This bridge repair project clearly portrays the art and science of civil engineering in terms of technical excellence. It demonstrates how innovative answers were found for the challenging demands of structural stability, safety, and economy. The repair technique implemented is unique and displays engineering ingenuity in the practical, efficient and structurally sound approach followed. The bridge is an important link between the Free State, Transkei and Eastern Cape that is now restored to its original historic glory. Notwithstanding the fact that the bridge had to be closed for traffic during repairs, good public relations ensured that no incidents took place.
Description Of The Project
Bridge No. 422 is located on Route P53/1 (R726) between Zastron and Sterkspruit where it crosses the Orange River near Sterkspruit on the farm Mayaputi. It was constructed in 1934 when pre-manufactured steel was imported from the United Kingdom and erected in South Africa. These are historic bridges and concerted efforts should be made to maintain them for posterity.
The superstructure consists of two simply supported, steel truss spans of 61 m (200 ft) each supported by a concrete substructure with a roadway width of 3 m (10 ft). The truss members consist of laced, riveted sections. The bridge was seriously damaged as a result of vehicular impact to certain of its structural members.
THM Engineers Free State cc was appointed to evaluate the deficiencies, find structural solutions and compile contract documentation for the urgent repair and rehabilitation of the bridge.
Since this is a narrow single lane bridge, the bridge had to be closed for the whole construction period. The contractor had no other space than the bridge deck to work from for scaffolding, rigging equipment, welding machines etc. Closure of the bridge would have had a major impact on the local community and long-distance haulers. Therefore, the point of departure was to complete the repair work in the shortest possible time.
In a statically determinant truss bridge, the removal of any compression or tension member for repair will lead to an immediate and complete collapse of the bridge. The traditional engineering solution would have been to construct very costly coffer dams and prop the complete bridge 30 m high from the river bed to make the removal of the damaged members possible. Considering the risk of the known frequent flash runoff of the Orange River the traditional approach was not an option.
THM displayed innovation and engineering ingenuity with a unique “repair in the air” design solution satisfying the challenging demands of structural stability, safety and economy. The challenge was to find a repair method that allows for the safe removal and replacement of damaged structural members while the damaged bridge supports the contractor’s workers and construction loads.
The following “repair in the air” technique was followed to replace all damaged vertical compression members while they were carrying the full loads due to the bridge’s own dead load and applied construction loads. Two temporary compression struts capable of supporting the full load due to dead weight and the construction loads were installed adjacent to each damaged member. To ensure that the full load was carried by the temporary struts a simple though effective and functional support bracket was designed to pre-stress the struts in a very controlled manner before the damaged vertical laced member was removed. Due to the complexity of the riveted connections, it was not possible to install a replacement member as a unit. The four angle iron legs of the laced member had to be built up one by one.
The diagonal laced members are tension members and do not need composite action for stability as each angle iron leg of the member is in tension. Tension force influence lines that were obtained from analyses were used to cleverly position heavy construction loads such to reduce the forces in the member under repair. The result was that it was possible to replace the damaged diagonals by removing and replacing the legs one by one without the use of expensive temporary tension cables while they had to resist the influence of the full dead weight and construction loads.
An elegant steel solution
Due to the fact that the bridge is a single lane structure with limited workspace, it was, from the outset, envisaged that repair could not take place under traffic conditions. Long distance traffic could preplan for this eventuality; however, it was problems associated with the locals that proved a thorny issue with potential political implications.
Many locals travel between the two towns on a daily basis due to work commitments as well as for the purchasing of goods. The only practical alternative which could be considered was to accommodate pedestrian traffic over the bridge at certain stipulated times and under certain conditions as determined by prevailing construction activities.
This arrangement allowed for Taxi operators from one town could transport their passengers to the bridge; passengers could then walk across the bridge; taxi operators from the other town could then pick them up to complete the journey. Persons making use of their own transport could leave their vehicles at the bridge and arrange for transport on the opposite side.
Being fully aware of the generally aggressive attitude adopted by Taxi Associations, the Engineer and the Contractor realised that this issue had to be sorted out prior to the closing of the bridge. A meeting was arranged with the Taxi Associations of Zastron, Sterkspruit and Aliwal North, during which all the implications and eventual advantages of the Project were discussed at length and in detail.
By observing the daily “crossings” of the bridge one became amazed at the vital role a bridge such as this plays in the lives of a local community.
This was indeed an out of the ordinary technical project, but also one which had close ties with the local community.
Developer/ Owner: Department Public Works, Roads & Transport, Directorate Land Transport Planning
Structural Engineer: THM Engineers Free State cc
Project Manager: THM Engineers Free State cc
Main Contractor: Tanekk cc
Steelwork Contractor: Tanekk cc
Other: Rand Sandblasting Projects (Pty) Ltd