SAP2000 DESIGN OF PILECAP

[neukcm]

Hello,

Does anyone here using SAP2000 for pilecap design? I was able to model and analyze the pile cap with piles but is confused in the ff items:

1) How to obtain vertical spring value for pile with known pile reaction? How about with only SPT data?
2) After running the analysis, how do you incorporate the Resultant forces and shell stresses in designing the pilecap reinforcement?

Thank you for the comments and advice!

Neukcm

 

[KootK]

It may be time to break out your pencil and calculator. Or, at the very least, a spreadsheet application like excel or MathCAD. In my opinion, FEM is not an appropriate tool for the design of routine pile caps of normal proportions. If you have access to a CRSI manual, there's some first rate guidance there.

i recommend saving yourself some time and just modelling the piles as rigid vertical supports for the sake of cap design.

The recommendation above assumes that you've got a single load on a small group of piles. If this is a giant concrete raft in a sea of piles etc, some FEM modelling may indeed be the way to go. What is your particular situation?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[neukcm]

Hi KootK,

Your response is very much appreciated. Thanks.

I am currently designing a pilecap 6.5M x3.5M x0.5 M depth which will support a 150 TONS of weight applied with an eccentricity of 840mm from the edge of pilecap. The dimension is fixed due to limited space. Initially, i had calculated the required no. of piles each limited to 325kN and analyze the pilecap using SAP200. But i had doubts in my assumptions as below:

1) Pile Capacity/Resistance: I am in doubt with my pile resistance as i don't have any idea how to relate SPT-N value from soil report to pile skin and bearing resistance. Found some direct method using meyerhof and used it but still confused. Any advice?

2) Pilecap Analysis: - Using sap2000, i need to model my piles as spring support but again unable to find some reliable method how to convert pile capacity to spring stiffness, using t-z curve? Any Idea on this? And lastly, given the resultant forces and stresses in my pilecap modeled as shell element, how to come up with proper reinforcement. I'm thinking to do it manually and it will likely be that way. Any advice?

Thank you once again, I'm designing shallow foundation and this is my first time designing a pilecap based on Eurocode specs.

Cheers,
neukcm

 

[KootK]

Can you post a plan view sketch of your pile, cap, and load?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[neukcm]

Please see image below.

Pilecap details (6.5m x 3.5m x 0.5m THK) supported by 15 piles 400mm diameter and 9m deep. Load will be coming from a hydraulic jack but is not a long term loading. In short this is a jack foundation and the jack will be placed at the center in the first jacking and will move in the second jacking (as shown).



Thanks,
NEUKCM

 

[KootK]

Interesting problem.

1) I would seek help from a geotechnical engineer for the pile capacities. I've seen some methods in textbooks but, personally, I always leave that to the pros. I noticed that you posted a related comment in the foundations forum here. That's a great idea. We have a dedicated geotechical forum as well with some really sharp dirt guys that participate.

2a) Again, I would be seeking geotechnical assistance regarding the springs. It's important to recognize that you'll never know the spring stiffness with much accuracy and, particularly if these piles have an end bearing resistance component, the spring stiffness values may be markedly different in compression and tension. Do you know if the ends of your piles will reach bedrock?

2b) If you're happy with your SAP results, you could integrate your moments and shears over suitable design strips and then reinforce accordingly for those design actions. The devil is in the details though. Where the cap meets the piles, you'll need to pay careful attention to the localized connections issues such as bearing stress, punching shear, and tension pile rebar detailing. Strut and tie principles could be used as could the CRSI manual recommendations.

Because it's difficult to know the parameters with much accuracy, I would favor a very simplistic model. I might assume that all of the compression resistance to the load takes place uniformly in the two rows of piles closest to the load. Then I'd add in pile tensions in the remainder of the piles until I had enough to rectify the moment created by the eccentricity between the load and the centroid of the compression pile group. This is crude work. You really can't hope for too much more than an equilibrium solution.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

Also, given the uncertainties that will be involved in the load distribution, I would be wanting to create a more rigid cap element than you have proposed (500mm). Perhaps something in the 1000 - 1500 mm range. I suspect that your concrete design will take you in that direction anyhow.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[neukcm]

Thanks for that masterful advice!

1) Yes. Luckily someone replied to me immediately giving some prescriptions on my problem.

2a) No. My pile is embedded until 9m only below ground level and will not rest on bedrock. Do i need to consider tip bearing resistance if it's not resting on bedrock and shall consider the skin friction only as total pile capacity? As of now i only have the raw data, and to tell you honestly i'm a bit reluctant to ask directly how to interpret soil report based on SPT on this forum as it is a general question to be. SPT value convert to Pile Resistance? I think to seek an advice from a Geotechnical people (as per your advice) is the best way i can do. Thank you!

2b) Unfortunately i don't have access on CRSI manual but i'm used to manually calculating reinforcement sometimes with the aid of personally generated spreadsheets. I'm used in designing shallow foundations but pilecap is a totally different story specially when considering the interaction between the piles and pilecap and the + & - moments, shears it will generate that's why i tried to model it using an FEA software.

Cheers,
neukcm

 

[KootK]

You're most welcome neukcm.

Do i need to consider tip bearing resistance if it's not resting on bedrock and shall consider the skin friction only as total pile capacity?

1) If your piles won't extend to bedrock then no, you probably don't need to pay too much attention to the effect that end bearing would have on stiffness.

Unfortunately i don't have access on CRSI manual but i'm used to manually calculating reinforcement sometimes with the aid of personally generated spreadsheets.

As I mentioned above, just keep an eye on punching shear and local bearing stresses etc. The determination of basic flexural reinforcement is usually the easy part. You really should consider picking up a CRSI handbook, even if it's only for future work. The old versions have pile cap design information and you can pick them up for next to nothing online: Link.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[neukcm]

Hi KootK,

You are absolutely right. I did a check, the tip bearing resistance is too small to consider in clay strata and hence will not consider it in pile resistance.

Thank you for the link also i will sure find a way to have a copy of CRSI manual. It's a great pleasure for me to spend meaningful discussion with you.

Thank you once again!

Neukcm

 

[neukcm]

Hi KootK,

I'm sorry to extend this thread a bit, i just have one more question regarding SAP2000 and i think asking here will be straightforward rather than posting another thread in Computer and Structures section.

Can SAP2000 design plate elements (thin & thick) just like ANSYS, ABAQUS and other FEA softwares? I know we can model plate and have it mesh, apply loads and obtain resultant forces, stresses & displacement but how about determining if a plate pass a certain code requirements say EUROCODE 3?

Thanks,
Neukcm

 

[KootK]

I'm sorry to extend this thread a bit.

It's no trouble at all.

Can SAP2000 design plate elements (thin & thick) just like ANSYS, ABAQUS and other FEA softwares?

You bet.

how about determining if a plate pass a certain code requirements say EUROCODE 3?

Not that I know of. At least not well. Most people use applications like CSI SAFE or RAM ConcePT for that. Note that none of these software packages will handle the detailing in the connection areas. Those often need to be addressed by hand.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Leftwow]

KootK, have you ever used a modeling structural software such as staadpro to analyze pile reactions and moment? We had an engineer come in that showed us you can model a 1 foot by 1 foot plate with the thickness of the concrete and set your reactions where the piles lie, then it shows you the reaction on each pile and a stress analysis in the concrete. I'm still wondering if this approach is accurate enough.

 

[KootK]

KootK, have you ever used a modeling structural software such as staadpro to analyze pile reactions and moment?

You bet.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.