CONCRETE PIERS AND ANCHORS (Continued)
constructed in advance of the time of conduit installation, when the support fixtures are grouted in place. The base of the pier should be placed below the frost line.
.34 For conduits supported above ground or in open tunnels, anchors are required
at all bends subjected to forces exceeding the resistance of the weight of the
section of conduit and water supported at the bend. Anchors are also required
about midway between expansion joints where there are two or more expansion
joints between two bends requiring anchorage. This condition occurs when the
distance between any two bends requiring anchors is in excess of about 500
feet, as it is desirable to limit the distance between expansion joints to a value
that will keep the longitudinal forces at the anchors within practicable limits.
They may be of the type shown in Figure 43 or they may completely encase the
conduit as shown in Figure 44.
Conduits placed underground usually require anchors at short-radius horizontal
bends with intersection angles which will produce forces exceeding the fric-
tional and compressive resistance of the soil, at vertical bends at summits,
and at bends adjacent to power plants and pumping plants. Where buried con-
duits are laid with horizontal and vertical curves of very large radius, anchors
usually are not necessary.
The forces at an anchor may be estimated by use of the formulas in Figure 45.
These forces should be calculated for both the expanding and contracting con-
dition with the pipe both full and empty. Wind and earthquake loads should also
be considered where conduits are above ground. The resultant of all forces,
including the reaction at the anchor and the weight of the anchor, under the
most unfavorable conditions should intersect the base within the kern to avoid
tension in the concrete (see Paragraph 2.33). The vertical component of the
resultant of all forces should not be less than the horizontal component of the
resultant of all forces divided by the coefficient of sliding friction at the base
of the anchor. The friction coefficient may vary from 0.35 to 0.65 under dif-
ferent soil conditions. Where conduits are buried the position of the resultant
of all forces is not important, and a friction coefficient of 1.00 may be used.
Anchors should preferably be placed on a rock base if possible. The bottom of
the anchor should be extended below the lowest frost line in areas subject to
freezing. As actual forces may vary on both sides of an anchor from the com-
puted forces, such possibilities should be considered in proportioning the
anchor. At points where computations indicate a balance of forces and where
theoretically no anchors are required, it is desirable to provide at least a
nominal size anchor to prevent displacement due to reasons stated above.
FABRICATION AND INSTALLATION
.35 Steel conduits are usually fabricated in courses, each course having a length
equal to the net plate width after trimming. Figure 46 shows courses for a 15-
foot penstock during fabrication. A number of courses are welded together to
form laying sections of the lengths required. End plates of pipe sections should
be prepared for field welding in accordance with the specifications. The lengths
of the courses are usually made optional with the fabricator. The number of
longitudinal joints in each course should be as small as practicable, taking into
consideration plate lengths available. Fabrication and welding procedures
should be in accordance with the applicable requirements of the API-ASME
Code unless departures from the Code are desirable in special cases. All shop
welding is required to be performed under full procedure control on automatic
welding machines as shown in Figure 47 where possible. Hydrostatic pressure
tests at a pressure equal to about 1-1/2 times the working pressure are usually
required. Butt-welded longitudinal joints in penstocks and large pump-discharge
lines, are usually radiographed in accordance with the provisions of the Code.
Figure 48 shows a portable X-ray machine used for the radiographic inspection
of shop welds.
FABRICATION AND INSTALLATION (Continued) Transportation problems usually determine whether conduits are fabricated in the contractor's shop or at the site of installation. As transport by rail of conduits over 12 feet in diameter is usually not feasible, conduits of such size should be fabricated in a field fabricating plant near the site of installation, either from plates shipped directly from the mill or from plates prefabricated at the contractor's home plant. Figure 49 shows a portion of the field fabricating plant used for the 22-foot penstocks at Davis Dam. The plates were prefabricated, cut to size, edged and rolled, then transported to the field fabricating plant for completion. Internal spiders were used during fabrication to keep the pipe courses circular until the stiffeners were welded on.
.36 The installation of steel conduits is frequently performed under a separate contract. Where this is the case it is necessary to define the length of the pipe sections and the edge preparation for the field joints so that the amount of work to be done under each contract can be determined. It is also essential in such cases to specify the permissible tolerances in pipe lengths, trueness of ends, out-of-roundness, clearances in bell-and-spigot joints and in expansion joints, spacing of supports, etc.
