Materials & Design Stresses
A. Hoists are operated by means of a motor and oil pump with the directional flow of oil controlled by valves that are actuated by electric contacts from any desired position.
B. The lifting capacity of the hoist will be the sum of the following:
Weight of intermediate stems, piston, and gate stem
Wheel and track friction
Hydraulic downthrust, or unbalance.
The operating pressure in the hydraulic cylinder should be limited to about 750 psi and the cylinder should be designed for 1,000 psi.
C. The cylinder is made of rolled plate or pierced forging with forged flanges
welded to the ends of the cylinder. The heads are made of cast steel. The
piston stem if of monel metal and the intermediate and gate stems are
low-carbon or alloy-steel forgings.
The working stress in steel castings should be limited to about one-third of the yield point of the material.
The stress in the cylinder wall and stems should be limited to about
one-third of the yield point of the material when loaded to the capacity
of the oil-pump relief valve.
D. A typical design of a hydraulic hoist is illustrated in the following drawing:
15' x 19.05' Penstock Coaster Gate Hoist--
Shasta Dam (Figure 55)
.34 Lifting frames or lifting beams are used as grappling devices for placing or removing bulkhead gates, trashracks, and stop logs, and are designed to operate below the water surface, utilizing the guides or slots provided for the equipment being handled. Trashracks are generally left in place after the initial installation, for periods of 10 years or more; therefore, lifting frames are considered maintenance equipment and are designed at the project if needed.
A. Wherever possible, large stop logs and bulkhead gates should be picked up
from a single point, with a single hook. Where two hooks are used, the
possibility always exists for only one hook to engage with the resulting
probability of dropping the load. When designing either a lifting beam or
lifting frame it is desirable to provide spears or pilot rods to guide the
hook into proper position for grappling and to prevent rotation of the gate
or stop log when raised above the guides. To reduce the possibility of
binding in the guides, the height of a lifting frame should not be less than
six-tenths of its length, in which case a single-rope connection from the
lifting frame to the hoist or crane is considered sufficient. A lifting
beam consisting of a single horizontal member requires considerably less
fabrication and headroom than a lifting frame, and by providing small
rollers on the ends the possibility of binding in the guides is practically
eliminated. To provide greater stability, lifting beams should be
equipped with two hoisting ropes or a suitable sling.
MISCELLANEOUS METALWORK (Continued)
B. Typical designs of lifting frames or beams are illustrated by the following drawings:
.35 Trashracks are used to protect turbines, pumps, and valves from objectionably TRASHRACKS large debris. The usual practice is to provide as large a clear opening between trashbars as is consistent with the features to be protected, to consider hydraulic head loss through the rack only when this loss is important, and to consider the ease of replacement in proportioning the rack so as to build long life into those racks which may not be readily replaced.
A. The details and general construction of trashrack installations vary with the service required, depth of water, and accessibility for replacement. Trashracks may be divided into types according to their constructional features and the methods required for their installation. The types are described as follows:
(1) Type 1--Removable-section racks which are installed by lowering the
sections between side guides or grooves provided in the trashrack
structure so that the section may readily be removed by lifting it
from the guides. This type may be side bearing or end bearing;
however, the end-bearing type of racks formerly used on installations
such as the Grand Coulee Dam outlet works have been abandoned, for
economical reasons, in favor of the side-bearing type. Reference
drawings for side-bearing racks are:
Types & Reference Drawings
(2) Type 2--Removable-section racks in which the individual sections are
not installed between guides in the trashrack structure, but are placed
adjacent to each other laterally and in an inclined plane to obtain the
desired area. Since small lightweight rack sections may easily be
displaced, consideration should be given to securing them in place
with bolts located above the water line. Reference drawings are:
MISCELLANEOUS METALWORK (Continued)
(3) Type 3--Trashrack sections which are bolted in place below water line in locations which may be readily unwatered, or are accessible seasonally. Reference drawings are:
B. The appropriate type of rack for an installation is dependent upon its accessibility for painting or replacement, the size and quantity of trash expected, and the requirement for raking. The type of rack selected must be suitable for the trashrack structure, and the design of the rack, including spacing of the bars, should be discussed with the engineers responsible for the design of the structure and the equipment being protected. Trends in trashrack-type selections are indicated in the following:
(1) All major trashrack installations where at least a portion of the racks are deeply submerged have been Type 1 side-bearing racks, installed in a vertical position.
