Gantry Construction

Precasting the box girder segments speeds up the construction of the bridge structure in two ways: (1) It permits the segments to be fabricated elsewhere (precast) while the substructure (i.e. piles, footings, pier columns and pier segments) is being built; and (2) It allows rapid assembly of the segments when building the cantilevers since the delays in waiting for newly cast concrete to harden are eliminated.

The decision to precast obviously requires a method for transporting the segments to the bridge site and also a means for lifting the segments in place to assemble them on the structure. Oftentimes, when bridges are constructed over water, the segments can be shipped to the site on barges and lifted into place by equipment positioned at the end of the cantilevers. For the Roosevelt Bridge, the St. Lucie River presented a special challenge. Most of the river is not deep enough to readily permit barging without "dredging", or lowering a portion of the river bed. In the interest of preserving the integrity of the river in downtown Stuart, it was decided that dredging should not be permitted. The solution chosen to meet these special conditions was to build the bridge entirely from above using a specialized piece of equipment called a "gantry" and to transport the segments over the previously completed superstructure as shown n Figure 10(a).

The massive size and length of the gantry is not so much for lifting the segments but to allow it to move from pier to pier as it completes one section of the bridge and moves on to the next. It normally sits on supports near the center and the rear of the gantry while lifting segments to build a cantilever [Figure 10(a)]. With the aid of temporary props, the gantry can be raised so the supports can be moved to the next pier. The gantry then follows by sliding along the top of the supports as demonstrated in Figure 10(b).