The crew then shifted one complete quadrant and erected two more oppos- ing pairs. With these four pairs of cables attached, the loads were effectively balanced and the erection of the re- maining cables proceeded with the crews working on opposite sides but not necessarily setting opposing pairs simultaneously.
With the cables all in place and adjusted to a uniform drape, the ten- sion ring was decentered by lowering the shoring jacks until the tension ring hung free on the cables. The drape was again checked and necessary adjust- ments were made by turning the adjust- ing nuts inside the tension ring. The cables were now running almost straight between the tension and compression ring.
As the load, in the form of concrete ribs, penthouse roof and gypsum roof deck, was applied to the cables, they were elongated under the stress. The total elongation was of the order of 6 in. At the same time, the stress in the compression ring shortened the circum- ferential length of the ring. Under full load the diameter decreased 1 in.
Parabolic curve
Both the lengthening of the cables and the smaller diameter of the com- pression ring lowered the position of the free-hanging tension ring. However, as loads were applied to the cables, the drape of these changed from an almost straight line between the tension and compression rings to a pronounced pa- rabolic curve. The overall effect made the tension ring rise up more than 30 in.
The radial concrete ribs that rest on top of the cables are I-shaped with a lower flange that matches the final shape of the cable. The upper flange is a series of straight lines. Flanges vary in width from 12 to 14 in. and the web is 2-1⁄2 in. wide. In depth, the ribs range from about 30 in. deep at the tension ring to a maximum of 96 in. and back to 54 in. at the compression ring.
Each rib has a continuous slot in the centre of the bottom flange. Semi- circular steel cable shoes are embedded in the slot. The cable shoe rides on 6 in. lengths of Teflon tubing placed around the cable.
Each of the radial ribs spanning from tension to compression was pre- cast in two 90 ft. long sections together with the projecting portions of the diaphragms.
To erect the inside 90 ft. long ribs, a mobile tower crane was placed in the centre of the arena with its tower pok- ing up through the tension ring. One of the roof cables was then removed to
Plan
Tension ring
Concrete ribs.
Cable
Roof details
make room for hoisting up the rib sec- tions. As they were erected, adjacent pairs of ribs were joined together by cadwelding the projecting reinforcing bars of the diaphragms. Only every second joint in the annular ring of diaphragms was tied together at this time, allowing for additional deforma- tions of the roof as more load was added.
When all the inside rib sections had been erected, the crane was moved to the outside of the arena. It then raised the outside 90 ft. section of the ribs over the compression ring and swung them into place on the cables. The same connection procedure followed, with key connections being omitted until all the roof dead loads were in place. At this point, a final adjustment of the cable length and the shape of curvature of the ribs was made.
With the ribs in place, the penthouse roof structure was erected and the gypsum roof poured. Finally, the re- maining diaphragm connections, and the connection between inner and outer sections of ribs and ribs to ten- sion and compression rings were cad- welded and poured. The delay in making these connections allowed all the initial roof load to be taken by the
Concrete compression ring
cables while the live load on the roof was resisted by the radial ribs, acting as an inverted dome.
The analysis of the cables was done manually. Since the loads for the cable system are symmetrical, the outer ring is under uniform compression and the inner ring under uniform tension. The radial ribs were analyzed, by computer, as a space frame using a variety of loading conditions.
The drainage system for the roof is highly unusual. Because of the inverted shape, no natural runoff is possible. Instead, the rain water is collected at the outer wall of the penthouse and fed by gravity to pumps located inside the penthouse on top of the tension ring. From here it is pumped back up the sloping roof to the drainline at the compression ring. In case of pump fai- lure, the roof is designed to store up to 160,000 gallons of water. Any more rain than that will be dumped in over- flow on to the arena floor below.
The arena, designed for indoor sports meetings, ice hockey and cir- cuses, has a seating capacity of up to 15,000. It adjoins an underground ex- hibition hall which leads to a 50,000- capacity football stadium. The entire complex cost some US$30 million.
Far East BUILDER, May 1969
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