How to Read Shaw Precast Solutions Manhole Shop Drawings
To manufacture a manhole for a specific project, certain information is required to develop shop drawings which are used by our production and shipping staff to manufacture and deliver the appropriate pieces which make up the manhole. Information which is necessary to develop a shop drawing normally comes from a set of engineering drawings containing a plan and profile of the sewer line.
The minimum amount of information to determine required manhole layout:
- Finished Grade Elevation (or top of concrete cover if manhole extends above grade)
- Size and type of pipe entering the manhole (nominal diameter and pipe material)
- All pipe inverts (elevation at inside bottom of the pipe taken at the manhole wall)
- Angles between piping (measured clockwise from the outlet pipe which is taken as 0º)
- Size of manhole (nominal inside diameter)
Additional information which should be provided would include:
- Sump depth (if required)
- Base configuration (with or without benching, with or without bottom slab)
- Opening type (water tight rubber gasket, smooth or rough cut hole, doghouse opening.)
- Gasket type for joints / confined rubber o-ring, “Ram-Nek” butyl rubber strip, no gasket
- Special allowances for grade adjustment
- Type of access hatch or cover to be used (cast iron frame and cover, aluminum access hatch, etc.)
- Size and location of access opening in concrete cover
- Special items to be cast into manhole sections (lifting davits, access frames, tie downs)
Once this information has been received, a standard shop drawing form is filled out for each manhole in the project.
First, the total height of the manhole (laid height) is determined, by calculating the difference in elevation between the top of concrete cover and the lowest pipe invert (or bottom of sump). The top of concrete cover elevation is typically set as 300mm below finished grade, to provide an allowance for final grade adjustment once the manhole has been installed.
Next, the pipe angles, size and type are determined and laid out. Angles and pipe inverts are checked to ensure sufficient clearance between adjacent pipes. All angles are measured clockwise from the outlet opening which is taken to be 0 degrees. For a manhole with two or more inlet pipes, the angle for each pipe relative to the outlet is determined.
Next, the distance to the center of each pipe opening is determined ( + to CL). All distances are measured from the outside bottom of the manhole section in which the opening is located. Typically all openings are in the base section, unless there is a large change in elevation between pipes. If this is the case, the height of the base section, and intermediate sections will be adjusted to ensure that there is sufficient clearance between the opening and the manhole joints.
Finally, the manhole sections are selected and listed, giving the laid height of each piece and any special requirements for each section. Opening types are listed for each pipe, and any special instructions are noted.
Unless otherwise specified, shop drawings are sent to the contractor for review and approval by the project engineer. Any revisions required by the engineer or contractor are made, then the final approved shop drawings are issued to our production staff for fabrication.