Lateral Capacities of Steel H-Piles

[jdahnke]

I am trying to design a pile bent for a large prefabricated steel truss bridge. This bridge has a rather large (on the order of 55k) lateral wind load. This wind load is causing excessive stresses in my H-piles based on my calculations. The piles are oriented with the strong axis parallel to LC bearing of the bent. I am trying to avoid adding additional piles if at all possible. The geotech report gives values for the Lateral Modulus of Subgrade Reaction, but I have never used these before to determine lateral pile capacities. Does anyone have any information or advice on this matter?

Would changing the orientation of a couple of piles be an option? If so, how does one analyze this situation, by averaging the Section Modulus and Moment of Inertia of the newly oriented piles with the remaining piles in the bent?

Also, can someone explain to me how battering a pile would help me? Would battering reduce the stresses in the piles and if so how?

Sorry this post is rather long with lots of questions, but this has been bugging me for a while now. I appreciate any help anyone may have no matter how trivial it may seem.

Sincerely,

J

 

[wiktor]

jdahnke
Based on the information provided in your post, I’m assuming that you are calculating the stresses in your piles, assuming some point where the piles are fixed, without calculating their lateral resistance.
You should calculate the lateral resistance for group of piles, based on the geotechnical information provided, or have geotechnical engineer engaged to do it.
A book “Pile Foundation in Engineering Practice” by Prakash & Sharma has a nice chapter on the subject, and design examples. There are also several programs, which could compute the lateral capacities for you, but I’ll strongly recommend using geotechnical engineer for this task, as the subject is sometimes tricky.
That’s on the subject of the lateral load capacities.

Rotation of some of the piles will definitely reduce the stresses, but the procedure to calculate the stresses should be somehow different than the one you’ve described, as the stiffer piles will attract more load. Again, some advice from the experienced geotechnical engineer will be needed.

You can try to analyze the pier together with the piles with the horizontal springs, which properties will be based on the lateral modulus of subgrade reaction. But the calculations involved in this kind of modeling are rather extensive and not commonly used in bridge engineering practice.

The battering of the piles (if you are not restricted in your footprint), in both directions is a common practice. In the past (and bridges with the piers designed by this simple method are still standing), the battered piles were solely used to carry lateral loads. You should note that for these battered piles to be effective, the inclination should be 1 to 7 or more, but I’ve seen plans with 1:10 specified. This you have to check with the piling contractor, as it varies with different types of pile drivers.

To calculate forces in your pile bent, create a simple 2-D or 3-D model with the inclined piles. Assume fixed condition for the piles approximately 6D from the top of bearing layer of soil. This will give you approximate forces in the piles, which could be latter refined by using more sophisticated model.

 

[jdahnke]

wiktor,

I appreciate your input. Yes, I am assuming a point in the ground at which point the piles become fixed in order to calculate the stresses in the piles. In my case I am assuming 10' below the bottom of the bent. I could reduce this number and that would reduce my bending moments in the piles, but I have not found any information to support a good value to use. You say 6D, so I assume that D is the depth of the H-pile? (By the way, that book you recommend is expensive!)

On lateral load capacities, is the lateral load capacity of a battered pile the horizontal component of the axial load capacity? I currently have the inclination at 1:3. Isn't calculating the bending stress in a pile taking into account the lateral load capacity of a pile to an extent? The pile resists lateral loads through bending and pressure exerted against the soil. Does the lateral load capacity of a pile refer strictly to the lateral pressure a pile exerts on the soil?

I realize the stiffer piles would attract more of the load. Would it be reasonable to use the ratio of the pile stiffnesses to proportion the amount of load to each?

You mention building a model, what do you mean by this? In a structural analysis program?

Again thank you for your input, it is much appreciated!

j

 

[ghghghgh]

It depends on your soil condition and loads. For rock or a true end bearing condition, then the lateral capacity will simply be the horizontal component if your battered pile's axial capacity and you don't count on the soil providing any help. In your case, with such high lateral loads, using the spring analysis that jdahnke suggests will be necessary.
Also, I'm not sure about 1:3 batter, that seems like it would be difficult to drive, check with your state DOT to see if they recommend a max or min batter.
Check with the geotech before raising the POF, they should provide that.

 

[SlideRuleEra]

The US Army Corps of Engineer has recommendations on pile batter at this site:

http://www.hnd.usace.army.mil/techinfo/ti/818-02/chap5.pdf

(Maximum batter in the 1:3 or even 1:2 range)- However as ghghghgh says even 1:3 is hard to do. For cost effective construction I suggest that you not exceed 1:4.

If all the pile tops are "locked" together in a pile cap, then the deflection of all piles (at the top) will be the same. If they are all assumed to be cantilevered from the same fixed depth, then the load on each is directly proportional to each members Moment of Inertia. Take a look at Beam Diagram #22 in the AISC Manual of Steel Construction, and solve the "Max Deflection" equation for "P".

 

[Focht3]

[blue]wiktor[/blue] gave great advice - follow it. While you can batter the piles - this is not as good as performing a nonlinear analysis using p-y curves. Have an experienced geotechnical engineer do the analyses for you -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[wiktor]

D is for the diameter of the pile. In case of H-piles, you can assume the width of the pile, for commonly used sections.

On lateral load capacities, is the lateral load capacity of a battered pile the horizontal component of the axial load capacity?

Yes, the lateral load capacity is the horizontal component of the axial load capacity for the inclined (battered) piles, together with the lateral resistance of the soil.

I currently have the inclination at 1:3.
The 1:3 inclination seems excessive, as the driving of the piles could be unpractical. Check with the pilling contractor, what are the limits of their equipment.

I realize the stiffer piles would attract more of the load. Would it be reasonable to use the ratio of the pile stiffnesses to proportion the amount of load to each?
Using of the ratio of the piles’ stiffness could be misleading, as the whole system will interact (the piles are connected by the footing), so a model will be more appropriate.


You mention building a model, what do you mean by this? In a structural analysis program?

Yes, any structural analysis program with spring supports will suffice. I’ve used GTSTRUL and Staad, with good result. The stiffness of your horizontal springs will vary with the depth and the type of soil. The number of the springs to be used could be limited – use your judgment and geotechnical advice.

 

[jdahnke]

First off, I would like to thank everyone for the advice and information. It is a great thing that one can use the knowledge and experience of so many engineers and that those engineers are willing to provide it. Again thank you to you all.

To determine the lateral load capacity of a pile could you ignore the soil below the bent and just assume it uses only bending to resist the lateral loads? This was the assumption I was going with and the soil was providing a point of fixity. This would make SlideRuleEra's (and mine) explanation seem reasonable.

Thanks again to all.

j

 

[VoyageofDiscovery]

When all your piles are vertical you should do this. However assuming there is no soil will result in excessive deflections.

VOD