(2) Type 2 racks have been used for canal headworks, and for pumping plants where a single rack section extends from water surface to the bottom of the rack-protected area. This type of rack is generally installed in an inclined position.
(3) Type 3 racks are well suited to installations where power rakes may be required, and are particularly adaptable for completely submerged intakes.
C. In small installations of deeply submerged outlet works, racks should be designed to fail at approximately 40 feet differential hydraulic head. Racks which are submerged 20 feet or less are usually designed to fail at a hydraulic head equal to two-thirds of the maximum depth of submergence; however, if these racks are intended to serve as supports for flashboards, they should be designed for safe stresses under maximum loading conditions.
D. Trashrack bar sections are usually rectangular, but if considerable
tonnage of racks is purchased at one time bars having rounded edges may
be used. The depth of a trashrack bar section should not be more than 12
times its thickness, nor less than 2 inches. For racks which may require
raking, the distance from the face of the rack to the spacers or other
horizontal members should be at least 1.5 inches. The laterally unsupported
length of a trashrack bar should not exceed 70 times its thickness.
Trashrack bars should be assumed to fail when the stress in the bar
reaches the following value:
where L the laterally unsupported length of bar and t = the thickness of the bar. Similarly, safe working stresses for trashrack bars used to support flashboards should not exceed the following value:
MISCELLANEOUS METALWORK (Continued)
E. Members used as horizontal beams in Type 1 side-bearing rack sections
do not require stress reduction to compensate for lack of lateral support.
These members are assumed to fail at yield point stress; but calculations
should include stress due to the dead weight of the beam members and
trashrack bars. To insure rigidity during handling, the lateral deflection
of the beam members due to dead load should not exceed one three-hundreths
of their span.
F. Several large trashrack installations, notably at Friant and Marshall Ford
Dams, have racks fabricated from a low-alloy steel. However, whether
the higher yield point justifies the increased cost or whether the life of the
rack is actually prolonged by the low-alloy content of the steel is prob-
lematical. The current procedure is to provide racks of structural steel
with 3/8-inch thick wearing bars welded to the surfaces of the rack, which
are in contact with the concrete grooves. The wearing bars prevent the
protective coating, given the racks proper, from coming into direct contact
with the concrete, and the subsequent injury to the coating by abrasion.
An example of this construction is indicated on Drawing No. 351-D-2014.
The thickness of metal to be used in trashracks is to some extent dependent
upon the nature and probable permanence of the rack installation. However,
the minimum thickness often used for Type 2 and Type 3 racks is five-
sixteenths inch. It is customary to use 1/2-inch minimum thickness on
deeply submerged racks. In some instances an analysis has shown economy
in the addition of metal to the calculated thickness requirements of trash-
bars, the added metal thickness thus materially increasing the life of the
trashrack. This alternative should be considered only for trashracks to
be used in connection with high-pressure outlets. Racks protecting power
intakes are usually designed to fail at 20 feet differential hydraulic head.
These racks constructed without added metal for corrosion will have a very
long life, as any clogging of the rack will be noticed and corrected before
an appreciable loading is developed due to head loss through the rack. Thus,
these racks, having a very high safety factor when new, will still be service-
able with a reduced safety factor after considerable corrosion has occurred.
G. In proportioning welds for trashracks it should be recognized that the
corrosion of the weld is a factor of greater importance than the strength
of the weld, also that a fillet weld will corrode from one side only, whereas
a trashrack bar will corrode on two sides. It is recommended that the
minimum size of fillet weld be the same as the thickness of metal in the
trashrack bars where these bars are three-eighths inch or less in thickness
and that 3/8-inch fillet welds be used for connections of heavier trashrack
bars. It is also recommended that the horizontal beam members of Type 1
side-bearing rack sections have 1/2-inch welds at their end connections.
