MEMORANDUM
DATE: February 26, 2008
TO: Homayoon Pirooz, P.E.
Manager, Project Management Unit
FROM: Michael G. Nearing, P.E.
Senior Project Manager, Project Management Unit
RE: 2006-2007 Bridge Inspection Program
Reduced Weight Postings for E. Stadium Boulevard over S. State Street
File No. 2006-014.17 (mgn)
We are writing to inform you of the recent investigation and
analysis that was performed on the E. Stadium Boulevard Bridge over S.
State Street. We are also reporting on the condition of the bridge and
providing information regarding possible short- and long-term solutions
for the E. Stadium Boulevard Bridge over S. State Street and the E.
Stadium Boulevard Bridge over the Ann Arbor Railroad tracks.
Attached, please find a Traffic Control Order (TCO) that
recommends a reduction in allowable gross vehicle weights for the E.
Stadium Boulevard Bridge over S. State Street in accordance with the
limits contained in this memorandum.
Background
On December 29, 2007, our Field Operations forces were called
out to the E. Stadium Boulevard Bridge over S. State Street as
medium-sized pieces of concrete were falling off of the bottom of one
of the box beams that support the roadway. These pieces of concrete
were large enough to damage passing vehicles, if they were to strike
them. Our Field Operations and Safety Services personnel closed one
lane of traffic at a time and removed the remaining loose concrete from
the underside of the box beam. No injuries due to the falling concrete
were reported.
As a result of the concrete removal, we reviewed the structure
again and were concerned about the condition of one of the box beams.
In early January 2008, a meeting was held on-site between Project
Management Unit, our bridge inspection consultant, and bridge design
personnel to review the condition of the bridge and (1) determine what
possible courses of action could be taken to help prevent further
damage to the bridge and (2) identify possible repair strategies.
We directed our consultants to re-analyze the bridge structure
given the section loss that had just occurred. As you may recall,
effective January 1, 2008 we lowered the weight limits due to section
loss that was discovered during the biennial inspection that was
performed in late October 2007. Based upon the new analysis that was
performed in January 2008, it is recommended that we reduce the weight
limits on this structure again.
Condition of the bridge
The existing bridge is composed of 16 pre-stressed concrete box
beams that are laid side-by-side. They are “tied” together with steel
post-tensioning rods that connect pairs of beams together. The beams
are overlaid with an asphalt wearing surface. There is a concrete
sidewalk that has been cast on top of the two northernmost beams. The
bridge does not provide sufficient vertical or horizontal clearances
and is considered structurally deficient and functionally obsolete.
One of the box beams has developed a severe longitudinal crack.
This is the beam that our field personnel had to hand-chip to remove
the spalled concrete in late December 2007. There are other beams that
are developing longitudinal cracks, as well. Other beams have several
broken pre-stressing strands and the abutments that support the beams
are in poor condition. Note that the pre-stressing strands are the
primary elements of the bridge beams that provide structural strength.
Based on these deficiencies, the bridge, viewed as a whole, is
considered to be in poor condition. The beam that is severely cracked
is considered to be in serious condition.
Upon review of the “as-built” drawings of the structure, we
believe that it is not possible to repair the severely cracked beam in
any manner that can restore its structural integrity. This is due to
the location of the beam itself, and the location and configuration of
the post-tensioning rods that tie the box beams together. The severely
cracked beam is the fifth beam in from the south side of the
structure. Due to the manner in which the bridge was constructed, it
is not possible to replace only this beam. In order to repair the beam,
we would have to cut the post tensioning rods on each side of the
beam. We would not be able to access the needed areas of the beams to
re-install the post-tensioning rods, however, effectively leaving the
beams to function individually. This would be unacceptable. There is
also one other beam that contains small cracks that currently are not
as severe as the beam described above, but we believe that it is likely
that it, too, will crack in a similar manner. AS A RESULT, WE BELIEVE
IT WOULD BE NECESSARY TO REPLACE AT LEAST FIVE BEAMS ON THE SOUTH SIDE
OF THE BRIDGE, AND POSSIBLY AS MANY AS EIGHT.
Exacerbating the problem is the condition of the remaining
beams. We believe that it is likely that the tops of these beams have
begun to spall. It is apparent to us that salt-laden water has seeped
between the asphalt layer covering the beams and the beams themselves.
Water routinely leaks around the beams and runs down the abutment
walls. Over time, this moisture and salt will cause the concrete to
disintegrate. Given the length of time this has been occurring, we
have reason to believe that it is possible that the tops of the beams
are damaged to the point that they may not be able to be repaired, if
we were to attempt it.
We also believe that any attempt at performing a repair itself
would be expensive. We do not have detailed costs at this time, but we
estimate that it would cost at least $250,000 to attempt to replace the
five to eight south side bridge beams. Also, it would take at least
one month to perform the needed construction.
