Star treatment for a mouse

Engineers accommodate threatened species in Colorado

Bridges Article November 13, 2003
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The design and construction of the 560-ft bridge taking
Fifth Street over I-25 and Plum Creek in Castle Rock, Colo., came up against
severe environmental constraints. Plum Creek is the habitat of the Preble’s
Meadow Jumping Mouse, which is a threatened species protected since 1998 by the
Endangered Species Act. Among other things, this act makes it unlawful to
significantly modify the habitat in a way that would disrupt normal behavior
patterns of the mouse, such as breeding, feeding or sheltering.

This federal constraint:

* 
Restricted the period of construction work in the habitat area to
October through mid-April--the hibernation period of the mouse;

* Implied minimal size of construction equipment; and

* Called for longer spans to minimize the number of
piers in the habitat area.

Two additional constraints affected the bridge design. The
state did not permit a pier in the median of I-25, which was scheduled for
rebuilding in the future. And the city wanted to keep the maximum grade of
Fifth Street to 5% to avoid disruption to businesses and to make it possible
for the handicapped to travel the pedestrian way via wheelchairs. This latter
constraint required shallow girders to keep the bridge relatively low over the
highway.

According to Douglas Wellock, project manager of the
bridge’s structural design for Range Engineering (now SEH Inc., Cheyenne,
Wyo.), all of these constraints pointed toward steel as the material of choice
for the bridge.

“We selected a three-span welded plate steel girder
bridge with two end spans of 170 ft and a middle span of 220 ft,” he
said. “The girders are hybrid systems since we used HPS-70W
high-performance steel on the flanges over the piers and M270 grade 50W webs.

“Additionally, we located one of the two piers in the
wetlands of Plum Creek since the wetlands are not considered the habitat of the
Preble’s Meadow jumping mouse.”

Wellock added that the design firm requested preliminary
designs for the bridge from the National Steel Bridge Alliance in Chicago,
which recommended certain design, detailing and fabrication measures to
minimize the cost of the bridge. “Based on NSBA recommendations, we ended
up with a four-girder bridge with girders spaced at 10 ft 9 in.,” said
Wellock. “The web depth over I-25 is 60 in., giving us the required 16 ft
6 in. clearance. Integral abutments eliminated the need for costly expansion
joints. Installed cost of the steel was $0.91 per lb, a very good price that
can be credited to the NSBA suggestions. The total bridge cost came to
$76.40/sq ft.”

The new HPS-70 weathering steel offers high strength,
ductility and weldability--a great combination for steel bridges. The
steel is slightly more difficult to drill, requiring sharp bits and ample
lubrication.

Mill scale, which can be difficult to remove, must be
sandblasted so that the steel weathers uniformly, especially in the dry Western
climates.

“This was one of the first high-performance steel bridges
in Colorado to open,” said Wellock. “The lighter girders allowed
more fabricators to compete for the project based on reduced crane capacity and
shop space requirements. It also decreased shipping and erection costs. Our
longest shipped piece was 116 ft, and each girder has four bolted field
splices.”

Wellock cited the following design criteria:

* American Association of State Highway &
Transportation Officials Standard Specification for Highway Bridges, 16th
edition;

* Load Factor Design; and

* HS-25 truck live load with sidewalk deflection
limited to the span length divided 1000.

Because of the limits on deflection, the bridge required
haunches over the piers. “Normally we don’t want haunches because
they increase the fabrication costs,” said Wellock. “But to control
the deflection caused by the heavy HS-25 truck we had to put haunches over the
piers.

“We needed to get the steel up before the mice came
out of hibernation. Egger Steel delivered in February, and it was up in a few
days.

“To raise the girders, the erector first bolted two
girders together--side by side--and then lifted them in pairs for
increased stability.”

About the author: 
Information provided by SEH Inc., Cheyenne, Wyo.
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