×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Piles and Pile Caps

Piles and Pile Caps

Piles and Pile Caps

(OP)
Hello fellow engineers, i have a question regarding the structural design of piles and pile cap. I know that piles are structurally designed as columns but do they follow the same classification " Slender/Short" in other words what is the effect of the surrounding soil? and how it could be modelled in softwares such as Etabs & Safe ? . How piles are connected to pile cap will the connection transfer moment to the pile?.

Finally, it's my understanding that pile caps are most likely to be like deep beams that is it should be designed using truss analogy not the conventional bending theory, is this right? and if so do programs such as csi safe take this into account?

RE: Piles and Pile Caps

The soil will act as a series of springs acting perpendicular to the column, typically varying in stiffness from very stiff a the pile tip to nearly zero stiffness at the surface (or 12" down, or at the frost line - depends on the situation). There are programs that can analyze this pretty accurately, but I've never played with them. The geotechnical firms typically had them and would provide me the information I needed if asked.

That said, I've had to do a few analyses without the benefit of the geotech's help (smaller firm without the software, no geotech hired, etc.). The way I'm most comfortable with finds the "Depth of Fixity" of the pile. This is based on the horizontal subgrade reaction, which is in turn based on the lateral stiffness of the surrounding soil (like the springs I mentioned above). It's not perfect, but it's pretty well documented in the literature and it has worked so far. Despite "fixity" I generally assume that point as a pinned support unless I have a whole lot of pile below it, and even then I'll lower it down several feet before switching it to a fixed condition. That's partially due to the soil conditions in my area - mostly soft clays and silty sands for the first 50 to 90 feet below the surface.

Yes, pile caps should be designed using strut-and-tie methods (truss analogy). I can't speak to specific programs. It should be discussed in the program's documentation. If you can't find it, the company's customer service should be able to help you.

RE: Piles and Pile Caps

To answer a few of your questions:

Any reasonable soil (e.g. not marsh or marine clays) will have enough support to brace piles against buckling for their embedded length. Piles are only designed for buckling under axial loading if they extend above grade by some height.

If you have lateral loads on your piles, then you would go into the "soil spring"/"P-Y"/Lpile analysis to determine depths of fixity, bending and shear below grade, etc.

Piles are most commonly connected to the pile cap by embedment, and considered a pinned connection. Developing a moment connection is possible, but generally extremely difficult and cost prohibitive.

----
just call me Lo.

RE: Piles and Pile Caps

(OP)
I can"t thank you enough this is very helpful

RE: Piles and Pile Caps

Your Geotech should have given you a soil stiffness, varying with pile diameter, that you can use in a simple FEA model to determine pile moments and shears.

RE: Piles and Pile Caps

Quote (Lo)

Piles are most commonly connected to the pile cap by embedment, and considered a pinned connection. Developing a moment connection is possible, but generally extremely difficult and cost prohibitive.

I agree with your other statements, but I have to disagree with you here. Full scale testing of pile caps (I believe it was by TXDOT) showed that 12" embedment of an HP12x53 into a cap with confining reinforcing provided sufficient restraint to develop the full plastic moment capacity of the pile. While it requires some thought and some extra reinforcement to provide confinement, I wouldn't characterize it as difficult or expensive.

Generally, the pile cap itself is embedded in soil, which is typically adequate to resist the small lateral shear forces using passive soil resistance on the side of the cap.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Piles and Pile Caps

BridgeSmith - while possible, it usually isn't relied upon in building construction.

1) The analysis is easier if you ignore fixity (and is generally conservative).
2) Accounting for the fixity could tempt an engineer to ignore the usual lateral restraint rules (3+ piles, 2 piles and grade beam in one direction, or 1 pile and grade beams in 2 directions). I could see that not passing the standard of care smell test if something went wrong.
3) No pile driving exercise in building construction is ever terribly accurate. I don't think I've ever gotten a pile survey back with every single pile in spec. Being able to fall back on the fixity provided can be a nice buffer to not have to change the design of the foundation.

RE: Piles and Pile Caps

Fixed head piles are usually required in cases the lateral displacement is excessive, or the structure is sensitive to lateral displacement.

RE: Piles and Pile Caps

Another way building construction apparently differs from bridge construction, I suppose, phamENG. Those all look like good reasons to not count on fixity.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Piles and Pile Caps

Quote (phamEng)

Yes, pile caps should be designed using strut-and-tie methods (truss analogy). I can't speak to specific programs. It should be discussed in the program's documentation. If you can't find it, the company's customer service should be able to help you.

I wanted to point out that CRSI's Design Guide for Pile Caps (available in pdf format) does not recommend using the strut and tie method as it 'can be unconservative' in certain conditions and CRSI does not illustrate the method in the guide because of their research. Something to look into.

RE: Piles and Pile Caps

Thanks, Boiler106 - I was unaware of that. Not sure what other method you could use, unless they've fit some design equations to research results. I'll look into it.

RE: Piles and Pile Caps

I personally stayed with the old ways - design the pile cap as one way beam, or two way plate, depending on the pile layout. Strut and tie method should be good for simple pile cap without continuity over several spans.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

eBook - Manufacturing the Cars of Tomorrow
In this ebook, we'll explore how additive manufacturing is going to transform the way cars are made. This includes commentary from thought leaders such as Ford's CTO, Ken Washington, Customer case studies of ways 3D printing is being used today, and a variety of part examples where 3D printing is already impacting how automobiles are made. Download Now
White Paper - Smart Manufacturing for Semiconductor
New technologies and approaches present great opportunities for semiconductor manufacturers to achieve high levels of innovation, yield and improvement. This white paper explores some of these cutting-edge technologies and how they can be applied effectively in the semiconductor industry. Read about how Smart Manufacturing is transforming the semiconductor industry. Download Now
White Paper - Analysis and Simulation in Aircraft Structure Certification
Organizations using simulation and analysis tools effectively see the benefits in their ability to achieve certification faster and with drastically less total cost than those who do not maximize these tools. Read this White Paper to learn about how digital tools such as analysis and simulation help in aircraft structure certification. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close