Click for next page ( 11

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement

Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 10
11 CHAPTER FOUR ERECTOR PRACTICES AND VIEWS This chapter discusses erector practices and views related to for curved members. Their specific discussion of the calcu- steel bridge erection, as reported in the questionnaires and lation of pick points based on certain criteria suggests that follow-up telephone interviews that were a part of this syn- this is the extent of their analysis of girder stability. One thesis. Information is based on responses from four erectors. erector calculates the sum of moments in the transverse direction along the member length when picked to ensure that the girder remains level. The erector also reported that multi- ERECTION PROCEDURE PROVIDED BY OWNER ple cranes or shoring are used. The second erector dis- cussing this topic reported that curved girders can be picked Two of the erectors have not received erection procedures with a single crane using a correctly sized spreader beam or from the owner. The other two have, but they have diametri- by using two cranes. The location of the pick points can be cally opposite opinions on the procedure's worth. One believes calculated so that the girder is picked straight without roll. that a required erection procedure from the designer has a That erector applies the rule of thumb that picking at two positive effect because the designer must think through the points usually eliminates any later stability problems, as scheme and the associated forces to erect the structure. The long as a line between the pick points runs through the cen- second believes that this practice does not improve the qual- ter of gravity of the girder. ity of the erected structure. APPROACH TO ERECTION SEQUENCE FLANGE SIZING REQUIREMENTS FOR STABILITY AND LOCATION OF FALSEWORK Two of the erectors reported that an L/b of 60 or less between The responses to questions regarding approach suggest that unbraced points provides stability during transportation and a rigorous analysis is not employed. Rather, erectors depend erection. One of them went on to report that an L/b value of on experience and intuition. 60 to 80 may be adequate, but further stress calculations need to be verified, and values of more than 80 require temporary One erector indicated that the need to provide temporary support (falsework or holding cranes) to provide stability. support through falsework was based on bridge type, number Another erector indicated that it is desirable that the flanges of spans, and span lengths, indicating that long spans gener- be sized so that each individual girder piece can laterally sup- ally require multiple falseworks to control geometry. Another port itself when erected in a simple span or cantilever condi- erector pointed to site limitations and the size and strength of tion, depending on the erection sequence. With longer spans the individual girder pieces as discriminators on the need for and smaller flanges, temporary lateral support trusses made and location of falsework. Finally, one erector cited the rule of angles and wire rope are often required until adjacent gird- of thumb of using falsework near splices and locating them ers are erected and permanent crossframes and lateral brac- under stiffeners, as well as the need to provide jacking reac- ing are connected. The final erector simply stated that the sta- tion capabilities in falsework to adjust as necessary to main- bility of single girders needs to be addressed. tain proper elevations. An erector suggested that it is often possible to ground assemble two adjacent girders with the lateral bracing and WHERE DOES RESPONSIBILITY crossframes, and then erect them as a single unit. In that way, FOR STABILITY LIE? temporary bracing is not required. One erector raised the important issue of responsibility for bracing for stability, noting that certain states require the con- STABILITY WHEN LIFTING CURVED MEMBERS tractor to determine if lateral bracing is required for the bridge. The problem is that bracing for wind and steel erection issues Two of the erectors discussed maintaining individual girder may not suffice for deck forces or sequence of pour. The ques- stability through the choice of pick points (crane lifting points) tion remains: Who then is responsible at what stage?