loads on multiple piles under a cap subjected to moment

[rittz]

looking for a text which deals with loads on multiple piles under a cap subjected to moment and vertical load

 

[csd72]

Just treat the pile cap as a rigid body and calculate the pile loads in proportion of their distance from the centroid.

What size pile cap, how many piles, where is the vertical load applied? Can you take lateral load into the piles or do they need to be skewed?

 

[rittz]

Thanks csd72 I remember now from your reminder It's 30 years or so since I last did a bridge pier.

 

[edwin17th17]

if it's a big mat we use, in our company,
staad. we model plate elements and use spring support
for piles.

i dont really know which one is preferable
as plate element is a flexible approach..
while manual/spreadsheet calculation use rigid approach..

can anyone explain when is it proper to use rigid or flexible?

 

[csd72]

Just from logic I would suggest:

If your length is greater than about 10 times your depth then definately flexible, less than 5 then definately rigid body, in between up to judgement.

 

[Tmoose]

Some big FD fans were installed on rule-of-thumb-heavy-enough but relatively tall slabs/blocks poured on long steel pilings years previous. The fans were plagued with high vibration and required frequent balancing. Basic vibration measurements showed the concrete blocks were moving with the fan bases as rigid bodies. Witnesses to the original construction "recalled" The tops of the piles were featureless and smooth.

A technically curious friend relocated to Egypt for a few years on business. He noted that the rebar they were using in construction was smooth.

 

[DaveAtkins]

The funny thing about pile caps and piles is--we are taught that all the piles share the vertical load equally (this is also how the CRSI Tables are set up). However, if you do a finite element analysis, you will find this is not the case.

So I would be careful about trusting a finite element analysis alone. I would double check the results using the method recommended by csd72.

DaveAtkins

 

[rr45]

Thanks guys, for discussing this topic. I have a similar problem at hand.

csd72, based to your criteria, should I still consider a 400 ft X 200 ft slab flexible even if it has a thickness of 12-15 ft?

Thanks.

 

[csd72]

rr45,

That is a huge slab! What are you designing?

I would imagine you could save a lot of money if you analysed it as flexible in an FEM analysis program.

DaveAtkins,

Did you include an allowance for the flexibility of the pile supports or did you treat these as 100% rigid. The flexibility of the piles will even out the loads a little.

Even FEM is not perfect even if you include everything as it treats everything as elastic materials. The method I described above has been used for many years and has been found to be satisfactory for small pile caps that can be treated as rigid.

 

[DaveAtkins]

I do not include allowance for axial shortening of the piles. This would tend to even the loads out somewhat.

DaveAtkins