		2008 NZSEE Conference
	Abstracts

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

The Damage Avoidance Design of tall steel frame buildings - Fairlie Terrace Student Accommodation Project, Victoria University of Wellington.

Sean Gledhill, Geoff Sidwell and Darrin Bell

Recent advances in Seismic Engineering have focused on a “Damage Avoidance Design” philosophy, whereby a structure is designed to withstand a major seismic event with minimal and repairable damage. This typically involves incorporating mechanisms in the structure that can control loads and sustain large deformations without causing damage.

At the request of Victoria University of Wellington, Connell Wagner was tasked to undertake the design of the new high-rise student accommodation Buildings at 74-87 Fairlie Terrace, to incorporate a “Damage Avoidance” philosophy.

The challenge to create damage avoidance design features was complicated by the building form. Typically short, stiff heavy buildings with low periods are suited to base isolation, but few options are available for tall, relatively light, flexible steel buildings. Damage avoidance features available on the market are often viewed as expensive and complicated and have not been widely utilised.

To meet the challenge we developed a cost effective new system for damage avoidance applicable to high-rise steel framed buildings. The system utilised research conducted by HERA and the University of Auckland.

The Damage Avoidance system featured coupled concentrically braced frames with prestressed Ringfeeder Springs and sliding hinge joints between columns and foundation. The system also incorporates steel beams with Sliding Hinge Joints which we refined in conjunction with HERA.

This paper outlines the possible damage avoidance design solutions considered for this project and reviews and outlines the concepts and application of the chosen system. The paper does not address the methods of analysis or detailed design techniques used to refine the system.

Paper P63: [Read] [Presentation]

Plastic shear strength of continuous reinforced beams

Col Gurley

This paper addresses the theoretical, rigid-plastic yield-line collapse-mechanism analysis of continuous, reinforced concrete beams considered as two-dimensional, plane-stress problems including mixed shear/bending mechanisms. It provides a range of collapse mechanisms that may be sufficient for ‘exact’ analysis using simple calculations of yield segments as free-body static equilibrium problems across the range of ductile-frame beams from long-span, gravity-dominated examples to deep coupling-beams. The analysis assumes that beams are continuous with stronger columns or walls of equal width/thickness thereby avoiding issues related to bearing details. The methods proposed do need to be calibrated to experimental data and, perhaps, they will also assist experimental researchers.

A related issue is that plastic design of steel structures has long been restricted to elements appropriately detailed to ensure ductile failure, for example, by limiting members to sections that are ‘compact’ enough to prevent premature local buckling of webs or flanges under plastic rotations. Perhaps the plastic design of concrete structures should be more explicitly restricted to elements appropriately detailed for ductility despite the tensile weakness of concrete. This suggests stronger rules for minimum reinforcement content in both directions in the web/mid-depth of concrete beams. This issue is discussed in Appendix A, and Appendix B summarises the relevant suggestions of M.P.Nielsen 1999. Appendix C describes an extension to elasto-plastic analysis where there do seem to be some problems.

Paper P19: [Read] [Presentation]

Finite Element Analysis of Old Steel Buildings in NZ

Majid Naderi

In the early 20th century, steel frame buildings were built to different standards from those used in modern construction. Riveted, built-up members were used instead of rolled sections, with joints and members encased in concrete for ﬁre protection. These early steel buildings were designed based upon observations of past building performance rather than through detailed calculations and predictions of structural behaviour. The walls were infill masonry and floors were typically reinforced concrete. The strength and stiffness of the semi-rigid connections and masonry infill as well as the effect of floor slabs integral with their supporting beams are not well documented.

Although riveted stiffened seat angle connections are not designed to resist moments, they can develop a considerable moment capacity and exhibit a relatively ductile hysteretic behaviour which could be beneficially considered when evaluating frames built of these connections and subjected to small and moderate earthquakes. Structural engineers have found it challenging to make realistic predictions of the seismic performance of these buildings, many which are quite prestigious, in full service and often enjoying heritage protection. Examples include Auckland’s Britomart Station and Guardian Trust Building, and Wellington’s Tower Corporation, Prudential Assurance and Hope-Gibbons Buildings.

