C R E AT I V E C O N C R E T E C O N S T R U C T I O N

Prestressed Concrete
Girders Achieve
Record Lengths
by Cameron West, Concrete Technology Corporation

Concrete Technology Corporation (CTC) and the design-build team of
Atkinson Construction and Jacobs have partnered in Tacoma, Wash., on
the Interstate 5 (I-5) Portland Avenue to Port of Tacoma Road southbound
high-occupancy vehicle project, which is pushing the length limits of what
is currently achievable with prestressed concrete girders. This bridge over
the Puyallup River and multiple rail lines requires long span lengths and
extreme skews. The longest girder, spanning five existing and future rail
lines, has a plan length of 223 ft and approximately 55-degree end skews.
This girder now holds the record as the longest single-piece prestressed
concrete girder made in the U.S. Other important features of the project
include the use of lightweight concrete to meet handling and shipping
requirements and the use of a modified Washington State Department of
Transportation (WSDOT) WF100G girder section with a widened top flange
to improve stability.

Limiting Girder Weight
An early challenge in the project was reducing girder weight to ensure
that it could be hauled to the jobsite by truck. Because available hauling
The 223-ft-long modified Washington State
Department of Transportation WF100G girder
after the side forms were removed.

56   ASPIRE Fall 2019

The 223-ft-long girder before side forms were set. All Photos:
Concrete Technology Corporation.

 equipment in the region currently limits the maximum girder weight to
270 kip, lightweight concrete was chosen. On the I-5 Portland Avenue to
Port of Tacoma Road project, the use of lightweight concrete with a fresh
density of 0.125 kip/ft3 and a reinforced density of 0.138 kip/ft3 reduced
the girder weight by 20%, to 247 kip. This concrete mixture met all project
criteria and was able to reach a compressive strength of 8.4 ksi for transfer
at 15 hours and 10.0 ksi at 28 days. CTC had previously used the same
lightweight concrete mixture in several girder projects, including the I-5
Skagit River Bridge Replacement (see the Winter 2014 issue of ASPIRE® and
the January–February 2015 issue of the PCI Journal) and State Route 162/6
Puyallup River Bridge (see the Summer 2016 issue of ASPIRE).

Girder Stability and Prestressing
Special consideration was given to girder stability and stresses during
plant handling, hauling, and erection. This analysis resulted in the
selection of a modified WSDOT WF100G girder cross section, where the top
flange was widened from 4 ft 1 in. to 5 ft 1 in. for this project to increase
the weak-axis stiffness. The modified girder cross section was only used
for one span with the longest girders; other spans used standard girder
cross sections. To meet handling and hauling criteria, the design also
required the use of ten 0.6-in.-diameter pretensioned temporary top
strands, pick points at 26 ft from each end, and hauling bunk points at
25 ft from each end.
In addition to the record-setting length, this girder has a very high
amount of prestress. The design specified 46 straight, 35 harped, and
10 temporary top 0.6-in.-diameter strands, which results in an initial
tensioning force of nearly 4 million lb. This force, when combined with the
girder length and lifting locations, required a concrete compressive strength
of 8.4 ksi at transfer. The prestressing force in combination with the severe
skew required strand debonding and supplemental mild reinforcement in
the acute corner of the bottom flange to minimize cracking.
__________
Cameron West is the chief engineer with Concrete Technology
Corporation in Tacoma, Wash.

Modified girder cross section. Figure: Washington State
Department of Transportation.

The 223-ft-long, 247-kip girder
being transported into storage
across the five-lane Port of
Tacoma Road.

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