EXTERNAL LOAD CALCULATIONS FOR DIRECT BURIED CONDUIT
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EXTERNAL LOADS ON DIRECT BURIED PVC OR HDPE CONDUIT OR DUCT
FALL INTO TWO CATEGORIES, DEAD LOADS (EARTH LOADS) AND LIVE
LOADS. DEAD LOADS ARE BASED ON THE WEIGHT OF THE COLUMN OF
SOIL ABOVE THE BURIED CONDUIT OR DUCT. LIVE LOADS ARE FROM
DIFFERENT SOURCES SUCH AS HIGHWAYS, AIRPORT RUNWAYS OR
RAILWAYS. LIVE LOADS HAVE LITTLE EFFECT ON CONDUIT
PERFORMANCE EXCEPT AT SHALLOW DEPTHS (4’ OR LESS FOR
HIGHWAY LOADS).
THE TOTAL LOAD IS THE SUMMATION OF THE DEAD LOADS AND OF THE
LIVE LOADS. THE TOTAL LOAD SHALL BE LESS THAN THE LOAD
(MAXIMUM LOAD) REQUIRED TO CHANGE THE INSIDE DIAMETER OF THE
CONDUIT OR DUCT BY ARBITRARY DATUM OF 5% DEFLECTION. THIS
DEFLECTION, CAUSED BY THE TOTAL LOAD, IS FAR BELOW THAT WHICH
WOULD CAUSE DAMAGE TO THE PVC WALL OF THE CONDUIT OR DUCT.
THE MAXIMUM LOAD (W) CAN BE CALCULATED WITH THE FOLLOWING
FORMULA:
1
W = [∆∆∆∆X *(0.149*PS + 0.061*E
)]/[D*K]
W = MAXIMUM LOAD (lb/in2)
∆∆
∆∆
X = DEFLECTION (%/100)
PS = PIPE STIFFNESS (lb/in2)
E_ = BEDDING SOIL MODULUS (lb/in2)
D = DEFLECTION LAG FACTOR
K = BEDDING CONSTANT
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EXTERNAL LOAD CALCULATIONS FOR DIRECT BURIED CONDUIT
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DEFLECTION (∆X) IS THE RATIO OF THE INSIDE DIAMETER (LOADED)
VERSE THE INSIDE DIAMETER (UNLOADED) OF THE CONDUIT OR DUCT
(5% = 0.05). THE INSIDE DIAMETER (I.D.) CAN BE CALCULATED BY TAKING
THE OUTSIDE DIAMETER (O.D.) AND SUBTRACTING TWO (2) TIMES THE
WALL THICKNESS (t). I.D. = (O.D. - 2*t).
1
BEDDING SOIL MODULUS (E
1000 lb/in
2
FOR TYPE III SOIL.
) SHALL BE 2000 lb/in2 FOR TYPE I SOIL AND
A DEFLECTION LAG FACTOR (D) OF 1.5 IS A COMMONLY USED VALUE.
A BEDDING CONSTANT (K) OF .1 IS A COMMONLY USED VALUE.
PIPE STIFFNESS (PS) OF THE CONDUIT OR DUCT IS EITHER
PREDETERMINED OR CALCULATED BY USING THE FOLLOWING
FORMULA:
3
PS = E*I/(0.149*r
PS = PIPE STIFFNESS (lb/in
E = MODULUS OF ELASTICITY IN TENSION (lb/in
)
2
)
2
)
I = MOMENT OF INERTIA (in3)
r = MEAN RADIUS (in)
MODULUS OF ELASTICITY (E) OF PVC FOR THE CONDUIT OR DUCT IS
EQUAL TO 500,000 lb/in2. HDPE CONDUIT OR DUCT MODULUS OF
ELASTICITY (E) IS EQUAL TO 150,000 lb/in2.
EXTERNAL LOAD CALCULATIONS FOR DIRECT BURIED CONDUIT
Gross Automation (877) 268-3700 · www.carlonsales.com · sales@grossautomation.com
THE MOMENT OF INERTIA (I) EQUALS THE WALL THICKNESS (t) CUBED,
DIVIDED BY TWELVE (12). I = t3/12.
THE MEAN RADIUS (r) EQUALS THE OUTSIDE DIAMETER (O.D.) MINUS
THE WALL THICKNESS (t) DIVIDED BY TWO (2). r = (O.D. - t)/2
2
PIPE STIFFNESS FOR DB60 IS EQUAL TO 60 lb/in
AND FOR DB120 IS
EQUAL TO 120 lb/in2. 4” SCHEDULE 40 CONDUIT WILL HAVE A PIPE
STIFFNESS OF 346 lb/in2 (MINIMUM WALL THICKNESS).
