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AASHTO H-20, HS-20. . .

AASHTO H-20, HS-20. . .

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.

RE: AASHTO H-20, HS-20. . .

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.

RE: AASHTO H-20, HS-20. . .

Thank you very much

RE: AASHTO H-20, HS-20. . .

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

RE: AASHTO H-20, HS-20. . .

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

RE: AASHTO H-20, HS-20. . .

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

RE: AASHTO H-20, HS-20. . .

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.

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