I have been asked to provide the structural details of a concrete
protecting an existing 33" irrigation line.
sketch was drawn by a civil/civil, so don't be too critical.
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
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.
As you note, I, also, would try to have the wheel
load evenly distributed over a "footing". As it shows now, there will be
a net moment on the base, which may cause a "depressed hat"
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 side.
Makes sense, however, if you rely on passive pressure
as the restraining force, the repetitive nature of the loading may require
investigation of the ground so that a bump doesn't occur over the
Also, I don't think it's a good idea to lay the road immediately
on the concrete if there is any chance of settlement around. A cushion
of 6" of basecourse ...?
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
In line with my first comments?
issues which I have not considered.
Anyone wishing to share how they
would address the above issues and solve the problem would be
T. William (Bill) Allen, S.E. (CA #2607)