Testing Equipment

The testing equipment used in this project was purchased from Bridge Diagnostics Inc. (BDI). Included with the BDI materials were strain gauges, wiring, software to record strain data on a portable laptop computer, and a device to track the load truck location. Additional equipment such as the load vehicle and traffic control was supplied by VTrans on the day of testing.

4.1.1 Strain Gauges

The BDI manufactured strain gauges utilize a full Wheatstone bridge with four 350 foil gauges to measure both tension and compression along its axis of orientation, within two percent accuracy. The BDI strain gauge shown in Figure 4.1 is 3 inches (72.2 mm) long between mounting holes. Each gauge is reusable and has an individual serial number used to correlate recorded strain data during analysis.

Typical strain gauges are attached to structural members to record the strain between the two points it is mounted. In this study, at least two gauges are placed at the same cross section of a member in a pair. The ideal placement of gauges is at both the top and bottom of the cross section to record maximum strain values, either in tension or compression. Assuming a linear strain profile, a strain diagram of the member can be created and moment calculated at the gauge pair location as is discussed in Chapter 6.

The holes at each end of a gauge allow attachment to structural members with

14 inch (6.4 mm) bolts. A one-sixteenth inch (1.6 mm) pad under the mounting holes

ensures the gauge does not contact the structural member and that measured deformation occurs entirely over the across 3 inch (72.2 mm) gauge length.

Figure 4.1: Picture of BDI Strain Gauge

BDI also manufactures aluminum gauge extensions for testing reinforced concrete structures, which increase the gauge length beyond 3 inches (72.2 mm). A gauge extension is useful when testing heterogeneous materials such as reinforced concrete, in which flexural cracks occur under loading. Because deformation concentrates around flexural cracks in the concrete, the extensions allow an average strain over a longer distance. The BDI extensions extend the effective length of the gauge in 3 inch (72.2 mm) increments between 6 and 24 inches (152 to 610 mm). The actual strain can be determined during data reduction by dividing the measured strain by one-third of the extended gauge length.

4.1.2 Strain Gauge Attachment

Gauges can be attached to structural members using either C-clamps or a bolt and nut through the holes of each gauge as shown in Figure 4.2. The selection of an attachment method is governed by the material of the attached member, ease of installation, and access to structural members. C-clamps, which are easiest to install and remove, can be only be used in steel structures where they can be attached to a beam flange or side of an angle section. As noted in Appendix A, no significant differences in measured strains were observed during tests in the laboratory with different attachment methods.

For reinforced concrete or steel structures where clamps cannot be attached, the alternative is to pass ¹₄inch (6.4 mm) bolts through the mounting holes on the gauge and secure with a nut. The bolt can then be attached to a structural member using either an adhered tab or mechanical anchor drilled into concrete. When using a strain gauge extension, one end of the gauge is bolted to the extension while the other end is adhered to the member using a bolt. A second bolt then attaches the gauge extension to the structural member.

a) Clamped Strain Gauge b) Adhered With Tab

c) ¼" Mounting Tabs d) Gauge With Extension Figure 4.2: Attachment of Strain Gauges 4.1.3 Recording Strain Data

Gauges are wired to a single power supply box that interfaces with a portable laptop computer. The computer operator must balance all strain gauges before load testing begins and can control data recording. The frequency that strain data points are recorded can be set with software. All tests with this project were conducted at 33.33 Hertz, that is 33.33 points were recorded per second. During a test, continuous strain data is recorded from all gauges connected to the system.

4.1.4 Load Vehicle

Testing requires the use one or more heavy trucks to generate a response from the bridge structure. As the test is non-destructive, the truck weight is necessarily below the service limit. It is important to record each axle weight, or if possible the load applied at each tire to calibrate an analytical model with recorded strain data.

Both of the load tests conducted for this project utilized a load vehicle provided by VTrans. An official from the Vermont motor vehicle enforcement unit used portable

For the majority of runs, the truck was driven across the bridge at approximately 5 mph (8 km/hr). This speed eliminates effects of dynamic impact and simulates a semi-static loading. Additional test runs at higher speeds were also conducted for comparison purposes.

4.1.5 Auto Clicker

Calibration of a finite element model also requires knowledge of the truck position on the bridge paired with each strain history. To track the longitudinal truck position along the bridge, BDI developed the AutoClicker, a device shown in Figure 4.3.

Figure 4.3: Photographs of AutoClicker Attached to Load Vehicle

Using a reflective paddle attached to the truck tire, an infrared camera mounted over the wheel senses the paddle at each tire revolution and emits a signal with a portable radio. The signal is instantaneously received at the power supply box and the time is marked on the raw data recorded by the laptop. Knowing the distance traveled per revolution of the truck tire, the longitudinal position of the vehicle can be calculated during data analysis.