supporting the cantilever beams were designed to rest on spread footings. These footings extend into rock a minimum of 12 inches. From each of these footings, four 1-3/8-inch reinforcing bars were embedded into rock a minimum distance of 8 feet for the two bars nearest the canyon rim and a minimum of 15 feet for the other two bars. These bars develop the maximum strength in the bedrock by "stitching" through possible fissures in the rock.

(b) Structural Floors, Stairwell, and Shaft.-The design of the structural floor was based on dead loads and the following live loads:

100 p.s.f. for rotunda, corridors and lobby, and concession and storage room.

50 p.s.f. for office, toilet rooms, first aid room, guides room, and stairwell.

Floors in the audiovisual room and in the basement were placed on grade.

The combined stairwell and elevator shaft were designed for dead and live loads, the weight of the elevator equipment, that portion of the room framing into the shaft, and seismic loadings.

(c) Design Codes and Data.-The following data and codes were used in designing the visitor center substructure:

(1) "Reinforced Concrete Design Data," Engineering Monograph No. 10, Bureau of Reclamation.

(2) "Moments and Reactions for Rectangular Plates," Engineering Monograph No. 27, Bureau of Reclamation.

(3) "Design Standards No. 9, Buildings," Bureau of Reclamation.

(4) "Design Standards No. 11, Housing and Community Facilities," Bureau of Reclamation.

(5) "Building Code Requirements for Reinforced Concrete," American Concrete Institute (ACI 318-56).

(6) "Influence Lines for Horizontally Curved Fixed-End Beams of Circular-Arc Plan," University of Missouri, Bulletin No. 35.

(d) Allowable Unit Stresses.-The visitor center substructure was designed for concrete having an

ultimate compressive strength of 3,000 pounds per square inch at 28 days, and an allowable working stress in the reinforcement of 20,000 pounds per square inch in flexure and 16,000 pounds per square inch in web reinforcement.

SUPERSTRUCTURE

BUILDING

120. DESIGN. The building is a steel frame structure of welded and bolted construction, having in plan a 133by 60-foot auditorium and utility wing, and an attached 92-foot-diameter rotunda (this diameter includes the exterior concrete panels). The auditorium and utility wing is a beam and column semirigid frame structure designed for vertical and lateral loads as noted below. Three intersecting welded plate roof girders, with clear spans of 81 feet 6 inches, form the main framework for the rotunda and were designed to carry all the vertical loads. The lateral loads are carried through a horizontal truss system into the three large concrete piers.

(a) Loads.-The auditorium and utility wing was designed for a roof dead load of 20 pounds per square foot and a roof live load of 30 pounds per square foot. The rotunda was designed for a live load of 35 pounds per square foot and a dead load of 35 pounds per square foot plus the weight of the overhanging precast concrete panels. Windloads used in design were a compression load of 20 pounds per square foot plus a tension of 10 pounds per square foot acting on exposed vertical surfaces; earthquake forces are 10 percent of gravity loads.

The central system consists of air-handling units with water-cooling coils, electric heaters, filters, outdoor and return air connections, electric reheaters, supply air ducts, outlets, ventilating ceiling, fans, and hermetic refrigeration machine. Air conditioning of the visitor center was provided for comfort and protection of personnel and visitors, the distribution and removal of heat, the relief of dampness, and the disposal of contaminated air.

The heating load design temperatures were an outside ambient temperature of 0° F. and an inside temperature of 72° F. The cooling load design conditions were an outside ambient temperature of 95° F. dry bulb and 65°F. wet bulb, and an inside temperature of 75° F. The heating and cooling loads were calculated in accordance with the methods outlined by the American Society of Heating, Refrigeration, and Air-Conditioning Engineers.1

The system is designed to automatically provide required ventilation, remove contaminated air, and maintain nominal room temperatures of 75°F. for cooling and 72°F. for heating.

ENTRANCE

122. TOP OF DAM AND STRUCTURE. The entrance structure is located in block 26 at the top of the dam. A concrete slab was placed on top of the existing top of dam and parapets were placed with the slab. Epoxy-bonded concrete was used to connect the walkway slab to the existing walkways and top of dam. The entrance structure consists of canopy and roof for sheltered entrance to the tunnel, and end-of-dam parapets. A 4- by 4-foot shaft was provided to connect the pipe chase to the existing service adit in the top of the dam. The exposed canyon wall around the portal entrance was rock bolted prior to tunnel excavation. One row of 3/4-inch expansion-type anchor bolts at 5-foot centers was used to stabilize the rock and to prevent slabbing. The details and reinforcement of entrance structure are shown on figures 236 and 237.

(a) Structural Design.-The design was based on concrete having a compressive strength of 3,000 pounds per square inch at 28 days. The allowable working stresses are shown on figure 71.

Canopy and roof were designed for a live load of 40 pounds per square inch. Sidewalks and slabs were reinforced to control cracking--5/8-inch bars at 9-inch spacing each way were used. Parapets were designed for a load of 150 pounds per lineal foot acting at the top of the

parapet. Horizontal temperature reinforcement (5/8-inch) bars at 6-inch centers) was used.

123. ELEVATOR SHAFT AND TUNNEL. An 8by 8-foot 6-inch lined tunnel from the end of the dam in block 26 and a 10-foot 6-inch by 20-foot 4-inch lined elevator shaft connect the top of the dam with the visitor center complex. A pipe chase below the floor of the tunnel was provided to carry utility lines for the visitor center. Minimum thickness of lining for the tunnel and shaft was 9 inches. The details and reinforcement of elevator shaft and tunnel are shown on figures 238 and 239.

(a) Structural Design.-The design was based on concrete having a compressive strength of 3,000 pounds per square inch at 28 days. The allowable working stresses are shown on figure 71.

The elevator shaft was reinforced using 3/4-inch bars at 12-inch centers horizontally and 3/4-inch bars at 24-inch center vertically. One and one-eighth-inch anchor bars were installed on the centerline of the shaft on each side to stabilize the concrete lining. The anchor bars were spaced at 5-foot centers and embedded 8 feet into the rock. Three rows of 1-inch rock bolts spaced at 5-foot centers were installed at the junction of the elevator shaft and tunnel lobby. The rock bolts were embedded 10 feet into the rock.

The roof of the tunnel was designed as a simply supported beam with a 3-foot rock load. The tunnel was reinforced using 3/4-inch bars at 12-inch centers (transverse) and 5/8-inch bars at 18-inch centers (longitudinal). Rock bolts were installed in the roof of the tunnel as required.

124. RETAINING WALLS. The retaining walls were designed as gravity structures and located to conform to the existing rock surface along the canyon side of the visitor center. The base of each wall has a maximum slope of 5 to 1 with a 24-inch minimum bench. A 5-foot-wide walkway and a parapet were provided at the top of the retaining walls. The sidewalk has a minimum thickness of 18 inches. The parapet is 12 inches wide and 3 feet 4 inches high. The details of the retaining walls are shown on figure 240.

(a) Structural Design.-The design was based on concrete having a compressive strength of 3,000 pounds per square inch at 28 days. The allowable working stresses are shown on figure 71.

Figure 237.-Visitor center-Reinforcement for top of dam and entrance structure.