|Figure 1: Bridge girder erected by two mobile cranes||Figure 2: Bridge girder erection using twin gantry cranes, safer and cost-effective|
Precast girders used in bridges/ flyovers are heavy and can weigh up to 200 Tons. These are usually cast away from the site and are carried to the span of installation on trailer. The girders are handled by mobile cranes. Hosting the heavy girder requires careful spotting of the cranes and synchronized lifting. In addition the ground has to be prepared to reach and locate in the span.
Cranes are to be brought to site for short intervals from their parking places and returned after lifting a few girders as it is difficult to deploy them continuously. These facts make the operation very expensive for projects with large number of spans. Typical lifting of the girders by double mobile cranes is illustrated in the accompanying picture.
It is to overcome these difficulties a system of two gantry cranes straddling the bridge on rails laid on either side is designed. The scheme is illustrated in the 3D model Fig. No. 2 shown below.
These cranes can be operated synchronously together or separately. The cranes are electrically driven can be used independently for form work and concrete handling. Girders can be cast between piers in the span itself. They are cheaper and safer to operate and cost effective.