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13
CHAPTER FIVE
REPORTED PROBLEMS ENCOUNTERED IN THE FIELD
DISTORTION OWING TO DECK CANTILEVER versely adjacent part of the deck [Stage 2]." Problems develop
BRACKETS owing to the transverse differences in elevation between the
Stage 1 deflected position and the undeflected position of the
Deck cantilever brackets are used to support deck forms and Stage 2 members before pouring the Stage 2 concrete. Cross-
the screed. If the bracket's diagonal strut does not intersect the frame connections between Stage 1 and Stage 2 girders require
flange, but instead the girder web at a point above the bottom
special considerations. These problems may be magnified in
flange, there is a potential for web deformation or fascia girder
curved or skewed structures.
rotation, or both. The bracket rotation is in proportion to the
actual deformation of the web as well as to any twist that may
Eight owners reported problems as a result of unantici-
develop in the girder as a result of the cantilever load. The dis-
pated lateral movement and rotation of girders during deck
tortion will depend on the force in the diagonal, location of
pour, including the installation of crossframes between stages.
the diagonal/web intersection relative to the flanges, web
In addition, two fabricators cited problems arising from stage
thickness, how close the bracket is to a transverse web stiff-
construction. One specifically reported problems with con-
ener or crossframe, and any temporary support or stiffening
necting crossframes between stages.
provided.
Nine owners reported problems as a result of the displace- GIRDER STABILITY
ment at deck cantilever brackets during the concrete pour.
These problems included insufficient deck thickness and poor The stability of girders during shipping, lifting, and erect-
deck profile, resulting in poor riding characteristics and/or ing, and before completion of placement of the deck, is an
the potential for ponding. important concern. As noted in the responses, there are a
number of factors (e.g., b/t and L/b of flanges, crossframe
One owner, Wyoming, cited a rule of thumb that web dis- design and erection practices, wind loading, temporary sup-
tortion becomes a problem when the overhang length exceeds ports, and length of cantilevers) that need to be addressed to
the girder depth. ensure stability of the individual members.
Five owners reported problems with, or at least concerns
THERMAL DISTORTION OF SUN HEATING
ERECTED MEMBERS about, maintaining girder stability during the various stages of
construction, up to the final condition. One owner reported that
The expansion and contraction of parts of individual steel a girder fell because not enough crossframes were installed
members owing to thermal effects from the sun can cause the before releasing the crane. Two owners reported problems
horizontal and vertical alignment of the member to change specifically with the ends of girders cantilevered from a pier to
continuously during the course of a day. a field splice. Also, four owners reported specific stability
problems as a result of winds during construction.
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, UNANTICIPATED DISTORTION
or both, owing to thermal expansion caused by heating from
General
the sun.
Two erectors agreed that there are generally few problems
UNANTICIPATED RELATIVE DISTORTION with the alignment of straight girders. Problems occur with
BETWEEN CONSTRUCTION STAGES deflection, web verticality, and elevation on certain highly
skewed or curved and skewed bridges. The issue pertains to
As noted in chapter one, in the section on terminology, for pur- at what load condition the webs should be vertical and what
poses of this document, stage construction is defined as "The tolerances are applied to "vertical." Because skewed and
construction condition where the deck on part of the bridge curved girder bridges rotate under dead loads, the "desired
has been poured and cured [Stage 1] prior to pouring a trans- plumb" condition must be established in advance or ignored.
