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Figure 13.--Spillway gate structure--Crest, pier and bridge details.
Figure 13. --Spillway gate structure--Crest, pier and bridge details.
7-foot-wide by 25-foot-high inclined openings and a 6-foot by 16-foot opening on the top. Each of these openings is equipped with metal frashracks (sec. 23).
The base of the gate and bridge supporting structure forms the crest of the spillway. The crest has a length of 126 feet and an elevation of 2743.00, and was designed to conform with the lower nappe of a sheet of water flowing over a sharp-crested weir, inclined downstream on a 1 to 1 slope.
(a) Design Considerations.— The gate and bridge structure is divided by keyed and grouted contraction jointsinto three parts--a middle section and two outer sections. These three sections were analyzed separately for base pressures and stability against overturning, but were considered acting as a unit for stability against sliding. The structure is designed to be safe against sliding and overturning, and for resulting pressures on the foundation material to be within safe allowable limits and distributed so that no unequal settlement of the foundation will result. The following loading conditions were assumed:
(1) No water in the reservoir and no uplift.
(2) Reservoir water surface at elevation 2773.0 and no uplift.
(3) Reservoir water surface at elevation 2773.0; uplift ranging uniformly from a full hydrostatic head at upstream edge to zero hydrostatic head at the downstream edge and acting over entire base area.
(4) Reservoir water surface at elevation 2785.0 and gates open; uplift corresponding to a reservoir water surface at elevation 2773.0 at the upstream edge, decreasing uniformly to zero at downstream edge, and acting over entire base area.
(5) Reservoir water surface at elevation 2785.0 with the gates open and no uplift .
(b) Reinforcement Design.— The reinforcement for the cutoff at the upstream edge of the gate structure was designed for a horizontal load in excess of that absorbed in friction between the base of structure and the foundation. In a direction transverse to the direction of flow, reinforcement was provided in the crest structure to take moments in 1-foot strips considered as fixed beams at the piers. The reinforcement in the piers was designed for combinations of loads that would result in most severe stress conditions. These included reactions from gate loadings at various stages of operation together with
a hydrostatic loading on the pier, and a differential loading produced by one gate open and the adjacent gate closed.
Reinforcement for the river outlets and river outlet gate chambers was designed for computed stresses. The reinforcement for the float wells and gate wells was designed for the maximum differential hydrostatic head resulting from any operating condition. The river outlet works trashrack structures are separated from the gate structure and the inlet works by contraction joints, and were designed for embankment and hydrostatic loads. The trashrack structure was designed for an assumed differential hydrostatic head of 20 feet.
18. Chute and Stilling Basin. - The spillway chute has a length of 694 feet and a uniform bottom slope of 0. 010 for the first 474 feet (fig. 9). The bottom slope of the remaining length is that of a 130-foot vertical curve connecting with a 3 to 1 slope at the stilling basin. The chute width increases from 142 feet at the upper end to 266 feet at the stilling basin. The design was based on a maximum discharge of 133,000 second-feet and the unit discharge in the stilling basin was limited to 500 second-feet. The length of chute was determined by economic studies of chutes and stilling basins complying with the stated requirement.
A stilling basin joins the spillway chute at about station 37+75. This structure is of reinforced concrete construction, having a width of 266 feet and a length of 125 feet, and was designed to create a hydraulic jump at all flows. Dentated sills are provided at the upstream and downstream ends of the structure. The floor is 4 feet 6 inches thick at the upstream sill and 24 inches thick at the downstream sill. Walls extending to a maximum height of 57 feet are provided at the outer edge of the floor slab. Wing walls extend from the downstream end of these walls to the right and left, and at right angles to the spillway centerline.
An outlet channel having a width of 266 feet, 2 to 1 side slopes, and a length of about 6,000 feet carries the spillway discharge to the river.
(a) Ftoors.— Floors of the spillway were designed for stability under all anticipated loads. The stilling basin floor was designed for full uplift for normal tailwater elevation 2680.0 combined with the hydraulic jump water surface for maximum spillway discharge. The spillway chute floor and approach channel lining (floor) was reinforced for shrinkage and temperature stresses with 3/4-inch round bars, spaced both ways at 12-inch centers in the top face. The percentages of reinforcement in concrete
of the spillway chute floor and the approach channel floor are 0.20 and 0.24, respectively.
(b) Vails.— For the design of the walls, studies involving sliding, overturning, stresses resulting from various loading conditions, and base pressures were made. The stability and design computations were made by using 1-foot-wide vertical strips.
The inlet walls were designed to retain the embankment of the inlet channel of the river outlets. Uplift was assumed to act over the entire base area and was assumed equal to the hydrostatic head measured from the saturation level at the heel and varying linearly to the hydrostatic head at the toe of the wall. The following loading conditions were assumed:
(1) Embankment dry with no water in the inlet channel.
(2) Embankment saturated with water in the channel to elevation 2710.0 (invert of river outlets).
Studies indicated the need for heel cutoffs on several wall panels to prevent sliding under the most severe loading condition. These cutoffs were designed for a horizontal load in excess of an assumed load absorbed by friction between the base of the wall and the foundation material.
