Siesmic Design of Piles 1

[samwise753]

Hello ladies and gentlemen,

Some of you may recall my inquiry on seismic design of bearing pads. Now, I am dealing with the seismic design of piles. Specifically, these are driven piles supporting supporting footings that support multi-column bents (one pile group per column). I originally designed the pile groups to resist axial force for the Strength case only with no tension allowed. I have now been told to design the pile group for the Seismic forces. I have stopped design and have come to this forum because the pile group has gone from a 9 pile group (3x3) to a 25 pile group (5x5). The 25 pile group almost meets the compressive axial demand due to seismic, but it does not come close on reaching zero tension in all the piles (150+ kips (T) on some piles). With a 4-ft pile spacing, the footing is already 23ft x 23ft for the 25 pile group; this does not seem practical as it is, and the footing dimensions and number of piles will have to increase to take the tension out. Should the piles be designed to resist axial forces due to Seismic?

 

[Qshake]

"Ladies and Gentlemen" - Boy are you mistaken!!

ok, down to the serious stuff now.

You've probably already done this but please make sure your owner (DOT as the case may be) precludes any tension in the footing. As an example, a long time client I've worked with has, for LRFD, withdrawn their policy on tension in pile from the LFD days. Recognizing this could save time and money.

If you can have tension, then get the permissible tension load from the geotechs and design accordinlgy.

If you can't you may wish to reconfigure your bent arrangment. In many years of designing foudnations in zones of moderate but infrequent seismicity, I see a lot of footings much larger than can or should practically occur. As an example, taking a step back from your footing as is 23' by 23' you might have a 3.5' diameter column on top of this. In such a case, the question is how do I get loads from the 3.5' column face (even the equivalent face) to the outermost pile? Reversing that for the case of the footing reinforcing, how do I get the pile loads back to the column using conventional reinforcing and good footing thicknesses. There are rules, geometric, that govern, for a specific distance and footing thickness whether or not the behavior is linear elastic or not. So if you're at 23' and don't have a 5' thick footing (whatever the ratio is) you may be violating the assumptions that allow for conventional reinforced concrete.

A better idea, if you must have no tension, is to consider a differnt foundation or footing system. You may wish to consider the columns framing into a small wall that sits atop one footing. Or you might consider a seismic beam at the top of the footing spanning from column to column. This won't help too much in the longitudinal direction but will significantly reduce the transverse direction demands.

Drilled shafts are another alternate but then also add the issue of stability to the matter.

I hope some of this helps.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[VoyageofDiscovery]

The philosophy of seismic design of piles with Caltrans has changed over the years, with racked piles almost prohibited.

The reason being that raking the piles increases the stiffness which attracts more load as evidenced by pile cap failures.

The seismic data now reflects 2500 yr return so I believe loads have gone up. The new philosophy is to design the piles as beam columns with lateral springs. This approach should solve your problem.

HTH

VOD

 

[samwise753]

I think I can give you the benefit of the doubt on the "Ladies and Gentlemen" in my introduction; my last post, I only included Gentlemen; Just wanted to make sure all the female engineers out there are covered.

To the problem at hand; it may help to understand how I am applying the loads to the piles. I am using WinSeisab to get the loads at the top of the column. This includes axial force, lateral forces (longitudinal to bridge and transverse to brige), and moments (about bridge longitudinal axis and about bridge transverse axis). These loads are applied to the tops of the columns in our substructure design program (RC-Pier). It is the moment about the bridge transverse axis that is causing the most demand making the longitudinal direction the most imperative.

So far, I am trying to keep the footing thickness to 4 feet, however, I have not manipulated that very much since my main objective has been to get a pile arrangement that is not overloaded and has 0 tension (MS DOT isn't allowing any). But, a 23' by 23' is starting to get out of proportion with 4' footing thickness. Also, we are not using raked (battered?) piles. Just haven't heard the term "raked" where I am.

How would using the approach of beam columns and lateral springs help with getting a more practical pile and footing design? Also, do you know of any example configurations of pile footings in seismic areas?

 

[Qshake]

Seisab or any program should give you the loads at the bottom of the column, which are more of interest than the top for the footing. Even still you'll have to generate another moment on the pile group caused by the shear at the bottom of the footing.

Using the springs as a means of characterizing the pile will generally lessen the load due to the flexibility. This, of course, assumes that you've analyzed the pile group as a rigid body and doing the P/A+-My/I thing. To check if it is a benefit you should try to model your pile foundations as springs in SEISAB, which you can do either as a group (set of three springs or more) or you can do this for each individual pile (3 to 6 degrees of freedom). Perhaps you've already done this and so there is no benefit to be found.

Another matter may be the springs you're using at the abutments. For short bridges (<200') in length it is important to push as much energy into the soil behind the abutments as you can. If you're using integral construction this is easy, again, use the springs to characterize the soil and check that the forces are not unrealistic and therefore violating the elastic assumption. If you have an open seat abutment well, it's a bit harder because you have to account for the gap (expansion joint) before the superstructure actual engages the backwall adn thus the soil. Either way you'll see that the force going into the soil will decrease the force on tbe bents. This is so because the forces are distributed proportionally to the stiffness and so the higher stiffness at the abutment will atract more load and less at the piers.

