Demolition of the Grace Memorial and Silas N. Pearman Bridges over Cooper River, South Carolina, USA

Demolition of the Grace Memorial and Silas N. Pearman Bridges over Cooper River, South Carolina, USA — A Case Study

Singh, Sarbjeet / Mirzakashani, Mehrdad / Hagh, Abdol

Engineers have always been challenged when it comes to developing an economical, efficient, and safe demolition approach for an existing long span truss bridge spanning over a major navigational channel. The technical reasons are simply a large number of site specific and structural specific parameters. This paper outlines the issues encountered during demolition process and adopted methods of dismantling 2-miles long two bridges over Cooper River, named "Grace Memorial (built in the year 1929) and Silas N. Pearman Bridge (built in the year 1966)". The demolition process was started in the middle of the year 2005 and was completed in April 2007. Both the bridges were consisted of various types of superstructure systems such as (i) Multi span steel truss (ii) Concrete deck on steel girder framing and (iii) Concrete deck on prestressed and precast concrete girders system. Since they were spanning over water and land, therefore, it was very important to maintain the stability of the remaining portions of the bridges during demolition process. One of the major challenges was the structural modeling of the section under demolition consideration, based on existing condition as well as adjacent span's behavior. At every demolition stage various dismantling options were studied, including imploding steel truss by blast, lowering down portions of a span with hydraulic machines, dropping the deck framing members on the ground, and removal of members piece by piece. Structural vibration and its propagation was the main concern and therefore, every span of the bridges was analyzed. Temporary support systems like shoring towers, temporary trestle etc., were designed wherever needed. Superstructure and substructure were checked for construction loads from the equipments e.g. excavator and cranes. The elements, unable to support these construction loads, were strengthened before using the equipments. The coordination and discussion between design and construction engineering teams was very important for overall cost-effectiveness.