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|Federal Highway Administration > Publications > Public Roads > Vol. 58 · No. 2 > Lincoln Builds First Heated Pedestrian Viaduct|
Lincoln Builds First Heated Pedestrian Viaduct
by Milo D. Cress and Al Imig
When, in the summer of 1990, bridge inspectors in Lincoln, Neb., identified a significant structural problem with the 131-meter (430-feet) through-girder viaduct that was constructed in 1909 to carry 10th Street traffic over several railroad tracks, officials of the Nebraska Department of Roads (NDOR) and the city of Lincoln decided to replace it with the nation's first heated pedestrian viaduct and a separate vehicular viaduct.
The viaduct is located just north of the central business district and adjacent to the University of Nebraska at Lincoln (UNL) campus. It serves as the primary pedestrian and bicycle access route to UNL for the large number of students and football fans who live or park north of the railroad tracks.
This structure was a particularly appropriate candidate for a heated pedestrian deck because of Lincoln's severe winter conditions, the bridge's central location in the city, its heavy use by bicyclists, and it's potential use by people with physical impairments. After consulting with the Federal Highway Administration (FHWA), NDOR and the city requested funds to install a deck heating system in the new 10th Street viaduct.
The pedestrian viaduct is a precast, prestressed, post-tensioned concrete superstructure with a 165-millimeter-(6.5-inch-) thick cast-in-place deck. It features a 3-m (10-ft) clear-width walkway for bicyclists and cantilevered rest pads providing protected rest areas for people in wheelchairs.
The project was let to contract in August 1992, and construction began the following October. The new pedestrian viaduct was opened on May 28, 1993; however, the boiler-pump house for the heating system could not be completed until a substantial amount of the north approach for the vehicular viaduct was in place. Construction of the vehicular viaduct began in March 1993, and the deck heating system was charged with heating solution in May 1994.
The original pedestrian viaduct construction contract totaled $1,153,206. The deck heating system change order added $258,646 -- or $161/m2 ($15/ft2) of deck surface area. The city of Lincoln, the Nebraska Center for Infrastructure Research, the University of Nebraska, Delta-Therm Corporation, A.M. Cohron and Son Inc. (prime contractor), Wells Engineering (design consultant), NDOR, and FHWA joined in a unique partnership that provided matching funds for federal-aid money. This money was provided under the Intermodal Surface TransportationEfficiency Act of 1991 (ISTEA), Section 6005(a), which provides funding to identify and promote technologies designed to improve highway, transit, and intermodal transportation systems.
Deck Heating System and Operation
The hydronic system includes a gas boiler that heats a propylene glycol-water solution, which is pumped on cue through hoses encased in the deck and keeps the concrete deck surface warm enough to prevent accumulation of snow and ice. A summary of viaduct and heating system features is in table 1.
Table 1 -- Features of Pedestrian Viaduct and Deck Heating System
The deck heating system was installed from end to end across the viaduct including the approaches. The system has eight major components:
The 10th Street viaduct is divided into 13 independent heating zones, each about 121 m2 (1300 ft2). A natural gas boiler heats the 35-percent propylene glycol and water solution to about 322 degrees Kelvin (120 degrees Fahrenheit). In-line pumps move the solution through the system under a pressure of about 207 kilopascals (30 pounds per square inch). The solution moves from the boiler into the 152-mm- (6-in-) diameter PVC distribution line, which extends the full length of the viaduct for distribution to each heating zone.
At each heating zone, the solution is transferred by a 25-mm- (1-in-) diameter hose from the PVC distribution line to a copper distribution manifold that is encased in the concrete.
The manifold distributes the solution to 10-mm (0.38-in) hydronic rubber hoses spaced at about 114-mm (4.5-in) centers within the concrete slab or deck. These hoses run parallel from the distribution manifold to the extreme boundary of the heating zone and back to the collection manifold, which is located adjacent to the distribution manifold.
The solution is then transported from the collection manifold to the PVC collection line by a 25-mm- (1-in-) diameter hose. Finally, the solution is returned through the PVC collection line to the boiler to be reheated and recycled.
