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3.b Structural Engineering

Non-linear equivalent frame modelling - Assessment of a two storey perforated unreinforced masonry wall

C.L. Knox & J.M. Ingham

The highly non-linear behaviour of unreinforced masonry walls makes linear static analysis methods inadequate and inaccurate and therefore for both academics and practicing engineers non-linear analysis of masonry buildings is required. Although accurate predictions of the structural response and cracking pattern can be generated by complex finite element (FE) meso models, the computational skill and high time cost often discounts this approach for everyday use. Alternatively, Equivalent Frame models are able to represent the essential characteristics of perforated wall response with minimal computational expense and can evaluate the key design parameters of ultimate strength, maximum displacement and failure mode.

The main features of the Equivalent Frame model used to represent the non-linear behaviour of unreinforced masonry perforated walls in SAP2000 are detailed. Closed-form solving of sectional equilibrium equations to evaluate the flexural strength of pier and spandrel components using a stress-strain relationship specific to New Zealand URM material behaviour which incorporates strain softening, is presented and used to define the coupled axial-moment hinge. Spandrel failure modes are developed and equations to capture the shear strength for each mode are presented. Finally the comparison between the modelled force-displacement response, and experimentally obtained force-displacement response for full scale sub-structures and a two storey perforated wall previously experimentally tested is discussed.

[Paper 076]

NEES Integrated Seismic Risk Assessment Framework(NISRAF)

S.L. Lin, J. Li, A.S. Elnashai & B.F. Spencer

The paper presents an integrated and extensible framework for assessment of the impact of earthquakes on the civil infrastructure systems, particularly buildings and bridges. The framework, referred to as NEES Integrated Seismic Risk Assessment Framework (NISRAF), is developed with a focus on improving the reliability of earthquake assessment results. The components are structural fragility assessment using measured data and hybrid simulation, hazard characterization by free-field site response analysis, and integrated impact assessment. The hazard and fragility components are refined by employing nonlinear site response tools and model updating techniques, respectively. Components are tailored to achieve seamless integration and to arrive at an operational system. The novelty of the developed framework is primarily the integration of the various components of earthquake impact assessment, which have not been deployed in such an application before. The framework has been built and demonstrated via applications to a test bed in California. Earthquake impact assessment results using the generated hazard map and fragility curves correlate well with field reports, indicating the efficacy of the approach.

[Paper 083]

Structural efficiency by analysis: The Fonterra Dryer Plant, Darfield

N.J. Brooke, T.J. Stuart, B.J. Davidson, S. Blain & P. Clark

The Fonterra Darfield Dryer Plant is a large reinforced concrete industrial facility located near the town of Darfield in central Canterbury. The structure consists of precast concrete walls and columns supporting heavily penetrated, irregularly located concrete floor plates.

Dairy dryer buildings are typically founded on shallow foundation beams. Initial elastic analyses indicated that this was not feasible for the Darfield Dryer building due to the large size of the structure, and in particular due to its relatively high aspect ratio. It was thus initially concluded that piled foundations would have to be provided, adding significantly to the cost of the structure.

As an alternative it was suggested that the dryer building should be allowed to rock, thus removing the need for piled foundations. Adoption of the rocking solution required non-linear pushover and time history analyses to be undertaken, which showed that rocking was a viable mechanism for the structure. Despite the relative complexity of these analyses significant cost benefits were obtained, in addition to a superior structural solution being provided.

Design of the Darfield dryer plant occurred in the period after the September 2010 and February 2011 earthquakes but before the revision to zone factors for the Canterbury region. Comment is made regarding the effects of loading changes on the design of the structure. This paper also includes comments comparing the results obtained from the time history analyses with results obtained for the same structure using a recently proposed simplified design approach for rocking structures (Kelly 2009, 2011).

[Paper 111]

Understanding cladding damage: A numerical investigation into a Christchurch earthquake case study

A. Baird, A. Palermo & S. Pampanin

In order to better understand the damage sustained by cladding systems in the earthquake that struck Christchurch on the 22nd of February 2011, a numerical investigation of the effects of cladding-structure interaction has been undertaken.

The numerical investigation focuses on a typical reinforced concrete multi-storey building located within the Christchurch CBD and analyses its seismic performance with and without the interaction due to cladding panels. The cladding panels are precast concrete panels that are attached to the frame using two fixed connections at the base and two flexible tie-back connections at the top. Static push-over analyses are used to determine the change in strength and stiffness of the system. Results show that when cladding interaction is taken into account, the frame has a higher stiffness, strength and earlier onset of collapse. Dynamic analyses are performed which involve subjecting each system to fifteen earthquake records, each scaled to both design basis and maximum credible earthquake intensities. The maximum inter-storey drift and subsequent cladding connection damage is inferred. Results confirm the high influence of cladding systems upon the seismic behaviour of multi-storey buildings. Also revealed is the significant variation in possible levels of cladding damage throughout a building.

[Paper 121]

Improving the Seismic Resilience of Lifeline Tunnels

V.S. Romero & R.J. Caulfield

Earthquakes subject tunnel linings to transient strains due to ground shaking, and can cause localized permanent strains in zones of fault rupture. These issues are discussed with respect to seismic analyses and design strategies. Most underground structures are flexible in relation to the ground, and these structures will be subject to the same strains as the ground in which they are embedded, yet such strains often do not drive structural design. However, without design detailing to create strain compatibility, large changes in structural flexibility can create undesirable changes in strain due to seismic shaking. Similarly, while it is impossible to design a tunnel lining to resist discrete fault movement, measures can be taken to minimize damage and facilitate repair following a major earthquake. Two case histories illustrate these concepts: (1) the Bay Tunnel in Santa Clara County, California, a new water supply tunnel in soft ground near active faults and subject to severe seismic shaking; and (2) the Claremont Tunnel Seismic Upgrade in Berkeley, California, retrofit of a water supply tunnel crossing an active fault.

[Paper 064]

Experimental Modal Identification of Structures under Earthquake Excitation

Sherif  Beskhyroun, Quincy T. Ma, Liam  Wotherspoon & Barry J. Davidson

Micro-electro-mechanical system (MEMS) based accelerometers are now frequently used in many different parts of our day-to-day lives. It is also increasingly being used for structural testing applications. Researchers have had reservation of using these devices as they are relatively untested, but now with the wider adoption, it provides a much cheaper and more versatile tool for structural engineering researchers. A number of damaged buildings in the Christchurch Central Business District (CBD) were instrumented with a number of low-cost MEMS accelerometers after the major Christchurch earthquakes. The accelerometers captured extremely high quality building response data as the buildings experienced thousands of aftershocks. This data set was amongst one of only a handful of data sets available around the world which provides building response data subjected to real ground motion. Furthermore, due to technological advances, a much larger than usual number of accelerometers has been deployed making the data set one of the most comprehensive available. This data set is utilised to extract modal parameters of the buildings. This paper summarises the operating requirements and preference for using such accelerometers for experimental modal analysis. The challenges for adapting MEMS based devices for successful modal parameters identification are also discussed.

[Paper 066]