round cover/plug for septic tanks

[adk48ist]

Most precast septic tanks, and other tanks have openings in their top slabs; usually 24"diameter, but some upto 48" diameter; these openings need to be covered by concrete cover/plugs same thickness as the top slab; my question is such; is there a need for circumferential reinforcing around the periphery of the cover; usually a mesh of bottom steel is satisfactory to resist H-20 loading; I am concerned about cracks forming due to shear;
a general question in this subject:
in slabs where there is no shear reinforcing how to prevent shear failure resulting from a moving truck load for example; this load will act as an impact load of magnitude 20 kips in a localized area on the slab; here slab is the top of a rectangular tank slightly ( about 8" ) buried in ground. appreciate your toughts expertis ein the above subject. thanks

 

[RiBeneke]

I cannot recall seeing circumferential reinforcing in a slab like this, but it is possibly a good idea as the orthogonal reinforcing is not always well cut and fitted to the circular shape of the hole.
I have seen diagonal (to the main orthogonal reinforcing) bars used around a circular hole. This may be a more practical solution.
Shear reinforcement is seldom used in thin (less than 150 mm = 6'') slabs, and some research has shown that shear reinforcing would be of little value. The shear strength of a thin slab is achieved by a conbination of concrete compressive strength, good aggregate interlock, and the correct amount of tensile reinforcing steel.

 

[jsnowe]

How do you account for the the shear capacity of the tensile reinforcing steel? In ACI 318, shear capacity is the sum of the concrete capacity and the shear reinforcing capacity.

Can the tension steel that passes through the shear plane be used in the calculation of shear capacity?

 

[blake989]

jsnowe, I believe RiBeneke is referring to the shear-friction concept located in Section 11.7.4 Committee 318.