February_1967 — Page 30

new span is erected, to give, on com- pletion. two sets of cables in each span, each set overlapping the other by one span length.

There are several merits in adopt- ing this system: the top and soffit ar- rangement of anchorages permits the use of a cable profile which never crosses that of any other cable in plan or elevation, thus considerably simplifying both the detailing and construction. The first set of cables is sufficient to support the dead load of the leading complete span, allow- ing the staging to be advanced before that span receives its second set of prestressing cables.

The majority of the prestressing cables are approximately 210 ft. in length and are tensioned simultane- ously from both ends to a stress equal to 80 per cent of the maker's guar- anteed ultimate tensile stress (U.T.S.).

To achieve this a force of 218 ton is required on each end, and on an- choring off from this load the an- chorage draw-in with associated negative friction within the duct, im- mediately reduces the maximum stress in the cable to just over 70 per cent U.T.S. Further losses due to relaxation, elastic deformation, creep and shrinkage eventually re- duce the maximum stress in the cables to about 60 per cent U.T.S. and the minimum to just under 50 per cent U.T.S.

A few prestressing cables shorter than 100 ft. are stress- ed from one end only.

The prestressing cables are posi- tioned within the two webs of the spine beam and are equally divided between the two. In both webs, the

cables lie in two distinct horizontal layers, each layer being one set of prestressing cables.

In the three lane section, a total of sixteen 12/0.6 in. diameter cables are used, eight in each set with two layers In the ramps. of four in each web. two layers of three cables at each side of the solid spine make up to the total number of twelve 12/0.6 in. cables.

The two lane section differs slightly from the other two in requiring fourteen 12/0.6 in. cables, or seven in each web. Now seven cables can- not be equally divided between the two webs and to stress seven cables in the one set would therefore pro- duce an undesirable, although tem- porary, imbalance of forces between the two webs.

are

To overcome this, six cables stressed in one set followed by eight cables in the next, or vice versa, final- ly providing the required fourteen cables equally divided between the two webs. During stressing, the out- of-balance force between the two webs is never allowed to be greater than that from one cable.

Articulation

The elevated structure is fixed at only one point in its entire length, at the foot of both western intermediate ramps by Cambridge Street. The re- mainder of the structure is free to move, following changes in tempera- ture and due to the effects of creep, shrinkage and elastic deformation.

While temperature movement must be accommodated throughout the life

of the structure the other factors pro- duce their maximum effect during construction and early life. It is there- fore necessary to incorporate an initial offset to sustain such movements, while maintaining the ability to ac- commodate the temperature changes. This is achieved by offsetting the bearings by an amount determined by computer from the general stressing sequence, the age from casting and the rate of construction.

All bearings are placed paralled to the centre lines of columns with later- al guide plates on approximately every fifth column.

Expansion joints are fitted at all extremities of the structure and be- cause of their simplicity and reliabi- lity, cantilever comb units are used on all carriageway joints while slid- ing cover plates are used in the edge The fingers beams and median strip. of the spheroidal graphite cast iron units are 14 in. long by 1⁄2 in. wide and spaced at 11⁄2 in. centres; to re- duce production complications all the joints are basically identical and can accommodate a 9 in. range of move-

ment.

This maximum, at the western ab- utment, is made up of 4 in. from the mean position due to temperature and humidity changes and I in. resi- dual creep and shrinkage after the units are placed. The expansion joint at the western abutment will not be placed until some time after the last span has been stressed, when it is estimated that approximately half the final movement due to creep, shrink- age and elastic shortending will have taken place.

RESTRAINING PLATE

SHEAR KEY

48

T

O

-TOP

PLATE

PTFE LAYER

BLOCK

ANCHOR BOLT

TOP PLATE

BEALING

BING

-SLIDING PLATE

CLAUR BOLT

NEOPRENE SEAL

---

:

25-

550 TON

ROTAFLON BEARING

550 TON

ROTA

BEARING

ARRANGEMENT OF BEARINGS

POT

BOTTOM (PLATE

·

:

Far East Architect & Builder February, 1967

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