2.11

PRELIMINARY STUDIES (Continued)

operate in reverse as a runaway turbine. The maximum increase in head at the pump occurs while the pump is operating as a turbine, except where check valves are installed at the pump; in this event the pressure upsurge resulting from the starting of the pump may exceed the rise in pressure following a power failure. Decreases in head or negative pressures in the pipe line occur prior to the time that the pump starts to operate as a turbine. The negative pressures produced may be of sufficient magnitude to cause the collapse of a discharge pipe if the pipe is not properly designed.

TYPES OF .11 The types of conduit construction used are largely determined by physical CONSTRUCconditions at the site of the installation. In some cases, however, two or more TION types of construction would appear to be equally satisfactory or appropriate. In these cases, costs, safety features, operating requirements, and other pertinent items should be considered before selecting the type or types of construction to be used. Where conduit sizes are such that the pipe can be shipped by rail or truck, it is usually desirable to have the pipe fabricated in an established fabricating plant in lengths suitable for shipment and installation. If the size of a conduit is such that it is impracticable to fabricate it in sections small enough to ship by rail or truck, the establishment of a field fabricating plant at or near the site of installation may be necessary. In either event, the spacing and type of field joints should be such that the fabricated pipe sections can be transported, unloaded, and installed under the conditions prevailing at the point of installation.

The different types of steel-conduit construction involved in Bureau of
Reclamation projects usually fall within one of the following general
classifications:

A. Conduit supported above ground in a manner similar to that shown in
Figure 2, or carried on cradles similar to the one shown in Figure 21.

B.

C.

D.

Conduit placed in open tunnels and supported in a manner similar to that shown in Figure 4, or on sliding-type supports similar to the one shown in Figure 20.

Conduit embedded in a large mass of concrete similar to that shown in • Figure 1.

Conduit placed in tunnels or other passageways and backfilled with
concrete after installation.

E.. Conduit placed in open trenches and backfilled with earth as shown in
Figure 3.

Either welded joints or sleeve-type couplings may be used for circumferential
field connections with any of the different types of construction noted above.
The use of sleeve-type couplings is limited to pipe with diameters which are
within the range of coupling sizes furnished. Sleeve-type couplings are a com-
mercial product, a typical section of which is shown in Figure 11. They con-
sist of a middle ring, with pipe stop, follower rings, gaskets, and connecting
bolts. They are furnished without pipe stops if desired, to meet special require-
ments. The couplings may be used to advantage for installations where it is
desirable to eliminate all field welding, and for pipe lines crossing rivers or
canyons on suspension bridges requiring a flexible construction which will
adjust itself to the changing catenary of the suspension cables, with pipe filled
and empty. Figures 12 and 13 show the 31-1/4-inch diameter Ogden Canyon
Siphon supported on a suspension bridge of 360-foot span. The pipe was made
in 20-foot 3-inch lengths, and connected in the line with sleeve-type couplings.
A stiffener ring was provided at the center of each pipe section which was
bolted to the bridge framing, and alinement saddles were provided near each
end, as shown in Figure 14.

2.12

PRELIMINARY STUDIES (Continued)

Typical welded field girth joints for conduits to be installed under varying circumstances are shown in Figure 15. When there are no clearance restrictions or other features precluding their use, double-welded butt joints as shown at (a), (b), or (c) should be used. Type (a) joints with exception of the backing weld should, in most cases, be welded from the inside. Where it would be difficult to weld on the outside of the conduit on account of close clearances and where digging of bell holes at girth joints would otherwise be necessary, joints of the types shown at (d) and (g) may be used. Joints of the types shown at (e) and (f) may be used where sufficient clearances can be provided for outside welding. The type (e) joint is limited to plate thicknesses which are not beyond the range of equipment available to produce bell ends. The pipe taps may be omitted if soap tests are not required. The bell joint provides the desired flexibility for the installation of long pipe lines to comply with ground profiles or to adjust discrepancies in length. Types (f) and (g) joints may be used as closing joints in long tangent lengths after the temperature in the pipe has been normalized by backfilling.

DESIGN OF STEEL CONDUITS

PIPE SHELL

.12 The type of steel selected determines the permissible design stresses. For satisfactory welding, the plates used in the fabrication of conduits should be of structural, flange, or firebox quality with a carbon content not exceeding 0.35 percent. Plates meeting these requirements are available in either carbon- or alloy-type steels. Limited experience in the welding of alloy-type highstrength steels retarded their use for conduits constructed by the Bureau of Reclamation.

.13 Joint efficiencies assumed for design purposes vary for different kinds of joints, and different methods of inspection and testing. According to the APIASME Code, joint efficiencies also vary for different types of steels. Where field girth joints differ from shop joints, the kind of longitudinal joint used will usually govern since working stresses in girth joints are usually less than those in longitudinal joints. Under these conditions, and where double-welded butt joints are used, joint efficiencies should be assumed as follows, for all types of steels:

90 percent for unradiographed welds or for welds spot-radiographed,

100 percent for welds completely radiographed and weld defects repaired. Radiographic tests are usually limited to longitudinal joints.

.14 A design stress of one-half the yield point should be used in the design of carbon-steel conduit shells, stiffener and support rings, and other attachments under assumed normal operating conditions. Where the conduit is embedded in concrete with a minimum cover equal to one-half of the static head, a design stress equal to two-thirds of the yield point may be used. Under emergency conditions expected to exist for short periods of time and at infrequent intervals, a design stress of two-thirds of the yield point may be used. For alloysteels having a small margin between yield point and tensile strength, a design stress of either 50 percent of the yield point value, or 30 percent of the tensile strength, whichever is smaller, should be used.

