June_1967 — Page 18

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while that for the unevenly distributed live load, wind load and seismic load of the whole structure becomes more complicated. For the sake of good appearance arches of constant depth are used. The width is 60 cm for T's and 30 cm for both TX's and TY'S. The actual bar area of any section is correctly furnished by vary- ing the number and size of round bars as the stress analysis requires.

Rebar arches are shop-fabricated. The main arch T's are in six sections while the secondary TX's and TY's are in two to facilitate handling and truck travel. They are assembled and welded on site.

Girder Wall

The loading of the transit shed is wholly supported by the caissons so that no consideration of soil bearing capacity is necessary. The girder wall section is designed as deep beams of 45 metre and 50 metre spans. Roof loads and truck loading account for only a small portion of the loading on the girder wall; a substantial part comes from its own weight.

A girder wall section is composed of three reinforced concrete beams with reinforced concrete wall in fill- ing. BI in Fig. 8 is a parabola whose upper boundary coincides with the bottom chord projection of the main diagonal arches. The wall ends over the caisson supports are enlarged to

make them into square

gular

or rectan-

section columns. From column to column run ten stiffener columns at intervals of about 4.5 metres. The function of Bl and B3 in the girder wall section is like that of the top and bottom flanges in I-beams, whereas B2 takes up the horizonal thrust from the roof arches.

All three beams are called upon to resist wind and earthquake lateral forces, B2 being stressed the most. The 3 metre canopy together with the B2 beams are designed to resist such lateral forces.

The girder wall designed as a deep beam acts as one only when it is constructed and gets enough strength from concrete setting. Before this stage the original filling ground has to be relied upon to sustain the dead weight of the wall proper.

The design of the girder wall sec- tion investigates the wall portion up to the intermediate flange as a partially finished beam. When the concrete of the beam and canopy reaches an age of 14 days, it is able to take its own weight and releases the ground of its pressure. The pressure

so created amounts to 900 p.s.f.. assuming that anything from the beam and canopy down is fresh concrete.

The upper part of the wall above the beam and canopy creates a bearing on the ground much less than the 900 p.s.f.

Expansion joints are as shown in Fig. 9. Square keys are provided to

resist horizontal shears from lateral forces.

In Fig. 10 are shown the tension rods on top of B2's of compartment 2 and 4; they are to take the horizon- tal thrust of the roof arches of the compartments because the B2's are no longer continuous at the expansion joints.

The roof consists of wood sheath- ing, built-up asphalt roofing and aluminium sheets. J-bolts are used to fasten the sheathing. The built-up roofing is generally a three-ply con- struction but on the flatter part at the top, in a valley or a basin around the top of leader pipes, a five-ply con- struction is adopted. MBV-and H14- treated aluminium sheets 3 ft. square are folded on all four sides to allow cleats to be nailed on to the sheathing for safety and weather-tightness.

No backwater flow on the roof sur- face is possible, because the cross- flow folding seams all face the lowest- level strainers. In order to prevent capillarity by accumulated dust in the sheet folds, Thiotight, a Folex poly- sulfide-base sealant, is used to caulk every folding seam.

The floor is a 5 cm asphalt pene- tration pavement with a base 25 cm thick,

There are 57 chain-operated steel rolling doors, 4 m. by 4 m. and 84 steel windows, 2 m. x 4.5 m., on all four sides of the transit shed.

Top left: Arch pin setting

at support

Left: Reinforcement

placing

Top right: Main arch

centre pin

Right: Tension rods with

turnbuckles

Far East Architect & Builder June, 1967