Equivalent Coefficient of Friction 2

[butch81385]

Hello,

I have weight (140 kip +) that will be sitting on a precast plank (yet to be designed). The end goal is to have a system with an "equivalent coefficient of friction" of 1.0, so that 140 kips of weight can resist 140 kips of lateral force. It was suggested to place plates at the end of the precast plank that protrude into the ground. Is there a way to design such a plate as part of an "equivalent coefficient of friction" as opposed to strictly relying on code maximum lateral bearing pressure (contact surface is 6" of 3/4" crushed aggregate on top of 18" of pit run, on top of existing soil with mostly unknown properties).

I am afraid that using the AASHTO friction factor for concrete on gravel = 0.6, and designing the plates for 200psf/f of lateral bearing, the plates will need to be way too large or too much extra weight will have to be added.

Any help is appreciated.

 

[dicksewerrat]

What are you building?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[RWF7437]

"I am afraid that using the AASHTO friction factor for concrete on gravel = 0.6, and designing the plates for 200psf/f of lateral bearing, the plates will need to be way too large or too much extra weight will have to be added."

The coefficient if friction is NOT a function of AASHTO, or any other organization. It is a property of both of the materials which are in contact with each other.

It also does NOT depend on the area of that contact.

 

[civilperson]

Two coefficients of friction are known: to initiate movement and to continue movement. 140 kips of horizontal force is enormous, from what does this force originate?

 

[FeX32]

I never heard of anything that has a coefficient of friction of 1.0 between the materials.
Also, how are the protruded plates part of the coefficient of friction? Aren't the plates all together separate from the friction between the weight and plank?



Fe

 

[butch81385]

Thanks for all of the replies. It is meant to be a system to resist lateral and uplift forces from a 235' clear span fabric structure. The lateral loads come from both wind/seismic as well as thrust.

RWF7437: I understand that the coefficient of friction is not a function of AASHTO, but for lack of proper testing we are using the common values of different materials against soil which is listed in a chart in AASHTO. We were looking at "Concrete mass on gravel" which is 0.55-0.6

FeX32: I know that nothing reaches the ideal coef. of friction of 1.0, but our task was to create this system that would have an "effective" coefficient of friction of 1.0 (providing the same result as a coefficient of friction of 1.0).

I guess what I am trying to say is this:
Having a the concrete block with plates on ends seems like it should be able to provide more resistance than just the friction of the plank plus the lateral bearing pressure on the plates. Is there something that I am missing? Can the whole be greater than the parts in this example?

Thanks again for any help.

 

[kslee1000]

You can embed the concrete deep in the ground, or add a rigid shear key below the plate, to utilize the passive pressure. Most importantly, consult with the geotech to determine soil properties and coefficients.

 

[steve1]

Add mass to the system to increase the resisting force without increasing, and possibly decreasing, the size of the block. Embed some solid steel plates (down low to keep seismic OT as low as possible) into your concrete block. A 12 inch thick plate 10'x 10' adds almost 50k. The plates should be perforated to allow the concrete to fully engage the plates. Perhaps two or three plates would do the trick.

 

[butch81385]

Thank you for your help. The concept that I was grasping for, but could not remember, was that I get to use the weight of this system as overburden when calculating the passive pressure. This is going to help to greatly reduce the depth of the plates.

 

[DFLewis]

Since no one has mentioned this: It is a common misconception that a coefficient of friction between two materials cannot exceed 1.0.

Examples being soft rubber on dry concrete (1.0 - 4.0) or copper on cast iron (1.05).

Just throwing this idea out there, but it seems on a micro scale that the soil/gravel act similar to ball bearings rolling against a smooth surface. Maybe you could prepare the surface with a sticky agent first.

Also, running tracks that use recycled rubber for their surfacing and pelletized rubber dirt used in artificial turf come to mind.

Hope this helps to spark some new ideas for you.


DFLewis

http://www.phoenix-engineer.com

 

[kslee1000]

How about super glue? Just joking. While many materials have been tested against steel, concrete, but rarely done on soil (clay, silt, gravel, sand, silty clay, silty sand...) if any.

 

[EddyC]

Another misconception is that area has no effect on friction. Its does have an effect and thats the reason why performance cars have bigger tires.

 

[RWF7437]

http://answers.yahoo.com/question/index?qid=20071024091446AASaUth

 

[SteelPE]

How much of the lateral pressure you are trying to resist is thrust?

Is it possible to attach the footings on opposite sides of the structure together to eliminate the thrust portion of the lateral load? Usually thrust is equal and opposite on the two sides on a structure. This can be done with steel rods or some other means.

 

[butch81385]

Unfortunately it is a 235' clear span and the ground inside the structure will be removed (the structure is acting as a cover/air-tight seal aruond the operation) so nothing can span side to side to tie the building together. There are two controlling cases with almost the exact resultant. One is entirely thrust due to DL, unbalanced snow load, and an internal suction. The other is a mix of deadload, wind load and its resulting uplift.

We will be looking at using (30) 4.1kip blocks stacked in rows (no higher than 4 so as to not over stress the soil), all resting on a precast concrete block, with 3 or 4 plates protruding the bottom of the block to use passive pressure (all located as close as possible without interfering with each other so as to use the most of the "overburden").

This is by far the most complicated and ridiculous lateral system I have ever encountered, but when you can't dig or use piles for lateral you have to come up with something...