UCII: On-Line Bridge Monitor

Monitor Hardware:

Data Acquisition Set-Up at HAM-126 (Hardware)

A MEGADAC data acquisition system from Optim Electronics was also used for this project. Its purpose was to monitor high-speed sensors installed on the bridge. An Optim MEGADAC is ideally suited to read high-speed sensors for several reasons. One reason is that it uses a GPIB interface that allows highspeed communication between the data acquisition system and a computer. It also has an aggregate 25 kHz sampling speed. The CR-10 system is incapable of reading sensors at such speeds and therefore is not used for traffic monitoring. Another reason was that although the MEGADAC can read virtually any sensor, it is much more expensive to implement than a CR-10 for a large number of channels. The most efficient and cost-effective solution was to use the MEGADAC for the smaller number of high-speed sensors and the CR-10 for the slow speed sensors. Finally, the Optim MEGADAC requires a dedicated computer that would have been more difficult to power and protect during the early stages of the project when only slow speed sensors were being monitored.

Several common sensor types (e.g., foil strain, accelerometer, wire potentiometer, LVDT, etc.) are wired directly to an Optim Electronics MEGADAC datalogger. Sample speeds of at least ten times the bandwidth of interest (i.e., up to 20 Hz) are required to completely visualize the bridge response and its properties. The system is rather competitive in price with other high-end products that provide the integrated components for excitation (up to 10V DC or square wave), amplification (x100), filtering (3-pole Butterworth at 100Hz), digitization (16 bit), and computer acquisition for a broad range of sensors. There have been over 100 sensors installed at HAM-126-0881 for various experiments and the MEGADAC system has handled them all. The provided software allows the user to specify different sensor types, a common sample interval (i.e., 200 Hz), and immediate download to the onsite computer via a General Purpose Interface Bus (GPIB, IEEE RS-488).

Optim Electronics MegaDAC High-Speed Data Acquisition System

Video camera

A BCR424II monochrome video camera was mounted on an aluminum pole next to the bridge approach for a wide and unobstructed view of the bridge traffic. The camera is wired directly to a National Instrument PCI-1408 framegrabber board that is installed in the PCI bus of the onsite computer. A real-time view of the bridge traffic is provided for remote actualization of the operating environment of the structure. Digital frames may be archived in the event of transportation or load violations. Windows-based utility software, is provided with the framegrabber board for picture display, adjustment, and frame acquisition.

The Video Camera Employed at HAM-126

Weigh-In-Motion scale

Several common transportation sensor types (e.g., embedded load scales, piezoelectronic tubes, inductive loops, etc.) are wired to an independent data system purchased by the Ohio DOT. The PC-126 was plan specified and installed at HAM-126-0881. This system is clearly advanced in the sense that: piezo sensors are employed in addition to loops for vehicle detection, a staggered scale is installed to measure the weights of both tires, a faster processor is used by the on-site computer, and both a digital (IEEE RS-422) and phone modem interface are provided. The provided software allows the user to access the immediate and statistical record of truck class, speed, weight, axle weight, axle spacing, and violation (if any). The accuracy of the WIM system is dependent upon several factors: truck class, speed, load, suspension; pavement roughness, alignment, and slope; environmental factors (e.g., temperature, wind); and calibration regimen. A procedure has been written to access this information via the local digital line through a National Instruments NI-182930E-12 interface board that is installed in the EISA bus of the onsite computer.

The Set-Up of the Weigh-In-Motion Control Cabinet and Scale

Sensors used at HAM-126 for the High speed Monitor

Selecting sensors to monitor the bridge responses during the various phases of construction was a challenging design issue due to the diverse variety in available sensor characteristics. These differences/variations made some sensors suitable for measuring one type of response but not another. For example, two types of strain gages, resistance strain gages and vibrating wire strain gages, were used to monitor components of the steel framing system. Resistance strain gages are superior to vibrating wire gages for short-term high speed measurements like traffic monitoring, but due to their propensity to drift, they are inferior to vibrating wire gages for measuring long-term slow speed phenomena such as environmental variations. Other considerations which added complication to the design process included the variety of bridge responses to be measured, differences in function of the various bridge components, and the diversity of the bridge materials and their properties.

This section briefly describes what sensors were used to instrument the bridge, for the High-speed monitoring scheme, the function of these sensors, and where they were used. Strain gages were the most numerous type of sensor used for the project. A strain gage measures its change in length over the length of the gage and converts that change to an output corresponding to the amount of strain. There are numerous variations on the construction and characteristics of strain gages. For the most part, strain gages are inexpensive, readily available, simple to install, and reasonably reliable. The strain gages used for this project are classified according to their construction as:

Resistance Strain Gages used on Steel

All the gages that are used for the High-speed monitor are of this type. The resistance gage selected for instrumenting and continous monitoring and rating the bridge was Texas Measurements Model AWC-8B. This model is designed specifically for use in harsh environments, such as those which exist at a bridge site, and for long-term survival. It is constructed of a special alloy-foil-strain element enclosed in a stainless steel tube. The steel tube is attached to a metal base that can be spot welded to a steel specimen. This gage was used for high-speed measurements, such as for load tests and traffic-induced responses. It was also used to monitor beam areas not exposed to heat during heat cambering.

Resistance Strain Gage for Steel

Resistance Gages Used for Concrete Components

The Micro-Measurement EGP Series strain gage was embedded in the reinforced concrete bridge deck. This gage is specifically designed for measuring mechanical strains inside concrete. The transducer is encased in a 5-inch long outer body composed of a proprietary polymer concrete. The outer body contains a series of circular indentations to ensure adequate bond with the concrete matrix. The outer body also helps to prevent mechanical damage to the sensing element, minimizes reinforcement of the gage structure, and provides protection against moisture and corrosion attack. This gage was primarily used for high-speed measurements, such as for load tests and monitoring traffic responses.

Resistance Strain Gage for Embedding in Concrete