Steel Bridge Erection Practices (2005)

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Owner-specified or preferred practices related to steel bridge erection were reported in the questionnaires and follow-up telephone interviews. These practices are categorized by design, fabrication, or erection. DESIGN Flange Dimension Requirements Owners restrict flange dimensions to limit unwieldy flexibil- ity during handling and distortion after welding. Only one owner, Maine, restricts the width-to-thickness ratio (b/t) of flanges to between 12 and 20, with a preferred ratio of 16. Three owners limit the width of the flange to a minimum of 300 mm (12 in.). These owners also limit the thickness to a minimum of either 19 or 22 mm (3/4 to 7/8 in.). Kansas has minimum flange dimensions that are a function of span length. Texas, although having no requirements, presents some preferences on the National Steel Bridge Alliance website (http://www.steelbridge.org-texas-Preferences-Nov- 2000.doc). Although Washington State has no specific lim- its, officials there believe that some in-house guidelines and AASHTO specifications would be helpful. It should be noted that in Section 6 of the AASHTO LRFD Bridge Design Specifications (1), an upper bound of 12 is placed on b/t for tension and compression flanges. Member Length-to-Flange Width Ratio (L/b) Two owners require that L/b for compression members be less than 85. Minnesota requires an L/b ratio of between 80 and 85. Maine prefers that L/b for welded beams not exceed 90, but up to 110 may be used if economy dictates. Again, Texas pre- sents some recommendations on its website. In-House Steel Specialist or Advisory Group Fifteen owners have a steel specialist or advisory group within their organization that reviews and provides assistance to designers developing steel bridge plans. Analysis Methods for Complex Structures A wide range of computer software and analysis methods were reported as being employed when there were concerns about 6 actual deflections and rotations owing to structure complexity. These methods ranged from three-dimensional, finite-element analysis through simple one-dimensional, line-girder analysis. This range represents a tremendous variation in analytical sophistication and accuracy in capturing system behavior. The state DOT respondents expressed varying opinions that the impact of the sophistication of the analysis method on erection procedures varied from minimal to total impact. FABRICATION Shop-Assembly Methods The AASHTO/National Steel Bridge Alliance Steel Bridge Fabrication Guide Specification allows either full girder or progressive girder assembly (2). A review of specifications submitted in response to the questionnaire shows that seven owners require full girder assembly unless contract docu- ments specify otherwise. Fourteen owners and the AASHTO LRFD Bridge Construction Specifications (3) either specify or allow progressive girder assembly as a first choice. Five of the owners apparently have no shop-assembly require- ments, as no such provisions appear in their specifications. There are various requirements for progressive girder assembly, whether first choice or an option to full assem- bly. They range from minimal numbers of girders (two or three) or spans (one or two) per assembly to no specified minimums at all. Alternate Shop-Assembly Methods Allowed Most owners allow alternate shop-assembly methods. Vari- ous additional requirements noted were: • Must be approved, • Should be equal or better than specified methods, • Should be based on fabricators’ performance, • Must give credit based on cost differential, • Used only if stresses and tolerances are within design limits, • Must be in writing, • Must meet minimum specifications, • Only be considered if framing is simple, CHAPTER TWO OWNER-SPECIFIED OR PREFERRED PRACTICES

