Highway Bridge LiftlMonitoring System

J.W. LO WRY, Electrical Engineering, Auburn University, Alabama G.E. RAMEY, Civil Engineering, Auburn University, Alabama

Due to road resurfacing, many highway bridges through- out the state of Alabama now, or will in the near future, violate underneath clearance requirements. Solutions to this problem are: (1)cut out and lower the roadway be- neath the bridges, or (2) raise the bridges. In most cases, interstate roadways pass under these bridges; and hence, the first solution necessitates the cutting out and replace- ment of a considerable width and length of roadway. The cost of this approach, along with its potential drainage problems and inherent traffic interference, makes it some- what unattractive. A bridge liftinglmonitoring system was developed as part of the second solution, i.e., raising the bridges. It is briefly described below.

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Bridge Lift System Since many bridges will require lifting, it was felt that an

economical and efficient means of accomplishing these lifts should be investigated. The lift system and procedure developed emphasizes simple, reliable, rapid, uniform lifting of continuous bridges at a minimum cost. The general criteria to be met in the design of the equipment for the system were that it must:

-uniformly lift continuous-span reinforced-concrete

-be reusable and lightweight, -be adaptable to varying bridge geometries,

bridges

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Fig. 1-General layout of bridge lift system

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Fig. 2-Bridge displacement sensor Fig. 3-Displacement read-out panel

-be capable of being emplaced and raising bridge

-provide a quality lift and hence minimize bridge without stopping or detouring underneath traffic, and

damage. The lift system consists of, (1) a jack support and lift hardware system, (2) a hydraulic system, and (3) a displace- ment monitoring system. These systems are shown in the general layout drawing of Fig. 1 and were tested on an experimental test bridge on the Auburn University campus. The displacement monitoring system developed is the topic of this paper.

Displacement Monitoring System

The bridge-displacement monitoring system was designed to provide displacement feedback during jacking and hence allow immediate hydraulic adjustments to assure a uniform and quality lift. The system consists of 10 electrical displacement sensors connected to a central displacement readout panel. The sensors are powered by two 12-V batteries and their output signals are filtered, processed, and calibrated to give vertical displacement on the dial indicators of the central monitoring board. This board is located adjacent to the hydraulic control panel for con- venience in monitoring and regulating the lifting of the bridge. The displacement range of the sensors and readout meters is 12 in., which is compatible with the stroke of the lift jacks. For lifts exceeding 12 in., the sensors and meters require retracting and rezeroing at the same time that the jack cylinders are retracted.

The displacement readout meters are calibrated to read 0 to 5 in. with 10 divisions per inch. The meters can be offset by 0, 2, 4, 6, and 8 in. to give a meter range of up to 13 in., which exceeds the 12-in. range of the sending units. This offset feature allows meter readings with better resolu- tion than could be obtained for 0 to 12 in. without offset. The offset also permits the use of accurate midrange meter readings (2 in. to 4 in.) for most of the lift. For example, a lift of 7.5 in. would read 3.5 in. with an offset of 4 in. A lift of 8.5 in. would read 2.5 in. with an offset of 6 in.

The readout meters have vertical moving pointers and are mounted side by side for convenience in achieving a uniform lift. When all 10 points on the bridge have been lifted the same amount, the meter pointers are aligned in a horizontal line. The operator need only keep the pointers aligned horizontally to assure a level lift. Figures 2 and 3 are photographs of a displacement sensor and the readout panel, respectively.

Figure 1 shows the location of the displacement sensors on the bridge. The operating principle of the displacement sensors is illustrated in the schematic drawing of Fig. 4. As indicated earlier, the sensors can operate for lifts of up to 12 in. before requiring resetting. It was anticipated that the measuring line shown in Fig. 4 (which runs from the dis- placement sensor on the bridge to the ground) would vibrate due to wind disturbances from passing vehicles. The displacement readout unit was designed to dampen out these anticipated sensor output flucations.

A detailed setup/zeroing procedure for using the system is as follows.

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Fig. 4-Schematic of bridge displacement sensoi

(1) Locate the sending units on the bridge at the same points where the jacks are located. Connect the electrical cables between the sending and main unit. The sending units are numbered and must be connected to the corres- pondence numbered input on the main unit. The meters are also numbered to correspond to the sending units.

(2) Extend the measuring line from each sending unit down beside the bridge and secure it to the ground directly below the sending unit.

(3) Adjust the length of each measurement line until the slide in the sending unit is off of its stop and somewhere in the first inch of its 12-in. travel.

(4) Connect the batteries to the main unit and check the battery meters on the main unit to assure adequate charge.

(5) With the range offset adder set on " + O " , adjust each meter to read zero using the zero-adjustment knobs.

(6) Move each slide on the sending unit and check to see that the meter moves correctly.

(7) Lift the bridge and keep the meter pointers in a straight line. When all of the meter readings reach 4 in., change the range offset to give a reading of 2 in. plus a 2-in. adder. Keep the meter readings between 2 in. and 4 in. using the range offset switch.

(8) Disconnect and periodically recharge the batteries when the unit is not in use.

m e as u r i n g 1 i n e

Testing of System The displacement monitoring system was refined by

laboratory testing while it was being developed and it per- formed very well under those conditions. After all lift systems were developed, they were fabricated and tes!ed on a three-span continuous-concrete test bridge on the Auburn University campus. The displacement monitoring system performed nicely in this testing and the meter readings were very stable and accurate.

Since the experimental bridge testing, the displacement system has been used by the Alabama Highway Depart- ment in lifting a highway bridge near Birmingham, AL. The system again performed very well. Wind disturbances on the sensor-to-ground lines caused some line vibrations. These vibrations, in turn. caused some readout-needle flutter. However due to internal electrical damping in the meter, this flutter was insignificant.

Acknowledgments The bridge liftlmonitoring system discussed in this paper

was developed as part of an Alabama Highway Department Research Project, 930-092. Their support of this project is greatly appreciated.

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