By Claude Goguen, P.E., LEED AP
Round manholes are the most widely used maintenance utility structures to provide access to buried pipelines for inspection and cleanout. Manholes are typically used for connecting two or more converging storm or sanitary sewer lines, or pipes, which may be of different diameter and may enter the manhole at various angles and elevations. It is important, therefore, that sizing design ensures structural integrity for the required number of pipe openings for a given junction. Manhole sizing design includes not just the outer diameter or dimensions of the structure itself, but also the thickness of its walls and the distance, or leg, between pipe openings.
Manhole design must be flexible enough to permit pipe-size changes, accommodate abrupt changes in alignment or grade, and allow for direct surface-flow interception. The largest impact on the design size of these structures is the diameter and angle of entrance of the intersecting pipe. This article provides a minimum guideline for sizing round manhole structures for various sizes and angles of incoming pipe.
ASTM standards and design considerations
The two main design criteria in designing manholes are:
- Manholes must be large enough to accommodate the maximum intersecting pipe size.
- The minimum leg, or structural length, between adjacent pipe openings in a manhole must be maintained.
ASTM C 478 does not specify the minimum leg distance between pipe openings, but proof-of-design testing methods are addressed in ASTM C 497.1 For this reason, industry design tolerance guidelines based on accepted test methods are useful.
For manholes, the minimum pipe opening is assumed to be the pipe’s inside diameter plus its wall thickness. The minimum structural leg, as a practical industry guideline, is taken as 6 in. (150 mm). Anything less than a 6-in. minimum leg between pipe openings may reduce manhole structural strength and allow cracking to occur that can lead to leakage, structural distress or durability concerns. Some state departments of transportation and other regional agencies may specify a greater minimum distance between entering pipes in manholes.
In designing manhole pipe blockouts, an additional oversizing of no more than 4 in. (100 mm) greater than the outside pipe diameter is typically used for the cutout. The final cutout or blockout opening, therefore, includes all of the aforementioned design provisions with respect to all other pipe openings, relative pipe elevations and vertical clearances. In complying with the two main manhole design criteria, these tolerances are guidelines:
- As a general rule, the minimum concrete leg between openings for adjacent pipes in manholes is 6 in. (150 mm), based on the outside diameter of the intersecting pipe.
- Blockout or cutout diameters are based on the outside diameter of the pipe plus a distance not to exceed 4 in. (100 mm).
The type of pipe entering the structure, and the connection method (boot, compression or mortar) must be known to accurately determine the required hole size and, consequently, the manhole size (See Table 1 below). When possible, avoid pipes entering into manhole structure joints and, for square manholes, avoid pipe entry at corners; this intersecting geometry may compromise the structural integrity and watertightness of the manhole.
It should be noted, however, that pipe entry at structure joints and corners may be unavoidable for certain installations; in these cases, design considerations should be left to the discretion of an experienced precast concrete manufacturer. Always consult your local precast concrete manufacturer and connector supplier for recommended design requirements and product specifications.
a. Maximum pipe size for a given manhole diameter is based on normal pipe alignment (no deflection) to a deflection angle of 45° deflection
b. Maximum pipe size for a given manhole diameter is based deflection angles of 45° to 90°, without any additional pipe connections
The manhole sizing recommendations in this article provide a quick and easy resource for estimating the optimal size for manholes, depending on the size and number of incoming pipe and type of connections being used. Ultimately the manhole design is the responsibility of the engineer, who must ensure the angles of deflection are correct. The contractor must also measure and confirm the as-built pipe deflection angles in the field. If the pipe is installed at deflection angles other than normal or 90°, new calculations must be made.
Claude Goguen, P.E., LEED AP, is NPCA’s director of Technical Services.
1 ASTM C 478 “Standard Specification for Precast Reinforced Concrete Manhole Sections” provides requirements for precast manhole manufacturing and purchasing. ASTM C 497 “Standard Test Methods for Concrete Pipe, Manhole Sections, or Tile” describes product testing and acceptance methods.
MSA Mihraj says
Manhole minimum -starting size 48″ internal diameter.
I am using 36″ inner diameter RC precast manhole.
Please explain to me why 36″-3′ manhole is not included.
Thank you very much.
I would appreciate if you could provide a copy of your software.
