October_1966 — Page 71

the barrel. Only one line of rein- forcement was used in the 60 cm. pipe.

cm..

The standard strength culvert pipe was cast of concrete having a mini- mum strength of 245 kg sq. while the extra-strength pipe used concrete of 315 kg/sq. cm. Minimum shell thickness varied from 6 cm. for the 60 cm. diameter extra-strength pipe, to 13 cm, shell thickness for the 120 cm. diameter standard strength pipe. In certain locations 30 cm. in- side diameter reinforced concrete standard strength culvert pipe was used to carry utility lines under the highway, or along the shoulders in some instances.

A concrete barrier rail was design- ed and installed wherever the 5-metre median strip was used on the four- lane pavement.

Major Structures

A total of 24 bridges, all reinforced concrete were built.

In order to comply with the clear- ance requirements it was necessary to design lift-spans at two locations. To avoid the unsightly appearance and the expense of permanent towers, a set of portable towers and a jacking system was designed at a fraction of the cost of constructing a permanent installation at each bridge.

The alignment of the improved highway followed the existing road, which made it necessary for most of the new bridges to be constructed at approximately the same locations as the existing bridges. Use of the exist ing bridges in the improved highway was not possible because of their poor condition. At thirteen bridge sites the required locations of the new bridges prevented use of the existing bridges, even for detour purposes.

After a survey was made of the necessary bridge and span lengths a cost comparison was made between reinforced concrete and prestressed concrete which disclosed little reason to select either type of span on the basis of cost alone. However, pre- stressed concrete spans have a longer

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Moveable span bridge

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maintenance-free life expectancy as they are not so subject to open cracks under normal loading. The decision was made to use prestressed concrete for the bridge spans and reinforced concrete for the abutments, pier caps and sidewalks.

A study was then made using estimated costs of construction for piles, caps, and spans, to determine the most economical span lengths for the bridges. Results of this study in- dicated it would be economical to use span lengths of 6, 10, 12 and 14 metres and to utilise combinations of these span lengths in providing the necessary length of bridge at each location.

The number of piles per abutment ranged from four piles for an abut- ment supporting a 6 metre span to six piles for an abutment supporting a 14 metre span.

Stub. or spill-through type abut- ments, were used for all bridges in order that forces on the abutments caused by backfill and surcharge loads might be held to a minimum. The two end piles of each abutment were battered on a slope of 1:4 normal to

the abutment face to resist these forces, except at bridges 6 metres in length, where these longitudinal forces could be safely carried by the deck beams.

To save concrete and reduce abut- ment dead load, the abutment back- walls were made as thin as practicable. By locating the backwall between the foundation piles and the approach fill. it was possible to utlise the piles for horizontal support along with the lower portion of the backwall, and then design the wall as a horizontal beam. The basic backwall thickness of 12 cm. was stiffened by adding a beam near the bottom of the wall, to resist the cantilever moment at the end piles. Caps on the piles were

Far East Architect & Builder October, 1966

Above: Preparation of vertical jute drains

Left: Preparation of drain field before bridge approach embankment was placed. The 30 cm. wide and deep trenches were filled with sand to provide for the passage of water which rose through the vertical jute drains

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