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Precast
Inverted T Beam
The Need
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Current systems are either costly or time
consuming to erect and have limitations. Cast-in-place slabs have become too expensive and
time-consuming due to extensive field formwork. Precast solid slabs aren't economical for
spans longer than 9 meters. I-girders with concrete deck slab topping can span longer than
other systems, but require field formwork and significantly higher clearance. The new
precast concrete product for short to medium span bridges was developed by the University
of Nebraska. The Nebraska Inverted Tee (IT) can span up to 26 meters with a total
structural depth of 725 mm. Most U.S. highway bridges have short to medium span
lengths. And most bridges needing rehabilitation or replacement are short-span. A recently
developed type of pre-stressed concrete beam is helping slash 20% off the weight of a
bridge being built in Florida between the cities of Boynton Beach and Ocean Ridge. |
Developed in Nebraska, a precast
inverted T-beam saves up to 20% in weight off standard I-beams |
The Technology
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The Nebraska Inverted Tee
(IT) system was designed for use by small contractors in sparsely populated areas where
relatively modest erection equipment exists. The ITs were developed in "hard"
metric units, i.e., using round-figure millimeters. The ITs are 600 mm (23.5 in) wide.
They vary in depth as needed for design, using a single set of formwork. Through
analytical and experimental testing, researchers have shown that the system can span up to
25.9 m (85 ft) with a total structural depth of 725 mm (28.5 in). Researchers conducted a
full-scale test of two 400 mm (15.7 in) deep ITs with CIP topping at the Structures
Laboratory at the University of Nebraska-Lincoln. Test results confirmed that available
design procedures are sufficient for the design of this new system. It's an I-beam without
a top phlange. The goal was to build precast short-span bridges with shallow profiles,
eliminating need for heavy equipment and casting on site, yet still load-bearing. |
Tests is carried out in the Lincoln
campus structure labs |
The Benefits
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No other existing precast concrete or
cast-in-place conventionally reinforced system has this capability. The only CIP system
that has a similar span/depth ratio is post-tensioned slabs, which are relatively
complicated to design and expensive to build.
- Cost-competitive: Less expensive
- Esier to erect than traditional concrete slabs.
- Does not require site formwork.
- Faster to build than a cast-in-place conventionally
reinforced slab bridge, while the structural depth was maintained in the same range as for
slab bridges.
- Its weight is suitable for county bridges where contractors
do not have heavy lifting equipment.
- Favorably compete with existing systems used for short to
medium span bridges, especially cast-in-place slabs.
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Status
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The inverted T-beam was initially developed at
the University of Nebraska under a fellowship four years ago in conjunction with the Nebraska Dept. of Roads. Nebraska has since built six and Iowa
has built three. The three-span 111-meter-long
Boyton Beach bridge in Florida may be the longest yet to use T beams. The Florida Dept. of
Transportation decided to use the T-beams to replace the nearly century-old bridge. Nebraska Department of Roads' and UNL engineers designed a 120 ft
inverted tee bridge over Wolf Creek near Syracuse, Nebraska. This new three-span structure
is a twin bridge that was constructed next to an existing bridge. Construction was
complete in 1997. Lamp Rynearson & Associates, Omaha, is designing the Dahlman Avenue
Bridge for the City of Omaha using the IT system. |
Points of Contact
- Dr. Maher Tadros, Engineering 129, 60th & Dodge Streets,
Omaha, NE 68182-0178, Tel: (402) 554-2985 Fax: (402) 554-3288, <mtadros@unomaha.edu>
- Mr. Mike Beacham, State of Nebraska, Department of Roads,
P.O. Box 94759, Lincoln, Nebraska 68509-4759, 1500 Highway 2, Lincoln, Nebraska
68502, Tel: (402) 471-4567, Fax: (402) 479-4325
- Mr. Lyman Freeman, State of Nebraska, Department of Roads,
Tel: (402) 479 4701.
References
- Engineering News Record, July 19, 1999, p16
- Center of Infrastructure Research, University of Nebraska -
Lincoln
Disclaimer Statement
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Neither the Construction
Industry Institute nor Purdue University in any way endorses this
technology or represents
that the information presented can be relied upon without further investigation. |
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