Multi-Modal Natural Frequency Response of Utility Transmission Tapered Wood Poles Under Various Soil Foundation Conditions

Studied herein is the multi-modal natural frequency response of utility transmission tapered wood poles under various soil foundation conditions. Strong winds and hurricanes in various parts of the world have resulted in collapse of such utility poles and have resulted in the disruption of electrical distribution systems in addition to creating hazardous conditions for the public. To avoid the development of resonance under such dynamic loading, the multi-modal natural vibration of the utility poles first needs to be understood in the presence of practical soil foundation conditions. To capture the soil-structure interaction effects on the multi-modal frequencies, a SAP2000 dynamic finite element model is created in which the foundation soil stiffness is characterized by means of a series of ‘soil springs’ below the ground level. The properties of the soil springs vary with types of foundation soils and depths. Three types of foundation soils are considered, namely sandy, clayey soils and Granite (Rock). The results are compared to a standard fixed base model. It is found that the fundamental natural frequencies decreased by 52%, 37%, and 3% for sandy, clayey soils and granite, respectively, when compared to fixed base model. It was observed that there was an increase in the frequencies of the embedded utility poles in clay and granite, when compared to those with the fixed based after the 1st mode whereas, poles embedded in sandy soils showed increase in modal frequencies after the 3rd mode. The 10th mode appears to be a starting point of modal frequency convergence, while an apparent convergence occurs after the 20th mode. The convergent modal frequency was about 740 Hz for the Class H1 utility pole. However, there was a significant increase in the higher modal frequencies such as nearly 55% at the 20th mode, in all soil types when compared to the fixed base model.