Testing a 21 ft Model Steel Bridge: Vibration Testing, Destructive Pullout-Testing, Full Scale Cyclic Load Testing

The UC San Diego Steel Bridge Team designs, analyzes, optimizes, and fabricates a model steel bridge. The team optimizes for high stiffness, low weight, and fast constructability.

Extensive testing was performed on the model bridge for the 2022 UC San Diego Steel Bridge Team. The testing was performed to insure sufficient strength of members and connections, to accurately predict local load-deformation behavior of connections and global load-deformation of the whole bridge, and to perform a vibration system identification of the bridge.

1. Vibration Testing

Vibration testing was performed with a home-made setup of accelerometers and Arduino micro-controllers in an attempt to identify system properties. First natural freq. of 7.9 Hz aligns fairly closely with predicted 8.8 Hz from analytical dynamic FEA model. Significant variation observed in higher modes.

Notes on instrumentation:

• 3-axis accelerometers stick to bridge with magnets (right pic)
• Arduino nano micro controllers (left pic) communicate with up to 3 accelerometers
• Computer with C++ and MATLAB to coordinate microcontrollers and compile/postprocess data.
• Karl Johnson, a UCSD electrical engineering student, coded and integrated the accelerometer system, teaching UCSD steel Bridge members how to sauder and wire circuit boards in the process, and how to communicate with Arduino microcontrollers with laptops. Open source code is published here: GitHub Accelerometer Code
• Smartphones can also be utilized to record vibration data. Saul and teammates created a document with MATLAB code for how to remotely control your phone as an accelerometer through the MATLAB app, and graph acceleration data. The document is here: Acceleration from MATLAB phone app

2. Destructive Pullout-Testing

The destructive connection testing was used as part of the connection design process. A robust set of design, analysis, prototyping, and testing techniques were used, including:

• Hand calculations based on AISC § J3 to check for bearing capacity exceedance, net section failure, and pull out failure
• Finite Element Analysis models using “worst case” loading scenarios for connections with complex geometries (left picture).
• Destructive prototype testing (center, right) to verify results of hand calculations, Finite Element Analysis, to determine weld strength and loading behavior. Force-deformation results were then applied to “weak members” in a SAP 2000 to accurately estimate bridge deflection, both from local connection deformation and global elastic deformation.

3. Cyclic Loading Tests

We performed 3 full-scale loading and unloading tests prior to competitions to (1)insure sufficient strength and (2)measure experimental deflection and compare to analytical linear-elastic FEA model.

The right figure shows loading curves of the bridge. The pink test was performed after stiffness-increasing
updates were made in preparation for nationals.

The bridge does not fully rebound to initial position, indicating irreversible “play” or other inelastic deformation in the connections. This “play” was quantified through the destructive pull-out testing, allowing the team to accurately model the inelastic behavior of the bridge in SAP 2000. Connection are being built for the 2023 steel bridge with heat-treated chromoly, in order to reduce local yielding of connections and thus reduce inelastic behavior of the bridge. Carefully consideration is being used to balance the goal of local stiffness with the need for ductility.