HS20 Wheel Load directly atop a concrete wall

We typically assume distribution of load through concrete at a 45 degree angle. There are several provisions in the AASHTO spec. that support that assumption is conservative. You can add the tire patch dimension to the distribution width, also (10" long x 20" wide).

If the trench is perpendicular to traffic on a 2 lane or larger roadway, at that height, you'll have to consider multiple truck loads, also. So you'll have 4 wheel loads spaced at 6', 4', and 6' (2 trucks w/ 6' axle gage, 4' apart). So in your case it should be the pressure of 4 wheel loads over 26' applied to the bottom slab.

Luckily for you, some of the earth pressure will resist the moment due to eccentricity of the grate. We use 36pcf earth pressure for resistance for that load case, and 72 pcf + live load (traffic) surcharge for the case where the truck is adjacent to the trench, but not on it.

Rod Smith, P.E.

Thanks HotRod 10

My trench is only 12 feet long so the most I can get on the trench is 3 wheel loads. As the wheel load is distributed downward, are the pressures additive at the bottom once the pressure triangles intersect? Is it appropriate to use the 1.33 dynamic load allowance for the wheel loads (16 x 1.33 = 21.28 kips)?

Tony Gravagne, PE, SE

For the case of the eccentric moment of the wheel load on the grate, to be conservative, I would just use the tire patch width for the first try and see if the wall is adequate. I suspect it will be. If not, you'll have to take a closer look at the distribution of load across the grate to see how much you can spread it out.

I'm not sure where/how to check the required development of the vertical reinforcement to resist the moment due to eccentricity. We would use a minimum 8" wall thickness with stirrups. We'd also likely extend the grate, support it and anchor it at the centerline of the wall, to eliminate the eccentricity moment and eliminate reliance on the concrete 'lip' to keep the grate in place.

Another option to consider is attaching angles to the bottom of the grate, so the vertical legs fit inside of the walls to 1) lock the grate in place, and possibly 2) use the grate to provide a top support to the walls to resist the lateral loads. If you're going to use the grate in this way, the tolerances on the distance between the inside faces of the walls have to be fairly tight.

Rod Smith, P.E.