HomeMy WebLinkAboutWATERFIELD FOURTH FILING - FDP190009 - SUBMITTAL DOCUMENTS - ROUND 2 - SUPPLEMENTAL INFORMATION (3)MANHOLE SIZING RECOMMENDATIONS
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Introduction
Round manholes are the most widely used maintenance utility
structures that provide access to pipelines for inspection and
cleanout. Manholes are used for connecting two or more
converging storm or sanitary sewers, permitting pipe size
changes, accommodating abrupt changes in alignment or grade
and allowing for direct surface flow interception. The largest
impact on the size of these structures is the diameter and angle
of entrance of the intersecting pipe.
This document provides a guideline for sizing round manhole
structures for various sizes and angles of incoming pipe.
Sizing Considerations
Two main design criteria in designing manholes, are that they
must be large enough to accept the maximum pipe size, and
the minimum structural leg width between pipe holes must be
maintained.
The minimum pipe opening is assumed to be the pipe’s inside
diameter plus the wall thickness. The minimum structural leg is,
as a practical minimum, 6 inches. Anything less than this width
may allow cracking, which can lead to leakage, structural distress
or durability concerns.
A typical additional over sizing of no more than 4 inches larger
than the outside diameter of the pipe is used for the cutout. The
final cutout or opening, therefore, includes all of these design
provisions with respect to all other pipe openings, relative pipe
elevations and vertical clearances.
The type of pipe entering the structure, and the connection
method (boot, compression or mortar) must be known to
accurately determine the required holes size and consequently,
the manhole size.
When possible, avoid pipes entering into structure joints and
corners, as this may compromise the structural integrity and
watertightness of the structure. However, this practice may
be necessary for certain installations and should be left to the
discretion of an experienced precast concrete manufacturer.
Always consult your local precast concrete manufacturer
and connector supplier for exact design requirements and
product specifications.
Design Method
The following is a method that can be used to determine
minimum manhole size for as many as three incoming
pipes. (For installations where more than three pipes
enter a manhole, the design equations in the Appendix
must be used. )
The formulas are:
• One Pipe: 180° > K
• Two Pipes: 180° > X° > (K1+K2)/ 2
• Three Pipes: 180° > X° > (K1+K2)/ 2 and
X° + (K2+K3)/ 2 < Yº < 360° - (K1+K2)/ 2
Where K represents the K Factor that can be found in the
tables that follow for different types of pipe.
Xº and Yº are the angles between the pipes
Reference Pipe
Pipe
Diameter
(inches)
Manhole Diameter
48 in 60 in 72 in 84 in 96 in 108 in 120 in
72 - - - - 154 120 102
66 - - - - 129 106 92
60 - - - 142 112 95 83
54 - - - 119 98 84 74
48 - - 131 102 86 75 66
42 - 154 109 88 75 66 58
36 - 118 91 76 65 57 51
33 180 106 84 70 60 53 47
30 136 96 76 64 55 49 44
27 117 86 69 58 51 45 40
24 103 77 63 53 46 41 36
21 90 69 56 48 41 37 33
18 79 61 50 43 37 33 29
15 68 53 44 37 33 29 26
12 59 46 38 32 28 25 23
RCP AND HDPE PIPE ‘K’ FACTOR
Pipe
Diameter
(inches)
Manhole Diameter
48 in 60 in 72 in 84 in 96 in 108 in 120 in
64 - - 169 117 97 83 73
60 - - 140 107 90 78 69
54 - - 124 98 83 72 64
48 - 142 104 85 73 63 57
42 - 115 89 74 63 56 50
36 135 95 76 64 55 48 43
30 105 79 64 54 47 41 37
24 83 64 52 44 39 34 31
20 70 55 45 38 33 30 27
18 64 50 41 35 31 27 25
16 58 46 38 32 28 25 22
14 53 42 34 29 26 23 20
12 47 37 31 26 23 20 18
10 42 33 27 23 21 18 16
8 37 29 24 21 18 16 14
6 31 25 21 18 15 14 12
PVC AND DUCTILE IRON PIPE “K” FACTOR
Page 2 of 4 • NPCA MANHOLE INSTALLATION GUIDE • precast.org
Pipe
Diameter
(Inches)
Manhole Diameter
48 in 60 in 72 in 84 in 96 in 108 in 120 in
72 - - - 141 111 94 83
66 - - - 121 99 85 75
60 - - 138 106 89 77 68
54 - - 116 93 79 69 61
48 - 134 100 82 70 61 55
42 - 110 86 72 62 54 48
36 128 92 74 62 53 47 42
33 113 84 68 57 49 44 39
30 101 76 62 52 46 40 36
27 91 69 57 48 42 37 33
24 81 63 51 44 38 34 30
21 72 56 46 39 34 30 27
18 63 50 41 35 30 27 24
15 55 43 36 31 27 24 21
12 47 37 31 26 23 20 18
10 42 33 28 24 21 18 16
CORRUGATED METAL PIPE ‘K’ FACTOR
Design Tables
Since the wall thickness for the design of reinforced concrete
pipe and high-density polyethylene pipe are similar, a standard
precast concrete C-wall pipe thickness was used for the
development of the tables for these products. For inlet pipes,
which have greater total outside diameters than standard ASTM
C76 C-wall pipes, you must use the design equations in the
appendix to determine the acceptable manhole diameter.
