of the individual piles - depends on the soil properties the point bears on.
The piles shown on the spread sheet have a combination of point bearing and friction (cohesion)... possible, but tricky.

4) "Too much of a simplification."

5) Same answer and question 4). Making a pile cap thick is an important step in proper distribution of load to all piling, but there is more to it than that.

6) Don't use it. In it's current state: Information In - Unreliable Results Out.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[dik]

With item 5, there are some foundation manuals that recommend a minimum thickness of 4' (1.2m approx). On a recent project I reduced this thickness to 3'+ (<1m approx). Doing a finite element study of the cap, the load transfer to different piles was less than 10% due to the reduced stiffness. I had excess pile capacity and chose the thinner cap. The reason for doing this was to avoid additional thermocouple testing requirements for mass concrete, 'triggered' by the thickness. In general, thick, stiff caps, as SRE noted, are the way to go. Pile spacing was so close that reinforcing and shear was not an issue.

Should have added for Item 3, that I never use a combination of friction and bearing due to the different load-deformation curves...

Dik