Live load distribution through rigid pavement

[yjung]

Hi all again,

I have a question regarding Live load distribution through rigid pavement. The AASHTO code only talks about the live load distribution through earth fills for buried structure.

I have about 1.5 ft concrete pavement on top of the 4 ft earth fill. Should i still follow the AASHTO CODE even though i have rigid pavement on top of the earth fill? I think the infinitely long rigid concrete pavement will redistribute the wheel loads and disregard the Live load effect. Am i right?

Thanks always.

Young

 

[ishvaaag]

Some concentration of stresses under the wheel or truck in any case there will be. If worth of consideration is other thing, since for transient loads it is usual (specially if the allowable stresses on the soils and safety factors have been kept conservative) to accept 25 or 33% excess on the allowable compressive stress for the soil. My guess is that one ordinarily wouldn't expect any problem with a 45 cm deep slab, except soil or fill is bad and the coefficients of safety have been downgraded to a (too risky) minimum in the calculations.

Think a moment if pumping of the ground of whatever under the slab is likely. This is again more difficult with the thicker slabs like this, yet if so is the case you may go for corrective measures (encasing, soil treatment, pile mats, etc).

Soils as every structure will deal naturally in the most (available) effective way with the loads standing till its final capacity. This is good for your problem, since this means that the adaptation between slab and ground or fill under it will turn plastic when ongoing for final capacity. Excluded any pumping action, this would mean more or less that one may compare the final allowable stress with the sum of live and dead loads acting. For rigid footings, compressive action from the edges will soon arch towards less loaded parts of the subsoil. Rigidity of an ample slab around the load will involve significant area of the soil in response, diminishing the impact on the subsoil stresses of whatever live load exists. As well, for (too!) flexible footings (which seems here is not the case), the deformation of the slab itself will be an initial contributor to the uniformness of the compression on the soil.

Since the maximum stress is to happen under the slab and wheel, it is easy to anaylze for a rigid plate on soil springs derived from the expected modulus of subgrade reaction. There are also closed form formulae for this.

 

[VoyageofDiscovery]

Hi yjung,

If my memory serves me correctly, Westergaard did some work in this area. Check with a local engineering library.

HTH

Regards

VOD