Parametric Investigation of Seismic Interaction between Precast Concrete Cladding Systems and Moment Resisting Frames Andrew Baird1, Riccardo Diaferia2, Alessandro Palermo3, Stefano Pampanin4 1
PhD Candidate, Dept. of Civil and Natural Resources Engineering, University of Canterbury, P.O. Box 4800, Christchurch 8140, New Zealand; PH (+64) 276961791; email:
[email protected] 2
Master student, Dept. of Civil Engineering, Politecnico di Milano, Campus Leonardo, Milano, Italy; PH (+39) 328-3934441; email:
[email protected]
3
4
Senior Lecturer, Dept. of Civil and Natural Resources Engineering, University of Canterbury, P.O. Box 4800, Christchurch, 8140 New Zealand; PH (+64) 212370407; email:
[email protected]
Associate Professor, Dept. of Civil and Natural Resources Engineering, University of Canterbury, P.O. Box 4800, Christchurch 8140, New Zealand; PH (+64) 212370407; email:
[email protected]
ABSTRACT This paper presents the results of a preliminary numerical investigation into the interaction between precast concrete cladding systems and moment resisting frames. Macro-scale models of cladding systems are implemented in existing lumped plasticity models for moment resisting frames. Different failure mechanisms and various configurations are considered in order to show the effect of the entire cladding system upon a structure’s seismic behavior. Several parameters are varied in order to establish their associated influence on the overall structural response. Results show that it is clearly more advantageous to have a failure mechanism governed by the connection than one governed by either the panel or the frame. An experimental program is now underway building on what has been learnt from the parametric investigation. The authors intend to continue the research to successively develop improved or innovative low-damage cladding-moment resisting frame systems. They also aim to produce simple design tools that provide easy inclusion of the seismic effects of cladding-frame interaction.
INTRODUCTION The response of non-structural components can significantly affect the functionality of a building after an earthquake, even when the structural members are left undamaged. The poor performance of non-structural components or secondary structural elements in past earthquakes has led to buildings being left unoccupied, resulting in substantial economic losses due to business interruption. Furthermore, damage to non-structural components, such as that shown in Figure 1, can pose a serious risk to the safety of people inside and outside the building (Charleson, 2008). Recent earthquakes in L’Aquila, Italy (6 April 2009), Concepcion, Chile (27 February 2010) and Darfield, New Zealand (4 September 2010) have further highlighted the severe impact that damage to non-structural elements can have upon the overall recovery effort. In order to develop and propose practical and efficient solutions that reduce the risk of damage to non-structural components it is necessary to understand how they interact with a structure. In addition, determining which parameters most influence this interaction is essential so that all possible damage and/or failure mechanisms are identified. This paper presents the preliminary results of a numerical study on the interaction between a precast concrete cladding panel attached to a reinforced concrete moment resisting frame. This is achieved using static push-over and cyclic push-pull analyses of a lumped plasticity model representing an interior single-storey, single bay of a multistorey building. In order to understand the different possible damage and failure mechanisms several parameters of the systems are varied. The results also confirmed the sensitivity of the systems’ behavior to such changes.
Figure 1: Example of cladding failures
BACKGROUND Precast concrete panels are widely used around the world as an exterior cladding for multi-storey buildings. Such cladding can be considered as non load bearing wall systems which are designed primarily to transfer their self-weight and out-of-plane (wind and earthquake) lateral loads to the supporting building structure. The contribution of the cladding system to the lateral stiffness of the building is often ignored in the structural design. However, experimental investigations on newly designed buildings have shown that claddings can contribute significantly to the lateral stiffness of the structure and that the panels can be subjected to significant in-plane forces (Goodno et al., 1988) which might cause unexpected structural failure. In order to avoid this unintended interaction, it is possible to isolate cladding panels, as shown by research using autoclaved lightweight aerated concrete (ALC) panels. ALC panels can be connected using sliding and rotating connections, as shown in Figure 2, such that they contribute very little to the stiffness and strength of the overall structure, even under a very large inter-storey drifts of 0.04 radians (Okazaki et al., 2007).
Figure 2: Sliding panel (left) and rotating panel (right), (Okazaki et al., 2007) Complete isolation with the structural system does however mean that the cladding is simply a dead weight. Consequently, investigations have been carried out into ways in which the structure can profit from having cladding panels attached. If the additional stiffness and strength that cladding panels provide is utilized during design then a savings up to 25% in the volume of steel used in the structure can be achieved (De Matteis, 2005). Cladding panels can also be used to provide passive control for the seismic behavior of buildings with the use of energy dissipative connections. Results show that energy dissipative cladding connections like that shown in Figure 3 could provide the total hysteretic energy required of the structural system (Pinelli et al., 1995).
Figure 3: Advanced energy dissipating connection (left) and hysteretic loop (right), (Pinelli et al., 1995) The main emphasis of research on cladding systems to date is on detailed research into isolated façade technology. This therefore leaves open the question of whether benefits found can be applied should some parameter be slightly different. This paper aims to help in providing a way to broaden the results of such research so it can be applied to a range of similar façade technology.
PERFORMANCE-BASED DESIGN and FAILURE MECHANISMS Design for seismic resistance has been undergoing a critical reappraisal in recent years, with the emphasis changing from ‘strength’ to ‘performance’. For most of the past 80 years (the period over which specific design calculations for seismic resistance have been required by codes) strength and performance have been considered to be synonymous (Priestley, 2000). However now, performance based engineering has become a standard norm for research, development and practice of earthquake engineering, particularly after the 1994 Northridge and 1995 Kobe earthquakes (Okazaki et al., 2007). The primary function of performance-based seismic design is the ability to achieve, through analytical tools, a building design that will reliably perform in a prescribed manner under one or more seismic hazard conditions. The performance,
or condition of the building as a whole, is typically expressed through qualitative terms, intended to be meaningful to the general public. These terms use general terminology and concepts describing the status of the facility (i.e. Fully Operational, Operational, Life Safety and Near Collapse) but should also be associated and linked to appropriate technically-sound engineering terms and parameters. These performance-based design criteria are applicable to both structural and non-structural elements. Table 1 provides descriptions of the damage associated with three performance levels for two façade elements. Table 1: Suggested performance levels (FEMA 356, 2000) and seismic design performance matrix (SEAOC Vision 2000 Committee, 1995) PERFORMANCE LEVEL Element
Collapse Prevention
Life Safety
Some Local crushing, Precast connection spalling at Concrete failure but no connections, but no Panels elements gross failure. dislodged. Severe distortion in Severe connections. damage to Distributed connections Cladding cracking, bending, and cladding. crushing and Many panels spalling of loosened. claddings
Immediate Occupancy Minor working at connections, crack width