Conduit sections are usually transported from the nearest railroad station or
from the field fabricating plant to the site of installation by truck, trailer, or
barge. Figures 50 and 51 show the transport by truck and trailer of 18- and
22-foot penstock sections at Grand Coulee and Davis Dams, respectively, and
Figure 52 shows the transport by barge of an 18-foot penstock section at Grand
Coulee Dam from field fabricating plant to dam. Upon arrival at the site of
installation the conduit sections are lifted in place by cableway, derrick, or
other means suited to the site. Figure 53 shows handling by cableway of a 15-
foot penstock section at Shasta Dam, and Figure 54 shows handling by derrick
barge of an 18-foot penstock section at Grand Coulee Dam. Figure 55 shows
handling by whirley of a 22-foot penstock section at Davis Dam, and Figure 56
shows its placement in the 27-foot octagon blockout in the intake structure.
After being set to line and grade on temporary supports, the conduit sections
are first tack-welded into the line, before the joints are completely welded.
Figure 57 shows the installation of the Shasta penstocks. All field welding is
performed manually. After welding, the girth joints may or may not be radio-
graphed de ending on the load carried by the girth joints or on the importance
of the conduit. For long-span crossings, as shown in Figure 58 of the Shoshone
River Siphon having a center span of 150 feet, radiographic inspection may be
advisable as the girth joints are subjected to very high beam stresses. For
conduits which are hydrostatically tested after installation and where girth
joints are subjected only to normal stresses, radiographic inspection may be
omitted. Special portable X-ray machines as shown in Figure 59 are used for
the radiographic inspection. The machines are provided with rubber wheels
and are moved from joint to joint on the inside of the conduit. For conduits on
steep slopes, special scaffolding is required to make the welds accessible.
Where a proof hydrostatic pressure test is called for, such test should follow
the radiographic inspection. A test pressure of 1-1/2 times the operating
pressure is usually applied by means of pumps and held for a sufficient time
to permit the inspection of all parts of the conduit.
Installations in tunnels or concrete blockouts are more complicated, due to
limited clearances around the conduit which may require that all welding be
performed from the inside of the conduit against a backing strip. If such con-
duits are to be backfilled with concrete, all welding, weld tests, and pressure
tests should be completed and the exterior surfaces of the conduit cleaned before
backfilling.
.37 Among the possible nondestructive tests and inspections some of the following
may be required by the Bureau of Reclamation to insure a safe welded job:
A. Physical and chemical tests of plates, at mills.
F. Strain-gage measurements of peening operations in special slip-joints.
G. Hydrostatic pressure tests of individual pipe sections.
H. Soap or compressed-air leakage tests.
I. Hydrostatic pressure test of entire installation.
In order to insure compliance with the requirements, all operations, both in shop and field, should be subject to careful and rigid inspection by qualified welding inspectors. Inspection requirements are covered in the Bureau's Arc-Welding Manual.
.38 Periodic painting will increase the useful life of steel pipe. The accepted
practice calls for cleaning of the pipe surface to bright metal by blasting, the
application of one coat of cold coal-tar primer followed by one coat of hot coal-
tar enamel on the inside and on the outside of pipe to be buried. The outside
of exposed pipe should be given two coats of synthetic red lead followed by one
coat of aluminum paint. The hot enamel coating should preferably be applied
by centrifugal casting where possible. About 9 inches of each end of pipe sec-
tion should be primed but left without enamel coating for field welding of the
joints. The paint and the method of application should be in accordance with
the American Water Works Association Standards 7A.5-1940.
.39 A cement-mortar lining of 1/2-inch thickness, applied on the inside of steel
pipe in the shop by spinning, may be used in lieu of painting. The lining should
be given a steel-troweled or smoothing bar finish. Before lining, the pipe
should be cleaned by blasting to bright metal. No steel reinforcement is nec-
essary. At each end of a pipe section about 6 inches of the lining should be
omitted for field welding of the joint. After the lining has been cured, internal
bracing should be placed at the uncoated ends and elsewhere if necessary to
limit out-of-roudness to one-half percent on the pipe diameter. Bends,
fittings, and specials, which cannot be mortar-lined by spinning, may be lined
by the gunite method. A gunite coating of 3/4-inch thickness, reinforced with
2 by 4, 12-gage steel wire mesh may be applied in the shop on the outside of
pipe to be buried, as shown in Figure 60. For highly corrosive soils gunite
may be applied over a coal-tar enamel coating. About 12 inches at each end
of pipe section should be left uncoated to facilitate field welding of the joint.
Tentative Specifications 7A.7 of the American Water Works Association give
further data on cement-mortar protective coating of steel pipe of sizes 30
inches and over.
Field joints where the protective lining and coating was omitted should be cleaned and painted or gunited, as the case may be, after welding and testing is completed.
|