February_1967 — Page 28

Preparation of moulds and reinforcement for a two-lane unit in the contractor's casting factory

ture is constructed with precast con- crete units of uniform cross section and separated by joints of in situ con- crete. The transition section between the two and three lane parts of the structure is formed with in situ con- crete, and includes four entry and exit ramps which provide intermedi- ate connexions to ground level. The ramps carry a single traffic lane and because of this narrower width the spine beam is solid except for an 8 in. diameter hole carrying a drainage pipe.

Dual carriageways are supported on identical twin structures linked to- gether by an in situ concrete median which also carries the lighting col- umns and has provision for a double- sided safety barrier if considered necessary in the future.

In situ concrete edge beams pro- vide a clean and continuous line along the outer edge to express the structural and aesthetic continuity of the finished structure; they also serve as emergency walkways with hand- rails and safety barriers.

The structural anchorages are built into the abutments of the western in- termediate ramps and are ap- proximately at the centre of the main structure. Comb type expansion joints are fitted at the remaining abutments. all of which, including the anchorage abutments, are of cellular construc- tion.

The superstructure is supported by sliding pot bearings except at the an- chorage abutments where rubber pads are used. The reinforced concrete columns are of rectangular solid sec- tion, tapering on the longitudinal sides and vertical on the transverse faces: they are monolithic with the pile caps and each one is founded on two bored cylinders which carry the loads down to the bearing strata.

The ramp columns are proportion-

ally smaller and are designed as rock- er columns with pot bearings at both top and bottom.

Erection

The design of the superstructure is largely influenced by the method of construction adopted, and because of the large number of basically identic- al spans the erection and stressing procedures follow a sequential order based on a module of one span length. The system adopted is founded on

ф

STAGING DESIGNED TO PERMIT LONGITUDINAL MOVEMENT

GROUND LEVEL ACCESS

both theoretical and practical con- siderations; staging is theoretically necessary for only one span length at a time, and the units are positioned with the leading end cantilevering be- yond the supporting column by one quarter of the span. (In practice, to ensure continuity of work, further scaffolding is needed to allow erec- tion, jointing and stressing operations to proceed simultaneously in succes- sive spans.)

This cantilever form of construc- tion relieves the midspan dead load bending moment and allows the struc- tural depth and number of prestress- It en- ing cables to be minimized. ables full structural continuity to be maintained over the supports and conveniently allows the prestressing cable anchorages to be positioned near the point of contraflexure; it al- so simplifies the setting up of the be bearings by allowing them to mounted on one precast unit seated centrally over the column.

Superstructure

The prestressed concrete elevated superstructure is designed as a beam, fully continuous over all internal sup- ports throughout the length of 3,232

ft. 6 in.

A specific method of construction and erection of the units was outlined in the tender drawings, and contrac- tors were invited to submit alterna- tive forms for consideration. No al- ternatives were offered among the seven tenders and the structure is

substantially

the constructed

by methods envisaged by the consulting engineers.

The normal 105 ft. span consists

STAGING ATTACHED TO STRUCTURE FOR

LONGITUDINAL CONTINUITY

STAGE A ERECT STAGING

+

STAGE B PLACE UNITS

MA

THIS

$OFFIT STRESSING

SPAN HAS FIRST STAGE THIS

PRESTRESS

SPAN MAS SECOND

PRESTRESS

STAGE

D

STAGE C - FIRST

STAGE

H

STAGE D- ADVANCE

TOP STRESSING

PRESTRESSING

THIS

SPAN SELF SUPPORTING

FOR

DEAD LOAD ONLY

+

STAGING & PLACE

UNITS

THIS SPAN KAS

HALF

THIS

SPAN

PRESTRESS

NOW HAS FULL PRESTRESS

с

STAGE E

- SECOND

46

STAGE PRESTRESSING

Far East Architect & Builder February, 1967