Vol. 31 – 35

President's Report

Jason Ingham

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Note from the editor

Stewart Hobbs

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Letters to Editor

Barry Davidson

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Simplified Capacity Design Procedure

Hamish Brookie

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This paper proposes a “Simplified Capacity Design” procedure (SCD) that enables designers to utilise modern analysis and design software to perform capacity design and feedback (via the SESOC journal) would be appreciated.
 

Seismic Assessment and Strengthening of the Majestic Centre, Wellington

J. G. White , H.S. McKenzie, A.E. Philpott , D.K. Bull , B.D. Galloway , R. van Ballegooy

Abstract
The Majestic Centre is located in central Wellington, New Zealand. Constructed circa 1991, the building comprises a 25 storey tower above a five storey podium. The tower has a dual lateral load resisting system; a perimeter reinforced concrete moment frame and two central shear cores. Following the Canterbury Earthquakes, the building’s owner, Kiwi Property, commissioned seismic assessments of their property portfolio. An Initial Seismic Assessment (ISA) of the Majestic Centre was followed by Detailed Seismic Assessment (DSA, 2011) of the structure using both modal response spectrum (MRSA) and non-linear time-history (NLTHA) analysis methods. NLTHA and performance based assessment methodology concluded a seismic assessment rating in the range 35-45%NBS which, whilst above an Earthquake Prone threshold, identified a number of critical structural weaknesses (CSW’s). This was considered inconsistent with the building’s ‘Grade A’ office status and posed a risk to the building occupants and the surrounding Wellington CBD. CSW’s identified included; L5 transfer beams, shear core foundations, non-ductile shear core walls, tower diaphragms, precast cladding panel connections, podium roof load paths and seismic displacements and tower floor related issues associated with 1980s precast flooring detailing. These issues are covered in further detail as part of this paper.
Design and construction of strengthening works progressed in parallel from 2012 whilst maintaining a fully tenanted building. The project was completed in late 2016. This extremely ambitious and challenging project, undertaken by Kiwi Property and their consultant and contractor team has served to demonstrate that seismic strengthening of large commercial buildings ‘in- service’ is possible provided owners, designers, contractors, regulators, and tenants are willing to work together. This paper aims only to provide an overview of the assessment, analysis, design and construction processes from a Structural Engineering perspective.
 

Performance of Panel-to-Foundation Connections in low-rise precast concrete buildings

Lucas Hogan, Rick Henry and Jason Ingham

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Low-rise precast concrete wall buildings represent a significant portion of the New Zealand building stock, but there is limited evidence of the seismic performance of existing connections between panels and other structural elements. An experimental program investigating the seismic response of dowel type panel-to-foundation connections was undertaken. The testing program consisted of over thirty singly reinforced concrete panels incorporating both details currently used in practice as well as alternative connection details that have been proposed to improve connection robustness. Specimens were subjected to out-of-plane, in-plane, and bidirectional actions. It was found that in the out-of-plane direction, current connection details utilising shallow embedded threaded inserts resulted in brittle joint failure and as such do not meet performance criteria in NZS 3101:2006. Improved joint behaviour can be achieved either through adding additional reinforcement to the joint area to force damage to occur in the panel outside the joint region or by providing sufficiently deep embedment of the starter bars into the panel. It was also determined that the use of anchor pull out equations in NZS 3101:2006 are inappropriate for the design of threaded insert connections as the connection is not in direct tension but instead fails through the propagation of a flexural crack behind the insert. The performance of the panel was similar when subjected to either in-plane or bi-axial loading due to the flexible out-of-plane panel behaviour resulting from the single layer of reinforcement
 

Adopt a Ductility for Steel Portal Frame Structures

M. Grant and S. Lanser

Abstract
Steel portal frames are a simple and commonly used structural form. It is also common to design portal frames to support heavy precast concrete cladding panels. This paper aims to outline the Steel Structures Standard NZS 3404 requirements for the seismic design of portal frames and show that the premise of ‘adopt a ductility’ can be irrelevant in highly seismic zones.
 

