Keynote Address Session 1 Session 2 Session 3 Session 4 Session 5 Session 6 Poster Session Session 8 

Determination of Acceptable Structural Irregularity Limits for the Use of Simplified Seismic Design Methods

Vinod Sadashiva, Greg MacRae, Bruce Deam and Richard Fenwick

Approximations to the exact response of a structure under design level excitation may be obtained by conducting a number of 3-D inelastic dynamic time history analyses considering all relevant effects and using the best information available. Factors that should be considered include foundation effects, floor diaphragm effects, and the likely variation of earthquake demand and structural capacity.

In general, this type of approximate analysis is too complex for design engineers, so simpler, and hence more approximate analysis methods are commonly used. These have been calibrated based on the response of regular structures. When it is felt that a structure is too irregular, some approximate analysis methods are not permitted to be used. The amount of irregularity permitted has generally been selected based on engineering judgment.

This paper describes a quantitative method to determine limits for structural irregularity for structures designed using different analysis procedures. The method considers both the design approach estimations and the response from inelastic dynamic time history analysis. Irregularity limits based on a specified level of confidence are selected and these are then proposed for the use in codes. The method is illustrated by an example.

Paper P14: [Read] [Presentation]

Base Shear Scaling

Barry Davidson

The base shear scaling requirements of NZS1170.5 are sometimes difficult to implement for the complex buildings for which they are intended. An alternative design approach, the implementation of the Modal Response Spectrum Method without base shear scaling, is proposed. The seismic response of four structures, designed without base shear scaling, is used to illustrate the “satisfactory” response of this approach. It is concluded that the additional strength required for irregular structures could be more rationally achieved through the incorporation of a modified dynamic magnification factor rather than base shear scaling.

Paper P58: [Read] [Presentation]

Feasibility and detailing of Prestressed Timber Buildings for Seismic Areas

Tobias Smith, Stefano Pampanin, Andy Buchanan and Massimo Fragiacomo

This paper describes the structural design and selection of construction detailing for low-rise multi-storey timber buildings using a new and exciting structural timber system. This system, originally developed for use with pre-cast concrete, combines the use of un-bonded post-tensioning techniques and additional sources of energy dissipation. This system eliminates residual displacement, while greatly reducing the damage to structural members during a significant seismic event. The paper shows how this new structural system can be used with large size structural timber members manufactured from laminated veneer lumber (LVL) or glulam timber, for use in multi-storey buildings, with lateral load resistance provided by post-tensioned structural timber frames or walls, separately or in combination.

An extensive on-going research program at the University of Canterbury has tested a wide range of beam-to-column, wall-to-foundation and column-to-foundation connections under simulated seismic loading, all giving excellent results.

As part of this contribution, a case study of the design methods, construction options, cost and feasibility of a six storey timber office building in a moderate seismic area is carried out. The structural design of this building allowed investigation of different methods of structural analysis, and the development of many construction and connection details offering feasibility of rapid construction. Total building cost was evaluated and compared to equivalent steel and reinforced concrete options.

Paper P53: [Read] [Presentation]

Experimental Study of Prestressed Timber Columns under Bi-directional Seismic Loading

Asif Iqbal, Stefano Pampanin and Andy Buchanan

Structural members made of laminated veneer lumber (LVL) in combination with unbonded post-tensioning have recently been proposed, which makes it possible to design moment-resisting frames with longer spans for multi-storey timber buildings. It has been shown that prefabricated and prestressed timber structures can be designed to have excellent seismic resistance, with enhanced re-centring and energy dissipation characteristics. The post-tensioning provides re-centring capacity while energy is dissipated through yielding of mild steel dissipating devices.

