Ground Bearing Pressure

[MikeAll]

If a smaller mat is laid on top of a larger one, do you calculate the GBP to each individual mat to get an overall GBP. ie GBP of 10t/sqm onto a 2m x 1m mat, then onto a 5m x 3m mat. Or is the calculation straight onto the bottom mat.

 

[LittleInch]

Depends on the strength of the bigger mat no?

If it is weak then the effective GBP will be higher as it deflects under the load.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EireChch]

Since your from England, i am gonna assume you are designing crane mats.

Typically the assumption is that the mats are sufficiently rigid that you just just the area of the lower mats.

 

[Panars]

While there might be details of a particular case that would lead me to do something different, generally I would just consider the area of the larger mat.

 

[le99]

I think the size differential needs to be considered. I would use 45 degrees spreading from the small pad if both are rigid.

 

[EireChch]

I think thats too conservative Le99, if the mat is even close to rigid the load will be spread over a much larger area. Considering 45 deg from bottom of upper mat only increases bearing area by the thickness of the mat which isnt much at all.

 

[le99]

@EireChch,

Between conservative, but saving time and uncertainties at bay, and saving a few dollars for concrete, I chose the former. It may not suit today's environment as the analytical programs are wildly available.

 

[geotechguy1]

I think it's pretty typical to use the area of the bottom mat; unless your crane mats are made out of floppy pool noodles (in which case you've got a different problem .

 

[dhengr]

MikeAll:
In your thinking process, you have to keep several things in mind. Your mats have to be properly proportioned w.r.t. each other, layer to layer, to work well together, you have to pay attention to alternating the spanning directions of the mats when they are made of something like individual 6x6 timbers, for example, and you have to consider the relative strength/stiffness of materials in the various layers. Your mat problem is really a gridwork of beams problem, a beam on spring supports problem, or a beam on an elastic foundation problem, and the engineering theory and thinking involved in these problems should color your approach. Look at some good Strength of Materials, Theory of Elasticity or Structural Design and Analysis textbooks, and/or Google those topics. The main ideas are to distribute the concentrated load to a larger bearing area with each successive layer downward, and that there must be compatibility of deflection from layer to layer for this to work.

 

[MikeAll]

Thank you all for your input.
I carry out the calculations one way and my work partner carries them out the other way. The results more often than not are very similar so I shall work on whichever is the greatest for safety sake.

Both mats would be solid and we occasionally have to use a one piece solid on the top and multiple mates on the bottom to increase the size. I'm guessing that in that case it would be calculated separately.

Once again thanks, you lot are great.