If you bridge is longer than 200' or so (it's not an exact number) then you might have a harder time reducing the longintudinal demand on the piers.

Again, if the transverse direction is problematic, try a coupling beam below grad connecting all the columns. This will reduce the transverse moments on the footings.

As for examples, you may wish to check out FHWA website. Or google FHWA and seismic design.

I have seen 23' by 23' and larger footings before in seismic areas but I generally don't think their an effective use of the owners money.

Even if you're in northern Mississippi (i assume this is what MS), you're not in a very high seismic zone, say moderate but very infrequent. So you don't need to put a lot of money in the ground.

Good luck.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[samwise753]

Qshake,

In order to model these piles as springs, where would I get the stiffness values? The geotechnical engineer seems to be the likely candidate. Also, all of the aboutments have been modeled as free in the longitudinal direction; Mississippi DOT uses expansion joints at the abutments as a standard; I'll have to talk with my project manager and relay this to the prime consultant to see if they want to take a different approach to the abutment. We have a meeting this morning, so I'll be bringing these things up.

Also, I realize the loads at the bottom of the column are the most applicable, but the substructure design program, RC-Pier, does not allow the input of moments on the column, only forces. Concentrated moments can be placed only on the cap, so I took the moments reported at the top of the column in SEISAB, and input those values on the location of the cap directly over the column in RC-Pier.

Another reason the loads are so high is that the DOT has specified the region as a level 4 Seismic Zone; this may be too high, but it is what they decided to use. It is my understanding that the area (Memphis and surrounding) is designing this way because they expect that whenever the next earthquake occurs, it will be a big one. The one in 1812 was devastating.

 

[Qshake]

I know all too well about the 1811 and 12 series of earthquakes, hence my handle name. I ply my trade on the north end of the New Madrid fault zone.

Yes, the geotech is the most applicable source for the springs. However, for years, we've used several sources for calculating them. Depends on your comfort level. I do a lot of seismic analysis and design and so use references were I can and if it's appropriate.

The most helpful, in your case will be the abutment springs to reduce the longitudinal load. If you're only dealing with a compression seal joint then you don't have much displacement to be concerned with before the superstructure engages the endbents. But just know that before the abutments are engaged the structure itself is resisting the inertial forces generated by the earthquake. So it's best when you can dump force into the abutment.

again, good luck.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[samwise753]

Qshake,

As an update, I have modified my seismic analysis in two ways. 1, I am using the cracked moment of inertia of my columns, and, 2, I am applying soil stiffness values at the bottom of the columns.

Using the cracked moment of inertia for the columns reduced the seismic load a little. A soil stiffness value of 84 kips/ft was applied to the lateral DOF's; the stiffness for axial and all rotation was sufficiently high enough to be considered fixed. The lateral stiffness value reduced the loads I was getting by about 65%. In your experience, is the 84 kips/ft reasonable? The soil is almost entirely stiff clay, with most above the water table. c=4800psf, e50=0.005. I know I need input from the geotech for anything to hold up in court, but I would like to know your opinion, and thank you for your time and attention on this.

 

[Qshake]

samwise753 -

For pile group in stiff clay this seems on the low end, but please realize that I'm a structural engineer and not a geotech. This, of course, is dependent on many factors, including the soil density (clay would be fairly high) and overall embedment of the pile.

One nice thing about SEISAB is that you can comment out the foundation springs and run the program with and without them to determine the boundary of the results. If you're using the pile stiffness as a baseline then this represents the most flexible the foudnation can be. The worst condition caused by fixing everything. As you've done check the percentage reduction in forces to see what's going on and where that load went too.

A general, very general, rule is that a pile might have a laeral stiffness of 40k/in. That's the pile by it's self. If you have a 3x3 group of pile you'd have 360 k/in of lateral stiffness alone in pile. Hence my reasoning that 84 is a bit small for both pile and soil.

Again, consider the abutment springs. By the FHWA criteria you can determine the amount of force its possible to remove from the bridge and put it into the soil by the following:

A soil may reach it's elastic limit at about 7.7 ksf. If the load is higher than this value, your inducing plasticity into the soil and no additional load is going in. So, as an example a two lane bridge with two 8' shoulders and two 16" barriers gives a out to out width (no skew) of 12'+12'+16'+2.3333' = 42.333' and presume the depth of the abutment is 3.5' for the abutment pile cap and a 4.5' girder depth so about 8' deep backwall. The total force that you can push into the soil is 7.7 x 42.33 x 8 = 2,608 kips. That's a lot of load to take out of a bridge.

I hope this helps.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[VoyageofDiscovery]

Wrt the soil this is entirely from a geotech.

Does your design lend itself to plastic hinges in the concrete? This would allow for some reduction of loading.

HTH

VOD

 

[VoyageofDiscovery]

Nigel Priestley speaks about effective bending and shear stiffness when you incorporate cracked properties.

I take it you have also included the R Factor.

VOD

 

[Qshake]

VOD - You make good points, but given the area and influence by the New Madrid Seismic Zone I would expect this client would prefer to resist the seismic forces in a completely "elastic" manner, save for the R factors.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.