49. Intake Structure. After a visual inspection of the rock face and topography in the vicinity of the inlet portal, it was decided to study the possibility of using a vertical towertype structure upstream and independent of the rock face. Such a structure was currently being considered for Trinity Dam of the Central Valley project, Calif. There are some advantages in using a vertical intake in lieu of an inclined structure placed on the abutment slope. Problems incurred from trash accumulation on the racks, wave action on foundation rock, and heavy overbreak in foundation excavation can largely be eliminated by using a vertical structure. Figure 29 shows the adopted structure. Accordingly, the general shape and proportions of the intake structure were made very similar to the corresponding structure at Trinity Dam. Many of the conclusions, results, coefficients, and studies prepared for Trinity were incorporated in this design analysis and are summarized herein.

Critical loading conditions were found to be those caused by earthquake, and the entire structure was checked for stability and maximum base pressures for a 0.05 gravity horizontal earthquake acceleration in any direction for both dry and submerged conditions. Since earthquake loads are inertial forces, the virtual mass of the structure when inundated in the reservoir was required. A model was constructed and the virtual mass determined by swinging the scale model as a pendulum in the air and in water and comparing the corresponding periods of swing. Details of these earthquake action studies together with magnitude and manner of application to the structure are given in three Bureau memoranda. 7/

The structure itself is made up of three portions, each with a different function; the trashrack structure, above elevation 5882. 5; the shaft, between elevations 5766.0 and 5882. 5; and the base, below elevation 5766.0. The loading conditions varied with each portion, depending upon its function as described below.

The structure above elevation 5882. 5 supports the trashracks and the steel bulkhead gate when it is stored in its normal raised position. This portion of the structure was designed to resist a 0.05 gravity horizontal earthquake load in any direction for both dry and submerged conditions. This load was considerably larger than the 20-foot hydrostatic head differential normally considered, and checked against here between the inside and outside portions of the structure.

The shaft section, between elevations 5766.0 and 5882. 5, supports the trashrack section and serves as the intake and vertical flow passage from the sill to the base of the structure. It was designed to support the trashrack section and to resist the full external pressure of the reservoir when the reservoir water surface is at the normal water surface elevation 6085. 0 (which is also the spillway crest elevation), and the outlet works is empty for inspection. It was also designed to resist the stresses induced by a 0.05 gravity horizontal earthquake load acting in any direction.

The structure below elevation 5766.0 serves as a base for the tower. In its initial stage of construction, it acted as the diversion intake for the outlet works; and it incorporates in its final stage an elbow and transition from the vertical shaft to the horizontal tunnel. The first-stage concrete was designed to withstand 300 feet of external water pressure and checked for fill loads during diversion closure. The second-stage concrete was designed to resist the entire external pressure with the reservoir filled to the normal water surface elevation 6085.0 and the interior of the outlet works empty. The joint between the first- and second-stage construction was pressure grouted to insure watertightness when the bulkhead gate is seated in position and the tunnel is unwatered for inspection or maintenance.

7/Intra-Bureau memorandum to Chief, Dams Branch, from Moody, W. T., Phillips,
H. B., and Allen, I. E., subject, "Virtual Mass of Intake Structure Under Earthquake
Action, Trinity Dam, Central Valley Project, " April 23, 1957 (unpublished).
Intra-Bureau memorandum to Chief, Dams Branch, from Crawford, C. C. and Donsky,
B., subject, "Earthquake Design, Loads for Intake Tower, Trinity Dam, Central Valley
Project, " March 13, 1957 (unpublished).

Intra-Bureau memorandum to Chief, Dams Branch, from Moody, W. T., Phillips, H. B., and Allen, I. E., subject, "Analytical and Experimental Studies of Stress in Intake Structure, Trinity Dam, Central Valley Project, " February 7, 1958 (unpublished).

The design incorporated a load accumulation and moment distribution method sometimes used for tall buildings. 8/

The following loads were added together and multiplied by the earthquake factor 0.05 gravity:

(1) The dry weight of the structure, including the trashrack, above elevation 5882.5.

(2) One and one-half times the net volume of water in a cylinder circumscribed around the tower.

Moments at the crest elevation 5882. 5 were computed for the above uniform horizontal loads applied midway between the top of the tower and the base. This total force was represented by a loading diagram, triangular in shape, with zero intensity at the base and maximum at the top, with a total moment equal to the uniform load moment. From this loading diagram, the concentrated load was determined at each of the horizontal struts and the shear loads were computed. Because of structural symmetry, only one-half of the structure was analyzed, using one-half of the concentrated shear loads at each strut.

The earthquake force was assumed to act directly into the nose of one column; therefore, the center strut had an inflection point at its center enabling the use of a pinned joint (by multiplying the beam stiffness by 1. 5).

Stiffness factors were computed for each frame member entering a joint and the load was distributed by percentage. With the hexagon shape of the frame, it was necessary to determine the stiffness of one strut on a diagonal. The stiffness factors of the members were considered as moments in the columns only and distributed through the structure.

