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| Erection schemes help the contractor verify details such as capacity and stability of temporary supports. |
Additionally, the contractor is often directed to provide a construction plan, stamped and signed by a professional engineer (herein referred to as the contractor’s engineer). The contractor’s engineer must perform a more in-depth investigation of the steel erection sequence to be used in the field, detailing the steel erection procedure and providing calculations that demonstrate the girders and bridge components meet stability and structural capacity requirements. The contractor’s engineer must thoroughly investigate each stage of construction, ensuring that there are no girder overstresses, excessive displacements and/or rotations, or instabilities. It is imperative that the design engineer review the erection procedure and calculations provided by the contractor’s engineer to verify that they are thorough, complete, and result in a structure meeting the intent of the original design.
General steel erection procedures
Procedures required for general steel erection of highway bridges are provided in the Steel Bridge Erection Guide Specification developed through the AASHTO/National Steel Bridge Alliance (NSBA) Steel Bridge Collaboration. This document highlights minimum requirements for the development of steel erection procedures, including steel erection drawings and calculations. For example, the steel erection drawings should provide the following:
- plan of the work area, including permanent and temporary supports, and obstructions such as roads and waterways;
- erection sequence for all superstructure components, noting the use of temporary supports, lifting cranes, and holding cranes;
- details of temporary support structures, tie-down devices, and blocking for the bearings; and
- details of jacking devices, spreader beams, and attachments, as well as the lifting weight of girder members, including weights of the rigging and lifting attachments.
The contractor’s engineer should be required to submit calculations that provide the basis for the details and procedures provided on the erection drawings. These calculations should verify the following:
- load capacity of the lifting and holding cranes;
- load capacity and stability of the temporary support structures;
- structural adequacy and stability of the girders for each stage of the erection sequence; and
- load capacity of spreader beams, beam clamps, stiffening trusses, and tie-down devices.
Finite element modeling
As a result of these requirements, the design engineer and contractor’s engineer now often need to create a finite element model of the bridge to investigate the step-by-step erection sequence. A model can be used effectively to estimate the girder deflections, rotations, and shears and moments that need to be examined and limited to ensure there are no problems during erection.
The finite element modeling associated with the investigation of the steel erection sequence will generally require a 2D grid or a full 3D finite element model. The type of finite element model employed depends on several factors, such as the complexity of the bridge, complexity of the erection sequence, requirements of the analysis, and requirements of the owner. Unless instructed by the bridge owner, the contractor’s engineer must determine the appropriate level of analysis to be used to investigate the steel erection sequence.
Regardless of the level of analysis, the contractor’s engineer should investigate each stage of the steel erection sequence. For each stage, girder stresses, vertical and out-of-plane girder displacements, cross-frame forces, temporary support loads, and girder stability need to be checked. These behaviors are required to determine the viability of the chosen erection sequence and methods.
For the analysis of the steel erection sequence, dead loads and construction loads need to be determined and applied to the appropriate elements in the analysis, including the following:
- dead loads, such as the self-weight of the girders and detail attachments;
- construction dead and live loads, such as deck placement machinery, contractor’s equipment, deck overhang brackets, formwork for the concrete deck, et cetera;
- wind loads; and
- temporary hold crane loads.
Girder stability during lifting
Once a girder section arrives at the construction site, lifting it can be accomplished by one of the following methods:
- using two cranes and picking the girder section at each end;
- using a single crane and picking the girder section at its centroid;
- using a single crane and picking the girder section with a spreader beam; or
- other methods, depending on the girder size and site configuration.
Each method results in several design checks that need to be considered because field pieces will be subjected to forces that are different from the final design forces, such as:
- girder cantilever at the end of the pick piece;
- positive bending between girder pick points; and
- girder rolling due to unbalanced loads and curvature.
In-place girder stability during erection
Even though a girder has been designed to accommodate all possible loading conditions in its final condition, the strength of the girder may not be adequate for temporary conditions that can arise during erection of the bridge. Temporary conditions can include cross-frame spacings several times larger than that in the final structure. In some cases, such as the first girder being erected, the girder will not have any functioning cross-frames within its span. The effect of the long unbraced length is that the buckling capacity of the girder is substantially reduced.
