Steel Bridge Erection Practices (2005)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

13 DISTORTION OWING TO DECK CANTILEVER BRACKETS Deck cantilever brackets are used to support deck forms and the screed. If the bracket's diagonal strut does not intersect the flange, but instead the girder web at a point above the bottom flange, there is a potential for web deformation or fascia girder rotation, or both. The bracket rotation is in proportion to the actual deformation of the web as well as to any twist that may develop in the girder as a result of the cantilever load. The dis- tortion will depend on the force in the diagonal, location of the diagonal/web intersection relative to the flanges, web thickness, how close the bracket is to a transverse web stiff- ener or crossframe, and any temporary support or stiffening provided. Nine owners reported problems as a result of the displace- ment at deck cantilever brackets during the concrete pour. These problems included insufficient deck thickness and poor deck profile, resulting in poor riding characteristics and/or the potential for ponding. One owner, Wyoming, cited a rule of thumb that web dis- tortion becomes a problem when the overhang length exceeds the girder depth. THERMAL DISTORTION OF SUN HEATING ERECTED MEMBERS The expansion and contraction of parts of individual steel members owing to thermal effects from the sun can cause the horizontal and vertical alignment of the member to change continuously during the course of a day. Although there were no questions asked directly about this problem, three owners reported horizontal and vertical girder movements on curved box-girder or I-girder bridges, or both, owing to thermal expansion caused by heating from the sun. UNANTICIPATED RELATIVE DISTORTION BETWEEN CONSTRUCTION STAGES As noted in chapter one, in the section on terminology, for pur- poses of this document, stage construction is defined as “The construction condition where the deck on part of the bridge has been poured and cured [Stage 1] prior to pouring a trans- versely adjacent part of the deck [Stage 2].” Problems develop owing to the transverse differences in elevation between the Stage 1 deflected position and the undeflected position of the Stage 2 members before pouring the Stage 2 concrete. Cross- frame connections between Stage 1 and Stage 2 girders require special considerations. These problems may be magnified in curved or skewed structures. Eight owners reported problems as a result of unantici- pated lateral movement and rotation of girders during deck pour, including the installation of crossframes between stages. In addition, two fabricators cited problems arising from stage construction. One specifically reported problems with con- necting crossframes between stages. GIRDER STABILITY The stability of girders during shipping, lifting, and erect- ing, and before completion of placement of the deck, is an important concern. As noted in the responses, there are a number of factors (e.g., b/t and L/b of flanges, crossframe design and erection practices, wind loading, temporary sup- ports, and length of cantilevers) that need to be addressed to ensure stability of the individual members. Five owners reported problems with, or at least concerns about, maintaining girder stability during the various stages of construction, up to the final condition. One owner reported that a girder fell because not enough crossframes were installed before releasing the crane. Two owners reported problems specifically with the ends of girders cantilevered from a pier to a field splice. Also, four owners reported specific stability problems as a result of winds during construction. UNANTICIPATED DISTORTION General Two erectors agreed that there are generally few problems with the alignment of straight girders. Problems occur with deflection, web verticality, and elevation on certain highly skewed or curved and skewed bridges. The issue pertains to at what load condition the webs should be vertical and what tolerances are applied to “vertical.” Because skewed and curved girder bridges rotate under dead loads, the “desired plumb” condition must be established in advance or ignored. CHAPTER FIVE REPORTED PROBLEMS ENCOUNTERED IN THE FIELD

External forces are needed to force a girder out of plumb dur- ing erection so that it will be plumb after the deck is cast. There will almost always be some distortion until all of the dead load is applied. At Supports Five owners reported problems resulting from unanticipated distortions at piers, abutments, or other supports. The unan- ticipated distortions cited at supports were either out-of- plane movement of girder webs or end rotations of girders or stringers. Those distortions resulted in a moveable bridge unable to close owing to dead-load stringer end rotations, concrete deck cracking as a result of dead-load girder end rotations, diffi- culty in fitting box girders to bearings and loss of bearing pin keeper plates owing to differential lateral movement of the girder ends. Although some of these problems have occurred at skewed supports, that is not always the case. In the Span Twelve owners reported problems resulting from distortions that have occurred “in span” rather than at supports. The 14 reported problems include webs not vertical, difficulty con- necting crossframes, buckling of K-frame members, poor alignment, a dropped girder, and bolts popping. The prob- lems often are a result of unanticipated differential deflec- tions between adjacent girders in sharply skewed or curved bridges, improper or inadequate use of falsework, poor hor- izontal and vertical alignment control, use of oversized holes, or detailing inconsistencies. Four fabricators reported problems as a result of unantic- ipated distortions in the span of bridges. Two specifically attributed problems to improper use of drift pins. GENERAL COMMENTS ON PROBLEMS One fabricator reported that most problems are the result of human error and are not technical problems. Such prob- lems include designers providing incorrect information on the plans, fabricators exceeding tolerances, or erectors not controlling geometry. Such reports suggest that more care is needed, not a change in practices. Another fabricator raised the issue of webs being cited as out of plumb and the question of what the effect on the bridge actually is.