Vibration modes of the Elizabeth Ashman Bridge, Madison, Wisconsin, USA

Nasser M. Abbasi

spring 2013   Compiled on December 29, 2018 at 5:28pm  [public]

The following are the first few vibration modes of the Elizabeth Ashman Bridge, generated using SAP 2000 software.


Figure 1: Picture of Elizabeth Ashman Bridge, located near UW Madison.

The following table is description of each mode. Clicking on the image plays an animated gif file of that mode.

mode 1

Bridge main body vibrates sinusoidally in the YZ plan with almost a full sin wave being described along the full length of the bridge. Ramp shows little vibration. Little motion in X direction.

mode 2

Similar to mode 1 but with larger amplitudes. Bridge vibration remained in the YZ plan. Ramp remains with little motion

mode 3

This is the ramp torsion mode. Ramp shows large twisting motion around the Y axis. Main bridge body now vibrates sideways moving in the X axes direction. The top of the bridge is tilting sidways more than the floor.

mode 4

Larger twists on the main bridge. Twist is around the Z axis where one half of the bridge swings to one side and the other half to the opposite side. Ramp has less torsion compared to mode 3.

mode 5

Both ramp and bridge now show large vibration. On the bridge, more twisting vibration are seen around the Z axis going through the middle of the bridge. Little vibration in the XY plan (up and down). Most of vibration is sideways. The half of the bridge connected to the ramp is vibrating in opposite direction to the ramp (out of phase with ramp).

mode 6

On the bridge, larger torsion vibration around the Z axis in the middle of the bridge. Ramp appears to vibrate less than in mode 5.

mode 7

Bridge floor vibration now in the YZ plane (vertically up and down) with larger vertical amplitude in the middle of the bridge. Almost two full sin wave can be seen across the full span of the bridge. Ramp appears to vibrate much less than it did in mode 7.

mode 8

Bridge has large torsional motion around the Y axis (Axis along its length). Bridge almost closes on itself near the top. The part of the Ramp attached to the bridge moves in phase with the bridge motion.

Table 1: Description of each mode

Reference: Main report