PROJECT SUMMARY
The existing bridge over the Kealakaha Stream on the island of Hawaii is scheduled for replacement. The new bridge will be a 645 foot long structure, designed and built to withstand all anticipated loads, including earthquake ground shaking. Because of the bridges location on the seismically active island of Hawaii, it represents an ideal opportunity for the first seismic instrumentation of a major bridge structure in the State of Hawaii.
The instrumentation program outlined in this proposal was based on observations from a number of CSMIP instrumented bridges. The procedure followed in developing the proposed instrumentation is detailed in a University of Hawaii Civil Engineering Report UHM/CE/96-04 (Stephens and Robertson, 1996).
The location of the structure in question is such that frequent seismic activity can be anticipated.
BRIDGE DESCRIPTION
The structural system suggested by the preliminary design of this bridge is typical of medium to long-span bridge construction in Hawaii. The structure utilizes KEEHI Type IV girders spanning between cantilever bents on hollow box piers. Spans range from 140 to 180 feet while the three supporting piers range in height from 42 to 114 feet.
The proposed seismic instrumentation consists of 41 seismic accelerometers, 6 relative displacement sensors and 3 data recording units. In addition, a system of optical fiber strain gages is proposed, both the determine the strains in concrete and steel elements of the structure, and to evaluate the suitability of optical instrumentation for this type of monitoring. All instruments and recorders will be interconnected and will have direct download capability to various research centers, including the University of Hawaii Civil Engineering Department, the UH Institute for Geophysics, and the Hawaii Volcano Observatory. Other organizations with an interest in the data would be given access based on State of Hawaii DOT approval.
The instruments will be located as shown in the Figures below. A detailed description of the locations and function of each gage is given in the UHM report mentioned earlier (Stephens and Robertson, 1996). The instruments will monitor and record the full motion of the structure, including free-field motion, pile cap translation and rotation, deck abutment accelerations, joint movement and column bent rotation. Embedded optical strain gages will record the concrete extreme fiber strains in one of the bridge piers and the concrete and prestress steel strains in selected precast girders.
PROJECT OBJECTIVES
The acceleration data collected from the instrumentation will be used for a number of important studies.
(1) To identify the structure's fundamental and most significant frequencies under ambient traffic loading. These will represent the elastic response of the structure.
(2) To identify the structure's fundamental and most significant frequencies under various levels of ground motion, depending on the seismic activity occuring on the island in the future.
(3) To investigate soil-structure interaction effects for local soil conditions.
(4) To compare the analytical model used in the design of the structure with the recorded motion of the structure. This study will provide valuable information regarding the structural assumptions that should be made when modelling similar structures in the future.
(5) To evaluate the forces and stresses induced in the structural elements, including pier axial load, bending and shear, and precast girder bending and axial stresses. This will allow for prediction of the performance of these elements when subjected to a design magnitude earthquake.
(6) To monitor the structural displacements and evaluate the adequacy of the bearings and expansion joints provided in the structure.
It is not anticipated that this structure will experience significant structural damage even under a design level earthquake, however, the response to smaller earthquakes will give a good indication of the likely behavior under the design conditions.
PROJECT SCHEDULE
The instrumentation program will be planned in concert with the design of the bridge structure. The final intrumentation plan will be appropriate for the final bridge structural system. The structural analysis program used to design the structure will also be used to make estimates of the elastic properties of the structure such as natural frequencies, mode shapes, and appropriate damping values.
The optical fiber instrumentation will occur in tandem with construction of the instrumented elements. The accelerometer and displacement instrument installation is anticipated to take from two to three weeks once the bridge structure is completed. The system will be commissioned and verified under ambient vehicle traffic. It will then be monitored at regular intervals by UH personnel to ensure working order at all times.
RECORDED DATA
In the event of ground shaking which exceeds the minimum trigger threshhold set for the instrumentation, the data loggers will record the readings on all instruments at 0.01 second intervals, with a back memory of 10 seconds prior to the triggering event. Recording will continue until the motion decreases below a predetermined switch-off threshhold. The instrumentation will be available immediately should aftershocks occur soon after the initial quake. Data storage capacity will be adequate to record in excess of 120 seconds of data from all instruments. The data will then be transfered by modem to each of the institutions with access to the data. The University of Hawaii will analyse the data with respect to the structural performance parameters listed earlier. A report of the recorded data will be available shortly after the event. A subsequent report detailing the performance of the structure will follow within a few months of the event.