With the cylindrical shape of the shaft, the external hydrostatic pressures with the shaft empty were easily taken by the concrete in the structure from elevation 5766.0 to elevation 5882. 5 without exceeding allowable concrete stresses. The earthquake loading was critical for design of reinforcement in the buttresses. Moments due to earthquake were determined at desired elevations, and the shaft and buttresses were assumed to act as a unit to resist them. Horizontal stiffeners were constructed between buttresses to prevent buckling under load. Beggs deformeter stress analyses9/ of several transverse nonconcentric circular sections were prepared, and the reinforcement requirement was verified by these analyses.

50. Transition Section at Upstream Portal. The conduit between stations 4+97 and 5+34 (fig. 28) is reinforced to withstand the external hydrostatic load of the full reservoir and was checked against a vertical rock load of 90 feet and a horizontal load equal to onethird the vertical load. Moments, thrust, and shear coefficients for conduits as established by Beggs deformeter stress analysis9/ were used for the design.

51. Tunnel Upstream of Gate Structure. The pressure tunnel upstream of the gate structure (fig. 28), except for a slightly larger section at the portal, was excavated to a minimum diameter of 23 feet 1 inch and lined with concrete 2 feet 2 inches or more thick to provide an 18-foot 9-inch diameter tunnel. Layers of shale encountered in excavation made it necessary to use structural-steel supports for much of the length. The concrete lining was reinforced with varying amounts of steel reinforcement throughout the length of the tunnel. The loadings assumed to act on the tunnel lining, for determining the reinforcement required, were as follows: From station 5+34 to station 10+08. 5 the external hydrostatic pressure was assumed to vary linearly from full head at the upper end to zero at the gate structure. At the gate structure, the internal water pressure was assumed equal to the full reservoir head and the pressure was assumed to decrease to zero at the upstream portal. The lining was reinforced to resist the external load at the upstream portal and the internal load at the gate structure and was arbitrarily reduced between these points. The section adjacent to the gate structure was checked against a water-hammer load equivalent to a reservoir head to elevation 6132.0. This did not increase the stress in the steel beyond the one-third allowable.

8/Grinter, L. E., "Theory of Modern Steel Structures, " vol. n, p. 137, ASCE Trans., vol. 99, 1934. p. 624.

9/"Beggs Deformeter Stress Analysis of Single-Barrel Conduits, " Engineering Monograph Bo. 14, Bureau of Reclamation, April 1952.

The rock surrounding the tunnel was assumed to take none of the load and the concrete lining was assumed to take no tensile stress.

52. Gate Structure and Shaft. For structural design purposes, the gate structure and shaft (fig. 30) was divided into a gate section, a gate chamber, and a shaft. The gate section includes the first- and second-stage concrete between stations 10+04. 5 and 10+74. 5 below elevation 5766. 0. The gate chamber is that portion of the structure between elevations 5766. 0 and 5778. 0. The shaft is above elevation 5778.0 and extends to the crest of the dam.

The first-stage concrete in the gate section served as a portion of the lining for the diversion tunnel and as the outside form for the second-stage concrete. The hoop steel reinforcement between stations 10+04. 5 and 10+74. 5 was designed for the bursting pressures in the joint between the first- and second-stage concrete induced by a water-hammer head to elevation 6132. 0, assuming no supporting force on the outside of the concrete ring and allowing a one-third overstress in the steel. The full hydrostatic force was assumed to act on the outside surface of the concrete section. The hoop reinforcement was designed to take the outward loading induced by the second-stage concrete plug when the 6- by 13-foot fixed-wheel gate is closed and the gate and plug are subjected to a hydrostatic pressure due to a reservoir at maximum water surface elevation 6101. 5. The full reservoir head was assumed to have penetrated the joint between the first- and secondstage concrete for this loading condition. The reinforcement was designed to resist the tension produced by the maximum water-hammer pressure on an area determined by subtracting the area of a circle averaging 17 feet in diameter from the area of a circle averaging 24 feet in diameter. The longitudinal reinforcement was designed to resist the force of the second-stage concrete in bearing against the annual face of the first-stage concrete, assuming the maximum thrust evenly distributed over the shear resistant rings. This load produced a bearing pressure of 133 pounds per square inch. Reinforcement in the stepped surfaces of the first-stage concrete was designed to take the downstream thrust from the second-stage concrete plug, assuming the 6- by 13-foot gate closed and the reservoir water surface elevation 6101. 5, with a maximum tensile stress of 20, 000 pounds per square inch.

The hoop reinforcement in the second-stage concrete in the gate section was designed for the external and internal loads acting simultaneously on a section normal to the outlet works centerline. The external load was the pressure which, acting in the joint between the first- and second-stage concrete, would not overstress the reinforcement in the firststage concrete. The first-stage concrete was assumed cracked and receiving no support from the surrounding rock. The internal load was taken as the maximum water-hammer pressure. The design was checked by using the results obtained from the photoelastic stress analysis made for Trinity Dam.

