Chapter 5 Analysis of the MMELS test
5.7. Conclusions
Civil Engineering
This copy is intended for use solely with Piping Design Layout Training.
For other purposes, refer to the original document available through Knowledge Online.
Other considerations that must be addressed in the selection phase of a thrust restraint system is available space, soil parameters, and whether the deflection is vertical or horizontal. A crowded utility corridor or small plant area may cause the design of a thrust block to be impractical because of space and excavation limitations. Also poor soil bearing capacities may require thrust blocks to become too large for the available space. Thrust blocks should not be used for vertical thrust restraint hat is in the upward direction.
Downward vertical and horizontal thrusts are appropriate directions for thrust block restraint.
Location
Location of thrust blocks:
Horizontal deflections greater than 10 degrees Downward vertical deflections
Under valves in asbestos cement systems (not used much anymore) At fire hydrants
The direction of thrust and the direction of the soil resultant reaction must be collinear to prevent an unbalanced moment from acting on the system. The depth to the bottom of the thrust block from the soil surface should be equal to or greater than two times the height of the block.
Sizing
Sizing of thrust blocks, as with all thrust restraint systems, must be designed for the highest pressure the pipe will experience throughout its service life. Typically, the highest pressure will occur during testing of the pipe line.
Calculation of thrust resulting from pipe deflection is determined using the following formula:
A = Cross Sectional Area (Square Inch) of pipe
= Pipe Deflection Θ
Values calculated from the above formula have no factor of safety.
Values of thrust forces have been tabulated by the CIPRA (Cast Iron Pipe Research Association) and are presented in Attachment 01, Table 1. Values in Attachment 01, Table 1 are higher than those calculated using Equation 1. CIPRA values may contain a factor of safety that has been included in these values.
As stated above, thrust blocks will be sized on the basis of test pressure in accordance with Attachment 01, Table 1, or the calculated value, and the bearing capacities of the soil.
Bearing capacities are readily available from the project soils report. For preliminary sizing, Practice 670 210 1211 Publication Date 20Sep95 Page 2 of 6 FLUOR DANIEL
THRUST RESTRAINT DESIGN
Civil Engineering
This copy is intended for use solely with Piping Design Layout Training.
For other purposes, refer to the original document available through Knowledge Online.
Attachment 01, Table 2 shows typical soil classifications with bearing capacities. Again, these values should only be used for preliminary sizing of blocks.
Knowing the force and soil bearing capacities, the thrust block size can be calculated. Block width usually varies from one to two times the height. Again, the height of the block should be at least as high as the outside pipe diameter and at least as deep as the block is high.
Reinforcement placement, sizing, and spacing for large blocks should be reviewed by structural engineering for adequacy.
The placement of thrust blocks should be at a 45 degree angle to the soil bearing surface and should not cover any bolts or fittings of the piping.
Restrained Joints
Restrained joints are specially designed joints that together with soil friction transfer forces at pipe bends. Restrained joints are predominantly used where thrust blocks are not economical or practical due to limited space, access, unstable soils, or possible disturbance by future excavation.
When restrained joints are used, the pipeline becomes its own thrust block. By restraining a length of pipe near bends and along the pipe line, the thrust force is transferred to the surrounding soil by the pipe.
Unbalanced Forces Horizontal Bends:
The length of pipe to be restrained is calculated by the formula L= SfPAK
KFs+DPp
Equation 2 where:
L = length of pipe to be restrained (feet) Sf = safety factor
P = internal pressure (psi)
A = cross sectional pipe area (square inch) Fs = pipe to soil friction (pounds per feet)
= deflection angle Θ
K = 4 tan
Θ 2
Pp = passive soil resistance (psf) D = pipe diameter (feet)
The length (L) calculated specifies the length of pipe that is required to be fitted with restrained joints to prevent the pipes from separating. Within this length, frictional and bearing forces of the soil will resist the thrust forces imposed from the pipe line deflection.
To calculate pipe to soil friction, Fs, certain soil parameters are required.
Fs = Ap C + W tan δ
Practice 670 210 1211 Publication Date 20Sep95 Page 3 of 6 FLUOR DANIEL
THRUST RESTRAINT DESIGN
Civil Engineering
This copy is intended for use solely with Piping Design Layout Training.
For other purposes, refer to the original document available through Knowledge Online.
where:
Ap = pipe surface area (SF/LF) C = pipe cohesion (psf) W = normal force on pipe (plf)
δ = fφφ
= soil internal friction angle (degree) φ
= pipe friction to soil friction ratio fφ
Also:
C = fc Cs where:
fc = pipe cohesion to soil cohesion ratio Cs = soil cohesion (psf)
Typical values of soil parameters are presented in Attachment 02, Soil Friction and Cohesion Factor, from Thrust Restraint For Underground Piping Systems by R. J. Carlsem. The soil parameters to be used for the friction calculation should be for whatever soil is in direct contact with the pipe. For example, if the pipe is surrounded by bedding material and the trench then backfilled with native material, the bedding parameters should be used for the calculations.
