		2007 NZSEE Conference
	Abstracts

Keynote Address 1 Defining Acceptable Performance Criteria Planning for Expected Performance / Improving Existing Performance I Improving Existing Performance II Keynote Address 2 Developments Within Design I Developments Within Design II Developments Within Design III / Understanding the Inputs Assessing Existing Performance Poster Papers

Experimental Investigation of Existing Hollowcore Seating Connection Seismic Behaviour Pre and Post Retrofit Intervention

J Jensen, Des Bull and Stefano Pampanin

In the recent past a number of issues regarding the seismic performance of typical existing hollowcore floor systems have been raised. The most concerning of these is the vulnerability to loss of vertical support of the floor system at the end floor to seating beam connection. This vulnerability arises due to incompatibilities between the floor system and intrinsic deformations of the neighbouring seismic frames.

In a previous contribution by the authors (Jensen et al 2006), a conceptual retrofit strategy for existing hollowcore seating connections was proposed. This paper provides an experimental validation of that strategy through quasi-static cyclic testing of alternative seating connection configurations, adopting varying seating lengths.

In general, unfavourable performance was exhibited by the existing seating connections, resulting in loss of vertical support of the hollowcore unit. In contrast, when additional seating and selective weakening retrofit approaches were implemented, a higher level of seismic performance leading to collapse prevention was achieved. In conclusion, issues and uncertainties associated with the evaluation of the likely failure mechanism, as well as the definition of an appropriate retrofit intervention are discussed.

Paper P12: [Read]

Seismic Testing of a Model Structure with Semi-active Resetable Devices

Roberto Franco-Anaya, Athol Carr, John Mander, Geoffrey Chase, Kerry Mulligan and Geoffrey Rodgers

Semi-active resetable devices have recently been considered to reduce the seismic response of civil engineering structures. Resetable energy dissipation devices are fundamentally hydraulic or pneumatic spring elements that possess the ability to release the stored spring energy at any time. Instead of altering the damping directly, resetable devices nonlinearly alter the stiffness of the structure. This paper describes a series of shaking table tests of a four-storey model structure subjected to seismic excitation. The model structure is a one-fifth scale steel moment-resisting frame and aims to model a typical reinforced concrete frame building. Two semi-active resetable devices were installed in the lateral bracing of the model structure to reduce the seismic response. The devices modified the stiffness of the model structure by following a control algorithm that utilised the measured dynamic response of the structure. The results of the shaking table tests are presented and interpreted.

Paper P13: [Read]

Development of a Non-tearing Floor Solution for Jointed Precast Frame Systems

Alejandro Amaris, Stefano Pampanin, Des Bull and Athol Carr

The effects of beam elongation in precast frame systems have been recently demonstrated to be a potential source of un-expected damage up to failure of precast floor systems, unless adequate detailing accounting for displacement incompatibilities between the lateral resisting systems and the floor are provided. In this contribution, an innovative “non-tearing floor” jointed connection solution with re-centering capabilities is proposed. An efficient floor-to-lateral-load-resisting connection can be obtained able to minimise the problems associated with beam elongation effects while maintaining the low-damage and re-centering characteristics of jointed ductile connections (e.g. PRESSS-Technology). The preliminary results of experimental and numerical investigations on the interaction between diaphragm and lateral resisting frames adopting non-tearing jointed ductile connections are presented. A series of 2/3 scaled beam-column subassemblies have been tested under uni-directional quasi-static loading regime. Both numerical and experimental results provide satisfactory confirmation of the unique flexibility and potentiality of the proposed solution for the development of the next generation of seismic resisting buildings. A large scale test on a two storey 3-D frame system implementing jointed ductile connections with the proposed non-tearing floor connection solution is under preparation at the University of Canterbury.

Paper P14: [Read]

Experimental Behaviour of Exterior Beam-Column Joint Subassemblies Retrofitted using GFRP Composites

Umut Akguzel and Stefano Pampanin

Most of the experimental studies available in the literature on the seismic behaviour of poorly detailed beam-column joint subassemblies prior to and after retrofit intervention have concentrated on the 2-dimensional response, thus subjecting the specimen to uni-directional cyclic loading. In addition, the attention has been typically given to interior (fully or partially confined) beam-column joint subassemblies, with limited information on the response of exterior 2D or 3D corner joints.

