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Protective Slab Design

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I have been asked to provide the structural details of a concrete cap
protecting an existing 33" irrigation line.


(if the hyperlink doesn't work for you, there's an underscore between the
"20325" and the "SK08a.jpg")

The sketch was drawn by a civil/civil, so don't be too critical.

The design criteria includes a fire truck with a double axle load of 60k. I
*think* that's the total load for both axles. I'm trying to get more detail
(spacing of axles, tire size, etc).

My concerns are as follows:

Distribution of wheel load? IIRC from my bridge days, it's 0.8 x span of the
slab. If someone references ASHTO, my copy is so old, I'm afraid to tell you
what version it is :o).

Soil pressure, especially when the wheel is right over the vertical wall. I
think I can turn the vertical wall into a footing and distribute the load
much further than the distribution width I use for the slab design.

Surcharge, when the wheel rolls off the slab turning the vertical wall into
a retaining wall. I think I need to take this load out via frame action of
the upside down "U" frame and resisted by passive pressure on the other

At first glance, it appears to me that the bottom, outward legs of the "hat"
(2'-6" wide) are not necessary if the vertical legs are sized to accommodate
the soil pressure and the Styrofoam is extended to line the inside face of
the vertical walls. But, maybe I just haven't thought about it long enough.

Other issues which I have not considered.

Anyone wishing to share how they would address the above issues and solve
the problem would be welcome.


T. William (Bill) Allen, S.E. (CA #2607)
San Juan Capistrano, CA

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