One of the most difficult reviewing engineer comments we get on submittals is to verify manhole walls around pipe penetrations are structurally adequate. How close can pipe penetrations be placed together without compromising the structure. How much concrete above or below a hole is needed. Can an opening be placed across a joint.
Two problems with this issue are that there is not any common industry analysis method for openings in round structures and that the shear volume of structures all with different opening sizes, configurations and loading makes analysis of individual structures prohibitive. Precasters can be left looking for answers that do not exist.
Manholes vary between 4’ and 10’(+) in diameter. Depth also ranges considerably.
ASTM C-478 is silent on openings in the barrels of manholes other than to caution that manhole bases with multiple or large openings may need special consideration for handling.
To help solve this problem, we ran a fictious 8’ manhole in RISA, a finite element software, to get an idea of what goes on around openings. We placed 2 large openings next to each other in a monolithic base. What we found was that there was no problem with excess stressed between openings when the column remaining between the openings was about a wall thickness wide. The distance between the opening and the horizontal joint was set at 2/3 the wall thickness in our model and it had no overstressing. We would assume this generality would translate for other manhole sizes as well.
Some state DOT’s have specific requirements for distance between pipe openings both horizontally and away from a joint. In absence of such standard requirements, Delta feels a manhole wall thickness between openings and 2/3 the wall thickness between an opening and a joint would be adequate in most situations.
We have recently performed additional modeling of openings in round manhole walls. As a result, we have determined that the rule of thumb for minimum concrete between openings and above openings to the manhole joint is 6”. This rule of thumb works as long as a minimum of two additional #4 bars are tied into the cage between and above openings to assist in stress distribution around the openings.