January_1971 — Page 47

or by mobile cranes. The former, al- though slow to use, could be assem- bled on site by those contractors who did not have mobile cranes of suffi- cient capacity for lifting the beams. With the exception of Lai Chi Kok Bridge, all beams were lifted and placed directly into position.

Erection of beams at the Canal Road flyover required detailed pre- planning by the contractor to ensure that the two mobile cranes could man- oeuvre safely over the canal and be- tween existing buildings, without becoming trapped as work proceeded. This was done in co-operation with the police, who also allowed beams to be taken across public main roads during daytime off-peak traffic hours after trials which showed that this could be done without serious delay to traffic.

Except for the Lockhart Road and Hennessy Road spans, this eliminated night working and minimised distur- bance of local residents. Standby faci- lities were arranged to cater for possi- ble failures of either the prime movers or the bogies when transporting beams across public roads.

Following the final placing of each beam it was necessary to set timber wedges so as to prevent the beam sliding on its bearings before the epoxy adhesive had cured. Accidental disturbance when placing an adjacent beam or due to self-weight was not easily seen, especially when working under artificial light at night-time.

Contractors' proposals for lifting and placing beams, including details of all equipment, were approved in ad- vance and it was found important to ascertain exactly who was to be res- ponsible for providing and fixing lift- ing slings. Working at night an accident occurred at Chatham Road Flyover when the approved lifting bar was re- placed by a longer bar which projected from the side of the beam. This shear- ed against the adjacent beam during lowering into position. Fortunately the beam at the time was within inches of its bearing and serious harm did not result.

During the contracts, a precast beam of each type was load-tested for shear and bending. All contractors chose to provide kentledge against which the beam was jacked to apply the required loading, and movements were checked by dial-gauges or by measurement to a fixed string-line.

During shear tests, it was necessary to provide the correct bearing area at beam ends in order to avoid the crack-

Far East BUILDER, January 1971 Page 47

Erecting beams on Canal Road flyover extension at the Wan Chai reclamation area. bridge column and capping beam in foreground ing of non-structural concrete at the beam faces.

Bearings for precast beams

pre-

To ensure accuracy of fit, all cast beams were checked dimension- ally after casting for direct comparison with face-to-face dimensions between supporting capping beams. For beams skew on plan, sloping in the longitu- dinal direction and supported on hori. zontal bearings, end notches were an added complication and special means of checking were devised.

Prior to lifting each beam, a con- trol line was marked along either side and the distance from each line to the soffit at the beam ends was recorded. Dimensions were then taken of the distance between the control line and the bearing face at each notch. Subse- quently, these control lines were simu- lated by fixing taught stringlines be- tween the faces of capping beams at the relevant precast beam position; string-line to capping beam soffit dimensions coincided with control-line to precast beam soffit dimensions. Finally the control-line to bearing face dimensions were checked against against string-line to rubber bearing dimen-

Foot

sions while seating the rubber bearings. By this means it was possible to obtain full-face contact between each beam and its rubber bearings.

References

1. Design and Construction of Lai Chi Kok Bridge by W.A. Maslowicz and G. Wilson: Engineering Society of Hong Kong, March 1969.

2. Concrete Technology and Prac- tice by W.H. Taylor, Hal-Stead Press, 13.2.2. Grouped Frequency Method.

3. Asphalt Work for the New Hong Kong Airport by F. J. Parker and J.K. Holt: Paper No. 10 of the 1960 ICE Conference on Civil Engineering Prob- lems Overseas.

This feature is based on the paper 'Some practical aspects of road and flyover construction', by R.J. Blake, resident engineer, Scott Wilson Kirk- patrick & Partners. The paper was read recently before the Engineering Socie- ty of Hong Kong and is published by kind permission of the Society.

In Part II, to be published next month, Mr. Blake describes stressing methods used for flyover beams, and details the materials employed and problems met in road construction.

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