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17
CHAPTER SEVEN
CONCLUSIONS
The findings of this synthesis study on the erection of steel Respondents reported problems relating to girder stability
bridges are based on survey questionnaires and interviews. owing to wind, crossframe erection sequences, temporary
They are summarized in these conclusions. supports, and deck pouring sequence. Considerations toward
solving the problems could include:
The overwhelming majority of respondents agree that
most of the common problems that occur during the erection · Verification of stability in using the pouring sequence
of steel bridges can be prevented by the following: in positive moment areas,
· Checking for stability of the cantilever end of girder
· Verifying horizontal and vertical alignment before and field section from pier to field splice,
during erection; · Checking the member length-to-flange-width ratio
· Installing enough crossframes to maintain geometry and (several states provide guidance with preferable val-
girder stability during erection; ues between 80 and 90), and
· Properly using temporary falsework or additional · Evaluating the need for lateral bracing.
cranes; and
· Rigorously following pinning, bolting, and tightening Where there are differential deflections between girders at
procedures. the ends of crossframe connections, the girders will rotate
transversely as (1) the dead load of the steel is applied, and
(2) the concrete dead load is applied. Curved bridges and
FINDINGS FOR OWNERS skewed bridges represent the most common examples of
where that condition will occur.
In regard to the procedures used by the designer, states reported
that consideration is merited as to whether the designer should The designer should address the condition and should
include an erection procedure in the design. show on the design drawings whether the structure should be
detailed so that the webs are vertical in no-load, steel dead-
Many states have reported problems with the deck profile load, or full dead-load condition. Article 1.6.1 in the Guide-
resulting from the deflection, rotation, and translation of deck lines for Design for Constructability (AASHTO and NSBA)
cantilever brackets. These deformations can be controlled discusses this issue in detail.
through designer input on support locations to the contractor
or contractor-developed forming plans. Also, it is not uncommon for girders to be out of plumb,
and designers should evaluate the condition rather than spec-
Also, states reported that problems develop in stage con- ulating how much the out of plumb is problematic.
struction as the result of differences in elevation between the
Stage 1 deflected position and the undeflected position of the The twisting of box girders is another situation that needs
Stage 2 members before pouring the Stage 2 concrete. Deck to be considered if there is more than one bearing on either
alignment between Stage 1 and Stage 2 and crossframe con- end of the box. Because of the rigidity of the boxes, provi-
nections between Stage 1 and Stage 2 girders require special sion must be made to allow for field adjustments in the bear-
considerations. ing height to account for any twisting that will occur.
Successfully implemented strategies include the use of External crossframe connections can also be difficult
because of the rigidity of the boxes to both transverse and
· At least three girders in either or both stages to reduce twist movement. Article 3.9 of the Guidelines for Design for
transverse movement during deck pour, Constructability recommends that "If multiple straight boxes
· A closure or construction pour between the two stages, or or tub girders are adequately braced internally, external inter-
· Only a top and bottom strut between girders between mediate crossframes are not required. For curved multiple
stages. If deemed necessary, a strategy would be to add box or tub girders that require crossframes between mem-
cross bracing after the deck pour. bers, use permanent crossframes."
