Page 28 - Modern Steel Construction (April 2019)

welded connections similar to the spandrel
columns, so an increase in pier stiffness would
also result in increased moments due to thermal
expansion of the approaches. The foundation-
soil interaction became a critical part of the
structural design for two main reasons. First, if
foundation stiffnesses were overestimated, the
steel structure would still be able to accommo-
date all of the design forces—but the vibration
analysis of the structure would prove unconser-
vative. Second, if the foundation stiffness were
underestimated, the vibration analysis would be
conservative, but pier forces might exceed those
used in design. To deal with these competing
issues, both the upper and lower bounds of
the foundation stiffnesses were determined. In Courtesy of STV
addition to the multiple stiffnesses used in the above and below: Erecting the brige over Storrow Drive.
design, the team also investigated the possibility
of the ground freezing and imparting additional
forces into the piers due to the added restraint.
Analyses included running a portion of thermal
loads with the typical foundation springs and
a portion with fixed supports, and applying a
service live load since the design load of 120 psf
would be unlikely at a temperature of -30 °F.
The continuous tub girders at the deck level
presented multiple challenges for laying out
the framework of the bridge. Curved staircases
frame into the main span from either side and
provide a horizontal restraint to the main span.
As the framing for the stairs diverges from the
main span steel, the tub girders needed to be
split so that the exterior appearance would
remain consistent with the fascia plate while Courtesy of Newport Industrial Fabrication
maintaining the same relative location to the
tub girder. The tub sections start with two webs below: Cutting 18-in. HSS used for the support columns.
then split, ending with four webs. In working Courtesy of STV
with fabricator Newport Industrial Fabrication,
the design team decided that the flanges would
be cut to a shape that would provide a radiused
transition at the splits. The webs of the girders
would have to blend into the main girder web
if they were to follow the shape of the bottom
flange and would require welding the plates at
a sharp angle. To avoid this sharp-angled weld
and an abrupt end of the plate at the weld loca-
tion, it was decided to curve the web plate so
that it would return perpendicular to the main
tub web. Internal diaphragms and cross frames
were located to aid in transferring shear from
the incoming webs.

Challenging Curves Courtesy of Newport Industrial Fabrication
The curved tub-girder and stair members
were fabricated as built-up sections with
each plate cut to shape and were formed in-
house by Newport as needed prior to assem-
bly of the section. The 1-in. fascia plate was
challenging to form, as the slope in the fascia
plates and fascia girders mathematically cre-

28 | APRIL 2019