Decks are constructed of 32 foot long, hollow box segments having a maximum depth at the bridge centerline of 48 inches and tapering to a 24 inch visible profile. During assembly each segment is placed on steel falsework bents that allow longitudinal movement of the segments during placement and longitudinal post-tensioning. Ten inclined cable pairs support the longer span bridge deck and eight cable pairs support the shorter bridge. Main stay cables (provided by CBSI, Inc.) are 2 inches diameter with back stay cables of 3 3/8 inches diameter ASTM A586 Structural Strand.
Staged erection analysis
Analysis with LUSAS was required to model the complex erection sequence involving falsework construction in the river, post-tensioning of the precast concrete deck system, and sequential cable tensioning to lift the structure from its temporary supports to create the free-spanning cable-stayed spans.
Project specifications required that the bridge geometry be set-up at the beginning of its service life to obtain the target geometry after 10 years of service. To accomplish this, time-dependent effects due to creep and shrinkage in the post-tensioned concrete deck segments had to be evaluated. This was achieved by using the CEB-FIP 1990 creep and shrinkage material model in LUSAS.
tower erection each of the 32 foot long deck box segments were
constructed upon structural steel falsework bents and initially
post-tensioned together for continuity using four, 1" diameter
post-tensioning bars. Following the complete longitudinal assembly of
the segments, four 19 strand, 0.6 inch diameter tendons were
installed and tensioned to obtain the required compression in the deck
system prior to cable installation. The LUSAS post-tensioning wizard
was used to model the erection of the deck segments at each stage of
Preliminary tensioning of the stay cables in the LUSAS model was accomplished through initial strain loading of the nonlinear beam elements representing each cable. During the actual stay-cable installation, the longitudinal concrete deck system was to be lifted from its temporary supports through a carefully planned installation sequence. The falsework system was required to allow longitudinal movement and shortening of the deck system as well as unrestrained lift-off from the supports. Modeling for these effects was accomplished through the use of nonlinear joint element supports in LUSAS. Final tensioning of the stay cables was achieved by applying a negative temperature load to each cable pair to obtain the desire tension. Excellent correlation was obtained between LUSAS predicted cable tension values and on-site measurements.
3D solid modelling of box segments
of the box segments contained access openings in the top slab to
provide access for the installation of the longitudinal
post-tensioning system as well as for two tuned mass dampers (provided
by Motioneering, Inc.) to control pedestrian-induced vibrations.
Openings for the tuned mass dampers exceeded 15 foot in length and
created areas of discontinuity in the top slab during lifting
operations and longitudinal post-tensioning. To verify that each of
these locations provided adequate continuity, individual segments were
modelled using 3D solid elements and checked for both lifting stresses
and longitudinal stresses due to post-tensioning.
3D modelling of tower
The unique tower design of these bridges creates a highly confined compression and flexural zone immediately below the upper cable anchorage. To ensure adequate stress transfer brought on by the cable tensioning, a 3D LUSAS model of the entire tower and upper cable support was developed to determine the local zone effects in this region. Confirmation of the local stresses was obtained prior to proceeding with the complete analysis.
"We used LUSAS Bridge exclusively for the erection engineering of the Wichita Bridges and obtained excellent correlation between the on-site cable stayed measurements and the values that LUSAS predicted for lift-off"
Dr. David Byers, President, Genesis Structures.