To predict the complicated behaviour of riveted connections, 3D nonlinear finite element models of a sample clip-angle connection taken from drawings of the now-demolished Jean-Batten building in Auckland and a sample T-stub connection taken from drawings of the Hope-Gibbons building have been generated using the ABAQUS finite element software package. The joint models were used to investigate load-displacement, failure mode and energy dissipation under axial loads, shear loads, and combined axial loads plus shear loads.

Paper P06: [Read] [Presentation]

Assessment of material strain limits for defining plastic regions in concrete structures

Adam Walker and Rajesh Dhakal

The New Zealand Structural Loadings Standard, until its latest revision, used the structural ductility factor as a measure of the deformation demand of all potential plastic hinges in a structure. In the new version of New Zealand Standard for Earthquake Actions (NZS 1170.5:2004) the detailing of potential plastic regions is determined according to the local deformation demand in these regions. The change has been prompted by evidence that the structural ductility factor gives a poor indication of the demand on individual plastic regions. This new approach has also been adopted by the revised New Zealand Concrete Structures Standard (NZS 3101:2006) which classifies potential plastic regions into three categories (namely ductile, limited ductile and nominally ductile) based upon their inelastic deformation demand specified in terms of material strain limits. The material strain limits currently set in NZS 3101:2006 for the three categories of plastic regions are based on limited experimental evidence and need a closer revision. This paper tries to obtain more justifiable values of material strain limits based on experimental data. In this research, reversed cyclic loading tests of beams are conducted to compensate for a lack of data in the nominally ductile range of detailing. Based on the results of the tests conducted, curvature limits for nominally ductile plastic hinges are derived. Combining the experimental results collected from literature and the tests conducted in this project, updated material strain limits for the three categories of plastic regions are proposed. To unify the design process for all types of plastic regions, curvature limits for nominally ductile plastic hinges are also proposed as the multiple of first yield curvature (similar to the existing approach for the other two categories of plastic regions) rather than the existing approach of specifying allowable compressive (concrete) and tensile (rebar) strain limits for nominally ductile plastic regions. To further simplify the process, the representative value of first yield curvature is approximated as two times the yielding strain to the beam height ratio, thereby relieving the designers from having to conduct section analysis to estimate neutral axis depth.

Paper P09: [Read] [Presentation]

Seismic Performance Assessment of Inadequately Detailed Reinforced Concrete Columns

Al Boys, Des Bull and Stefano Pampanin

Existing New Zealand building stock contains a significant number of structures designed prior to 1995 with inadequate detailing of the internal or ‘gravity’ reinforced concrete (RC) columns. Typically these columns have insufficient transverse reinforcement; lap-splices in the plastic hinge region; and longitudinal bars that are ‘cranked’ at the end of the lap-splice. Columns with such details have been shown to perform poorly when subjected to seismic demand, losing axial load carrying capacity at drift levels less than the building is expected to be subjected to during a design level earthquake.

This paper outlines an investigative program to determine the susceptibility of these gravity columns to axial collapse. A drift based backbone capacity model for shear and subsequent axial failure is presented which has been verified by experimental testing performed to date. Such experimental tests have highlighted the susceptibility of these inadequately detailed columns to lose axial load capacity at drift levels significantly below the seismic demand on such structures due to a design level earthquake.

Paper P29: [Read] [Presentation]

Forced vibration testing of a thirteen storey concrete building

Faisal Shabbir and Piotr Omenzetter

Testing of structures to understand their behaviour under seismic conditions can provide an important source of information for safe and economical design. The need for testing the behaviour of full scale structures under dynamic loads stems from the fact that laboratory scale structures cannot account for all complexities involved. This paper describes forced vibration testing of a 13 storey reinforced concrete building having central lift core, shear walls and flat slabs, to find out its dynamic characteristics. Experiments have been carried out using the shakers and sensors within the NZNEES@Auckland Mobile Field Laboratory. Different positions of the shakers and sensors has been tried to determine optimal response of the structure. The field observations have been compared with finite element computer model. An effort has been made to synchronize a computer model with the field observations. It is emphasized that response of complex structural systems may be understood better by using the presented experimental and analytical tools.

Paper P08: [Read] [Presentation]

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