Affiliations: Instituto Superior Técnico, Lisbon, Portugal
Note: [] Corresponding author: C.S. Oliveira, Instituto Superior
Técnico, Av. Rovisco Pais, 1049-001 Lisbon, Portugal. E-mail:
csoliv@civil.ist.utl.pt
Abstract: Before establishing the priority settings for the reduction of
seismic risk of water supply infrastructures, it is necessary to understand the
dynamic behavior of elevated water tanks, which are components of those
infrastructures. Among other information, the main frequencies of vibration of
these structures must be estimated and the analytical models used in their
analysis and design should reproduce the frequency values obtained by in-situ
dynamic tests. This work focuses exclusively on reinforced concrete (RC)
elevated water tanks (200–750 m^3 of water at heights
of 30–40 m), which are very common structures in the water supply systems
in Portugal since the mid XXth century. This type of structures can also be
seen in many regions around the world. First, a nationwide survey was conducted to determine the most
common typologies in the country in terms of structural layout. Second, an
in-situ campaign using ambient vibration as input was performed for a group of
selected structures to determine the main frequencies of vibration and to
identify modal shapes and damping values. Third, a finite element model of
several different typologies was developed using the water simply as a
concentrated mass at the top; the elastic properties of the model of the
structure including the foundation were calibrated, so that the frequencies of
various mode shapes obtained by the analytical model would match the
frequencies of the real structure. Finally, an expression was derived to
estimate the fundamental frequency of a group of elevated water tank typologies
based on the total mass at the top of the supporting structure, which include
the water, the global lateral stiffness, and the height of the tank. This study, providing important information on the frequencies of
vibration of RC-elevated water tanks, contributes in a definite way to the
analysis and design of such water tanks.
Keywords: Dynamic analysis, RC elevated water tanks, identification of frequencies, analytical modeling
