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9This chapter discusses fabricator practices and views related to steel bridge erection, as reported in the questionnaires and follow-up telephone interviews that were part of this synthesis. GIRDER ASSEMBLY PRACTICES Girder assembly refers to the shop practice of assembling girders to drill or ream the field splices and in some instances test-fit the crossframes. Questions pertain to whether the gird- ers are assembled with the webs vertical or horizontal, the number of girders that are in an assembly, and whether the crossframes are assembled with the girders. Straight I-Girder Assembly Most of the fabricators, that is, 13 of 15, assemble the gird- ers with their webs in the horizontal position without cross- frames. The other two assemble them with the webs vertical, generally without crossframes. Several fabricators test-fit sev- eral crossframes if there is a large skew or complex geome- try. Six fabricators assemble bearing to bearing (several with a minimum of three girders). Nine assemble a minimum of three girders. One fabricator pointed out that the issue of the minimum number of girders in an assembly depends primar- ily on span lengths, individual girder lengths, and the radius for curved structures. Curved I-Girder Assembly Ten of the fabricators assemble curved I-girders with their webs in the horizontal position. The remaining five assemble them mostly with their webs vertical, the exceptions being when the radii are short. (The responses included limits of less than 1,000 ft, 600 ft, and 500 ft, depending on the fabri- cator.) Eight fabricators assemble bearing to bearing, and the balance use a minimum of three members. Of those that assemble the girders with the webs vertical, a large number will assemble some, many, or all crossframes for test-fit, depending on the complexity of the structure. Straight Box-Girder Assembly Of the 15 fabricator respondents, 13 fabricate box girders. Nine of these fabricators assemble bearing to bearing and/or a minimum of three girders. Two assemble pier diaphragms and two others assemble with crossframesâone only if they attach to the web and flange. One also may drill crossframe connections from the solid in assembly. Curved Box-Girder Assembly All respondents that fabricate boxes do bearing-to-bearing and/or a minimum of three-girder assembly. Five assemble curved box girders with crossframes. Two others assemble them with crossframes if the geometry is complex and one assembles them with crossframes if the radius is less than 500 ft. Two others assemble them with pier diaphragms only. One also drills the crossframes in assembly. FIELD CONNECTION PRACTICES The fabricatorsâ opinions on proper field connection prac- tices of erectors are briefly summarized in this section. Drift Pins The proper use of drift pins during erection was the most com- mon issue reported by the fabricators, who noted the following: ⢠Erectors do not use enough pins. ⢠Some erectors take the position that if they can install a bolt, they do not need to use pins. ⢠Erectors need to use full-size pins. Crossframes Sufficient crossframes need to be installed to stabilize the structure. Other comments provided by the fabricators rel- ative to crossframes include that progressive assembly of crossframes and bolted crossframes for skewed bridges are difficult to do. Bolt Tightening Bolts should not be tightened until a horizontal and vertical alignment of the members has been made and accepted. CHAPTER THREE FABRICATOR PRACTICES AND VIEWS
OTHER IMPORTANT ERECTION CONSIDERATIONS Substructure Alignment Horizontal and vertical alignment of the bearings and anchor bolts should be accomplished (by others) before erection begins. Adjustments for any errors in elevation or location should be made at this time. Ground Assembly When members are to be assembled on the ground, they should be properly blocked (in the no-load condition or the same condition as that used in the shop), pinned, and bolted. Also, the alignment should be checked before lifting the mem- bers in place. One fabricator suggested that the erector obtain records of the actual blocking dimensions recorded in the shop during the shop assembly. Sequence of Erection The sequence of erection has a great impact on the overall geometry. Owing to fabrication and erection tolerances and practices, member deflections, and bolting/pinning practices, both horizontal and vertical alignment and the stability of the structure are controlled by the sequence of the erection of the girders, as well as the attendant cross- frames. 10 Falsework Proper placement of the required falsework is essential. Ele- vations should be set to account for tolerances and to match shop blocking dimensions. Additional cranes may be used as a substitute for falsework, if necessary or desirable. KEY ISSUES FOR A PROPERLY ERECTED BRIDGE In the collective opinion of the fabricators, a properly erected bridge should involve the following: ⢠The designer should address constructability issues (e.g., differential deflection) and keep the design simple. ⢠The owner should enforce submittal and approval of an erection procedure prepared by the erector. ⢠The fabricator should understand the geometric features and how they affect erection, properly match-mark the splice plates, maintain appropriate sweep and camber tol- erances, consider complexity of the structure when deter- mining shop-assembly method, accurately drill holes, and properly detail the structure. The erector should provide an erection procedure that addresses measures that lead to the desired erected structure, keep the ends of the girders aligned and webs vertical, prop- erly orient match-marked splice plates, and use qualified erec- tors and experienced crews. In general, horizontal and vertical alignment verification of both the substructure and the superstructure at all stages of the process is critical to attaining a well-positioned and erected structure.