Cantilever walls extend between spillway stations 30+80.97 and 37+75.00, and were divided into panels 30 feet long except the first panel which was 24 feet long. Because drainage was provided on top of the heel slab of the walls, these walls were designed for dry fill conditions. The base pressures were limited to 6,000 pounds per square foot. The walls were designed for such height that a 5-foot freeboard will exist under maximum discharge conditions.
Counterfort walls extend downstream from spillway station 36+75. These were analyzed for the same stability and base pressure criteria as the cantilever walls. The uplift assumption used for the inlet walls was used in this study. The following loading conditions were assumed:
(1) Backfill dry and no water in chute.
(2) Backfill saturated to normal tailwater elevation 2680; water in chute to normal tailwater elevation.
(3) Backfill saturated to maximum tailwater elevation 2700.6; jump profile for maximum discharge in stilling basin or water surface profile in chute.
(4) Backfill saturated to a tailwater at elevation 2698.5 corresponding to reservoir water surface elevation 2773; normal tailwater elevation 2680.0 in basin. An increase of 25 percent in stresses was allowed for this condition.
(5) Left wall was investigated for a condition of surface runoff saturation of backfill to elevation 2705; normal tailwater at elevation 2680.
Because of large horizontal forces, the walls were designed with heel cutoffs . To reduce the high stresses in some reinforcement bars caused by runoff saturation of backfill (condition (5)) a drain was placed between the counterforts immediately upstream from the stilling basin.
(c) Droins. — Drains were provided under or near the spillway structures to relieve uplift pressures and prevent saturation and/or erosion of earth materials. Paved gutters, sewer-pipe drains, and sand and gravel materials were used for drainage facilities .
A paved gutter was provided along the left side of the spillway chute and stilling basin at the foot of excavated slopes. A system of transverse and longitudinal sewerpipe drains was provided under the stilling basin floor. This system is vented and discharges through outlets on the downstream side of the upstream dentates. A longitudinal system was provided for the spillway chute floor, and drains with slotted dome strainers were constructed behind the chute walls. These drains discharge into the chute upstream from the stilling basin.
A 24-inch sand and gravel blanket was placed on top of the heel slab of the abutment walls. Drainage is collected by an embedded sewer-pipe drain located near the toe of the heel slab and is discharged into the stilling basin.
19. 42- by 30-Foot Radial Gates. - A 42- by 30-foot radial gate (fig. 14) is installed in each of three openings of the spillway structure. (See sec. 15.) The gates rest on bottom sills located 4.95 feet downstream from the axis of the spillway crest. Stoplog slots in the piers immediately upstream from each radial gate permit bulkheading when servicing or repairing .the gate. Each gate is fully enclosed, of riveted construction, and consists of three main horizontal girders which connect to an arm diaphragm at each end of the leaf. The framework is interlaced with horizontal and vertical stiffening members, and upstream and downstream skinplates are fastened to the framework. The upstream skinplate is formed to a circular arc of 40-foot radius with its center point at the trunnion pin. The entire gate leaf is attached to two radial arms, one at each end, which carry the hydraulic load on the gate to the trunnion pins and pedestals. Each gate arm is equipped with a grease-lubricated bronze bushing. The base of the cast steel pedestal is permanently embedded in babbitt metal.
Each gate is normally raised and lowered by means of a motor-driven hoist with the assistance of a 163,000-pound counterweight. In an emergency flood stage, the gate will be automatically raised by the float system without the use of the motor-driven hoists .
End guide shoes at each gate control lateral movement of the gate. A 28-inchdiameter manhole located near the bottom of the downstream skinplate permits access to the interior of the gate. Built-in ladders and manholes in the interior girders and diaphragm provide access to different areas inside the skinplates . Each gate is provided with brass-clad rubber seals on each side and a rubber seal on the bottom.
(a) Design Stresses.— Each gate was designed for a hydrostatic head of 30.53 feet at the bottom sill. Fiber stresses in the girders and beams were limited to 16,000 pounds per square inch, and the average web shear was limited to 12,000 pounds per square inch. The combined longitudinal and transverse skinplate stresses were limited to 18,000 pounds per square inch. Unit stresses in the gate arms were limited to those permitted by the//r ratio, and allowable stresses for axial and bending loads were 13,000 and 18,000 pounds per square inch, respectively. A bearing pressure of 4,500 pounds per square inch was allowed on the bearing pin.
20. Radial Gate Controls. - A motor-driven hoist system and an automatic hydraulic system are provided for control of each radial gate. The motor-driven system (fig. 15) is normally used for regulation when the reservoir water surface is between elevation 2743.0 and 2773.0. When the reservoir water surface reaches elevation 2773.0, the automatic hydraulic system will function for gate control. Provision is made in this system so that for each additional rise of 1 foot in water surface above elevation 2773.0, the gate will rise 5 feet until the maximum open position with the bottom of the gates at elevation 2773.0 is reached. This method of operation is a safety feature for automatically discharging floods which could not be held in the reservoir. In an emergency, the combined effort of both gate control systems may be used for raising or lowering the gates.
Two floats, a counterweight, and a hoist assembly connected with wire ropes, constitute the basic components of the hydraulic control system. Each float operates in a