Bi-directional loading conditions could have severe effects on the response of the beam-column joint with poor details (limited transverse reinforcement and confinement in the joint, plain round bars, weak joint shear capacity). The assessment of the hierarchy of strength and sequence of events should thus properly account for the 3-D effects in order to guarantee an adequate retrofit intervention.

In this paper, the results of uni- and bi-directional quasi-static tests carried out at the University of Canterbury on four exterior beam-column joint subassemblies, representing typical pre-1970 construction practice, prior to and after being retrofitted using Glass Fiber Reinforced Polymer sheets. The experimental results, part of a more comprehensive research campaign, provided satisfactory confirmations of the efficiency of the proposed retrofit strategy and solution, aiming at protecting the panel zone region, while activating a flexural behaviour in the beam in order to achieve a more desired weak beam-strong column global inelastic mechanism.

Paper P15: [Read]

Experimental Investigations of a Selective Weakening Approach for the Seismic Retrofit of R.C. Walls

Matt Ireland, Stefano Pampanin and Des Bull

Current seismic retrofit strategies generally focus on increasing the strength/stiffness or upgrading the mechanical properties of a structure or element. A typical drawback of this approach is that the demand on structural and sub-structural elements can be increased. In a previous contribution by the authors (Ireland et al., 2006) a counter-intuitive but rational seismic retrofit strategy consisting of selective weakening techniques was proposed.

In this paper results of experimental investigations performed on benchmark & selectively weakened structural walls at the University of Canterbury are discussed. The experimental investigations consisted of quasi-static uni-directional tests on two benchmark and two retrofitted cantilever wall specimens. The first benchmark wall specimen was detailed as typical of pre-1970’s construction practice. An equivalent wall was retrofitted using a selective weakening approach involving a horizontal cut at foundation level to allow for a controlled rocking response. The second benchmark specimen represented a more severe scenario where the inelastic behaviour was dominated by shear. A retrofit solution involving vertically segmenting the wall to improve the ductility and retain gravity carrying capacity by inducing a flexural response was implemented.

The experimental results confirmed the viability and efficiency of the proposed retrofit technique towards improving the performance of structural walls. Constructability issues and suggestions for practical implementation of the proposed retrofit solution are also discussed.

Paper P16: [Read]

Improving the Seismic Performance of Existing Reinforced Concrete Buildings using Advanced Rocking Wall Solutions

Dion Marriott, Stefano Pampanin, Des Bull and Alessandro Palermo

Recent major earthquakes such as Northridge 1994 and Izmit Kocaeli 1999 highlighted the poor performance of existing buildings constructed prior to the early 1970’s. Low lateral seismic design coefficients and the adopted “working stress design” methodology (essentially an elastic design) lacked any inelastic design considerations, thus leading to inadequate detailing. Insufficient development lengths, lapping within potential plastic hinge regions, lack, or total absence of joint transverse reinforcement, and the use of plain round reinforcement and hooked end anchorages were common throughout the structure. The behaviour is generally dominated by brittle local failure mechanisms (e.g. joint or element shear failures) as well as possible soft-storey mechanisms at a global level. Amongst several possible retrofit interventions, a typical solution is to provide the structure with additional structural walls i.e. external buttressing or column in-fills.

Extensive developments on precast, post-tensioned, dissipative systems have shown promise for the use of rocking wall systems to retrofit existing poorly detailed frame structures. In this contribution, the feasibility of such a retrofit intervention is investigated. A displacement-based retrofit procedure is developed and proposed, based on targeting pre-defined performance criteria, such as joint shear and/or column curvature deformation limits. A design example, using the proposed retrofit strategy on a prototype frame is presented. A brief overview on experimental work ongoing at the University of Canterbury investigating the dynamic response of advanced rocking walls for retrofit purposes will be provided.

Paper P17: [Read]