Weight watcher

Danielle D. Kleinhans, PH.D., P.E. / May 14, 2008

An unprecedented bridge construction project began last year in New Orleans with the widening of the Huey P. Long Bridge over the Mississippi River in Jefferson Parish. The four main spans of the Huey P. Long Bridge extend nearly 2,400 ft.

This cantilevered steel through-truss bridge opened to traffic in 1935 and is owned and operated by the New Orleans Public Belt Railroad. The bridge currently carries dual rail lines between the trusses and two lanes of vehicular traffic cantilevered to the exterior of each truss. Based on the need to improve vehicular traffic flow and constraints due to uninterruptible rail traffic, the Louisiana Department of Transportation and Development (LaDOTD) decided to widen the bridge rather than replace it. A structural health monitoring program is included in the construction contract as a measure to assess whether the expected amount of load is being transferred from the widening truss members to the existing truss members.

TIMED event

Widening of the Huey P. Long Bridge is part of the Transportation Infrastructure Model for Economic Development (TIMED) Program, which was established in 1989.

The goals of TIMED are all that much more important in light of the destruction caused by hurricanes Rita and Katrina. Project delays and cost escalation have been the result of these storms and the necessary rebuilding that is taking place regionally. However, these challenges have only served to strengthen the commitment of TIMED to the economic growth of New Orleans and the state of Louisiana as a whole.

The main spans of the Huey P. Long Bridge consist of a three-span cantilevered truss with spans of 530 ft, 790 ft and 530 ft and a through-truss span of 530 ft. The roadway sits 135 ft above the Mississippi River. Over the river, the railway and vehicular portions of the bridge merge at the same elevation to form a combined structure. Away from the river the railway and vehicular portions of the bridge split in elevation primarily because of the strict grade requirements of the railroad and adjacent vehicular intersection constraints. The railway bridge structure is nearly 23,000 ft long and is the longest railroad bridge in the U.S. The vehicular bridge structure is just over 8,000 ft long with steep grades on each end.

Long on service

The Huey P. Long Bridge serves about 50,000 vehicles daily, as well as carrying a dual-track rail line. The dual rail lines are supported between the trusses, and the two lanes of vehicular traffic in each direction are supported by cantilevered floor beams outboard of each truss. The bridge widening will facilitate an increase in roadway width on each side of the bridge from its current width of 18 ft to 40 ft. This expansion will accommodate an additional traffic lane in each direction, widening of the traffic lanes and the addition of dual shoulders in each direction. By increasing the width of the bridge roadways from two 9-ft lanes in each direction to three 11-ft lanes with shoulders, officials expect to improve traffic flow in the area.

Eyeing the strain

The existing truss members will be monitored with an array of over 800 static and dynamic strain gauges designed to measure axial and bending load effects. Monitoring of the inclination of the piers also will be conducted with the use of tiltmeters.

The truss-monitoring system consists of two separate monitoring systems: a static-load-monitoring system and a live-load-monitoring system. The static-load-monitoring system utilizes vibrating wire strain gauges and is capable of a monitoring frequency on the order of one reading per second. This system will be monitored continuously throughout the project duration.

The live-load, or dynamic, monitoring system utilizes full-bridge resistance strain gauges and is capable of much higher monitoring frequency on the order of 100 readings per second. This system will be monitored on demand, such as during the load testing, to be able to capture peak measurements under moving loads.

The steel truss member types generally fall into the following categories: built-up box members, built-up I members, rolled I members, eyebars and double-angle members. Each member type has particular considerations for sensor installation. Additionally, in an effort to avoid the effects of the connections on the measurements, each sensor will be located no closer than two times the largest cross-section dimension of the member from the edge of a gusset plate.

Widening the bridge required modifications to the existing trusses, so specific attention had to be paid to avoid installation of sensors or other system equipment in areas where future work would be required on a member.

Once construction activities begin, daily comparisons will be made between predicted and measured strain values to determine whether the erection is progressing as expected and to alert the construction team to any possible problems or necessary changes in procedure. Limits have been established on changes in strain on a member-by-member basis for each stage in the erection process. It is expected that comparisons will be made taking into account dead load and erection load effects; for the predicted values, only dead load and erection load will be considered and for the measured values the reported values will be those taken during periods of minimal live load.

Temperature measurements also will be monitored at a series of locations throughout the bridge superstructure. The monitoring system will remain active as the instruments measure and transmit data about the bridge’s structural behavior through the completion of the project, which is currently scheduled for 2013.

In order to assess the existing dead-load stress state in the bridge members, CTLGroup, Skokie, Ill., will determine the existing forces in selected eyebar members prior to any construction activities. These initial dead-load stresses will be based on the frequency of vibration of the eyebar. Data will be collected in the time domain and processed in the frequency domain in order to ascertain the resonant, or natural, frequency of the eyebars.

A magnetically attached accelerometer will be used for the measurements. Data will be recorded under ambient conditions; no additional excitation of the members is expected to be necessary. Collected data will be proc­essed to determine existing forces in the eyebars.

As steel erection begins, traffic on the bridge is expected to be maintained throughout the widening proc­ess. The use of staged construction makes this possible, whereby extensions of the currently cantilevered floor beams and new portions of the roadway will be added in stages.

As of March 2008, work was being conducted on the modifications to the substructure and installation of the instrumentation on the superstructure. Completion of the monitoring-system installation is expected in mid-2008, with data collection to commence shortly thereafter.

Following completion of the monitoring-system installation, a load test will be conducted to calibrate the monitoring system and the contractor’s analysis of the existing truss. Locomotives of known weight will be used to load the bridge at predetermined locations while the structure is monitored.

About the Author

Kleinhans is a senior engineer and group manager of the Structural Engineering & Mechanics Group at CTLGroup, Skokie, Ill. Kleinhans can be reached via e-mail at [email protected] or via phone at 847/972-3188 for more information.

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