At your request, we could prepare a more thorough and detailed
estimate of the repair costs and other possible options that could be
implemented should funding for the replacement of the structure not be
available in the foreseeable future. This evaluation would cost
approximately $15,000 to retain the services of a structural engineer
to analyze and review various options. We also recommend that about
$10,000 be budgeted for our Field Services personnel to assist in the
performance forensic investigations on the bridge in order to better
determine the extent of deterioration of the existing bridge beams and
abutments.
We do not believe that it is possible to repair the bridge over
S. State Street cost effectively, however. This is due to the
extensive deterioration of the existing beams, abutments, and
asphalt-wearing surface. As mentioned previously, the bridge is
considered structurally deficient and functionally obsolete. These
deficiencies cannot be easily remedied. We will, of course, continue
to monitor the condition of the bridge. Should it become necessary, we
are prepared to repair the structure in order to allow the bridge to
remain in service.
Proposed Short-term Action
In order to help minimize damage to this structure and prolong
its life span to the greatest extent possible, we are recommending that
the weight limits be lowered. We have prepared the following table
that compares the existing and proposed weight limits for the three
classes of trucks that can use the structure:
Design
Vehicle Existing
Posted Load
(since January 1, 2008) Proposed
Posted Load Reduction in
Load carrying capacity
Single Unit Truck 31 tons 19 tons 39%
Two-Unit Truck 39 tons 24 tons 39%
Three-Unit Truck 44 tons 26 tons 41%
The proposed Traffic Control Order (TCO) recommends a reduction
in gross vehicle weight for the three types of trucks that can travel
on Michigan roadways without special permits. They are one, two, and
three-unit trucks. A single-unit truck is any truck without a
trailer. A single-unit truck can be any number of common vehicles.
Some examples of this would be a school or AATA bus, moving van, or in
the worst case, a fully loaded concrete truck. A two-unit truck can be
most easily described as a semi-truck with one trailer. Note, a
pick-up truck or car pulling an “ordinary” trailer (such as the
trailers that lawn maintenance companies would use) would not be
considered a two-unit vehicle. Finally, a three-unit truck can be most
easily thought of as a semi-truck pulling two trailers. An example of
this would be the large sand or gravel hauling trucks that bring
materials to construction sites.
The recommended gross vehicle weight reductions are
significant. Also, the overall condition of the bridge has declined
measurably over the course of the last calendar year. We expect that
the condition of the structure will continue to decline as the
structure is nearing the end of its useful life. Based on the rate of
deterioration that we have observed over the course of the last few
years, we believe that the structure has approximately three to five
years in which it will be able to carry trucks. We also expect that
additional weight limit restrictions will be necessary in the future.
Proposed Long-term Actions
As you may recall, we have begun a project to perform the
preliminary design of the replacement for this structure, as well as
the E. Stadium Boulevard Bridge over the Ann Arbor Railroad tracks.
The bridge over the Ann Arbor Railroad tracks is located about 300 feet
to the west of the bridge over S. State Street. We believe it makes
sense to consider replacing both bridges at the same time, although it
is also possible to replace the two bridge structures at different
times.
We have not able to move forward on the preliminary design of
the bridge over S. State Street or the Ann Arbor Railroad tracks
because we are waiting for the 4th Ward City Council members to
nominate and confirm a Citizens Advisory Committee to assist us with
the public engagement process. It has been our experience with
projects of this nature that it is important to consider and review all
aspects of the proposed design with the citizens in order to make sure
the project that we deliver meets community expectations. This process
will also help us to avoid conflicts later in the design process as we
will have already confirmed the design approach and aesthetics of the
project with the public and City Council. Finally, this preliminary
design process will allow us to better estimate the cost of the project
based on a more complete picture of all project elements.
We have estimated that it will take about 2 to 3 years to
prepare for bridge replacement including the public engagement and
internal project review process; the preliminary and final design of
the bridges and needed approach work; and the development of a funding
plan for the project. We have prepared a tentative schedule for this
project such that its construction could be started shortly after the
University of Michigan’s Football Stadium Renovations are completed.
We are also planning and coordinating this project so that it is
compatible with the planned improvements along W. Stadium Boulevard.
Attached, please find a drawing that we have prepared that locates all
of these planned improvements and their anticipated starting and
completion dates. Also attached, please find the tentative project
schedule that details the required tasks and estimated durations of the
required activities to complete the project’s design.
Summary
Fortunately, the reduction in weights that we are proposing does
not yet affect most of the everyday users of this structure. However,
we believe that when it is again necessary to reduce the posted weight
limits, both AATA and the Ann Arbor Public Schools will be impacted by
this change.
The proposed change will impact the trucking and construction
operations that use this corridor as they, most probably, will not be
able to route trucks through this portion of the E. Stadium Boulevard
corridor between S. Industrial Highway and S. Main Street. These types
of trucks will have to find an alternative route around this portion of
E. Stadium Boulevard.
We are also working with the Communications Office to prepare a
press release notifying the appropriate agencies of the proposed change.
attachments
HP:MGN:mgn (e stadium weight reduction memo 080225.doc)
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