.15 Shell thicknesses should be such that the maximum equivalent unit stress (see Figure 16), resulting from the sum of all circumferential stresses combined with the sum of all longitudinal stresses, will not exceed the design stress multiplied by the joint efficiency. Circumferential, or hoop stresses resulting from internal pressure may be computed from the formula,

TYPES OF CONSTRUC

TION

(Cont.)

TYPE OF
STEEL

JOINT EFFICIENCY

DESIGN
STRESS

SHELL THICKNESS

If a conduit is freely supported at a number of points, it will act as a continuous beam. If expansion joints are installed, the portion of the conduit between the center of the expansion joint and the nearest support is assumed to act as a cantilever. Reactions, moments, and stresses can be estimated by the use of appropriate beam formulas. Stresses resulting from frictional resistance at supports and in expansion joints when movement of the pipe is caused by temperature changes, should be estimated on the basis of the appropriate friction coefficients referred to in Paragraphs 2.18, 2.19, 2.20, and 2.24. At supports, circumferential and longitudinal stresses in the shell should be estimated by means of the formulas referred to in Paragraph 2.17. Where shell thicknesses based on stress considerations are less than thicknesses calculated from the following formulas, the latter should govern.

Where conduits act as beams, the maximum equivalent unit stresses should not exceed the critical buckling stress which may be estimated from the formula, 3/

Values of C, based on experimental determinations, vary from 0.20 to 0.26 for shell thicknesses of from 0.03 to 0.25 inch, respectively.

3/ Wilson and Newmark, "The Strength of Thin Cylindrical Shells as Columns," Bulletin No. 255, published in 1933 by the University of Illinois.

2.16

DESIGN OF STEEL CONDUITS (Continued)

STIFFENER RINGS AND SUPPORTS

.16 Stiffener rings may be required to avoid excessive shell thicknesses under the following conditions:

A. Where conduits are subjected to uniform external pressure.

B.

C.

Where conduits are placed underground and subjected to earth pressure or
concentrated loads.

Where conduits are embedded in concrete and subjected to pressure
resulting from buoyancy while concrete is being placed.

For conduits subjected to uniform external pressure, stiffener rings should be
designed on the basis of the formula, 4/

STIFFENER
RINGS

Conduits placed underground may be subjected to (1) concentrated vertical loads, (2) distributed vertical loads, and (3) distributed vertical and horizontal loads. Maximum bending-moment formulas are as follows: 5/

4/ "API-ASME Code for the Design, Inspection and Repair of Unfired Pressure Vessels for Petroleum, Liquids, and Gases," Fourth Edition, 1943.

5/ "Handbook of Culvert and Drainage Practice," published in 1931 by Armco Culvert Manufacturer's Association, Middletown, Ohio, pp. 42-76.

2.17

STIFFENER
RINGS
(Cont.)

SUPPORT
RINGS

DESIGN OF STEEL CONDUITS (Continued)

STIFFENER RINGS AND SUPPORTS (Continued)

Values of q vary from 0.37 for rigid conduits to 0.85 for flexible conduits depending on the degree of flexibility and the condition of the backfill in the conduit trench. Where rings are installed, a value of about 0.75 may be used for g. Values of W may be obtained from Figures 17 and 18. Where stiffener rings are used, Q and W are assumed as being equal to the load per unit length of conduit multiplied by the distance between rings. In these cases d 2R, the neutral diameter of the combined ring section illustrated in Figure 22. Stiffener rings for conduits embedded in concrete should be designed for the reactions resulting from the buoyant forces to be expected or the reactions resulting from the weight of the empty pipe, whichever is greater. Suitable structural steel supports to be attached to the rings may be provided with the pipe or furnished by the installation contractor.

=

.17 Support rings are required at the supports where conduits are carried on rocker- or roller-type supports, on sliding-type supports, or on saddle-type supports. Typical designs are shown in Figures 19, 20, and 21.

Circumferential stresses in support rings for use with both rocker- and
sliding-type supports can be computed from formulas and tables in Figures 22
and 23 when the columns carrying the vertical reactions are placed as shown.
These formulas apply when the conduit is precisely full or under pressure,
which are the usual normal operating conditions for which the support rings
are designed. However, stresses occurring in a partially filled conduit will
exceed those occurring when the conduit is full, the most unfavorable condition
being when the conduit is half full. To eliminate the possibility of failure
while the conduit is being filled or emptied, the half-full condition should be
considered and the support rings should be designed so that stresses under
this condition will not exceed the design stress by more than 50 percent. For-
mulas for estimating stresses when the conduit is half full and for all condi-
tions when the support columns are located other than as shown in Figure 22
can be found in Chapter III of the bulletin referred to in the figure. Longitudi-
nal stresses in the conduit shell may be estimated from the formula, 6/

and other notations are as shown on Figure 22. It frequently happens that the moments at a support necessitate a greater shell thickness than that required between supports. In a case of this kind, the minimum length of the heavier plate at the support should be calculated from the formula,

where,
L1

6

L1

=

= minimum length of the support section, and the value of q
is as shown in Figure 22.

6/ Schorer, Hermann, "Design of Large Pipe Lines," American Society of Civil Engineers Transactions, 1931.