H. Type 1 side-bearing racks are usually installed between guides formed in
the concrete of the trashrack structure, thus eliminating the necessity for
embedded metalwork, except for small bearing plates at the base of high
tiers of racks. Type 1 side-bearing racks are more economical than Type
1 end-bearing racks, the economy being more pronounced in racks having
the closer bar spacings. For power intakes, the actual economy of a
trashrack may depend upon low hydraulic head loss rather than upon the
initial cost of the rack. In these racks it is usual to use streamlined
supports, and to use bar sections instead of structural shapes for horizontal
beams of Type 1 side-bearing racks. However, streamlining of trashbars
is quite expensive and the added expense is generally not justified with
prevailing power sales rates.
I. Structural details, connections, etc., should at least provide for failure loads on the rack. Also, care should be exercised to avoid heavy loadings
Design of Horizontal Members
Economies of
Trashrack
Installations
Structural
Details
(Cont.)
MISCELLANEOUS METALWORK (Continued)
near the exposed edges on corners of concrete construction. Thus it is
usual to arrange the horizontal beam members of Type 1 side-bearing
racks so as to apply their loading near the inner part of the rack guide as
shown on Section E E, Drawing No. 351-D-2014. Where Type 1 racks are
used in tiers, they should be equipped with dowels of sufficient size to
insure proper alinement of the racks in their guides. It is usual to arrange
the trashrack bar length dimensioning to allow the use of bars having
standard hot-rolled bar length tolerances and thus avoid unnecessary
trimming by the fabricator. The dead load of a tier of rack sections is
usually transmitted from one section to the next lower section through
the dowel plates, and the side members of the lower rack sections thus
become struts which may not have sufficient rigidity to carry the dead
load without some lateral support. This support is obtained from the
guides of the structure after deflecting to take up the clearance in the
slots.
Shop fabrication specifications have been developed for Type 1 side-bearing racks (see Specifications No. 2537).
Stairways and ladders constitute an important means of access to different elevations in powerhouses and dams. The types in common use are inclined straight stairways and spiral stairways. Ladders are usually vertical or slightly inclined.
A. The preferred slope for gallery stairways is 9 inches vertical to 12 inches horizontal with 7-1/2-inch risers. The treads are usually fabricated from grating and the stringers from standard 8-inch channels, spliced in convenient lengths for handling in the galleries. Handrail should be 1-1/2-inch standard black pipe of welded construction, in accordance with Drawing No. 40-D-4315. Installation of stairways should be made with bolted connections and expansion anchors for fastening to concrete. Headroom, handrail height, and tread and riser data for varying slopes may be obtained from Drawing No. X-D-2002. In all instances treads should be designed for a loading of 100 psf of projected area.
B. Stairways may be typified by the following reference drawings:
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C. Spiral stairways should preferably be 6-foot 0-inch diameter; however,
lack of sufficient space may justify the use of a smaller size, in which
case a 5-foot stairway is used. The direction of the stairway should be
clockwise going down, with 12 treads constituting a complete turn. The
desired distance between platforms is 1-1/2 turns; this may be increased
when necessary, but the maximum should not exceed 2-1/2 turns. The
landing.platforms may be 60, 90, or 120 degrees, but 90-degree platforms
are preferred.
(1) When spiral stairways are used between the galleries of dams, it is
necessary to locate the platform and railing so that convenient access
from the gallery to the stairway at each floor level will be provided.
This is accomplished by selecting a suitable width of landing platform
and by varying the platform spacing. A 120-degree platform provides
sufficient headroom if 10-inch risers are used but should not be used
with 9- or 9-1/2-inch risers. The headroom at the lower entrance
to a stairway should also be investigated.
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