|Session 8.2 - Tank Structures|
A large portion of the concrete water supply tank stock have distinct seismic defects, while others provide a lower seismic force resistance than required in modern practice. Water supply tanks form a critical part of infrastructure and post earthquake water supply is an important part of any asset management plan. This paper reports actual construction experience of the seismic retrofit of concrete city water supply tanks by the authors, 13 in the authors' specialist company and seven while employed by a large consultant.
The tank seismic retrofit methods included external and hoop post-tensioning with greased and sheathed tendons at roof level outside pilasters on circular tanks, epoxy bonding angle shaped steel plates to transfer seismic shear forces at the base of precast circular tanks, use of epoxy impregnated carbon fibre to structurally connect components, and strengthening of 9,000 m3 rectangular concrete tank cells by post-tensioning with greased sheathed strand passing through the body of the tank cells.
The paper provides an overview of design philosophy methods, construction experience and the benefits for asset management of New Zealand infrastructure.
Keywords: tanks, retrofit, seismic, strengthening, reservoirs, concrete
J.A. Munshi and N.A. Legatos
The new ACI Code 350/350.R-01, "Code Requirements for Environmental Engineering Concrete Structures", has greatly expanded the seismic design provisions of the previous edition, ACI 350-89. The alterations have been in two ways: (1) through the adoption of Chapter 21 of ACI 318 ("Special Provisions for Seismic Design"), and (2) the drafting of detailed seismic analysis guidelines in a separate Standard, ACI 350.3/350.3R ("Seismic Design of Liquid-Containing Structures"). Chapter 21 covers the "resistance" side of seismic design, while ACI 350.3 covers the "load" side. This represents a departure from ACI's traditional practice, which has been to concentrate on the design (resistance) aspect, while leaving the derivation of the seismic loads themselves (seismic analysis) to other references. As a result of this innovation, the two documents, ACI Standard 350.3/350.3R, in combination with Chapter 21 of ACI 350, fill a real need for the profession. They equip the practicing engineer with a practical and reliable tool for analyzing and designing liquid-containing concrete structures of all types to resist earthquakes. The paper will concentrate on ACI 350.3/350.3R in two parts; The first part will offer an overview of the theory behind the "load" provisions of the document, will highlight the basic theory involved, and will outline the derivation of the seismic loads. The second part will lay out an overall "road map" of how to design a typical liquid-containing structure for specified seismic loads.
Keywords: seismic design, tanks, ACI 350, liquid-containing structures
Several of the main issues of concern in prestressed concrete tank seismic design are addressed. The most important of these is the application of structural analysis methods for predicting the reserve capacity above the elastic response force levels. Another of the important issues investigated was the influence of soil-structure interaction on the response of both the horizontal and vertical modes of vibration. An analysis procedure for estimating the reserve capacity of prestressed concrete tanks is presented and the method illustrated by the analysis of a tank of typical dimensions. Charts for estimating the influence of soil-structure interaction for the practical range of tank sizes and foundation conditions are also presented.
Keywords: concrete tank design, tank force-reduction, tank damping, tank codes