AASHTO H-20, HS-20. . . 1

[FloThru]

I am trying to design a box culvert cover to be traffic rated and heelproof. For the heelproof, I have found that a gap of 1/4" or less will meet the local standards. The "traffic rated" part has me concerned. I have looked at many manufacturers of grating and all will supply me a grate that is H-20 rated. When I ask (out of my own curiosity) what the actual loading is for H-20 in psi or load over area, they all say that it is based on a 32k axle. This does not tell me what the actual loading is. Is there a call out for the loading of one tire? What is the size of that tire in contact with the grate? I have searched the internet and have found nothing stating the tire size and tire load.

 

[Dinosaur]

AASHTO Loading of either H-20 or HS-20 is based on an axle load of 32 kips. This load is divided into two tires; that is a load at each end of the axle. The tire area contact is defined in AASHTO 3.30 Tire Contact Area

The tire contact area for HS 20-44 shall be assumed as a rectangle with a length in the direction of traffic of 10 inches, and a width of tire of 20 inches.

Therefore, the pressure would appear to be equal to 16 kips / (10" x 20") = 80 psi or 11.5 ksf. In case you need to know, these two tires on the axle are spaced six feet apart (center-to-center) transverse to the direction of traffic and the successive axles of an HS truck are 14 feet apart along the direction of traffic.

Good Luck.

 

[FloThru]

Thank you very much

 

[miecz]

I believe you would typically add a 30% impact to the above loads.

 

[khm337]

I believe impact factor is 30% max. It can be lower based on span length.

 

[bridgebuster]

If it's a true culvert (span < 20') the impact would be 30%.

 

[FloThru]

The span is 12'.

The reason for this is that I am very suspicious of the supplier. The design calls for 2-16"wide x60long" grated areas (1 in each lane) to drain directly through the top of the culvert to the waterway below. The supplier showed me a sample grate that was only 3/4" thick and said that it was rated for traffic loads.

I want to know what parameters that I can use to test these grates so that I know I am not putting grates in traffic lanes that will fail. The final dimensions of the grates are 16" wide x 20" long. They are resting on a frame that is poured into the culvert top.