The design tables for PVC and ductile iron pipe were based
on the outside pipe diameters specified in the AWWA C150
standards for ductile iron pipe. Profile wall PVC pipe that does
not conform to the maximum ductile iron outside diameters
may have to use either the RCP/HDPE or corrugated metal
tables depending on which is closer to its cross-sectional
thickness.
The corrugated metal pipe tables were developed using a 0.064
wall thickness and a 0.5-inch corrugation pattern for all sizes.
Since there are a great number of corrugation depths available
for this product, the designer should adjust the design values in
the table accordingly.
The equations presented in the Appendix of this document are
simplified in the tables as previously noted for each pipe type.
These tables are used to determine the minimum size manhole
required for one, two and three pipes entering a single manhole
structure at various angles. The “K” factors are based on the
pipe diameter including its wall thickness and an appropriate
supporting wall between the entering pipe for each manhole
diameter. Maximum vertical cutoffs are limited to an 8-foot
high riser.
Example
Given: Two pipes entering a manhole: 42-inch RCP and
36-inch PVC at 115°
Find: Smallest size manhole
Solution: For two pipes 180° > 115° > (K1 + K2)/ 2
Trial 1 (60-inch manhole) K1 (42-inch RCP)=154 (from RCP Table)
K2 (36-inch PVC)=95 (from PVC Table)
(154 + 95)/2 = 125 >115
Result: Not Acceptable
Page 4 of 4 • NPCA MANHOLE INSTALLATION GUIDE • precast.org
APPENDIX
The determination of acceptability of each entering pipe relative to all the other incoming
pipe is based on the reference pipe, which is arbitrarily selected by the designer. For ease of
design, it is best to use the largest pipe diameter as the reference pipe. The other incoming
pipe are determined by their angle, x and y, to this reference pipe. The following illustration and
corresponding equations are used for making these calculations:
Reference Pipe
Pipe Opening Pipe Angle Pipe Equation Angle Range
Reference Pipe
(d1)
0° A° = 2 [sin-1 d1/r] None
Pipe 2
(d2)
X° B° = 2 [sin-1 d2 /r] X° - B°/ 2 > A°/ 2
Pipe 3
(d3)
Y° C° = 2 [sin-1 d3 /r]
Y°- C°/ 2 > X° + B°/ 2
and
360 - Y°- C°/ 2< A°/ 2
DESIGN PARAMETERS
Where: d = [0.5 (pipe inside diameter) + pipe wall thickness + 5]
(all units in inches)
Trial 2 (72-inch manhole) K1 (42-inch RCP)=109 (from RCP Table)
K2 (36-inch PVC)=76 (from PVC Table)
(109 + 76)/2 = 93<115
Result: Acceptable
Summary
The procedures presented in this document provide a quick
and easy solution for estimating the optimal size for manholes
depending on the incoming pipe and type of connections being
used. The ultimate design is the responsibility of the engineer,
who must insure the angles are correct. The contractor must
also obtain these angles in the field. If the pipe is installed at
angles other than normal or 90 degrees to the manhole, new
calculations must be made.
Page 3 of 4 • NPCA MANHOLE INSTALLATION GUIDE • precast.org