The concrete and reinforcement in the blockout for the gate track installation were designed for the gate closed with a full reservoir head against it. An evaluation of the stress and distribution of the track loads to the supporting concrete was made and checked by comparing with the results of the photoelastic studies prepared for Trinity Dam.

The gate chamber lining was reinforced to resist full internal bursting pressures, assuming the shaft and chamber full of water to the dam crest, elevation 6108. 0, and no counteracting external load. This condition was considered extreme, and a steel stress of 30, 000 pounds per square inch was allowed in the gate chamber and shaft. Extra shear steel was required just above the floor elevation 5766.0.

The shaft was designed against inside loads induced by maximum water-hammer surges and assuming no support from surrounding rock and for a dry shaft with hydrostatic pressures acting on the outside face. A tensile stress of 30, 000 pounds per square inch was allowed for this unusual condition.

A working platform at elevation 6080.0 was designed as a grating to permit surge above that elevation, and an 18-inch-diameter air vent extending to elevation 6132.0 was provided back of the gate. The horizontal reinforcement in the lining of the shaft above elevation 6080.0, the location of the gate erection deck, was designed using normal stresses. The gate erection deck was designed for a downward live load resulting from a gate stem pull of 144, 000 pounds. The foundation structure for the stem storage rack has four concrete footings with nominal reinforcement.

53. Tunnel Downstream of Gate Structure. The thickness of the tunnel lining downstream of the gate structure (fig 28) was made equal to the thickness of the tunnel lining upstream of this structure (see sec. 51). With a circular section, no internal pressures, and uniform external loads, no reinforcement was theoretically required; however, to protect against uncertain loading conditions, reinforcement was specified. This amount

of reinforcement was reduced with distance from the gate structure to what was considered as minimum between stations 13+00 and 19+50. From this latter station to the outlet portal, the reinforcement was increased to resist at the portal a load produced by a 50-foot depth of earth material weighing 130 pounds per cubic foot. One-third overstress in the steel was permitted for this condition; however, a 40-foot depth of earth material weighing 120 pounds per cubic foot did not produce a stress which exceeded the allowable.

54. Diversion Channel. The reinforced concrete walls and base slab of the diversion channel were designed as a monolithic structure acted upon by vertical and horizontal loads. Horizontal forces due to backfill were computed on the basis of equivalent fluid load distributions. Sections at four stations were analyzed according to the following typical conditions:

(1) Chute as built with dry compacted backfill in place, no water in channel. Weight of dry compacted backfill 120 pounds per cubic foot (equivalent fluid load 40 pounds per cubic foot).

(2) Same as condition (1) with surcharge loading of 400 pounds per square foot (133 pounds per square foot uniform horizontal load on walls). A one-third increase in unit stresses was allowed for this unusual loading.

(3) Chute as built with saturated compacted backfill to elevation 5723. 0. Weight of saturated compacted backfill 135 pounds per cubic foot (equivalent fluid load 75 pounds per cubic foot). Uplift not considered.

(4) Chute filled with water to elevation 5737.0 and no backfill above elevation 5728.0.

Maximum moments, thrusts, and shears were computed for these conditions, and the required reinforcement to resist the controlling stresses was stipulated. The floor slab was designed for uniform and triangular base pressures.

55. Steel Pipe Manifold Anchorage and Valve House Substructure. The pipe branch and bifurcation were encased in concrete to provide a protective covering and serve as an anchorage to resist the unbalanced hydrostatic thrusts at the bends and branch. The encasement also provides stability to resist uplift forces of tailwater seepage under the anchor block. To further assist in stabilizing the structure, 8-foot-long anchor bars at 5-foot centers were provided to tie the encasement to the foundation rock. Reinforcement was added to sustain external loadings from backfill, and shear keys at 10-foot centers were provided in the joint between the first- and second-stage concrete of the floor and sidewalls. This anchor block also supplied, in part, a base for the valve house.

56. Control House. The outlet works control house for the 30-inch ring-follower gate and hollow-jet valve was supported on the stilling basin wall and counterforts. The 30-inch ring-follower gate and steel pipe were embedded in concrete which was reinforced to withstand full internal pressures plus water hammer with normal allowable stresses. In addition to normal design loads, the walls and brackets were designed to support a trolley beam for a 3-ton hoist which was provided to handle the 30-inch hollow-jet valve and other equipment.

57. Valve House. The house for the controls of the 72-inch hollow-jet valves and ringfollower guard gates is a reinforced concrete structure (fig. 32). The beams and columns, placed parallel to the centerline of the outlet works, are designed to support the main loads. The transverse beams and roof slab transfer the loads to the main frame members. Short beams were required between the transverse beams in which were embedded anchors for trolley beams for two 10-ton hoists for handling the hollow-jet valves. Snatch anchors were also embedded in the walls to facilitate use of handling equipment and reinforcement v as added in areas of stress concentration. The roof slab was designed for dead loads plus a live load of 50 pounds per square foot. Normal working stresses were used for concrete and reinforcement for all members.