Wood Pile Base Reactions When Forced Out of Plumb

[Ron247]

For purposes of this general question, lets assume I have a building on 10' tall wood piles that were originally plumb. Even when the building was built on the piles, the beams were centered on the plumb piles. Due to this impossible ability to have no initial eccentricity, there should not be any lateral reaction for the piles.

A car hits one of the piles forcing one or more piles out of plumb. When the car is removed, the piles are still out of plumb to some degree but there is no longer a lateral force on the pile system. I am trying to get a better handle on the reactions to the piles since there is no applied lateral load but there is still some distortion. Since there is some leaning on some piles, there will be some lateral reaction at them due to the Dead Load not being centered any longer.

Since the misalignment has caused the diaphragm floor to distort to some degree, what about the other piles? Do they have lateral reactions that add to zero, or do they have no lateral reactions or is there some other state they are in?

This is a general question for discussion, I do not have a specific case that I am investigating.

 

[1503-44]

Pile reactions must account for any "batter" angle induced in a pile out of plumb. Other resisting piles that do not have an opposing batter angle will probably be forced to resist any horizontal load transfer by induced bending moments.

 

[Settingsun]

You mean pile that extend 10' out of the ground, right? So they're also columns?

If so (and also if not I guess), this is basically the reason for the notional loads in direct analysis in the AISC steel design code. Plenty of background literature on that. But running a few 2D models is instructive.

(For equilibrium, the support reactions must be equal-opposite to the applied loads. If no lateral load, the lateral reactions sum to zero.)

 

[Ron247]

Yes, the wood piles extend 10' out of the ground.

I know the sum of forces in the X-reactions had to add to zero, but that can happen by each pile being zero or each pile having some magnitude but they add to zero or as usual, some combination of the 2 possibilities such as all piles that are still plumb are zero and the out of plumb ones add to zero.

 

[1503-44]

Although not the best use of piles, vertical piles can resist some small amount of lateral loads by bending strength.

Two piles, one vertical and the other battered will distribute the load between them according to their stiffness, however the battered pile will most likely pick up a much greater share of the load, because axial stiffness is typically much higher than bending stiffness. Even so, with 10 feet of potential "column" bending action taking place in both piles, there is likely to be significant bending deflection of each pile, which may tend to make the load sharing more equal.

If they were cut closer to the ground, where lateral displacements would be more limited, the battered pile would probably capture most all of any horizontal load.

 

[Ron247]

Even though this is just a generic question, I do know of several houses supported on 10' above ground cantilevered wood piles and they either had no X-bracing or it was removed. they have no battered piles.

I made my scenario with plumb piles so any deviation from plumb was attributed to the car. But when the lateral load is removed and one or more piles are out of plumb to some degree, how does that affect the lateral reactions to the piles (plumb and out-of-plumb).

 

[Settingsun]

The question is too general, so it depends. Is it one column or all of them? Have they all moved the same amount and in the same direction? Are you talking about a line of columns that has moved in plane, or out of plane? Are they braced?

Also depends on your definition of lateral reaction. If a leaning column has pure axial force (no shear force), do you consider it has a lateral reaction when resolved into the vertical-horizontal coordinate system?

 

[Ron247]

I agree it is a very general, but in my situation. I say some columns are still plumb and some are out of plumb some amount. The ones out of plumb have a lateral force since the axial load combined with the eccentricity creates some rotation and thus some lateral reaction. There is no X-bracing.

Lets further say the East end is still plumb while the West end is where they are most out of plumb due to the collision. Lets also say there is no linear relationship to how much the piles are out of plumb. I think from the change in location of the pile tops, the diaphragm has distorted for some reason or pulled loose from the piles. I think if the diaphragm had not distorted, only 1 pile could still be plumb while all others would have some linear relationship to the plumbness. The one pile that is still plumb would have some torque on it.

After the car is removed, do you think:
1. The plumb piles have no lateral force and the out of plumb piles have lateral forces that add to zero.
2. Most/All piles have some lateral force that adds to zero.
3. Some combination of 1 and 2.

 

[1503-44]

An out of plumb pile will have a bending moment. With no horizontally applied load, the bending moment at any point in the pile will equal to the vertical load P (in the diagram) x eccentricity at that point, e. e is the distance "out of plumb". M = P * e

Shear in the pile, S, at any point will be P * e / h, where h is the height of the pile from point of interest to the point of load application, ie. the top of the pile. Shear = P * e/h

The moment will be zero at the top of the pile and increase until roughly at ground level.
M1 = P * e1,
M2 = P * e2 etc.
The moment diagram of the pile is shown to the right.

Once the pile is penetrating the ground, the soil will be loaded oppositely to the shear and start picking up the shear load in the pile as it resists the rotation of the pile. Soil pressure is in accordance with the lateral resistance of the pile-soil action. That is termed the "p-y" curve for the pile. As the soil resists the shear load more and more with deeper penetration, the shear in the pile becomes less and less, as does the moment, until at some depth, the pile has only vertical load remaining which is carried t the bottom of the pile and into the soil below.

If there is a horizontally applied load "H", such as wind on the building, the shear in the pile will be increased by that load and the moment at any point in the pile will be increased by that same load x the height h1, h2, to the top of the pile, etc. M1 = M from above + H * h1, h2. etc.

If your out of plumb pile is too flexible it may sideway towards the building, in which case some lateral load may be transferred to the other pile, but only if the building's floor was strong enough to effect the transfer. In your case it seems the floor warped instead.