In addition, the deck is not present during girder erection, so any stability and strength provided by the deck cannot be relied upon. Although the loading may be many times lower than what the bridge will see in its final condition, the buckling strength may be reduced by an even larger amount. To account for this, there are some critical construction steps that should be considered including individual girder checks, girder stabilization, multi-girder checks, multi-girder global buckling checks, girder cross-frame requirements, designing for wind load and curvature effects, and support tie-downs. Each of these is explained in more detail in the full version of this article on www.gostructural.com.
Construction aspects
Crane access, capacity, radius, and boom length are items that should be considered by the contractor’s engineer while developing the steel erection sequence. The contractor and the contractor’s engineer need to verify job site access for the typically large cranes required to erect steel I-girders and determine the locations in which the lifting cranes will be placed throughout the erection process. Location of the cranes can be affected by the terrain, existing roadways and structures, and by the sequence of erection.
The crane locations should be shown on the steel erection drawings. Furthermore, the crane type and size, counterweight requirement, and boom length should be provided in the steel erection plans so the assumed crane set-ups for design can be verified in the field. The lifting capacity and pick radius for each erected piece should also be shown.
Curved steel I-girders depend on their connection to adjacent girders through bracing members for stability. Therefore, fully tightened rather than loosely connected cross-frames should be used during bridge erection to ensure girder alignment and stability. Additionally, the bolting requirements of field splices during steel erection need to be considered by the contractor’s engineer. In accordance with the AASHTO LRFD Bridge Construction Specification, Article 11.6.5, "splices and field connections shall have one-half of the holes filled with bolts and cylindrical erection pins (half bolts and half pins) before installing and tightening the balance of the high strength bolts." The contractor’s engineer should ensure that the number of bolts and/or erection pins used provides enough capacity for transfer of loads for the given stage of steel erection.
The alignment of girder field splices and cross-frames during steel erection is critical to ensuring that the process progresses smoothly. When girders cantilever beyond a pier, the position of the top and bottom flange tips can significantly affect erection of the next girder piece. The positions of the flange tips can be determined through the finite element analysis of the steel erection sequence—typically requiring a 3D analysis, as discussed previously. The displacements include both vertical displacement due to dead load and lateral displacement/rotation due to curvature effects.
Temporary supports
When employed, the load capacity and stability of temporary support structures should be investigated by the contractor’s engineer. The load that must be resisted by the temporary support is typically due to the self-weight load of the structure. However, the load applied to the temporary support structures will change as steel erection progresses and, in curved girder bridges, will be different for each girder at the same radial location. The loads that must be resisted by temporary support structures can be determined through analysis of the steel erection sequence. Typical components of temporary supports that require design include the tower itself, upper steel grillage, lower mat/foundation, and lateral bracing or tie-downs.
Conclusion
Bridge engineers need to be aware of engineering issues that are specific to horizontally curved, steel I-girder bridge erection and what should be provided with the erection drawings and supporting calculations. The contractor’s engineer should ensure that the bridge components are not overstressed or subject to instabilities during the steel erection sequence, and that vertical and out-of-plane girder displacements are not excessive such that significant connection misalignments occur. The design engineer needs to review the erection procedure and calculations provided by the contractor’s engineer to verify that they are thorough, complete, and result in a structure meeting the intent of the original design.
All photos are of the US35/I-64 Flyover Bridge in Putnam County, near Charleston, W.V., which is a 1,165-foot-long, six-span, continuous horizontally curved, steel I-girder bridge with a centerline radius of 940 feet. The owner of the bridge is the West Virginia Department of Transportation.
Brandon Chavel, Ph.D., P.E., is a structural engineer in HDR’s Chicago office. He can be reached at brandon.chavel@hdrinc.com. Shawn Tunstall, P.E., is a structural engineer in HDR’s Pittsburgh office. He can be reached at shawn.tunstall@hdrinc.com. This text was adapted from a paper prepared for the 2008 International Bridge Conference.
