 Case Study
Wacker
Drive Reconstruction - Phase 2 Design
- Demolition of 75 year old beyond-life viaduct
- Replacement with biaxially post-tensioned, high performance
concrete slab structure
- Vehicle load optimisation for irregularly supported deck
 Wacker Drive is a major
two-level viaduct bordering the north and west sides of Chicago’s downtown
"Loop." Owing to severe corrosion of the reinforcing steel and spalling of the
concrete cover, the Chicago Department of Transportation decided to replace the existing
75 year old viaduct with a biaxially post-tensioned, high performance concrete slab
structure. Phase 1 of the reconstruction work is scheduled for completion in November
2002. LUSAS Bridge analysis software is being used by Alfred Benesch & Company for the design
of an 1800 foot (550 metre) length of the second reconstruction phase.
 Overview and Re-construction Phases
The lower level of Wacker Drive provides access for service vehicles to
57 high-rise buildings. The upper level intersects with 20 streets and connects with
bascule bridges that cross the Chicago river. About 60,000 pedestrians, 180,000 vehicles,
and 20 bus routes use Wacker Drive every day. The first phase of the
re-building project is over 4000 feet long and has a typical width of 114 feet. It has a
construction cost of $200 million and construction is scheduled for completion in November
2002. The second phase, which is anticipated to start in early 2004, is over 2900 feet
long, 134 feet wide, and also has an estimated construction cost of $200 million.
 Phase 2 Design
Alfred Benesch & Company is responsible for the design of about 1800
feet (550 metres) of the second phase of the project. The cross-section of this phase of
reconstruction consists of three to five unequal continuous spans varying from 12 feet to
46.5 feet. Longitudinally, the structure typically spans 32 feet with some longer spans
across the lower level street intersections. Typically, a 2 foot deep, 4 foot wide
longitudinal rib runs along each column line. At some locations, the rib depth or width is
increased depending on the spacing between the columns. The deck slab between the ribs is
typically 13" thick and is post-tensioned in the transverse and longitudinal
directions. This design concept requires an increased effort on the designer’s part
in order to accommodate the post-tensioning tendons without conflict in both directions
while maintaining the stresses in the deck. This issue is solved by banding the profiled
tendons in ribs in the longitudinal direction with straight tendons in the slab between
adjacent ribs. Banding of the profiled tendons allows for providing uniformly distributed
profiled tendons in the transverse direction. In this manner, the deck acts as a one-way
slab in the transverse direction supported by the prestressed concrete beams formed by the
ribs and the banded tendons.
The geometry of the deck, and the adopted design concept presents some
unique modelling and analysis challenges. The lack of uniformity in the spacing of the
columns supporting the deck adds to the modelling complexity for live load cases. In
addition to traffic loading, there will be heavy planters on the roadway dividers and
pavements that also require modelling. Since the Chicago Department of Transportation had
instructed that planters could be positioned anywhere within the roadway dividers and
sidewalk, the planters are modelled as live loads to determine their maximum effects on
the deck.
 Modelling and
Analysis
The 1800 foot long viaduct structure is comprised of 9 sections with
expansion joints between each. As a result a separate LUSAS finite element model is being
created for each section. The geometry of the deck is modelled using 3D shell elements,
which enable the definition of different thicknesses for the slab and the ribs as well as
the defining of an eccentricity for offsetting the bending planes of the two geometries.
In order to model the bearing supports on columns, spring stiffness supports are being
used at 4 points along the perimeter to define the bearing area. Doing this smoothes out
the peak negative moments over the columns and results in an average 25 percent reduction
in value compared to using one-point supports. Post-tensioning is modelled by applying
equivalent loads that account for prestress losses. These loads are generated using
spreadsheet macros that automatically analyze the post-tensioning forces in profiled
tendons. Several static analyses are carried out on each section for dead and live
loadings.
Vehicle Load Optimisation
 The Vehicle
Load Optimisation facility in LUSAS, incorporating the well-known Autoloader software,
helps significantly in speeding-up the determination of the design live load moments in
accordance with the AASHTO Standard specifications. Using this facility, vehicle and other
live loading patterns are determined from influence analyses performed for selected
critical locations, and live load design moment envelopes are then generated. These
analyses have illustrated that using a standard AASHTO HS20 Truck on all three lanes
produces the controlling load for the structural configuration of the viaduct.
Alfred Benesch & Company’s
analysis work will continue for many more months on this project but the benefits of using
the LUSAS Vehicle Load Optimisation facility have already been
realised. As Project
Engineer, Dr Ihab Darwish, says: "Using the LUSAS Autoloader facility for load
optimization expedited the live load analyses for a highly repetitive task that would have
otherwise been extremely time consuming."
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Software Information
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