To calculate the normal force on the pipe (W), the weight of the pipe plus the weight of fluid in the pipe plus the weight of soil above the pipe are added together. The soil above the pipe may be simplified to:
We = HDω
The passive soil resistance, Pp, is calculated using Rankine Theory.
Pp= ωHcNφ+2Cs Nφ
To accurately account for all forces along the pipe length, an additional force for the added resistance resulting from pipe bells should be added to Fs to calculate F's.
F's = Fs + Fb
Practice 670 210 1211 Publication Date 20Sep95 Page 4 of 6 FLUOR DANIEL
THRUST RESTRAINT DESIGN
Civil Engineering
This copy is intended for use solely with Piping Design Layout Training.
For other purposes, refer to the original document available through Knowledge Online.
where
Fb = πPp
Db2−D2
4
Db = outside diameter of bell (feet) D = diameter of pipe (feet)
If the pipe is to be wrapped or encased with polyethylene, the value of Fs and F's must be reduced by 30 percent to account for slipping which may occur between the pipe and polyethylene.
Vertical Bends:
For unbalanced forces resulting in vertical uplift the following formula should be incorporated for design lengths:
L= SfKPA KFs+2W
Terms have been previously defined.
Dead Ends:
The required length for unbalanced forces resulting from a dead end is calculated as follows:
L= SfPA Fs
Passive soil resistance may be included if the soil is to remain undisturbed.
Tees
Tees in pipe line are capable of restraining quite a lot of force through passive soil resistance.
Lp = length of pipe adjacent to fitting (feet) General
When designing thrust restraint joints, one problem to be aware of is restraint length overlap and bend combinations. If two deflections are located near one another, the total deflection to be designed for may be the sum of the two deflections if bends are within each others restraint length. Also, joints should always be designed for test pressures if that is the maximum pressure that the pipe will experience.
Practice 670 210 1211 Publication Date 20Sep95 Page 5 of 6 FLUOR DANIEL
THRUST RESTRAINT DESIGN
Civil Engineering
This copy is intended for use solely with Piping Design Layout Training.
For other purposes, refer to the original document available through Knowledge Online.
REFERENCES
Carlsem, Roger J. Thrust Restraint for Underground Piping Systems. CIPRA (Cast Iron Pipe Research Association).
Handbook - Ductile Iron and Cast Iron Pipe. Cast Iron Pipe Research Association. Oak Brook, Illinois.
Kennedy, H., D.S. Shumard, C.M. Meeks. Ductile Iron Pipe Thrust Restraint Design Handbook. EBAA Iron Sales, Eastland, Texas.
ATTACHMENTS
Attachment 01: (20Sep95)
Table 1. Thrust at Fittings in lbs/100 psi Water Pressure
Table 2. Approximate Values of Soil Capacities for Preliminary Design Attachment 02: (20Sep95)
Soil Friction and Cohesion Factor
Practice 670 210 1211 Publication Date 20Sep95 Page 6 of 6 FLUOR DANIEL
THRUST RESTRAINT DESIGN
Civil Engineering
This copy is intended for use solely with Piping Design Layout Training.
For other purposes, refer to the original document available through Knowledge Online.
Table 1. Thrust At Fittings In lbs/100 psi Water Pressure
4 1,810 2,559 1,385 706 355
6 3,739 5,288 2,862 1,459 733
Note!!! To determine thrust at pressures other than 100 psi, multiply the thrust obtained in the table by the ratio of pressure to 100.
For example, the thrusts on a 12 inch, 90 degree bend at 125 psi is 19, 353 x 125100 = 24, 191 pounds
Carlsem, Roger J. Thrust Restraint for Underground Piping Systems. CIPRA
Table 2. Approximate Values of Soil Capacities for Preliminary Design.
Type of Soil Safe Load lbs/SF
Muck, peat, etc. 0
Soft Clay 1,000
Sandy Silt 3,000
Sand 4,000
Sandy Clay 6,000
Practice 670 210 1211 Publication Date 20Sep95 Attachment 01 Page 1 of 1 FLUOR DANIEL
THRUST RESTRAINT DESIGN
Civil Engineering
This copy is intended for use solely with Piping Design Layout Training.
For other purposes, refer to the original document available through Knowledge Online.
Soil Description Silt (Passing No. 200):
Dry
Practice 670 210 1211 Publication Date 20Sep95 Attachment 02 Page 1 of 1 FLUOR DANIEL