Morales-Brignac, Juan C.

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    Dynamic properties and seismic response of the cable structures and towers of the Arecibo Observatory
    (2006-05) Morales-Brignac, Juan C.; Suárez, Luis E.; College of Engineering; López-Rodríguez, Ricardo; Martínez-Cruzado, José A.; Acosta-Costa, Felipe J.; Department of Civil Engineering; Jiménez, Héctor
    This investigation was requested by the staff of the Arecibo Observatory to determine whether the towers and cables that support the suspended platform are capable of resisting a maximum-credible earthquake in the elastic regime. The Arecibo Observatory is one of the most important research centers in the world for astronomic and ionospheric studies and thus maintaining its structural integrity is a top priority. Modal analysis is performed on detailed three-dimensional finite element models to determine the natural response. Three different types of modes are revealed - platform modes, tower modes and cable modes - in addition to strong interactions between them. Very strong interaction modes are recorded in the range of periods between 1.8 and 0.7 seconds. Modal time-history linear analyses are performed to determine the seismic response. Nonlinear direct-integration time history analysis with large displacements yield nearly identical results which validates the modal time-history linear analysis. Five different earthquake records are considered in the final seismic studies. These represent the expected range of seismic hazards for Puerto Rico. The towers are nearly capable of resisting in the elastic regime the seismic loading specified by the UBC-97 building code. However, the top two segments of the towers narrowly exceed their elastic moment capacity. The towers do not tolerate, and could even collapse, if exposed to the higher seismic demands expected in the cities of Ponce and Mayagüez. A preliminary retrofit investigation shows that the use of Fiber Reinforced Polymers (FRP) is the best alternative to boost the moment capacity of the towers to a comfortable level. This alternative requires further research into the capacity, durability and cost of this option. Several alternative locations for the placement of accelerometers are recommended.