INCREMENTAL LAUNCHING METHOD OF BRIDGE CONSTRUCTION

INCREMENTAL LAUNCHING METHOD OF BRIDGE CONSTRUCTION

Pre-Requisites for incremental launching:

• Used only for straight bridges and curved bridges with fixed radius over entire length.
• Slenderness ratio i.e. span to depth ratio of beam sections must not be more than 17.
• Single cell box and double T are most suitable cross section.

Important measures taken during incremental launching of bridge segments:

• During the process of launching the segments, the leading edge of the superstructure will be subjected to a high hogging moment. In this kind of situation, steel launching nose having a length of about 0.6 to 0.65 times the span length is provided at the leading edge in order to reduce the cantilever moment. However sometimes, a tower and stay system are designed instead of using launching nose which also serves the same purpose.

• During the launching, superstructure will be subjected to alternative positive and negative moments. Usually, a central prestress is provided in which the compressive stress at all points of bridge cross sections are equal. In this way, it counters the possibility of occurrence of tensile stresses at upper and lower parts when it undergoes positive and negative moments respectively. After the process is complete i.e., when the whole superstructure is completely launched, depending on the bending moments in completed bridge condition, continuity prestressing is performed by providing the proper  location and design of continuity tendons. These continuity tendons supplement the central prestress

• Sometimes if the span of the bridge is too long, temporary piers may be provided at suitable locations to reduce the cantilever moment.

• Construction is done without using form work to avoid problem of passing over obstacles.