If you can read rim and invert elevations, you can size a precast structure straight off the plans. If you cannot, the whole takeoff is a guess. These two numbers do most of the work.
What the numbers mean
The rim elevation is the finished elevation of the top of the structure, where the lid or frame meets grade. The invert elevation is the elevation of the inside bottom of a pipe where it enters or leaves the structure. Both are given in feet above a fixed sea-level datum, not as heights off the ground.
That datum matters. Because every elevation on the plan is measured from the same zero, you can compare them directly. A rim at 512.40 and an invert at 505.10 are seven feet and change apart, and you did not have to stand anywhere to know it.
Where to find them
Elevations show up in two places. The structure schedule lists each structure with its rim and each of its invert elevations and pipe sizes. The plan and profile sheets show the same numbers in context, with the pipes running in and out at their inverts. When the schedule and the profile disagree, that is a flag to resolve before you build anything.
The height calculation
Here is the move that turns elevations into a structure:
Overall height ≈ rim elevation − lowest invert elevation + base allowance
The rim sets the top. The lowest invert sets how deep the lowest pipe sits. The difference is how much barrel you have to make up between them, and you add an allowance below the lowest invert for the base slab.
A worked example
Say a manhole schedule reads:
- Rim: 512.40
- Invert in (12 inch): 506.10
- Invert out (12 inch): 505.90
The lowest invert is 505.90. So rim minus lowest invert is 512.40 − 505.90 = 6.50 feet from the top of the structure down to the outgoing pipe. Add your base allowance below that invert, and you have the overall height the base, risers, and top have to add up to. Then each pipe becomes an opening placed at its own invert: the 12 inch in at 506.10, the 12 inch out at 505.90, each at the angle the plan calls out.
A tenth of a foot is an inch and a quarter
Elevations are in feet and hundredths, not feet and inches. 505.90 is not five foot nine. It is 505 and nine tenths of a foot. Mixing the two up is a classic way to build a structure that misses the pipe by half a foot. When in doubt, convert carefully.
Why it ends up on the shop drawing
Every one of these numbers has to land on the approved shop drawing, because that is what the plant builds to. Get an invert wrong there and the piece will not line up with the pipe in the field, no matter how good the concrete is. For how the whole structure comes together from a schedule, read Box Culvert and Manhole Takeoff, Step by Step.