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The original bridge possessed two 12-foot lanes
plus a 5-foot sidewalk on each side of the structure and expansion joints in
the deck at every other spandrel column. The expansion joints have been
leaking for some time, causing deterioration to the deck slab and supporting
spandrel columns resulting in the need to replace the superstructure. The
bridge superstructure, including the deck slab, sidewalk, and spandrel
columns was in need of replacement down to the arch ribs.
Rehab starts
The rehabilitation consisted of devising a
reconstruction staging scheme, designing the new superstructure to match the
original design while meeting current structural specifications, and, where
possible, improve drainage and geometric characteristics of the bridge and
approaches.
The structural characteristics for an arch bridge of
this type preclude the normal approach to its demolition and reconstruction.
This structure contains two arch ribs, or two main structural members for
each span, deeming the bridge non-redundant.
If the structural integrity of either arch rib in a
span is compromised structurally, impending collapse is possible. The
removal or addition of any part of the superstructure during the demolition
or reconstruction process drastically affects the stresses in the arch ribs.
Thus, an in-depth structural investigation was necessary to evaluate the
demolition and reconstruction staging processes.
A three-dimensional finite element model of the
bridge was created to evaluate the stresses in the arches of the bridge
throughout the construction process. Many possible staging schemes were
evaluated with each staging scheme requiring numerous finite element models.
A typical model would include representative loading patterns of the bridge
during a certain stage of the demolition or reconstruction process, and
construction vehicle placement and/or movements.
Environmental effects were also a consideration when
devising the staging scheme. To preserve the pristine Paint River and
immediate surroundings around the bridge, the staging scheme would have to
keep all large construction vehicles on the bridge or road approaches. This
requirement ultimately led to staging the reconstruction of one span at a
time due to limits on crane and concrete-pump truck reach.
Once a staging scheme was developed, plans were
created for the staging and included in the final plan set. The final scheme
was viewed as a starting point for the contractor to work from, requiring
future refinement and coordination between URS and the contractor.
|
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| The entire deck and
sidewalks are fixed at the pier and allowed to expand or
contract longitudinally from the pier, independently from the
rest of the structure. |
Structure improvements
The new superstructure was designed with several
characteristics to improve drainage, geometry, and structural performance of
the bridge. The existing concrete deck had leaking expansion joints located
about every 21 feet, 2 inches along the length of the bridge. These
expansion joints led directly to the deterioration of the bridge and were
considered a design flaw in terms of future maintenance. It was proposed to
remove them entirely for the new structure.
A new joint-less deck would create a problem, in
that expansion and contraction forces generated from thermal movements of
the deck would be transferred down through the spandrel columns into the
arches. The design solution to this dilemma was to separate the deck slab
from the spandrel columns and allow it to float on guided elastomeric
bearings atop the spandrel columns.
The entire deck and sidewalks are fixed at the pier
and allowed to expand or contract longitudinally from the pier,
independently from the rest of the structure. To prevent independent lateral
movements of the superstructure, galvanized steel pins penetrate galvanized
slotted plates between the elastomeric bearings at each spandrel column. The
slotted holes allow for only longitudinal movements of the superstructure.
The original profile of the bridge included a
vertical curve with the point of vertical intersection located at the pier.
The PVI for the new structure was raised, resulting in a top-of-deck
elevation at the pier, 3-inches higher than existing for improved water
movement off the structure. The new concrete deck cross-slope was also
increased to meet current standards.
The original structure carried two 12-foot lanes of
opposing traffic. A large portion of the current traffic consists of
oversized logging trucks or manufactured home carriers, which may cause the
typical motorist to feel somewhat cramped in certain traffic situations. To
help eliminate the tight, constrictive feeling at the bridge, 2 feet were
added to the curb-to-curb clear distance of the bridge deck.
