Guest Blogger: Michelle Riedman

by Michael Mullaney on July 17, 2012

I’m Michelle Riedman, a civil engineering (structures) graduate student working with Professor Christopher Letchford and Professor Michael O’Rourke. We are currently conducting a research project for the New York State Department of Transportation (NYSDOT) entitled “Determining the Remaining Fatigue Life of In-Situ Mast-Arm Traffic Signal Supports.” Harry White is the NYSDOT project manager while my two advisors are co-principle investigators. Undergraduate electrical engineering student, Vinh Nguyen, is also working on the project as part of RPI’s Undergraduate Research Program.

The project studies wind-induced vibrations of long cantilevered mast arm traffic signal structures. When the wind flows past the mast arm of the structure, low-pressure vortices are shed on alternating sides of the arm causing the mast arm to vibrate in a cross-wind or vertical response, as shown below.

If the frequency at which the vortices are shed (which is a function of both the wind speed as well as the diameter of the mast arm) matches the natural frequency of the structure, then vibrations with high amplitudes can occur. These vibrations cause structural stresses and strains to occur in a cyclical fashion which can lead to fatigue of the structure, and in some cases full collapse.

To study these wind induced vibrations, we are currently conducting a full scale experiment on a 25 meter cantilevered traffic structure in Malta, NY. An ultrasonic anemometer and two 3-component accelerometers were installed on the structure with the help of Jon LaPointe from the civil engineering department and data is currently being recorded at intervals of 23Hz through a data acquisition system. A picture of the traffic structure along with several pictures from the installation of our equipment is shown below.

While installing the equipment, pluck tests were conducted in order to determine the dynamic properties of the structure. To conduct these pluck tests, the free end of the mast arm was manually excited by a person from the research team with access via a boom lift and then let go so that the structure entered into free vibration. Using both the time history of the data collected during these tests and the corresponding Fourier transform, the natural frequencies and damping ratios were determined for both the in plane and out of plane directions.

These results compared well with the finite element model that had been developed prior to the full scale tests. A screen-shot of the finite element model in its first mode of vibration is shown below.

The natural frequency of the structure was calculated to be 0.52 Hz for the first mode of vibration. Knowing the diameter of this particular mast arm, it has been calculated that vortices should be shed at this same frequency when the wind speed is around 6 m/s or about 13 mph. In order to determine how frequently this wind speed of interest should occur, wind climate data for the Albany, N.Y. area was obtained from the National Climatic Data Center (NCDC) and a wind rose was constructed. A wind rose shows the frequency at which wind blows from a particular direction at a particular speed. The angle of attack of the wind relative to the mast arm is of interest as well since the vibrations are most severe when the wind hits near perpendicular to the mast arm’s longitudinal direction. When overlayed on a site map, the wind rose for the Albany, NY area shows that the majority of winds in the area should hit the arm in a near perpendicular fashion, which is good news for our research project, but bad news for the life of the structure.

The end goal of the project is to use the data collected through the full scale experiment, the finite element computer model, as well as the general climate data from the NCDC for various locations in New York state to come up with a general methodology that the DOT can use to assess the remaining fatigue life of these types of structures throughout the state.