By: Steven T. Hague, P.E., S.E., and Shyam Gupta, P.E., Contributing Authors
By the end of this year, southeast Missouri will be the home
of the newest cable-stayed bridge in the U.S. and the longest cable-stayed span
over the Mississippi River. The 3,956-ft-long Bill E. Emerson Memorial Bridge,
named for the southeast Missouri congressman within days of his passing, will
span 1,150 ft and provide more than 60 ft of vertical clearance over the
navigation channel.
This is not, however, simply another cable-stayed bridge.
Cape Girardeau, Mo., is located approximately 50 miles from New Madrid, Mo.,
the location of the most violent series of seismic events on record--and the
bridge is a candidate to experience a significant earthquake during its
100-year design life. During the winter of 1811-12, this region was rocked by
more than 2,000 total events, over 200 of which were considered to have been
moderate to large earthquakes; the three most significant earthquakes having surface
wave magnitudes (Ms) of about 8.6, 8.4 and 8.7. It is estimated that the
recurrence interval of magnitude 8 quakes in this region is between 550 and
1,200 years. Further complicating the design of the Bill E. Emerson Memorial
Bridge is the potential for liquefaction and lateral spreading at the Illinois
bank, the probability of deep scour and the potential of a barge collision with
any of the bridge piers.
In 1991, the Missouri Department of Transportation selected
HNTB Corp., Kansas City, Mo., to conduct a structure-type selection study for
the new Mississippi River bridge at Cape Girardeau. This study considered the
cost and construction methods for various bridge types including a
through-truss, a tied arch and a cable-stayed span. The study also included
variable span lengths, the location of the major foundations within the river
and the cost implications of several foundation types and locations combined
with the different superstructures and span lengths.
From this study, it was determined that the most
cost-effective solution would be a 1,150-ft cable-stayed navigation span with
conventional steel or concrete girder approach spans on the Illinois side of
the river. The structure has an overall length of 3,956 ft and is composed of a
three-span, 2,086-ft steel and concrete composite cable-stayed unit and 1,870
ft of conventional composite steel plate girder approach structure.
What lies beneath
Geologically, the bridge is located on the eastern edge of
the Ozark uplift and the southwestern boundary of the Illinois basin. The
bedrock formations at the site are mostly limestone, with minor amounts of
shale, upon which the new bridge is founded. On the west side of the river, the
limestone is very near the surface, dipping to a depth of over 100 ft near the
Illinois abutment. In Illinois, the limestone is overlain by a granular,
liquefiable material. Structurally the area is heavily faulted, and most faults
are considered to be normal faults. However, some movement is thought to have
been strike-slip movement. Although the faults are considered inactive, some of
the faulting is thought to be as recent as Holocene.
Although earthquakes occur in other parts of the eastern
U.S., the New Madrid region has been the most seismically active region east of
the Rocky Mountains. Since the events of the winter of 1811-12 there has been
significant research conducted and it has been determined that for a 90%
probability of not being exceeded in 250 years, the peak rock acceleration at
the site is approximately 0.36 g. Based on input from the project design team,
MoDOT selected this as the design event. Considering that Ms 8 or larger events
are anticipated every 550 to 1,200 years, the design earthquake is essentially
a repeat of the 1811 and 1812 events.
With this in mind, exploratory borings were made and shear
wave and compression wave velocity tests were conducted to develop three
separate spectrum compatible, site-specific acceleration time histories for the
seismic analysis of the bridge. Each earthquake time history was established
for two orthogonal directions giving consideration to the effect of spatial
incoherency and the directional uncertainty of the design event. Although the
vertical component of the design earthquake was not directly considered, it was
included in the model by application of a percentage of one of the horizontal
accelerations in the vertical direction simultaneously with the separate
horizontal components.
As previously noted, the Illinois side of the site consists
of up to 100 ft of primarily loose to medium-dense sands. Both the
comprehensive geotechnical investigation and the site-specific seismic
investigation revealed Standard Penetration Test blow counts as low as 4 with
only thin seams of material having blow counts above 16 in the upper 70 to 100
ft of alluvium. With these poor soil conditions and the high level of shaking
that is expected to occur during the design earthquake, widespread liquefaction
is anticipated to a depth of up to 70 ft below grade.
Because of the gently sloping banks, particularly between
the main channel and the levee on the Illinois shore, lateral spreading is
anticipated along with the liquefaction and the banks could flow as much as 10
ft toward the channel while in a liquefied state. Clearly, this would produce
exceptionally large horizontal forces on the bridge foundations at a time when
there is little lateral support.
Finding right foundation
Taken in combination, the site geology, the seismic hazard and
selected superstructure types had a significant effect on the selection of
foundation types in both the Illinois approach spans and for the cable-stayed
navigation unit.
Since liquefaction presents little problem for the
cable-stayed unit, it was determined that the most cost-effective foundations
for the cable-stayed spans were spread footings or dredged caissons on bedrock.
The eastern approach spans, however, would require a deep
foundation system of either driven piles or drilled shafts. Pier 2, the
westernmost main tower pier located on the Missouri bank of the river, is
founded on a spread footing keyed into rock; Piers 3 and 4 are founded on
dredged caissons driven to bedrock.
Dredged caissons were selected for Piers 3 and 4 for a
number of reasons, including the severity of the design earthquake. Had drilled
shafts been selected, the combined depth of the pile cap and tremie seal would
have left only a few feet between the top of rock and the bottom of the seal
causing the shafts to serve as short, stiff elements incapable of resisting the
lateral forces resulting from the anticipated seismic event.
The approach spans are considerably different. As noted
earlier, these foundations are located in an area with very deep, highly
liquefiable soils. When combined, the liquefaction and the depth of anticipated
scour eliminated spread footing type foundations from consideration. After
extensive studies of various soil improvement techniques, it was determined
that any soil improvement would be ineffective due to repeated degradation and
aggradation of the channel.
Thus, both spread footings and driven steel piles were
eliminated as viable foundation alternatives, which led to the selection of
large diameter drilled shafts socketed into rock.
It's hard to move
The initial bridge analysis included both dead-load and
live-load analyses to determine preliminary member sizes, although the final
dead-load runs were made by "building" the bridge via computer to
determine the locked-in erection dead-load forces. The remaining analysis
results, including seismic analysis results, were superimposed on the erection
results in order to determine group load stresses for design.
The initial earthquake design runs for the cable-stayed unit
indicated that without any longitudinal restraint at the tower piers, the
design preference, the bridge would experience movements up to 48 in. in each
direction at the ends of the unit.
It was determined that the most efficient method to resist
these forces was by providing longitudinal restraint at the tower piers through
the use of an earthquake shock transfer device. This device, composed of a
cylinder filled with silicon and a piston, is capable of transferring forces in
both tension and compression. Therefore, the double-action unit simplified the
design of the connections to the structure and permits transfer of earthquake
forces at the most efficient elevation. These devices, each capable of
transferring 1,500 kips of force, have the added advantage of reducing
wind-induced motion of the bridge while improving the stability of the bridge
in strong winds and minimizing longitudinal displacements under the various
live-load combinations. Reduced movements would then require smaller expansion
joint devices and relieve the required movement capacity of the side span tie
down devices.
The Bill E. Emerson Memorial Bridge is on track for
completion in late 2003. When open to traffic, the bridge will be a much-needed
improvement over the narrow two-lane bridge built in 1927 and will provide a
significant new link in the region's transportation system.
About The Author: Hague is an associate vice president with HNTB Corp. and is the project manager for the Bill E. Emerson Bridge project. Gupta is the state bridge engineer for the Missouri Department of Transportation.
