November_1970 — Page 40

Lowering the 132t precast units across main railway lines

AF55

Completed structure over railway

of temporary anchorages to control thermal movement enabled work to progress concurrently until the four parts were complete. Construction of the splicing bays and release of the temporary anchorages connected the four individual structures into one monolithic structure with the elevated roundabout as the permanent anchor. Thermal movement is taken by sliding bearings located in pairs or singly at the top of the columns and comb block expansion joints at the thermal abut- ments.

The units were transported and placed on temporary works by a special- ly designed 51t safe-working-load twin crane transporter, mounted on the superstructure on rail tracks. Generally the temporary works consisted of a pair of 43.8m long by 2m Autofab girders, braced together at 1.2m centres.

36

+

With a latticed launching nose, each set had an overall length of 54.6m and a weight of 55t. Two of these girder sets were provided, both designed to span 31.4m between steel trestling located at the columns and cantilever forward 12.5m when loaded with the 772t dead load of a span plus construc- tion loads.

To ensure that the final profile of a span was correct, the units were sup- ported off the girders on hardwood blocks of a predetermined height, which allowed for any deflection of the girders. Shear keys also connected the girder sets to the last completed span; thus the span under construction moved with the live structure and eliminated thermal movement prob- lems while in-situ jointing was in progress. Movement of the girders was accommodated by constructing the

trestles in two parts and allowing the upper trestle to rock.

Section 4 of the structure, 670m long, runs adjacent to a railway line embankment and is constructed from pretensioned precast concrete longi- tudinal beams, simply supported on rubber bearings on reinforced concrete crosswalls and columns. Each 15m span consists of 19 beams, giving a total of 817 beams placed.

Curved section

The 1160m long section 5 of the project curves at a radius of 436m to cross main railway lines at an angle of 45 degrees. Said to be the longest con- tinuous concrete structure in Britain from a single fixed anchorage, it con- sists of 19 spans, 62m long, with no intermediate expansion joints.

Thermal movement is taken by pairs of 2810t capacity anticlastic bear- ings located at the top of each column. The large reversible horozontal forces generated by the thermal movement of the structure are resisted by the anchor- age abutment at the east end of the section. The foundations of this anchor- age incorporate two large diaphragm walls which transmit these loads into the ground as shear forces. A Demag joint capable of accommodating 0.9m of movement is located at the west end terminal abutment.

The highway is constructed from transverse precast concrete units, post- tensioned longitudinally. The contrac- tors established a precast yard nearby to produce these units which are 29m long by 2.3m wide by 3.2m deep and weigh up to 132t. There are 26 units in each span and 400 units were pro- duced to a high standard. Seventeen casting beds were used in a phased operation to pace the typical erection cycle of approximately six weeks.

After transverse stressing, the units were lifted from their beds by a pur- pose-made 142t travelling gantry and taken to the limited stock yard at the west end of the precast yard. The units were transferred as required on a power- ed bogie to a lifting bay by the an- chorage abutment. Here the units were lifted up to deck level by a 142t self- propelled portal framed transporter. Carried forward on rail tracks over the completed work, the units were then laid successively, span by span.

During placing, and prior to longi- tudinal stressing, the 3040t dead weight of each span was supported on tempo- rary works consisting of girder sets mounted on trestles at the columns and

Far East BUILDER, November 1970

Page 40Page 41

third points. These in turn were found- ed on the column pile caps and tempo- rary works piles of up to 810t capacity at the third points.