pile cap design 1

[monchie]

Hello,

I was tasked to design a number of pile caps(to BS code), 6 pile piles cap up to 24 piles pile cap(all in standard rectangular lay-out). The good thing is that, all the reaction of piles(ULS) were already given. Now, I'm just wondering with the reactions being given, what is the best way to design them(we are using 450mm dia. pile and a target of about 900 mm deep pile cap). Any ideas will be greatly appreciated(if somebody can show me a sample detailed calc much better!)

Wishing you all the very best of this holiday season!!

 

[SteelPE]

The way I understand it. You design the caps for the capacity of the piles. If you have 50 ton piles and a 6 pile group, you will design the cap to resist the full 300 tons (600k in the US). If you have a cap that had 525kips you will just select the 6 pile cap and move on.

I once had a job with pile caps where the owner wanted the ultimate economy in the design. They even went to the extent of varying the pile capacity throughout the project to cut down on the pile lengths (by varying the depth of embedment of the piles). The job was a disaster because of this economy.... I will never let a client do this again.

 

[dik]

As SteelPE noted about economy of design, once a structural system has been chosen, within reason, there is little difference in between a 'general' design and a 'tight' design.

That said, it would be a simple matter to do up a spreadsheet for a strut and tie model to examine the various load combinations. The simple model could be general to accommodate edge piles and interior piles and select a couple of variations. Good luck and all the best in the coming New Year.

Dik

 

[monchie]

Thanks guys for your ideas,I'm just wondering,ie, if I have a 24 piles pile cap(arranged as 3 x 8 ), can I analyzed them as a continous beam?ie, continous beam with 8 supports in the long direction and 3 supports in the short direction(pile load reactions were converted as linear loads on the continous beam).

 

[UcfSE]

Typically the cap is simplified as a cantilever beam, not a continuous beam.

 

[monchie]

Can you not analyzed it as a continous beam with both edge span as cantilevered?

 

[shin25]

How many columns do you have per pile cap, more than one for a 24 pile cap? As the others said, for a single column cap, the cap will be a cantilever beam. However, if you have more than one column then the cap will behave like a continuous beam between columns. In order to determine moment and shears in the cap for this case, you will need to know the correct locations of the columns with respect to cap and loads in the columns. If your system is in static equilibrium (for this case, sum of all forces in vertical direction is zero)then determination of shear force and bending moment is not difficult.

 

[monchie]

What I have at the moment are rectangular pile caps(8, 10 ,16 & 24 piles pile caps) wherein the reactions at each piles were given(ULS) on all the pile caps. I am thinking whether it is possible to design these pile caps with only the "reactions" at each pile were given. I know for a fact that these reactions came from a number of column loads, wall loads or combination of both to various load combinations. I was briefed that the reactions at each pile were the most onerous one. So I do not have to worry about performing calculations for worst case scenario. So I am thinking of the following ideas to proceed in designing the pile caps.

1. Get the total summation of all pile reactions in a pile cap and divided by the pile cap area in order to get loadings in force/unit area(kN/m2, ksi etc)

2. Analyzed a row/column of piles as a continous beam with the end spans(both ends as cantilevered).


Any ideas?has anybody done this before?

 

[JacksPanic]

I would analyse the 24 pile cap (raft?) as a flat slab...
What do you have starting on this foundation? A series of columns? Walls/shafts?

 

[SteelPE]

There is a book in the US called CRSI Design Handbook that design charts for ton of different concrete elements. Included in this book are the charts for pile caps and the set of equations used to calculate these charts. I imagine there is a similar book in your country. You may want to ask your boss if your company has a similar manual.

 

[asixth]

I would go with option 2 that you nominated above. If you can develop a "strut" of 45deg or greater from the column or wall into the pile that you can design as per strut-tie otherwise you would need to detail as per a raft with the possible inclusion of shear reinforcement. 900mm thk cap seems rather thin unless you have a wall above it to transfer the loads direct to the piles.

If you are designing as per strips it will need to be detailed with strips in both directions.

Strut-tie design or strip design will normally come out with similar "tie" or "flexural" reinforcement but if the load cannot strut direct into a pile than shear reinforcement will be required.

 

[hokie66]

To adequately analyze a pile cap, you need not just the pile upward forces, but the downward forces as well. That should not be difficult for the building engineer to provide.

 

[UcfSE]

When you say you want to analyze as a continuous beam, could you be more specific or descriptive? What is acting as your support for your "beam", and what is acting as the load, and what kind of load is it creating? I don't think I agree with your idea, but I cannot quite tell.

 

[mijowe]

You would want to layout piles in as close to a square as possible. An 8x3 for 24 piles creates to long of a cantilever in one direction. The CRSI mentioned above calls for a 5x5x4x5x5 layout.

X X X X X
X X X X X
X X X X
X X X X X
X X X X X


Other things that come to mind are pile spacing, this is typically given to me by the geotech, but a rule of thumb is 3x the pile diameter.

Shear is as big of a problem ans bending moment, you typically try to get several piles under the column or inside the shear cone.

 

[BAretired]

I agree with hokie. You need the downward loads and locations.

BA

 

[dtex]

Regarding your second option you were considering:

Model the footing as a continuous beam (assumes that simple beam theory is 'close enough').

Apply an Axial Load, Bending Moment and Shear loads to the footing at the "assumed" location where a column connects to the footing (middle of the continuous beam model) ?

Put a support under the beam where each row of piles exist.

Model the supports as an axial spring (rather than a pin support).

K-axial = spring constant per row of piles
= (num of piles per row)*(pile area)*(pile material elasticity)/(pile length)
= n*AE/L
This assumes the pile is supported by a perfect pin at the bottom
and that there is no friction helping you along the length of the pile.
I think this is a very conservative assumption if the pile lengths have
been chosen well.

This type of analysis should give you a decent approximation of the loads on the piles (if you know what the true applied loads from the columns are that is). You can increase the column loads until your analysis shows pile loads in line with what you are expecting. If you know nothing about the columns... you could also try using a beam model that is fixed at the column location and apply loads to the beam where the piles connect to the footing.

Dan