Drilled Pier Boundry Conditions

[OCI]

One of my current projects is a 16' x 44' mat slab on drilled piers supporting 3 12'dia. x 32' x 162k (full) fermentation tanks (beer!). I had a geotech bore and provide me with recommendations (600psf skin for piers, K=50pci for mat). In one of my geotechnical books I found a method for computing short and long term settlement for the 18" drilled piers. Would you model these piers as springs or fixed? I would think some sort of spring would be correct but one of my co-workers dissagrees. My FEA model for the slab is greatly affected by what I set as boundary conditions. Thanks. This forum offers a wealth of knowledge and I will contribute to the best of my ability.

 

[DaveAtkins]

I would model the piers as pin supports. I am not sure why the geotechnical engineer gave you a subgrade modulus for the mat--it doesn't make sense, since the mat is supported on piles and will not settle.

DaveAtkins

 

[OCI]

The piers are only along the 44' edges to help with OM. Would you ignore the soil underneath the slab and assume a clear span (14')? Thanks.

 

[DaveAtkins]

You could model the piers as pins, and provide springs under the rest of the mat. Depending on how thick your mat is, the piers will probably still take most of the load.

DaveAtkins

 

[JAE]

You can also use the tried and true floor-ceiling method.

First analyze as fixed, then with springs - you know the answer is somewhere in between so if you design all components of your mat to meet both you are covered. A bit conservative, I admit, but "when in doubt, make it stout".

 

[OCI]

Thanks Dave and JAE. I checked it both ways and designed for the envelope. Big, tall and heavy tanks in earthquake country. Don't want to take any chances.

 

[civilperson]

Locating the piles under the CG of the tanks would be more cost efficient.

 

[OCI]

This is my pier / mat arrangement.

 

[hokie66]

Nice pun, JAE.

 

[beton1]

Don't spill the beer; agree with JAE, make the lager stout.

 

[JAE]

Stout is good, especially when you drink it.

 

[kslee1000]

I think it is a neat idea to model the support as springs, since you have a mixed support system - friction piles at the edge, and subgrade in between. I would say the result is more realistic, but not necessary the safe choice (soil behavior is everybody's guess). However, it is good for comparison purpose, and refine the design.

 

[haynewp]

I would try to get clarification if the geotech meant for the design to be based on both springs at the soil combined with piers, or if it was intended to be either or and not both. For the piers themselves of this size, I would also use the pinned then the fixed slab to pier case and design using the envelope.

I assume you are using pinned head assumption for lateral?

 

[OCI]

The geotech gave me a friction coefficent of 0.3 for the bottom of the slab. This worked for lateral movement of the slab itself, so I fixed the slab for this condition. The slab is 12" thick which worked for shear everywhere except right at the piers. I added an additional 10" drop panel between the slab and piers. Because of the additional restraint at the pier to slab joint I modeled them as fixed. I will also check as pinned.

 

[DaveAtkins]

Wouldn't it be better to model the connection of the mat to the pier as pinned, because only the upper portion of the pier will be reinforced? Put another way, will the unreinforced portion of the pier be able to resist the moment you are putting into it?

DaveAtkins

 

[kslee1000]

Make sure the slab has no problem with both one & two way shears. It seems too thin. You may not have choice but to design with edges fixed to eliminate the potential that the edge would curl up if there is no adequate restrain. However do take DaveAtkins's question seriously.

 

[miecz]

I don't get it. If the piers are rigid vertically, and the slab is strong enough to span over the soil, then how do you determine the friction at the bottom of the slab for lateral resistance?

 

[OCI]

miecz, good point. Originaly I had the piers as springs and the soil under the slab was taking more of the load. That may have been more realistic, but unconservative. I will use the piers for the lateral restraint and design them for the moment.

I am an EIT and my design will be reviewed by my superior. I am doing my best to provide him with the most complete design without his input upfront. Thanks for the help.