Distribution of wheel loads through Rigid pavement. 1

[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

 

[wiktor]

Young;
You are right about the redistribution of the wheel loads, but the code doesn't allow for it, so you have to use the wheel loads distributed as for buried culverts, without impact (old code), or with reduced impact (LRFD).
That applies to the final condition. What about the construction? How you'll compact the fill on the top? The load from the compactor could be much higher, with less cover to distribute

 

[yjung]

Dear Wiktor;

Thanks. I did not think of the construction phase. You are right the compactor will generate higher load even though
the load will last only few hours.

Is there any way to design the slab if i have 6 ft earth fill with about 2 ft of LL? The precast option is so expensive.

Thanks

 

[wiktor]

Young,
RC slab approximately 1:25 span to structural depth ratio could be used. You can reduce the volume of concrete by creating voids - typical metal AC ducts could be used. Check the difference in the total cost - it could be not much less than the solid slab.
The slab could be designed as one way slab for the typical strip.
Check for the principal stresses at the edges, where you will need diagonal reinforcement.
This could be quickly done using influence charts for slabs, of any FE software.

 

[yjung]

Hi again, wiktor;

My span is 56 ft. I need 27 inch thick slab according to 1:25. I could put the two layer of the steel but concrete stress was too much. It requires very high strength concrete (7000 pci). Here in AZ we are using Allowable stress design. I am not concerning about using high strength concrete for buryed structure and the cost of the high strength concrete.

I am not sure that the strength design method will work for this case or not even though allowable stress design did not work.


Thanks again wiktor!

 

[wiktor]

Young,
I did undervalue the magnitude of the moments due to the soil on the top. It is obvious that you will need or deeper section or so called "stiff reinforcement" i.e. to use embedded W shapes as reinforcement. (it is basically typical multistriger span with the RC deck lowered below the bottom flange).
I'm estimating the magnitude of the moment (simple supported) to be approx. 530 kft, so for 24" internal arm and w/ regular A36 steel you will need approx. 10-12 inch sq/ft, making this rather unfeasible.