MATERIALS SUITABLE FOR FOUNDATION AND EMBEDMENT ARE
DEFINED ACCORDING TO THE UNIFIED SOIL CLASSIFICATION SYSTEMS
(USCS) IN ASTM D2487, “STANDARD METHOD FOR CLASSIFICATION OF
SOILS FOR ENGINEERING PURPOSES”. BEDDING SOIL MODULUS (E
1
)
SHALL BE 2000 LB/IN2 FOR CLASS IA AND IB SOILS AND 1000 LB/IN2 FOR
CLASS III SOIL. CLASS IA MATERIALS PROVIDE MAXIMUM STABILITY AND
PIPE SUPPORT FOR A GIVEN DENSITY DUE TO ANGULAR INTERLOCK OF
PARTICLES. CLASS III MATERIALS PROVIDE LESS SUPPORT FOR A
GIVEN DENSITY THAN CLASS I MATERIALS. HIGH LEVELS OF
COMPACTIVE EFFORT MAY BE REQUIRED UNLESS MOISTURE CONTENT
IS CONTROLLED.
DEAD LOAD (DL) IS EQUAL TO THE UNIT WEIGHT OF THE BACKFILL (γ)
TIMES THE HEIGHT OF COVER (H) DIVIDED BY 144.
DL = γγγγ * H/144
2
DL = DEAD LOAD (lb/in
)
25701 Science Park Drive
Cleveland, Ohio 44122
216-464-3400
1-800-3CARLON (322-7566)
www.carlon.com
EXTERNAL LOAD CALCULATIONS FOR DIRECT BURIED CONDUIT
Gross Automation (877) 268-3700 · www.carlonsales.com · sales@grossautomation.com
γγ
γγ
= UNIT WEIGHT OF BACKFILL (lb/ft3)
H = HEIGHT OF COVER (ft)
120 lb/ft3 IS A COMMONLY USED VALUE FOR THE UNIT WEIGHT OF
BACKFILL.
LIVE LOADS (LL) CAN BE CALCULATED BY USING THE BOUSSINESQ
FORMULA FOR A POINT LOAD AT THE SURFACE OF THE SEMI-INFINITE
ELASTIC SOIL OR BY USING THE PREDETERMINED VALUES IN THE
FOLLOWING TABLE:
LIVE LOADS ON PVC OR HDPE CONDUIT OR DUCT
(LIVE LOADS TRANSFERRED TO CONDUIT lbs/in2)
HEIGHT OF
COVER
H (ft) H20
1 12.50 * * 14 * 4.17 3.06
2 5.56 26.39 13.14 16 * 3.47 2.29
3 4.17 23.61 12.28 18 * 2.78 1.91
4 2.78 18.40 11.27 20 * 2.08 1.53
5 1.74 16.67 10.09 22 * 1.91 1.14
6 1.39 15.63 8.79 24 * 1.74 1.05
7 1.22 12.15 7.85 26 * 1.39 *
8 0.69 11.11 6.93 28 * 1.04 *
10 * 7.64 6.09 30 * 0.69 *
12 * 5.56 4.76 35 * * *
1
SIMULATES 20 TON TRUCK TRAFFIC + IMPACT
2
SIMULATES 80,000 lb/ft RAILWAY LOAD + IMPACT
HIGHWAY RAILWAY AIRPORT HEIGHT OF
COVER
1
23
E80
H (ft) H20
40 * * *
HIGHWAY RAILWAY AIRPORT
1
25701 Science Park Drive
Cleveland, Ohio 44122
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23
E80
EXTERNAL LOAD CALCULATIONS FOR DIRECT BURIED CONDUIT
Gross Automation (877) 268-3700 · www.carlonsales.com · sales@grossautomation.com
3
SIMULATES 180,000 lbs DUAL TANDEM GEAR ASSEMBLY. 26 INCH
SPACING BETWEEN TIRES AND 66 INCH CENTER-TO-CENTER SPACING
BETWEEN FORE AND AFT TIRES UNDER A RIGID PAVEMENT 12 INCHES
THICK + IMPACT.
* NEGLIGIBLE LIVE LOAD INFLUENCE.
ONCE THE DEAD LOAD AND LIVE LOAD ARE KNOWN THEY SHALL BE
ADDED TOGETHER TO OBTAIN THE TOTAL LOAD (TL = DL + LL). THE
TOTAL LOAD SHALL BE LESS THAN THE MAXIMUM LOAD.
SECTION 300-5 OF THE NATIONAL ELECTRICAL CODE (NEC) SPECIFIES
THE MINIMUM COVER REQUIREMENTS FOR CABLE, CONDUIT AND
OTHER RACEWAYS UP TO 600 VOLTS. TABLE 300.50 SPECIFIES THE
MINIMUM COVER REQUIREMENTS FOR APPLICATIONS OVER 600 VOLTS.
THE FOLLOWING IS A SAMPLE CALCULATION.
