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/297000
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 Sliding and rolling bridges into place after assembling them nearby has gained adherents as efficient construction techniques for saving time and costs and minimizing traffic disruption. Owners, designers, and contractors are quickly learning as much as possible about these new concepts so they are ready to discuss options when opportunities arise. This series of articles looks at some of the key considerations when using these approaches to bridge construction. Part 1 of this series considered key design considerations, including allocating duties among the construction team, superstructure and substructure concerns, and evaluating sliding forces. This article considers falsework and related design issues. The next article will review necessary field activities during construction. The use of self-propelled modular transporters will be addressed in a subsequent article. Falsework Design The design for temporary falsework depends on the application, but in any form it represents a key element to ensuring the structure is safely fabricated offline and out of the critical path, allowing for accelerated bridge construction (ABC). Building falsework next to the existing bridge location allows assembly of the new components to be completed while other key elements of the construction also are underway (such as piers and abutments). Extensive falsework and shoring requirements must be considered when sliding or rolling bridge superstructure systems into place. The falsework's constructability is critical to ensure the falsework provides the various functions required to build the bridge components and then move them into place. This means every aspect must be detailed in the shop drawings to ensure the falsework is built to perform as the specifications require. The temporary shoring and permanent abutment must be checked for large temporary lateral forces during launching. However, if a problem arises (such as excessive settlement) during the transfer of the bridge from the falsework to its permanent support, the shoring system has to be able to accommodate and possibly support greater lateral construction loads. For most projects, settlement of the falsework as it carries the load (during launching) of the bridge doesn't become an issue. The falsework will deflect slightly but not enough to be a concern. Even so, it is preferable to consider and evaluate this possibility to ensure the falsework is sufficiently rigid. If the falsework does deflect too much, it will be necessary to jack up the bridge once it reaches its final landing point or transfer point if using the permanent structure to support the slide rails. A counter concern is that there can be difficulties if the falsework is too high as well. In some cases with push/pull jacks, contractors have investigated the use of the falsework as an anchor point. This generally has not been done, as the falsework needs to be kept as light (and therefore economical) as possible. It is better to push against or pull toward something more rigid (that will ultimately act as a component of the permanent system), such as the abutments. In some cases where space is limited, these lateral bridge- supporting devices can be built on the other side of the existing bridge from the new bridge, allowing that old bridge superstructure to be pulled out of the way and the new bridge to be moved into place behind it. Sliding Forces With a static coefficient of friction of 0.12, the required force to slide a 1.3-million-lb concrete superstructure is 156 kips (1,300,000 lb × 0.12/1000). This can easily be achieved with two 100-kip jacks, which are readily available. Although a concrete bridge typically weighs about 50% more than a steel bridge, and thus a higher force is needed to move the bridge, the associated equipment and costs are not directly proportional—that is, the cost to rent the hoses, pumps, gauges, and a set of two 100-kip jacks is comparable to renting a complete system with two 60-kip jacks. Transfer to Permanent Supports Moving the structure from the sliding shoes to the permanent bearings is a critical step in construction that must be considered very carefully. In many cases, the bridge will be transferred from the sliding rails onto jacks and then lowered onto new permanent bearings. In some cases, the bearings under the beams can remain in place during the transfer. In any case, the bearings and guide rails should be horizontal to avoid moving the bridge uphill or downhill. Depending on by Craig A. Shutt Sliding and Rolling Bridge Solutions–Part 2 On the West Mesquite Interchange at I-15 in Mesquite, Nev., hydraulic jacks were used to push the bridge into place. All photos: Jackie Borman, HDR. ASPIRE , Spring 2013 | 19 AspireBook_Spr13.indb 19 4/1/13 11:02 AM