The design was based on the following allowable unit stresses in pounds per square inch: Tension and compression: Elliptical head and stiffener ring (Net section based on a joint efficiency of 95 percent and factor of safety of 5 on 60, 000 pounds per square inch ultimate) 11,400 Rolled shapes 15, 000 Shear: Rolled shapes 10, 000 Ribbed bolts 15, 000 Bearing: Ribbed bolts, single shear 24, 000 Tension: Lifting stem (12 percent chromium, and a factor 75. Trashracks. (a) Trashracks for Main Intake Structure. —Eighty-four trashracks are required for the main outlet works intake structure to protect the 30-inch hollow-jet valve in the control house from oversize trash. Each trashrack section is approximately 11 feet 10 inches wide by 8 feet 1 inch high, as shown on figure 44. The trashracks were furnished under specifications No. DC-5056. The trashracks are installed in vertical slots of the intake structure. The racks consist of 2- by 5/8-inch trash bars supported by 10- by 1-1/4-inch horizontal loadcarrying plates which in turn transmit the load to the side members. The 3-3/8-inch clear opening between the trash bars is considered sufficient protection for the hollowjet valve. The estimated weight of the 84 trashracks is 428, 000 pounds. The trashracks were designed for a differential head of 40 feet at a stress of 33, 000 pounds per square inch. (b) Trashrack for Bypass Pipe. —One trashrack is required for the main outlet works intake structure bypass pipe to protect the 36-inch-diameter cast iron slide gate from oversize trash. The trashrack section (fig. 45) is 5 feet 10 inches wide by 9 feet 10 inches high and was furnished under specifications No. DC-5056. The trashrack is installed by means of anchor bolts. The rack consists of 1-inchdiameter rods attached to 6- by 1-inch flat bars. The 3-1/2-inch clear opening between trashbars is considered sufficient protection for the slide gate. The estimated weight of the trashrack is 1, 600 pounds. The trashrack was designed for a 40-foot differential head at a stress of 33, 000 pounds per square inch. 76. Ice-Prevention Air System, (a) Description. —A compressed air ice-prevention system is installed at the intake structure. Compressed air, discharged through nozzles below the reservoir water surface around the main intake structure, lifts comparatively warm water to the surface. The warmer water inhibits ice formation and thus relieves lateral ice thrusts on the intake structure. The ice-prevention air system was furnished under invitation No. DS-5118 and installed by the contractor under specifications No. DC-5056. It is shown on figures 29, 30, and 31. The system essentially consists of a compressor, steel piping, copper tubing, and nozzles. The compressor, located in the valve house about 1, 645 feet downstream from the intake structure, furnishes compressed air through the 1-1/2-inch steel piping and tubing to the nozzles on the periphery of the intake structure. 11-5 EHT Pos ELEVATION SECTION B-B INSTALLATION NOTES Welding symbols apply to the joints of all members of similar identification Trashrack may be modified to suit UNITED STATO DURGAU OF REOL AMATION COLORADO RIVER STORAGL PROJECT NAVAJO DAM OUTLET WORKS INTAKE STRUCTURL Figure 45. --Trashrack for main outlet works intake structure by pass. (b) Design. —The compressor at elevation 5740 delivers approximately 44 cubic feet of free air per minute at a pressure of 125 pounds per square inch or 57 cubic feet per minute at 10 pounds per square inch. Normal maximum operating pressure for the lower set of nozzles at elevation 5882 will be about 50 pounds per square inch and for the upper set at elevation 5980 about 20 pounds per square inch. Under normal operating conditions, each of the 18 nozzles at one operating level is supplied about 3 cubic feet of free air per minute. Each nozzle is designed to maintain an ice-free area from 10 to 20 feet in diameter. Water is expected to seep through the nozzles and fill the air pipe when the compressor is not operating for a period of time. To force this water out of the air pipe, above normal compressor pressures are necessary. A 270-foot maximum static head, equivalent to a pressure of 117 pounds per square inch, can be developed when the reservoir reaches elevation 6010. The pressure switch control for the constant-speed, unloading-type compressor can be set to unload at about 130 pounds per square inch and reload at about 120 pounds per square inch. At maximum static head a differential pressure ranging from 13 to 3 pounds per square inch will be sufficient to force the water from the air pipe within 30 minutes. Copper tubing conforms to Federal Specification WW-T-799a, type K, hard drawn; and solder fittings conform to ASA-A-40. 3. Other materials are standard commercial quality. Pipe hangers supporting the steel pipe in the tunnel are attached to the concrete with expansion anchors, studs, and hexagonal nuts. Sleeve-type couplings joining the steel pipe were used for ease of installation and to provide for linear pipe expansion. 77. 6- by 13-Foot Fixed-Wheel Gate, (a) Description. --One 6- by 13-foot fixed-wheel gate is located in the main outlet works gate shaft at station 10+38. 50. It is required for emergency closure of the conduit in case of damage to the conduit or outlet works and for inspection and maintenance. Normally, the gate will be closed only after flow through the conduit has been stopped by closing the hollow-jet valves (see sec. 81). However, in an emergency the gate may be closed with water at full reservoir head flowing through the conduit. The gate was furnished under invitation No. DS-5375. The general arrangement and details of the gate installation are shown on figures 46 and 47. The gate is a flat structural-steel assembly with brass-clad rubber seals mounted on the upstream skinplate and a rubber seal on the bottom edge of the skinplate. The seals prevent leakage by contacting seal seats embedded in the concrete on the upstream side of the gate slot and across the bottom of the tunnel conduit. Wheels at each side of the gate bear on tracks embedded in the concrete on the downstream side of the gate slot. Guide shoes on each side of the gate bear against guides embedded in the concrete of the gate slot to center the unloaded gate. A series of steel stems connects the gate to a 14-inch-diameter hydraulic hoist located at the top of the gate shaft. The gate stem passes through a packing in the watertight steel bonnet, which is located immediately above the gate. The gate travel is 13 feet 3 inches. The estimated weights of the gate and the embedded frame are 85, 000 pounds and 25, 000 pounds, respectively. (b) Design. —The gate was designed for a static head of 374 feet, assumed to be uniformly applied over the skinplate. The hoist connection was designed for the maximum hoist capacity at the gate. Friction force due to loaded wheels, bushings, seals, etc., were investigated to be certain that the gate would have sufficient weight to close by gravity under a 374-foot differential head without the aid of an externally applied force. Bending stresses in the horizontal and vertical beams were limited to 15, 000 pounds per square inch tension and 14, 000 pounds per square inch compression with the web shear limited to 9, 500 pounds per square inch. Combined stresses in the skinplate due to beam bending and skinplate bending were limited to 20, 000 pounds per square inch. A portion of the skinplate was considered to be an integral part of the upstream beam flanges. The skinplate was also assumed to act as a haunched continuous beam with the supports on the center line of the webs. The stresses in the tracks and wheels were investigated by a method of calculating stresses due to the pressure of one elastic solid upon another. The wheels have bushings with self-lubricating graphite inserts. Bearing pressure on the projected areas of a bushing was limited to 4, 000 pounds per square inch. |