Sara Geer says
Thanks for the comment Mihraj. Eric Carleton, P.E., vice president of Technical Services here at NPCA, provided the following response: “The reason the discussion and charts begin at 48” internal diameter is due to the nature of the product description, “Manhole”. This has been traditionally thought of as a buried structure which a person (in the past a man) may enter a sewer, drain, etc. typically for maintenance. The key concept is the ability for ingress and egress of a person. The 36” diameter structure you describe may provide a connection of a pipe system, but would not provide adequate space for a worker to enter the structure and conduct normal activities. Consequently, I would consider a 36” diameter structure as a junction structure, inspection portal, or if it has a grate with stormwater, a circular inlet.
Certainly, the principles of structural adequacy still apply to the smaller diameter structures. However, the minimum permissible distance between pipe openings may be modified as determined appropriate for the reduced structure size and corresponding wall thickness. Additionally, similar to the larger manhole sizes if resilient rubber pipe/manhole connectors are being used the minimum hole spacing shall depend upon specific manufacturers’ recommendations.
The sizing software describe in the above 12/25/2012 blog response was developed and offered by Mr. McKibben through his own company, and is not a product of the NPCA. He will need to be contacted directly to obtain or provide further use of that product. Thank you.”
MSA Mihraj says
The information I got reading your publications are interesting & excellent for an engineer with limited experience in the pc industries.
I would be pleased to know following information which I could not get any reference from text books:
Maximum number of pipe entries to 36″ sewer pipe manhole if I am using 160 mm& 300 mm internal diameter PVC pipe?
I’m using 72″ dia PC manhole and two 42″ PC pipes at 180 degree. According some reccomendation this is OK, but other one don’t agree. Please comment …
Please explain to me what is manhole schedule
Sara Geer says
Thank you for your comment Newbe. Would you be able to expand on what you mean as “manhole schedule?” I’d then gladly forward what you need to know to our technical services engineers for a response.
what is the shortest joint allowed entering a manhole. I have less than 2 feet. inspector requiring me to pull out 36″ RCP. cut and pour collar and maintain a min. of 2′ from joint to face of manhole
Sara Geer says
Thank you for your comment Larry. I forward your question to our Technical Services engineers. The following response is from Eric Carleton, P.E. This response was also emailed to you directly.
There are manhole installation standards which show a short section of pipe to be used when exiting a manhole structure. This can be precast or cast-in-place. The concept is the structure will either settle slightly in respect to the pipe, or the soil overburden around the manhole will consolidate and settle along the plain of the manhole. Either condition could introduce a bending and or possibly load on the projecting pipe either entering or exiting the pipe. The short section of pipe, and importantly the joint between the two pipes will provide a spot to relieve the moment and/or shear by the permissible movement within the flexible connection joint. These are commonly called “rocker” pipes.
I am most familiar with this being shown on Corp of Engineer projects on gate wells placed in higher fill dams or levees. I have not seen it required within U.S. sewer projects within though a search may show it more common within Europe (particularly United Kingdom).
The contractor stated the inspector is requiring a two-foot minimum distance be maintained between the manhole outside face and the next pipe joint. Apparently, a longer pipe (say 6 feet) would be OK. Therefore, it appears the joint distance criteria is not related to the normal concern to allow for some pipe rotation. I am only guessing, but possibly a concern regarding a short section disjointing after installation?
It appears the contractor to meet the correct station placement of the manhole combined with the existing lengths laid pipe up to the manhole required to cut and place a pipe section, when installed, that would have a joint distance less than the two feet expected by the inspector. The remedy required of the inspector was to remove the short pipe, cut into the recently laid pipe to a point the next pipe into the manhole will be not less than two feet, remove that pipe joint (assume it is the bell), then set the manhole and new entering pipe, and lastly pour a concrete collar around the newly cut butt joint as a field connection. If this is accurate, then the remedy required to maintain the 2 feet length is not only expensive, but far worse than the original issue.
To answer the original question: I am not aware of any specific national requirements or recommendation stating minimum distances to be required between pipe joints and the adjoining manhole face. A study into this issue suggests that the first flexible joint outside of a structure should occur much sooner than 2 feet (1). However, there may be some jurisdictions which have developed some requirements and included them within their project documents. If this is the case for this owner, I believe a better solution to this specific problem would have been to have the contractor provide and install an external joint wrap which meets ASTM C877. This would provide an added protection to assure a quality joint much better than a concrete collar butt joint. Alternatively, another option would be to permit the contractor to slightly modify the placement of the manhole structure slightly further upstream such that the last pipe section to be laid into the manhole will meet the 2-foot requirement. However, this is only viable on straight through structures or those with minimal flowline deflection.
A. I. WIG says
Why deep manholes have small diameters?
Could send all different sizes of manhole and how it should be built