The system is designed to move solution through at 530 liters per minute (140 gallons per minute) and deliver 538 Watts per square meter (50 Watts per square foot) or 171 British thermal units per square foot per hour of energy to the deck surface. The rate of flow can be adjusted by turning one pump off or by changing the impeller size. The temperature drop between the distribution and collection rubber hydronic hoses is expected to be about 277 K (40 F).
Automatic operation of the system is accomplished by moisture and temperature sensors. The on and off operation is controlled by a moisture-sensing device (MSD). The system automatically turns on when the MSD senses moisture on the deck surface, the temperature of the deck surface at the MSD is below 273 K (32 F), and the ambient temperature near the boiler-pump house is below the temperature set by the system operator. The system turns off when the deck surface at the MSD reaches a temperature of 283 K (50 F). The MSD is heated sufficiently to create moisture during ice or dry snow conditions. The system can also be turned on and off manually.
Heating System Installation and Construction
No special construction equipment or personnel were needed to install the heating system. Construction personnel were trained on component installation, and the primary contractor enlisted specialized mechanical, electrical, and prefabricated building subcontractors. The heating system components were installed with reinforcement and concrete following the normal construction sequence.
Distribution and collection manifolds were positioned and tied securely in each heating zone on the approaches supported on mechanically stabilized earth. The hydronic hose was connected to nipples on the distribution and collection manifolds with circumference-type clamps and then stretched taut within the heating zone. The single layer of reinforcement was positioned on supporting concrete blocks over the hoses and tied. The hydronic hose was then tied to the bottom of the reinforcement.
Installation was similar for heating zones on the elevated structure, except that hydronic hoses were positioned between the two layers of reinforcement. When the reinforcement and hose were taut and tied, hydronic hoses were brought to 414 kPa (60 psi) air pressure, and concrete was poured in the forms, vibrated, finished, and cured following standard practice.
In the approaches, distribution and collection PVC lines had to be installed and connected to the manifolds before the concrete slab was poured. However, PVC lines under the elevated structure, including flexible expansion loops and valves, could be installed at the convenience of the contractor.
The PVC pipes are buried in sand on approaches and supported on hangers under the elevated structure. Pipes are fixed to prevent movement at the center of the elevated structure and can slide or rotate on other hangers. A flexible expansion loop is installed in each PVC line near each abutment to accommodate expansion of both the superstructure and the PVC during heating of the solution. The PVC is enclosed in 51 mm (2 in) of fiberglass insulation beneath the elevated structure and has no insulation in the approaches. Valves are installed in each of the PVC lines near each abutment to enable shutting off the solution to and from abutment areas at any time.
Monitoring and Evaluation of Heating SystemThe UNL Department of Mechanical Engineering has prepared and is carrying out a 12-month monitoring and evaluation plan. The monitoring equipment includes 18 temperature sensors and one fluid-flow sensor. Sensors are installed in five clusters of three. Each cluster includes a sensor at the top surface, center, and bottom of the concrete. Two clusters are installed in the north span of the elevated structure, two in the north approach pavement, and one in an unheated portion of sidewalk adjacent to the boiler building. Also, one sensor is installed in the distribution pipe to monitor fluid temperature; one sensor is installed in the collection pipe to monitor fluid temperature; and one is positioned to monitor ambient air temperature. Finally, a volume-flow sensor is installed in the collection pipe upstream from the boiler. The sensors are wired to a data-acquisition board in a computer located in the boiler building. The data will be transferred from this computer to floppy disks for analysis.
This project is the first application of its type in the United States. As such, it established that there are no specialized construction equipment or personnel requirements. The results of the monitoring program will be used to evaluate system and component size, efficiency, and effectiveness. Analysis of system components will influence future designs.
For information on the heated bridge technologies under ISTEA, contact Mike Hicks at FHWA, Office of Engineering, Bridge Division, HNG-33, Room 3303, 400 Seventh Street, SW., Washington, DC 20590-0001, or call him at (202) 366-4506.
Milo D. Cress is a bridge engineer for the Nebraska Division of the Federal Highway Administration.
Al Imig, P.E., is the deputy city engineer for construction for the city of Lincoln, Neb.
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