7• If requirements are generally upheld for complex geometries, • If it makes sense, • Provided that an unsatisfactory product will be cor- rected by the fabricator, and • If the fabricator assumes full responsibility for the procedures. Oversized Holes Sixteen owners allow oversized or slotted holes under some circumstances to facilitate fit-up of diaphragms or cross- frames. Another allows only vertical slots to permit differ- ential movement between girders during deck pour (staged construction or bridge widening, not staged deck placement). Ten owners prohibit the use of oversized holes. Load Condition for Detailing Crossframes Three owners indicated a need to research the appropriate condition at which to detail crossframes: no-load, dead-load, or full-load condition. One owner is developing a set of spe- cial provisions dealing with this issue. Another owner volun- teered that it requires crossframes to fit in the no-load condition (now a standard note on design drawing) for curved girder bridges, and in the steel-only dead-load condition for straight bridges. ERECTION Erection Procedures The vast majority of responding owners (27 of 32) do not require the designers to provide an erection procedure for com- plex structures. The other five owners specifically require the designer to provide an erection procedure as a part of the con- tract drawings. On the basis of its experiences with severe problems with erection of curved girders, one owner noted the following among its standard procedures: When designing curved girder structures, designers must inves- tigate all temporary and permanent loading conditions, includ- ing loading from wet concrete in the deck pour, for all stages of construction. Future decking must also be considered as a sepa- rate loading condition. Diaphragms must be designed as full load carrying members. A three-dimensional analysis representing the structure as a whole and as it will exist during all intermedi- ate stages and under all construction loading is essential to accu- rately predict stresses and deflections in all girders and dia- phragms and must be performed by the Designer. The designer is responsible for ensuring that the structure is constructible and that it will be stable during all stages and under all loading conditions. To achieve this end, the designer must supply basic erection data on the contract plans. This information must include, but is not limited to, the following: • Pick points and reactions at pick points for all girder sections; • Temporary support points to be used during all stages and loading conditions, and reactions for which support towers should be designed at all of these points; • Deflections to be expected in all girders under all con- ditions of temporary support and under all anticipated loading conditions; and • Direction pertaining to the connection of diaphragms to ensure stability during all temporary conditions. The opinions of the respondents on the value of erection procedures provided by the designer ranged from “a positive effect” to “a waste of time and money.” A smaller majority of the owners (17 of 32) require the erector to submit an analysis and erection procedure whether or not the procedure was performed by the designer. The com- ments of individual respondents suggest that their require- ments are not as rigorous, stating that the submitted erection procedures were for information only or record purposes, or both; required, but many times not actually submitted; and not necessarily based on analysis. A few of the remaining owners stated that the erector may be required to submit erec- tion procedures if specified in the special provisions or con- tract plans. Nineteen owners reported that they provide some sort of review (ranging from casual to thorough) of the erection pro- cedures if submitted by the erector, but they apparently do not go so far as to approve the procedures. Among those owners that stated that they did not review the procedures, the fol- lowing comments were added: “contractor’s responsibility,” “would if requested,” and “stay out of approval or checking.” Preferred Field Connection Practices Seven owners expressed a preference for field connection practices that lead to good final geometry. Shop assembly, good field inspection, verification of shop and field measure- ments, and use of experienced personnel are the most cited preferred practices. Texas uniquely indicated that bolting or welding of field connections work equally well when prop- erly executed. Oklahoma indicated a preference for direct tension indicators to aid inspection of bolted connections. Proven Methods for Erecting Complex Structures Eighteen owners reported that temporary supports and/or bracing have proved valuable in erecting complex struc- tures. Launching of girders, particularly box girders, was cited by three owners as a proven method. Four owners believed that full shop assembly is useful in ensuring more easily erected complex structures. Several owners cited novel methods for erecting unique complex structures. For curved

8bolts; a maximum of 15% pins and sufficient bolts to keep pieces together; and 25% bolts with the number of pins deter- mined by the engineer. Because the pins are important in setting the geometry, the stage at which the pins are removed (before or after bolts are tightened) may have an impact on the geometry. Three owners specify that all holes not containing pins be filled with tightened bolts before removing the pins. Two owners specify that all the holes be filled with snug-tight bolts, whereas one other owner specifies that all holes be filled with finger-tight bolts. Twelve owners’ specifications reflect several varying requirements as to when the bolts should be tightened. The major difference seems to be whether they should be tight- ened after full girder lines, at full structure, or as part of the structure has been erected, and if the horizontal and vertical alignments require verification. Several owners reported problems with field inspections and verification procedures. Problems ranged from inexperi- ence and failure to inspect to failure of inspectors or project engineers to require the contractor to follow the approved erection procedures. box girders, one owner prefers only one bearing at a support for a single box. Pinning and Bolting Procedures Inadequate pinning and bolting practices, including field ver- ification of horizontal and vertical alignment, were reported to be the cause of a number of problems. Conversely, many respondents listed good pinning and bolting practices as essential to achieving an effective erected structure. The use of pins during the erection of the structure is an important concern because the pins are very nearly the same diameter as the drilled or reamed hole. This situation allows very little movement, and consequently it is critical in setting the final geometry of the structure. The AASHTO specifica- tions and seven owners require an initial minimum require- ment of 25% pins and 25% bolts in each connection. Nine owners cited an initial minimum requirement of 50% of the holes filled with pins or bolts. Individual owners specified various requirements: at least two pins in extreme hole loca- tions; pins in extreme corners of splices; eight pins in each flange and web splice; 33% pins; a minimum of four pins; a preference for 25% pins, 15% pins, 50% pins, and 50% bolts in main splices; 50% pins and 50% bolts; adequate pins and