Reality-check and renewed challenges in earthquake engineering: implementing low-damage structural systems

From theory to practice

Earthquake Engineering is facing an extraordinarily challenging era, the ultimate target being set at increasingly higher levels by the demanding expectations of our modern society. The renewed challenge is to be able to provide low-cost, thus more widely affordable, high-seismic-performance structures capable of sustaining a design level earthquake with limited or negligible damage, minimum disruption of business (downtime) or, in more general terms, controllable socio-economical losses. The Canterbury earthquakes sequence in 2010-2011 has represented a tough reality check, confirming the current mismatch between societal expectations over the reality of seismic performance of modern buildings. In general, albeit with some unfortunate exceptions, modern multi-storey buildings performed as expected from a technical point of view, in particular when considering the intensity of the shaking (higher than new code design) they were subjected to. As per capacity design principles, plastic hinges formed in discrete regions, allowing the buildings to sway and stand and people to evacuate. Nevertheless, in many cases, these buildings were deemed too expensive to be repaired and were consequently demolished. Targeting life-safety is arguably not enough for our modern society, at least when dealing with new building construction. A paradigm shift towards damage-control design philosophy and technologies is urgently required. This paper and the associated presentation will discuss motivations, issues and, more importantly, cost-effective engineering solutions to design buildings capable of sustaining low-level of damage and thus limited business interruption after a design level earthquake. Focus will be given to the extensive research and developments in jointed ductile connections based upon controlled rocking & dissipating mechanisms for either reinforced concrete and, more recently, laminated timber structures. An overview of recent on-site applications of such systems, featuring some of the latest technical solutions developed in the laboratory and including proposals for the rebuild of Christchurch, will be provided as successful examples of practical implementation of performance-based seismic design theory and technology.

Ductility and damage: is this an unavoidable equivalency?
Recognizing the economic disadvantages of designing buildings to withstand earthquakes elastically as well as the correlated disastrous socio-economic consequences after a design-level or higher-than designed level earthquake intensity (e.g. as for example observed in the Great Hanshin event, Kobe 1995 and, most recently in the 22 Feb 2011 Christchurch Earthquake), current seismic design philosophies promote the design of ductile structural systems able to undergo inelastic reverse cycles while sustaining their integrity. The basic principle of this design philosophy, widely known and referred to as “capacity design” or hierarchy of strength, developed in the mid/late1960s by Professors Bob Park and Tom Paulay at the University of Canterbury in New Zealand, is to ensure that the “weakest link of the chain” within the structural system is located where the designer wants and that it will behave as a ductile “fuse”, protecting the structure from more undesired brittle failure mechanisms (Fig. 1). This approach would allow the building to sway laterally without collapsing in what in gergo is typically referred to as a “soft-storey” mechanism or, more simplistically a “pancake” collapse. Regardless of the structural material adopted (i.e. concrete, steel, timber) traditional ductile systems rely on the inelastic behaviour of the material. The inelastic action is intentionally concentrated within selected discrete “sacrificial” regions of the structure, typical referred to as plastic hinges. Until recent years, the development of inelastic action in traditional monolithic (or emulative) connections has been assumed to inevitably lead to structural damage, thus implying that “ductility = damage”, with associated repair costs and business downtime. As discussed later in the paper, following the introduction of recently developed, cost-efficient and high-performance technologies, under the umbrella of an emerging damage-avoidance or damage-control design philosophy, the ductility-damage equivalency is not anymore a necessary compromise of a ductile design.

Figure 1: A tribute to the basic concept of capacity design: the “weakest link of the chain” concept (left) and its implementation in a frame system with the protection of a soft-storey (brittle) mechanism in favour of a beam side-sway (ductile) mechanism (Paulay and Priestley, 1992).

What is an acceptable level of damage?
In response to a recognized urgent need to design, construct and maintain facilities with better damage control following an earthquake event, a special effort has been dedicated in thelast two decades to the preparation of a platform for ad-hoc guidelines involving the whole building process, from the conceptual design to the detailing and construction aspects. In the comprehensive document prepared by the SEAOC Vision 2000 Committee (1995), Performance Based Seismic Engineering (PBSE) was given a comprehensive definition, as consisting of “a set of engineering procedures for design and construction of structures to achieve predictable levels of performance in response to specified levels of earthquake, within definable levels of reliability” and interim recommendations have been provided to actuate it. According to a performance-based design approach, different (and often not negligible) levels of structural damage and, consequently, repairing costs shall thus be expected and, depending on the seismic intensity, be typically accepted as unavoidable result of the inelastic behaviour. Within this proposed framework, expected or desired performance levels are coupled with levels of seismic hazard by performance design objectives as illustrated by the Performance Design Objective Matrix shown in Figure 2.Performance levels are expression of the maximum acceptable extent of damage under a given level of seismic ground motion, thus representing losses and repair costs due to both structural and non-structural damage. As a further and fundamental step in the development of practical PBSE guidelines, the actual conditions of the building as a whole should be expressed not only through qualitative terms, intended to be meaningful to the general public, using general terminology and concepts describing the status of the facility (i.e. Fully Operational, Operational, Life Safety and Near Collapse) but also, more importantly, through appropriate technically-sound engineering terms and parameters, able to assess the extent of damage (varying from negligible to minor, moderate and severe) for the single structural or non-structural elements (ceiling, partitions, claddings/facades, content) as well as for the whole system. To give a practical example, according to the Basic Objective presented in this performance matrix, and associated to ordinary residential/commercial construction, a Life Safety damage level would be considered acceptable under a design level earthquake (traditionally taken as a 500 years return period event). This would imply that extensive damage, often beyond the reparability threshold (corresponding to a yellow/orange to red tag of the building), would be considered as an accepted/proposed target. Such implications might be clear and obvious to the technical professionals, but not necessary to the general public. It would thus not come as a surprise if users, residents, clients, owners/stakeholders of the building/facilities as well as the territorial authorities had a remarkably different opinion, based on a clearly different understanding of the significance and expectation from the behaviour of an “earthquake-proof” building. From the public perspective, not only life-safety and collapse prevention would be considered as “granted”, but also only a minimum level of damage would be actually expected so to require minimum repairing costs and disruption of the daily activities.

Figure 2: Seismic Performance Design Objective Matrix as defined by SEAOC Vision 2000 PBSE Guidelines, herein rearranged to match building tagging, and proposed/required modification of the Basic-Objective curve towards a damage-control approach (blue line, modified after Pampanin, 2010, Kam et al., 2011).

This Keynote was presented at the 15WCEE in Lisbon, Portugal, September 2012.