4” SCHEDULE 40 CONDUIT WILL BE BURIED 20 FEET BELOW A RAILWAY
IN TYPE I SOIL. 4” SCHEDULE 40 CONDUIT HAS AN OUTSIDE DIAMETER
OF 4.500” AND A MINIMUM WALL THICKNESS OF 0.237”.
1. CALCULATE THE MEAN RADIUS (r)
r = (O.D. -t)/2 = (4.500 in - 0.237 in)/2 = 2.132 in
2. CALCULATE THE MOMENT OF INERTIA (I)
I = t3/12 = (0.237 in)3/12 = 0.001 in
3. CALCULATE THE PIPE STIFFNESS (PS)
3
25701 Science Park Drive
Cleveland, Ohio 44122
216-464-3400
1-800-3CARLON (322-7566)
www.carlon.com
EXTERNAL LOAD CALCULATIONS FOR DIRECT BURIED CONDUIT
Gross Automation (877) 268-3700 · www.carlonsales.com · sales@grossautomation.com
PS = E*I/(0.149*r3) = [500,000 lb/in2*0.001 in3]/[0.149*(2.132 in)3] = 346.3 lb/in
4. CALCULATE THE MAXIMUM LOAD FOR 5% DEFLECTION (W)
W = [∆∆∆∆X*(0.149*PS + 0.061*E1)]/(D*K) = [0.05*(0.149*346.3 lb/in2+0.061*2000
2
lb/in
)]/(1.5*0.1) = 57.87 lb/in
5. CALCULATE THE DEAD LOAD (DL)
DL = γγγγ * H/144 = 120 lb/ft
6. DETERMINE THE LIVE LOAD (LL) USING THE TABLE
LL = 2.08 lb/in
7. CALCULATE THE TOTAL LOAD (TL)
2
TL = DL + LL = 16.67 lb/in
2
3
*20 ft/144 = 16.67 lb/in
2
+ 2.08 lb/in2 = 18.75 lb/in
2
2
2
SINCE THE TOTAL LOAD OF 18.75 lb/in2 IS LESS THAN THE MAXIMUM
LOAD OF 57.87 lb/in2 THE 4” SCHEDULE 40 CONDUIT WOULD DEFLECT
LESS THAN 5%.
4” SDR 13.5 CONDUIT WILL BE BURIED 4 FEET BELOW A HIGHWAY IN
TYPE II SOIL. 4” SDE 13.5 HAS AN OUTSIDE DIAMETER OF 4.500” AND A
MINIMUM WALL THICKNESS OF 0.333”.
1. CALCULATE THE MEAN RADIUS (r)
r = (O.D. -t)/2 = (4.500 in - 0.333 in)/2 = 2.084 in
2. CALCULATE THE MOMENT OF INERTIA (I)
3
/12 = (0.333 in)3/12 = 0.003 in
I = t
3
25701 Science Park Drive
Cleveland, Ohio 44122
216-464-3400
1-800-3CARLON (322-7566)
www.carlon.com
EXTERNAL LOAD CALCULATIONS FOR DIRECT BURIED CONDUIT
Gross Automation (877) 268-3700 · www.carlonsales.com · sales@grossautomation.com
3. CALCULATE THE PIPE STIFFNESS (PS)
PS = E*I/(0.149*r
3
) = [150,000 lb/in2*0.003 in3]/[0.149*(2.084 in)3] = 337.7 in
2
4. CALCULATE THE MAXIMUM LOAD FOR 5% DEFLECTION (W)
W = [∆∆∆∆X*(0.149*PS + 0.061*E1)]/(D*K) = [0.05*(0.149*333.7 lb/in2+0.061*1000
2
)]/(1.5*0.1) = 36.9 lb/in
lb/in
2
5. CALCULATE THE DEAD LOAD (DL)
DL = γγγγ * H/144 = 120 lb/ft
3
*4 ft/144 = 3.33 lb/in
2
6. DETERMINE THE LIVE LOAD (LL) USING THE TABLE
LL = 2.78 lb/in
2
7. CALCULATE THE TOTAL LOAD (TL)
TL = DL + LL = 3.33 lb/in2 + 2.78 lb/in2 = 6.11 lb/in
2
SINCE THE TOTAL LOAD OF 6.11 lb/in2 IS LESS THAN THE MAXIMUM LOAD
OF 36.9 lb/in2 THE 4” SDR 13.5 CONDUIT WOULD DEFLECT LESS THAN 5%.
DISCLAIMER:
THIS DOCUMENT IS FOR INFORMATIONAL PURPOSES ONLY. A
PROFESSIONAL ENGINEER SHALL BE RESPONSIBLE FOR ALL
SELECTIONS AND CALCULATIONS TO DETERMINE WHICH TYPE AND SIZE
OF CONDUIT OR DUCT SHALL BE USED.
25701 Science Park Drive
Cleveland, Ohio 44122
216-464-3400
1-800-3CARLON (322-7566)
www.carlon.com
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