An Overview of the Use of Glass Fiber Reinforced Polymer Bars as Reinforcement in Concrete Structures

V. Worner, H. Auman, A. Palermo, A. Scott

Abstract
This paper provides an overview of the use of glass fiber reinforced polymer bars as concrete reinforcement. Comparisons are made against steel reinforcement to highlight the key differences between the two materials, including differences in material composition, mechanical properties, behavior, design philosophies and applications. The results of tensile testing of GFRP bars at the University of Canterbury are also presented and discussed, which showed the typical linear behavior of the bars up to brittle failure.
 

Improving City Resilience - The Role of Structural Engineering

David Hopkins

Abstract
Concerns have been expressed at the scale of disruption, damage and demolition following the Canterbury earthquakes, particularly in the Christchurch CBD. More recently damage to Wellington buildings in the Kaikoura Earthquake, though at a much lesser scale, caused concern in terms of its impact on the city’s function and economy. With the increased emphasis on designing for resilience of cities it is timely to examine options available to structural designers and regulators to improve building resilience through changes in approach to structural
design and/or requirements for structural performance.
The paper examines three basic approaches: a) Retaining conventional design techniques but lifting the threshold of current regulatory requirements (for example requiring design to 2500-year shaking rather than to 500-year shaking); b) Adopting low-damage design techniques and c) incorporating seismic isolation. The effectiveness of each of these measures is discussed in terms of likely improvements in the Operational, Immediate Occupancy, Life Safety and Collapse Prevention states. Relative costs and benefits of each approach are indicated.
Indications are that seismic isolation holds the greatest promise in reducing both structural and non- structural damage – and thus reducing downtimes and cost. Low-damage design (of the structure) allows early reinstatement of the structure but non-structural damage remains potentially costly and disruptive. Lifting current regulatory thresholds reduces overall impact but does not remove the spectre of costly and time-consuming repairs, especially to the structure, or total replacement.
These broad comparisons highlight the need for a more comprehensive examination of the extent to which structural design approaches can improve the resilience of buildings and cities.
While the benefits of these approaches are clear in concept, achieving increased resilience in practice requires owners and their structural designers, to adopt them for the design of new buildings and the retrofit of existing buildings. This will require active promotion of the benefits of structural resilience amongst owners, tenants, users and the public. Development of awareness of the value of earthquake engineering in the property market is central to this and recent efforts in the US and New Zealand to establish earthquake rating schemes for buildings is encouraging in this regard. These should help develop and sustain this awareness with resulting improvement of building resilience over time. This will benefit owners, tenants, users and the community.
 

BRBF and CBF Gusset Plates: Out-of-Plane Stability Design using a implfied Notional Load Yield Line (NLYL) Method

B. Zaboli, G.C. Clifton, K. Cowie

Abstract
Gusset plates are a key component of braced frame systems, connecting the braces to the framing system. With traditional concentrically braced frames (CBFs), the braces are designed for controlled inelastic action, which involves brace buckling when the brace is in compression. Buckling restrained braces (BRBs) were developed in Japan in the late 1980s in order to avoid the undesirable effects of brace buckling. BRBs allow a brace to yield in compression without global buckling, thus making the brace of similar stiffness and strength when it is in tension and compression. Testing on individual braces has demonstrated that BRBs can perform very well, however such braces can also fail prematurely if their connections are not appropriately designed and buckle before the brace core yields in compression. Despite the importance of gusset plates, their behaviour has not been well researched, with engineers still using a design method originally proposed by Thornton (1984). This method uses a column analogy to describe plate behaviour and a number of recent studies have shown that while this method is too conservative in CBF connections, it could not be reliable in buckling restrained brace frame (BRBF) connections. This paper proposes a simplified notional load yield line (NLYL) model for both CBF and BRBF systems, which can adequately take into account the actual collapse mechanisms of brace-to-frame connections, ensuring gusset plate stability is maintained as required in each system. A comparison of existing experimental test results and those of the proposed method is made, which shows that the NLYL model is suitably conservative for application in both CBFs and BRBFs.
 

Book Review - Tall Wood Buildings - Design, Construction and Performance by Michael Green, Jim Taggart

Andy Buchanan

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SCOSS Report



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MBIE and Standards New Zealand Update



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News from the Regional Structural Groups



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The Institute of Structural Engineers Report



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