CHAPTER 4: CONCLUSION
4.3 Future Work
During the summer of 2015, a total of fifteen structural configurations were tested and preliminarily analyzed in the Multi-Function Dynamics Laboratory. In conjunction with this reinforcement, the study of a more traditional damage detection configuration has been initiated. Damage was introduced into the baseline configuration by adding rubber washers to beam and column connections. These washers were two inch by two inch squares of rubber cut from a neoprene sheet of quarter-inch thickness. A half-inch slit was cut diagonally in the middle in order to place them on the bolts. These neoprene inserts serve to reduce the
stiffness, or “soften,” the connections as with aging.
The rubber washers were sequentially added between stories. For the first
configuration (W1), a rubber washer was placed on each bolt between the angle bracket and the bottom beams. This means that there were four rubber washers at each bottom corner of the structure, as shown in Figure 4.1. For the second configuration (W2), additional rubber washers were placed between the middle beams and the angle brackets. The bottom washers were left in place. The same pattern was followed for the third configuration (W3) as the rubber washers were placed between the top beams and the angle brackets. The middle and bottom washers were left in place.
Note that the respective beams had to be removed in order to place the rubber washers: this disturbed the prior connections. After the washers were placed for each
configuration, the beams were replaced with the same outer washers and nuts. The nuts were tightened with a torque wrench to approximately 3 Newton-meters (N-m) until the rubber washer was intermediately compressed.
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Figure 4.1 – First story rubber washer connections. Left: Top view of southwest column.
Right: Inner view of northwest column.
The rubber washers are intended to reduce the stiffness of each joint of the steel structure. This reduced the stiffness of the entire structure from the baseline; no structural members were altered, and the joints were not tightened back to their original torque. By reducing the global stiffness of the structure relative to the minimal mass change, the overall frequency response of the structure should decrease. Employing washers as softeners has the benefit of adding no significant mass while decreasing signal noise. By increasing each joint’s damping, the washers prevent loose metal contacting, or “rattling,” that could interfere with sensor data processing
The more flexible structure’s mode shapes should be aligned in the same order as the baseline’s, considering no lateral bracing was added and the boundary conditions remained
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the same. This means that sway mode shapes should be seen at lower frequencies, followed by slight torsion, column bending, and localized beam bending. However, the mode shapes should appear at lower frequencies. The degree of frequency shift may vary based upon the type of phenomenon (torsion, bending, or beam bending).
For the three rubber washer configurations, testing and analysis was conducted identically to that of the baseline structure. As in Chapter 3, numerous frequency peaks were again identified for each configuration, and each corresponding mode shape was examined.
With the baseline mode shapes available, the rubber washer configurations were matched among each other and the baseline. Table 4.1 shows the baseline mode shapes that matched with a corresponding rubber washer mode shape and the frequency at which each mode occurred.
Table 4.1 – Natural frequencies (Hz) of mode shapes matched between the baseline and rubber washer configurations.
Baseline Mode 11 12 16 24 25 27
Baseline 16.953 18.672 35.820 63.477 73.711 87.852 W1 14.102 17.500 35.039 63.516 71.680 84.023 W2 15.625 13.867 35.742 63.086 67.148 84.805 W3 13.320 11.602 32.461 59.531 63.359 84.570
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Table 4.1 shows a general decrease in frequency across the decreasing stiffness configurations. This was the expected effect of the induced “damage” or decreased stiffness.
There are a few cases of a slight frequency shift upward between the baseline case and W1.
This could be attributed to the bottom beams not contributing much stiffness to the
superstructure. Another possibility is that a support could have slightly been moved inward, causing a shoring of the structure as a whole which reduced flexibility. Figure 4.2 shows a shows the natural frequency shifts of matched mode shapes among the baseline, W1, W2, and W3 configurations.
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Figure 4.2 – Waterfall frequency plot of matched baseline, W1, W2, and W3 modes.
Cumulative Amplitude
Frequency (Hz)
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Figure 4.3 shows an example of a progression of matching mode shape for the rubber washer structure configurations. The mode shapes have very similar global movements. The northwest and southeast corners move outward while the southwest and northeast corners generally move inward. This creates a coupled torsion and translation mode shape, or a
“pinching” action, as shown in the reinforcement cases.
Figure 4.3 – Mode shape progression from the baseline structure to the rubber washer configurations.
Future work for this structural configuration includes further examination of the matched mode shapes. Sequential and cumulative damage detection and evaluation of damage indicators will also be performed.
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APPENDIX A
Coordinated Mode Shapes 1-7
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APPENDIX B
Reinforcement Detection Resultant Plots
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Baseline to R1 Comparison – All Matched Modes Considered
(Sequential)
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101
102
103
104
105
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Baseline to R2 Comparison – All Matched Modes Considered
(Cumulative)
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108
109
110
111
112
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R1 to R2 Comparison – All Matched Modes Considered
(Sequential)
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