Overview of Seismic Assessment and Retrofit of Pre-1970s Reinforced Concrete Buildings
Principal Researcher: Stefano Pampanin
Staff involved: Athol Carr, Des Bull ,
Students involved:Kam Weng Yuen, Patricio Quintana Gallo,
Various retrofit or seismic rehabilitation schemes have been previously proposed and implemented for bc joints and rc frames (fib, 2003; NZSEE, 2006; ASCE-41, 2007). The majority of the established methods involve either the strengthening of the joint only or both the joint and column in order to induce plastic hinging in the beams. Alternatively, the demand onto the structure can be reduced by supplementary damping or base-isolation. While most retrofit techniques can theoretically achieve a targeted structural performance, excessive costs, invasiveness and constructability are still the main issues to be solved prior to wider implementation.
In this research, the experimental and numerical validation of a counter-intuitive seismic retrofit strategy, referred as “Selective Weakening”(SW) retrofit, (Pampanin, 2005) for rc exterior bc joint is seeked. The concept of weakening for retrofit of rc frames has been previously proposed with reference to the weakening of the beams in order to create a weak-beam strong-column mechanism(FEMA-356, 2000; Pampanin, 2005b; Viti et al., 2006).
Despite the variety of retrofit strategies and techniques in the toolbox (fib, 2003; NZSEE, 2006; ASCE-41, 2007) available to engineers, it is not uncommon to find global or local strengthening (Figure 1a) as the typical retrofit strategy. While adding obstructive braces or shear walls may seem structurally efficient, without proper engineering judgement, strengthening-only retrofit may generate failures elsewhere within the structural system such as the foundation. The use of composite materials such as fibre-reinforced polymers (FRPs) for jacketing has shown tremendous potential, though the labour intensity and invasiness of the retrofit techniques might be deterrent to its widespread application. Alternatively, for higher-end building owners, the reduction of seismic demand by the means of supplementary damping (Figure 1b) and/or use of base isolation system (Figure 1c) has been regular practice, as these allows higher performance levels while being less intrusive. Again, the issue of cost and time/space invasiveness of these common techniques has been the reason for its widespread application, particularly in private buildings. The effects of various retrofit strategies on the structural performance are illustrated in Figure 1 within an Acceleration-Displacement Response Spectrum (ADRS) domain, typical of a capacity spectrum method.
Increasingly, retrofit solutions focussed on deformation demand and capacity (e.g. curvature ductility, maximum and residual inter-storey drifts) rather than force/strength, as deformations are considered more effective measures of damage (Pampanin, 2005). In view of such a paradigm shift in the state-of-the-art of seismic retrofit (and design), the proposed selective weakening strategy aims to improve the global inelastic mechanism (deformation capacity) of the structure by first weakening, then upgrading specific/critical structural (or non-structural) elements. Conceptually, where by selectively weakening certain elements and/or re-strengthening the structure, the structure achieves higher deformation capacity with more desirable inelastic mechanisms as illustrated in Figures 1d and 1e.
(Left) As built pre-1970's exterior beam-column joint (Right) Pre-1970's exteriour beam-column joint retrofitted with beam weakening and external post-tensioning. Satisfactory seismic performance up to 4.0% inter-storey drift cycles.
See the following references for further information:
Kam, W.Y., Pampanin, S., Bull, D. (2009)"Experimental Validation of Selective Weakening Approach for the Seismic Retrofit of Exterior Beam-Column Joints." Proc. of 2009 NZSEE Conference, Christchurch, New Zealand. April 2009. pdf and ppt
Kam, W.Y., Pampanin, S. (2008) "Selective Weakening Techniques for Retrofit of Existing Reinforced Concrete Structures" Proc. of 14th World Conference on Earthquake Engineering (14WCEE) 2008 , Beijing, China. (Paper ID: 05-03-0074) pdf