Development of design guidelines for rocking structures

Abstract

Many new and existing buildings have insufficient weight to resist overturning loads due to earthquakes without uplift of part of their foundation. Uplift can be prevented by the use of tension piles, but these add significant costs and may cause larger loads on the structure above. Observations from past earthquakes suggest that local uplift and rocking will not be detrimental to seismic performance and may be even beneficial in limited forces transmitted into the structure. In some cases, rocking isolation systems have been implemented as a means of energy dissipation to improve earthquake performance.

The main impediment to permitting uplift is that the displacements, and the associated change in force patterns, cannot be quantified using conventional design techniques. Previous versions of the New Zealand structural design code allowed simplified procedures for the design of rocking structures provided that the ductility (upward displacement after uplift) was limited. The new loads code, NZS 1170, removed this exemption and requires that a special study be performed whenever energy dissipation through rocking occurs.

Many of the buildings with the potential for rocking are relatively small buildings, and the design fees and programme cannot support the time and costs of a special study, which typically requires a high end computer analysis. Research is under way in academia to investigate aspects of rocking and uplift but these will address the theoretical aspects in much more detail than required for design office use and the delivery time will not meet the pressing need for published guidelines.

The objective of this project was to develop and publish guidelines to enable structural engineers to design and evaluate buildings which are subject to local uplift under earthquake actions. The guidelines were to be sufficiently robust to substitute for the special study currently required by NZS 1170.

This report presents these tentative guidelines, which enable designers to estimate the effects of rocking on structures within the time and expense involved in a computer study. The guidelines form a starting point for further development but in their present form are suitable for relatively simple and regular structures with moderate amounts of uplift.