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Road Bridge over Nahoon River - David Lazarus
Abbotsford, East London, Eastern Cape

NINHAM SHAND and PARTNERS: Engineer
FOSATTI-GOLDSTEIN: Contractor

Date:1973
Type:Bridge
Status:Extant

 


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Coordinates:
-32.966444,27.920376
32°57'59.19" S 27°55'13.35" E

Councillor David Lazarus - after whom the bridge was named - was the Mayor of East London in 1967.

The David Lazarus bridge over the Nahoon River at East London
by C. J. Thompson, PrEng

THE first section of the new North-East Expressway, which will eventually link the National Route 2 with the Central Business Area of East London, is nearing completion. A notable feature of this expressway is the elegant structure over the Nahoon River, approximately 7 km from East London and 4 km upstream of the river mouth. At the bridge site the river has a wide flood plain on both banks and the foundation investigation revealed alluvial sediments which overlay shales and sandstones. During construction two geological faults were found to occur at the East London abutment and at one of the river piers.

After a comprehensive investigation and an economic study of schemes with various span lengths and forms of construction, the design illustrated in Fig 1 was selected. The bridge crosses the Nahoon River at a skew of 10° and has a 320-m radius horizontal curve on the Transkei side. The overall length of the structure is 260 m and each carriageway is supported on six piers at 39-m centres. Part of the structure is on a vertical curve while the remainder on the East London side is on a 5 per cent grade.

The bridge has two carriageways, each consisting of a 7,3-m travelled way with -2,45-m outer and inner shoulders. Each carriageway has a cross fall of 2 per cent down from the median on the tangent alignment and a maximum super-elevation of 9 per cent on the curved portion of the structure. The two carriageways are separated by a 750-mm central island which incorporates a back-to-back guardrail. This structure was designed in accordance with the Cape Provincial Administration's Code of procedure and design criteria for the design of road structures issued in January 1968.

Five of the piers and the Transkei abutment are founded on 250- and 100t piles respectively. The pile lengths were of the order of 6 to 15m and were socketed into the bedrock to a depth of 2,5 m. On the East London side of the structure the bedrock was relatively close to the surface and the abutment and first pier were founded on spread footings. Both the abutments were designed as open counterfort structures and were constructed in ordinary reinforced concrete.

One of the most striking features of this structure is the Y-shaped pier which springs from a solid plinth to support a hammerhead deck slab. The height of these piers varies between 15 and 25 m. The hammerhead deck slab which spans 6,5 m between the pier arms and cantilevers 3 m on either side, consists of a three cell box which was constructed monolithic with the pier arms.

The pier plinths and arms have been constructed in ordinary reinforced concrete while the hammerheads were prestressed with 32-mm Dywidag bars in both directions. The hammerhead decks were cast on temporary staging which was supported on the pier arms. During construction these arms were prestrained to compensate for the additional loads of this staging and the concrete. The average time of construction for one of these Y-shaped piers inclusive of the hammerhead was eight weeks.

The deck superstructure between the pierheads consists of three 26,25-m prestressed hollow box 'fish belly' girders which are tied together by means of an in situ deck slab. Apart from the two cross-beams at the beam seatings there are no diaphragms between the box girders. These precast girders have a depth of 1,5 m at midspan and 1,1 m at the support. The contractor elected to precast the 80t beams on a special steel truss system which was suspended between the hammerhead seatings. The box girders were cast in two stages, with a maximum delay of 24 hours between the casting of the bottom U section and the top slab.

The trusses were provided with a water ballast system which was drained during the casting of the top slab in order to counteract the flexibility of the truss and so avoid tensile tresses occurring in the bottom U section. On completion of the top slab, the top surface was sprayed with a fine water spray in order to roughen the construction joint between the precast beam and the in situ slab. The beams were prestressed five to six days after the casting of the top slab, and were lifted off the casting bed and lowered onto their bearings by means of a special 'goose neck' arrangement. After all the girders had been launched in a span the in situ deck slab was cast in a single operation which usually took about eight to ten hours. The success of the method used to construct the superstructure spans between hammerheads can be judged by the fact that one of these spans was usually completed in approximately six to seven weeks.

Two 610-mm x 335-mm x 70-mm rubber bearing pads were positioned at each end of the precast beams and were required to comply with a stringent performance specification. The transverse roadway expansion joints which have a total anticipated movement of 35 mm were formed using a neoprene cellular extrusion with epoxy nosings.

The consulting engineers, Messrs Ninham Shand and Partners, were appointed by the City of East London to carry out the design of the North-East Expressway and tenders were called for during March 1970. The contract was awarded to Messrs Fosatti-Goldstein, whose tender price for the construction of 2 km of dual carriageway and the related structures was R2 052 000.

The following are some interesting statistics of this bridge:
Final cost of structure including pro rata portion of preliminary item: R1 020 000.00
Length: 260 m
Width. 26,1 m
Overall cost. R150/m2
Total cost of piling R235 000
Cost of substructure inclusive of hammerheads R415 000
Volume of ordinary reinforced concrete 4 830 m3
Volume of prestressed concrete 2 730 m3
Mass of ordinary reinforcement 875t
Mass of prestressing reinforcement 150t

(The Civil Engineer in South Africa - August 1973 via Sabinet)

Submitted by William MARTINSON