To accommodate the width increase, a combination of
increasing the superstructure width 6 inches and decreasing the sidewalk
width 6 inches on each side resulted in the net increase of 1 foot for each
vehicle lane. The new sidewalks, with 6 inches less width, still meet
current design standards and were accepted for the minimum pedestrian or
snowmobile movements across the bridge. Use of current materials allowed for
a thinner concrete deck resulting in a wider superstructure, but no increase
in dead load, or weight applied to the arches.
Preserving details
Since the Paint River Bridge is on Michigan’s list
of historic bridges, the designation requires any rehabilitation or
reconstruction work to match the original with respect to appearance and/or
materials. Design of the new superstructure had to match the original, while
meeting current design standards.
The original structure included architectural
railing and light pilasters along the sidewalk perimeter, but due to safety
and maintenance concerns, the railing was replaced in the early 1970s with a
less attractive substitute.
The historic nature of the structure required that
any reconstruction match the original as closely as possible. Due to
structural and safety concerns, reconstructing the original barrier railing
was an issue.
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To
decrease forming and curing concrete time, precast concrete for
several super-structure elements was recommended. |
The solution to this quandary was to reconstruct the
railing with the appearance of the original, but with structural
improvements or additions, where needed. Structural steel tubes were added
spanning between the railing pilasters in front of the concrete railing.
These steel tubes act to transfer traffic accident “hit” loading to the
concrete pilasters located every 10 feet, 7 inches along the sidewalk, and
are invisible when viewing the bridge from the river or adjacent park.
The original appearance of the railing pilasters and
light pilasters were also recreated, but designed with appropriate
reinforcement to transfer loading down through the superstructure.
The non-functioning original lights and concrete
light poles atop the pier and abutments act as the main architectural focus
of the superstructure and seem to act as sentries for vehicles or
pedestrians crossing the bridge. The new lights and light poles were
designed to match the original in scale and appearance, but with a current
electrical system and materials.
Precast concrete
The schedule for rehabilitating the bridge in one
construction season was very restrictive for the contractor due to the
extreme staging sequence. To decrease forming and curing concrete time,
precast concrete for several superstructure elements was recommended.
The concrete transverse beams spanning between
spandrel columns, hundreds of individual railing balusters, and light poles
were all precast by the contractor, as recommended.
Precasting these elements also produced a superior
product because fabrication tolerances are greater, environmental effects
are lessened or eliminated, and forms may be used repeatedly when an element
is shop cast and cured.
The transverse beams, for example, were cast, cured,
and stored in a warehouse through the winter by the contractor. Thus, the
beams had a better finish and attained a higher strength before being
exposed to any loading due to curing the entire winter.
Approach improvements
The existing bridge approaches had a number of
geometric and drainage deficiencies. When the bridge was originally built,
the roadway standards were obviously developed to correspond to much lower
design speeds and hydrology requirements.
The western approach had a horizontal curve just off
the bridge with a deficient superelevation. The corrected superelevation of
the new approach provides for much safer driving conditions, especially
important for the large log trucks that tend to accelerate down the hill to
the west when approaching the bridge.
The eastern approach also had geometric deficiencies
in need of correction. The new approach was designed to provide adequate
length for the lane width transition and proper vertical alignment by using
an asymmetrical vertical curve.
During the design process the local MDOT and city
authorities expressed concern about the occurring flush overflows every now
and then in the vicinity of the bridge.
The bridge is located at the bottom of a steep
sloped valley and experiences significant and sudden rain flows. Some of
these have overflowed the curb line and caused erosion damage to the
southwestern approach.
The new design included redundant drainage
structures, spillway, and storm sewers, as well as full use of the existing
storm system so that the flush-flow problem is resolved with an adequate
drainage system.
 This
project demonstrates that it is possible to preserve the architectural
history of our infrastructure while meeting the standards of today. A
comprehensive study of this unique arch bridge was performed using the most
modern engineering software tools available. This, coupled with modern
materials and construction techniques not only reproduced significant
architectural details, they were made much more structurally efficient,
extending the life of this structure for decades.
Greg Garrett, P.E., is a
structural engineer with
URS Corporation,
Grand Rapids, Michigan.
Reprinted from Better Roads Magazine
August 2004 |