This paper summarizes an experimental investigation into the seismic response of LVL columns to bi-directional seismic loading, performed as part of a research programme on timber structures at the University of Canterbury. The experimental investigation includes testing under both quasi-static cyclic and pseudo-dynamic protocols. The results show excellent seismic performance, characterized by negligible damage of the structural members and small residual deformations, even under the combined effect of loading in two directions. Energy is dissipated mostly through yielding of external dissipators connecting the column and the foundation, which can be easily removed and replaced after an earthquake. Since post-tensioning can be economically performed on site, the system can be easily implemented in multi-storey timber buildings.

Paper P31: [Read] [Presentation]

Dynamic performance of timber diaphragms in the 1903 Nathan Building

Aaron Wilson, Claudio Oyarzo-Vera, Piotr Omenzetter, Nawawi Chouw and Jason Ingham

To date researchers and practitioners have assessed expected unreinforced masonry (URM) building response and formulated seismic retrofit solutions based on limited laboratory data, and inadequately validated modelling techniques. A problem exists that throughout the world almost no in-field testing has been undertaken to generate data with which to validate the accuracy of models and laboratory based studies of retrofit solutions. It is widely recognised that the behaviour of timber diaphragms in URM buildings is crucial to the response of the structure as a whole but little is known about the actual response and dynamic characteristics of these diaphragms in existing buildings. This includes both the properties of the diaphragm itself, and the dynamic characteristics of the diaphragm-wall connection when cyclically loaded either normal or tangential to the plane of the wall.

A current research programme has been initiated to investigate the dynamic behaviour of the timber diaphragms in the Nathan House, located in Auckland’s Britomart precinct. The timber diaphragm was tested in-situ using a dynamic mass shaker. The forced vibration test enabled determination of dynamic properties such as natural frequencies, damping ratios and modal shapes. These modal parameters were used to calibrate stiffness and boundary conditions of an FEM model of the diaphragm, enabling estimation of the stiffness of the diaphragm and diaphragm-wall connections.

The results of this test will improve the understanding of timber diaphragm response, including stiffness characteristics and connection performance to the URM walls. The data collected from this test will aid the development of effective retrofit solutions and is essential in the accurate finite element modelling of URM structures.

Paper P05: [Read] [Presentation]

Post-Disaster Benefits of Upgrading Residential Dwellings Foundations

Geoff Thomas and Jamie Irvine

A previous study by Irvine and Thomas completed in 2007 showed that approximately 70% of houses in Wellington City had foundations that would not be adequate to resist the design earthquake expected in Wellington. A significant proportion was so poor than they would be expected to fail in relatively minor earthquakes. This paper will discuss the indirect implications of this problem. With many of these houses being destroyed or damaged to such an extent that they are unsafe or uninhabitable then emergency accommodation must be found for the occupants. The difficulties and costs of providing such accommodation, indirect and other costs are explored and indirect cost savings could be as high as twice as the direct costs savings resulting in very favourable cost/benefit ratios for upgrading house foundations.

Paper P56: [Read] [Presentation]

Seismic performance of brick veneer houses

Stu Thurston and Graeme Beattie

Historically brick veneer houses have not performed well in earthquakes. However, modern construction using better brick-ties which are screwed to studs and the use of bricks with internal holes allowing some mortar mechanical interlock is expected to result in a better performance.

BRANZ cyclically racked two large brick veneer rooms (including ceilings) using a system which allowed the total load carried by the brick veneer to be measured directly. One room incorporated windows and had a door opening, while the other had fully separated brick veneer elements. Brick veneer cracking patterns were identified and rationalised. It was determined that in-plane brick veneer slip was significantly resisted by perpendicular walls and the mortar in brick veneer holes acted as dowels which resisted slip along horizontal mortar cracks.

Elemental tests were used to measured brick veneer tension bond and slip strengths and the in-plane brick-tie load versus deflection relationship. These were used to determine the theoretical veneer lateral load resisting strength and a model was calibrated by comparing with the measured room strengths. A new design philosophy is proposed.

Paper P11: [Read] [Presentation]

Keynote Address Session 1 Session 2 Session 3 Session 4 Session 5 Session 6 Poster Session Session 8