Allowable friction coefficient of a ground floor slab on a vapor barrier?

[abusementpark]

I am trying to figure out how I want to prove how the base shear in my building structure gets resolved. I am looking at the option of the distributing the load from each wind brace into the ground floor slab.

Does anyone know what the allowable friction coefficient would be for a ground floor slab cast over a vapor barrier on a 4" layer of stone over compacted fill? I would like to see how lateral resistance the slab will offer. I may have to distribute the load from the slab into other elements.

 

[MiketheEngineer]

I doubt it... Coefficient of friction would be almost impossible to predict for this.

 

[MrHershey]

You could try talking to the geotechnical engineer. Don't know if he/she will have much of an answer though. Seems like a somewhat unusual way of resolving your base shear.

Also important to remember that if you're trying to use the slab as a lateral element, you'll need to make sure it's reinforced enough to actually distribute all those forces. Have to treat it as a diaphragm just like an elevated slab.

 

[asixth]

I am also interested in this thread. Have you been able to find any worthy information? I have always used 0.3 based on textbook recommendations for friction at the base of retaining walls.

With the addition of the vapour barrier which acts as some form of bond breaker between the crushed stone and the slab I would feel comfortable with using 0.1.

 

[msquared48]

Seems to me that if there was any movement, the coefficient would gradually increase to the .3 or .4 area, as the visqueen will gradually be shreaded to smitherines. I would use the .3 regardless.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[dik]

Often use the foundation beams earth pressure, not the SOG friction.

 

[BAretired]

The vapor barrier below the slab on grade is intended to reduce friction so that the slab can shrink without excessive resistance. To use the small amount of friction remaining as a structural resistance is a fallacy. Don't do it. Find another way to accommodate your lateral forces.

BA

 

[rowingengineer]

This could be an interesting discussion if we were discussing a large raft on ground design. For an earthquake loadings it would make sense to design the building to slip at this surface. I know of a few building which are designed to have slip footings at foundation level. However this is reverse from a wind loading situation.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[slickdeals]

I think I will go back to what M^2 said. With the weight of the tall buildings supported on a raft, they barrier would "shread to smithereens" as it engages the frictional shear transfer mode. At which point, the purpose of the vapor barrier has been defeated

 

[abusementpark]

Seems to me that if there was any movement, the coefficient would gradually increase to the .3 or .4 area, as the visqueen will gradually be shreaded to smitherines. I would use the .3 regardless.

Interesting. Would you put a factor of safety on that?

 

[abusementpark]

Often use the foundation beams earth pressure, not the SOG friction.

I was actually trying to prove that the piles in the pile caps under my wind braces columns could directly resolve all the lateral loading in each brace. However, with consideration of the shadowing effect in pile groups and assuming the free head condition, the geotechnical engineer was reporting deflections of a little over an inch.

The truth is that there are many different mechanisms that will be engaged other than the piles to resolve the base shear in the structure: passive pressure on the pile caps and grade beams, side friction on the grade beams, slab friction, etc. It is just hard to numbers to the combined mechanisms.

I know the building isn't failing in base shear, especially if I provide a mechanism to develop the load into the ground floor slab. It is just the burden of the proof.

 

[abusementpark]

The vapor barrier below the slab on grade is intended to reduce friction so that the slab can shrink without excessive resistance.

I thought it was more to provide moisture control.

 

[BAretired]

Its primary purpose is to prevent groundwater from rising up through the slab. It also prevents water from the concrete mix from being absorbed by a dry subgrade during initial set, but it also controls cracking in the slab by decreasing frictional resistance between slab and subgrade.

BA