Pile group lateral resistance

[SayGoodDay]

Hi

Can anyone teach me how to find out the maximum moment at plastic hinge of the pile in pile group.

My case is, my abutment has 2 rows of pile spacing 1000mm, all piles size 400mm x 400mm. there are 4 piles each row(spacing 3m, 2.4m, 3m). The abutment beam is 1.9m wide x 1m depth. The abutment is resisting 1296kN braking load. say 2.5 pile passive coefficient, and 3.5 abutment beam passive coefficient.

So my question is, how can I calculate the maximum moment at plastic hinge. (I know the front row piles will resist more lateral loading, so I assume the max. moment happen at front load pile, right?). I know how to calculate the moment if only one row of pile, but don't know how to do it for two rows. Can anyone teach me.

Thanks.

Marco

 

[civilperson]

If piles are connected with pile cap, all piles will move the same and experience the same lateral pressure from the soil. Of what use is the plastic hinge of a individual pile?

 

[SayGoodDay]

The moment at plastic hinge is cause by the lateral force, and it must be smaller than the moment capacity of the pile.

 

[civilperson]

A plastic hinge assumes that all the area of a flexural member yields, not just the extreme fiber as detemined by section modulus. The use of a plastic hinge takes advantage of the resrve moment capacity at other locations. The moment capacity of the pile is when any portion of the pile surface achieves yield stress. The soil loading varies as modeled. Either elastic half space or homogenous over depth with constant horizontal subgrade modulus or other assumptions that approximate the soil/structure interaction.

 

[SayGoodDay]

Hi civilperson

I am not very understand what you are talking about, in my understand, two row of pile:

I I <--- lateral force
I I
I I
\ \
\ \ <--I want the moment at this point
I I
I I
I I

Thanks.

 

[Castigliano]

By definition, isn't the moment at the plastic hinge equal to the plastic moment capacity of the pile? Obviously, you need to adjust for the axial load in calculating Mp. Are you really asking where the plastic hinge occurs? Or are you looking for the max moment, and not necessarily expecting the piles to go all the way to Mp?

Castigliano

 

[SayGoodDay]

Hi Castigliano

The first thing I want to know is, how much soil resistance behind 1st row of pile, and how much behind 2nd row.

I read some papers, "pile spacing effects on lateral pile group behavior", I understand that the 1st row of pile resist more loads than rest, but I don't know how to calculate. I know the AASHTO suggest 3D spacing, but my case is less than that. Once I know how much lateral force go into first row of pile, I can find out the Mp at where zero shear.

But from another hand, I also agree with civilperson "If piles are connected with pile cap, all piles will move the same and experience the same lateral pressure from the soil".

So now, I am a little confuse. Can anyone explain more.

Thanks.

 

[hokie66]

In reading your last post, I think some of the confusion here was caused by your use of the term "plastic hinge", when you just really want to know the location of the maximum bending moment. You don't want a plastic hinge to form. Your question about the relative passive resistance of two rows of piles is really a geotechnical issue, and could best be directed to that forum.

 

[DanTex]

I agree 100% with hokie66. It is Highly unlikely that you want to form a plastic hinge in your pile as that essentially indicates a failure of your pile. The real question might be... at what location on each pile does the maximum moment due to the applied loadings occur and... what is the code allowed capacity of the pile at such points to resist the loading. Of course, how do you set up your model to get the moments that develop in the piles??? That probably depends largely on where the bridge is to be built. Each state in the US has it's own policy on how such analysis "should" be performed. For that one, I suggest you discuss the problem with some immediate coworkers. There are many different assumptions you can make in your modelling and you'll find that even the experts tend to disagree on which assumptions are best.

Dan

 

[buening]

Other can correct me if I'm wrong, but you will need to determine the approximate point of fixity for the piles. This varies from region to region, but in my area (Illinois) I believe we use around 6-10' below the abutment cap. This assumes that the abutment cap is at or below existing ground. If you have a profile raise and part of the piles are in new fill, this estimation is no longer valid. Once you know the point of fixity, you can determine the moment in the pile group analyzed as a cantilever. I believe this works for more than one row of piling. My experience is only with single rows though (integral abutments). Don't forget other lateral forces on the structure besides braking.

Think of it as driving a stake in the ground and then pushing on the exposed stake. The stake will deflect and push the soil out of the way near the surface, but there will be a point that the stake no longer deflects. This is the point of fixity and maximum moment location.