ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/297009
A C C E L E R AT E D B R I D G E C O N S T R U C T I O N Constructing bridges off line and moving them into place offers key benefits to owners and contractors. As a result, more bridges are being designed and built using these techniques. This article is part of a series looking at some of the key considerations when using accelerated bridge construction (ABC) approaches to construct bridges. It describes the use of self-propelled modular transporters (SPMTs), which are becoming more popular with owners, designers, and contractors as they understand the concepts and see the advantages of ABC. A single SPMT is a multiaxle platform operated through a computer-controlled system. Each axle line generally consists of four wheels arranged in pairs. Each pair of wheels can pivot 360 degrees around its support point. Consequently, an SPMT has complete freedom to move in all horizontal directions. These motorized vehicles, moving at walking speed, can lift and carry large and heavy loads, including entire bridge assemblies, from off-site locations to their final position. The SPMTs are then moved off site, allowing traffic to be restored within hours of completion. Otherwise, construction of the bridge is similar to that of a bridge built in its final location. This moving equipment often is used when construction sites are restricted, and prefabricated components need to be assembled off site and moved into place along a route that is longer than is practical for slide pads or rollers to navigate. More engineering is required for this type of move due to the added complications. SPMTs can be linked longitudinally or laterally to provide the number and configuration of axle lines required by the load. Linked units can be synchronized to a central computer, providing four basic commands: steer, lift, drive, and brake. The dimensions of SPMT units vary depending on the make and number of axles and wheels. When using SPMTs to move bridges, tolerances must be kept extremely tight. Minor deviations that can be corrected during typical bridge construction cannot be adjusted as the bridge is moving and being set. As a result, tolerances must be strict enough to avoid excessive stresses on the bridge yet reasonable enough to generate optimum moving speed. These tolerances must be specified, adhered to, and continually monitored. States Expanding use Some states, notably Utah, offer manuals that provide g u i d a n c e f o r u s i n g S P M Ts . U t a h D e p a r t m e n t o f Transportation's Innovate 80 program involved the replacement of 12 structures using SPMTs and made ABC techniques its standard in 2010. Utah projects have used SPMTs to carry bridges over distances as great as 1.25 miles and over grades as steep as 6%. SPMTs can provide a range of benefits. Most significant is the reduction in closure times, sometimes to as little as a few hours. This improves accessibility and reduces user costs while also improving worker safety. Additional benefits arising from the prefabrication of components can include the following: • Longer curing times for all concrete components • Control over the environment at the construction site by Craig A. Shutt SPMT Solutions For the Rawson Avenue Bridge projects, the two spans were constructed on opposite sides of the expressway. Photo: Henry G. Russell Inc. ASPIRE , Fall 2013 | 39 Book_Fall13ASPIRE.indb 39 9/6/13 12:05 PM