Civil Engineering Reference
In-Depth Information
Chapter 12
Vibration Monitoring and Load Distribution Characterization
of I-89 Bridge 58-N
Geoff May and Eric M. Hernandez
Abstract
The University of Vermont Structural Monitoring and Diagnostics Lab is collaborating with the Vermont Agency
of Transportation to instrument and monitor traffic induced vibrations in bridge 58N on interstate 89 in the town of
Richmond, VT. The bridge consists of an unorthodox concrete and steel deck supported on reinforced concrete piers.
A critical portion of the deck is strategically instrumented with 24 strain sensors and 10 accelerometers. The main objective
of the monitoring project is to use multi-scale vibration data to formulate a finite element model of the bridge deck that
allows engineers to accurately assess the structural capacity of the deck under heavy traffic loads. Two methods are
combined: (1) A direct method based on dynamic strain measurements at maximum stress locations in the deck and (2)
An indirect method based on an updated sub-structured finite element model of the bridge deck updated based on frequencies
and mode shapes identified from traffic induced acceleration measurements.
Keywords
Bridges • Dynamics • Vibrations • Instrumentation
12.1 Motivation
Interstate 89 is the busiest highway in the state of Vermont, for the last 5 years an annual average of 3.5 million vehicles
traveled through it [
1
]. The Vermont Agency of Transportation (VAoT) has expressed concerns regarding twin bridges 58N
and 58S in Vermont's I-89. The main concern is the structural adequacy of the deck stringers to sustain heavy traffic loads.
Material testing on the stringers shows a yielding strength that varies from 22 to 37 ksi, which is lower than the grade of steel
typically required in bridges. In addition to the lower steel grade of the stringers, composite action between the steel
elements (girders and stringers) and the reinforced concrete deck is uncertain since no information is available in the
structural drawings regarding the compressive strength of concrete nor the connection elements that would enable shear
stress transfer between the concrete and the steel beams.
Initial estimates based on standard codified procedures indicate a low rating for this bridge, however as it will be shown in
the next section, the deck's structural system is unusual and the possibility exists that the codified procedures are not
applicable to this unusual structural system. The VAoT is interested in gaining a better understanding of the static and
dynamic behavior of this bridge in order to accurately estimate the load distribution pattern under traffic loads. VAoT has
funded a project with the UVM Structural Monitoring and Diagnostics Lab to instrument, collect and analyze operational
vibration data for a period of at least 1 year. The monitoring began March of 2012.
The main research question addressed in this paper is the inverse problem of extracting the information contained in the
vibration measurements regarding the composite action taking place between the steel beams and the reinforced concrete
deck. This will enable us to create an accurate finite element model of the deck which allows for prediction of structural
response under heavy traffic loads.
