Vehicular Load: 3,000 lbs acting on an area of 4.5 in. by 4.5 in. Distribution Factor?

[xeneize10]

Good morning Ladies and Gentleman,

I would like to obtain assistance to determine the correct load to use on a one-way slab design in order to compute the live load moment for a project in California.

- ASCE 7-10 requires that floors in garages used for the storage of motor vehicles shall be designed for the following concentrated load: 3,000 lb acting on an area of 4.5 in. by 4.5 in.

- The live load moment per foot will be: P*L/4. how do i compute the correct P? What is the distribution factor to be used? I came across the following reference, but it appear to be for Canada.

Thank you in advanced for any guidance.

 

[TehMightyEngineer]

Is this an elevated garage slab or slab on grade?

Also, remember that vehicles not not just a single wheel. For residential slabs I typically consider 4 wheel loads in a 6' x 10' axle arrangement.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[tolchijb]

This loading is based on someone changing their tire using a bottle jack. Hence the small contact area. I generally use 2 times d (depth to reinforcing steel) plus 4.5" as my effective width, which is a 45 degree shear cone.

 

[xeneize10]

@ TehMightyEngineer: The city does not allow the slab to surcharge the soil due to existing soil conditions. I have to transfer the slab load to deep footings. Per@tolchijb the 3k load is based on using a jack. I understand the slab needs to be designed using a worst case condition (location of load at center) with the P concentrated load.

@tolchijb: are you assuming this is transfered to the soil? as i mentioned i cant transfer it to the soil. I am trying to determine what the correct P should be. From what I understand P will not be 3k, it has to be reduced by some factor.

 

[TehMightyEngineer]

This loading is based on someone changing their tire using a bottle jack.

Oh, very interesting. I always assumed this was just an assumed minimum tire contact area (similar to the 10"x20" contact area for AASHTO HL-93 loads), but the commentary in ASCE 7-10 does indeed indicate this is intended to be the load for a jack used to change a tire. Based on the number of times I've worked under my car I'd consider 2 jack points rather than just one.

Also discussed in the commentary, the distributed live load of 40 PSF is noted to be acceptable to capture the flexural effects.

To answer OP's original question, I would utilize Westergaard's equation for point load distribution if I wanted to verify that point load doesn't control over the 40 PSF.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[WARose]

For a one-way elevated slab, you are typically looking at about a 30 degree spread out (off a vertical line parallel to the span; for a total of 60) away from the point of application for moment distribution. But that changes with its position relative to the support.

Someone else mentioned Westergaard's method.....either that or a FEA approach will give you a precise answer.

 

[DETstru]

Found these in an ET thread a long time ago but I don't know what thread.

 

[briancpotter]

DETstru-

I believe those provisions are from BS8110 (or at least, that code uses the same formula).

Personally, I've only ever used the wheel load to check localized shear failure, and otherwise used the 40 psf garage load. But if your span is short enough, the 3k load would control for flexure as well.

Brian C Potter, PE
Simple Supports - Back at it again with the engineering blog.

 

[RPMG]

This is the thread, which contains some info about the equation's assumptions:

[URL unfurl="true"]http://www.eng-tips.com/viewthread.cfm?qid=394262[/url]

For load at the center, you can take b = 0.6 * L.

 

[rapt]

The method shown above is BS8110 and AS3600 also. I have tended to use half of the value from that method for suspended slabs. I have never been willing to accept that on a 10m span, I can assume 6m width will carry the point load applied at centre span.