Bulletin of the New Zealand Society for Earthquake Engineering

Seismic activity analysis of five major earthquake source segments in the Sumatra megathrust zone

2023-06-01T11:04:40+12:00

The Sumatra megathrust zone has five major earthquake sources, namely the Aceh-Andaman, Nias-Simeulue, Mentawai-Siberut, Mentawai-Pagai, and Enggano segments. This paper provides seismic activity analysis in these five segments via an unobserved process study of tectonic plate movements, which is conducted in two cases: each of the five segments independently (Case 1), and a pair of two adjacent segments (Case 2). To do this, two specific types of Hidden Markov Models (HMMs), i.e., Poisson-HMMs and Copula-HMMs, dealing with unobserved process issues, are applied. In practice, the data used is the annual frequency of mainshock earthquakes with a magnitude of >4.6 that occurred from 1971 to 2018. This data is obtained by working out the declustering process and estimating the magnitude of completeness from a particular earthquake catalogue. Due to the incompleteness of the data sets, the parameters of the two HMMs are estimated using the Expectation-Maximization algorithm. Results show that for Case 1, the model that fits the data for each of the five segments is the 3-state Poisson-HMM. The three states, in this instance, stand for the rates of seismic activity that correspond to the dynamic level of tectonic plate movements. Furthermore, in Case 2, the selected model for the Aceh-Andaman with Nias-Simeulue is the 2-state Gumbel Copula-HMM. Meanwhile, for the three groups remaining, namely Nias-Simeulue with Mentawai-Siberut, Mentawai-Siberut with Mentawai-Pagai, and Mentawai-Pagai with Enggano, the appropriate models are Gaussian, Gumbel, and Frank Copulas, respectively. In this case, the number of states represents the seismic activity association of two adjacent segments that corresponds to the association level of two adjacent tectonic plate dynamics.

Performance of rocking frames with friction tension-only devices

2023-06-01T11:03:49+12:00

The implementation of a new friction tension-only “GripNGrab” device attached to a rocking steel frame is described. The device, when subject to significant tension dissipates energy via sliding in the frictional component. When the device is loaded in the compression direction, almost no compressive force is carried, but displacement occurs in the ratchetting component. This absence of any significant compressive force within the dissipative system means that the rocking frame will always recentre after uplift from earthquake shaking. A 9 m tall 4.75m wide 3-storey steel concentrically braced rocking frame is designed for low-damage seismic performance. Restoring forces are provided by (i) gravity, (ii) friction “GripNGrab” (GNG) tension-only dissipation devices at the base, and (iii) beam-slab effects. The initial fundamental period of the structure was 0.16s. The initial structure used a 10mm GNG ratchet pitch, and had a GNG strength to not slide under serviceability level shaking. Elastic, pushover, cyclic pushover, as well as time history analyses, with different shaking intensities are conducted using OpenSEES software. The scope of work is limited to a single building and a single ground motion. Parameters varied included the presence of beam-slab effects, and the GNG device stiffness, strength and tooth pitch.

It is shown that the full behaviour of the frame could be understood considering cyclic pushover analysis. The peak uplift displacement was conservatively estimated from the peak roof displacement using rigid body mechanics and the tension-only device provided no resistance to full frame recentring. For the frames considered, cumulative uplift displacements, necessary to determine the inelastic displacement capacity of the tension only device, were up to 28 times the peak uplift displacement, not necessarily occurring at the maximum shaking intensity. Maximum frame base shear force demands were up to 1.43 times that from pushover analysis. When the beam-slab, connecting the rocking frame to the rest of the structure, increased the lateral force resistance, the base shear increased significantly, reduced peak roof displacements, and increased the effective number of peak uplift displacement cycles (N_PUDc). For large shaking intensities, yielding of the beam-slab occurred resulting in permanent peak roof and uplift displacements. The GNG device strength, stiffness and tooth pitch variations for the cases studied did not significantly affect the response. Initial stiffness, and secant stiffness, based methods to predict the response of rocking frames were non-conservative for these short-period structures with small energy dissipation, and a simple improvement to match the behaviour was developed for the case studied based on the R-T-m relationship for a range of shaking intensity.

Review of recently constructed concrete wall-steel frame hybrid buildings

2023-06-01T11:03:32+12:00

Around New Zealand there has been an increasing trend of ‘hybrid’ multi-storey buildings that combine reinforced concrete walls with structural steel framing systems. This study aims to characterise and understand this type of building, focusing on buildings constructed in Auckland and Christchurch from 2014 onwards. Drawings from a total of 50 buildings were reviewed, and their structural features were documented, including building use, building height, lateral load resisting system, ductility, wall configuration, wall construction method, steel framing system and suspended floor system. Meetings with structural engineers were conducted to validate the review findings and to further understand design principles and decisions that lead to these outcomes. A typology comprising five building types with distinct lateral load-resisting systems was proposed based on the building review. Results showed regional differences between Auckland and Christchurch, owing to building use and seismic hazard in the respective cities. Auckland buildings surveyed tended to be residential buildings five storeys or higher made of precast walls connected with steel beams. Christchurch buildings, on the other hand, were primarily commercial buildings three to seven storeys high with dual frame-wall systems. Structural connections between steel frames and concrete walls were also documented, showing that bolted connections with headed stud embedment were most common. The results can be used to identify critical aspects of these mixed structural systems for further investigation and to develop archetype building designs that can be used for modelling and testing.

Assessment of the torsional behaviour of hollow core slabs

2023-06-01T11:04:05+12:00

There are many applications in buildings in which precast pre-stressed hollow-core (HC) slabs are subjected to shear, torsion, or combined shear and torsion. Nonetheless, extruded HC units contain no transverse reinforcement, being inherently vulnerable to brittle failure modes due to shear and torsional actions. In previous work by the authors, a finite element (FE) modelling approach for HC units failing in shear was developed and validated against experimental test data. This paper aims to extend the applicability of the proposed FE approach to help improve the understanding of the torsional behavior of HC units. For this purpose, the developed model is further validated against experimental data available in the literature and then used to predict the torsional capacity of New Zealand-specific 200 mm deep HC units. Results suggest that the FE model is capable of predicting the capacity of HC slabs with and without eccentricity of the applied vertical load. Finally, the numerical results are used to evaluate the performance of available simplified analysis approaches for assessing the torsional capacity of HC units, which are found to be non-conservative if used with expected material properties.

Post-earthquake building assessments

2023-06-01T11:04:22+12:00

Major seismic events occurring around urban centres often cause widespread damage to the building stock. Engineers are then required to perform safety inspections of these buildings. This process may be time-consuming and can cause residents or businesses to be displaced for a considerable duration even if the building is safe to occupy. Furthermore, other post-earthquake recovery phases, such as repair and demolition/reconstruction works, may not even initiate until the building inspection phase is complete. As such, the disruptions caused by post-earthquake inspection need to be considered when modelling building occupancy/functionality downtime.

This study uses the data obtained from the 2011 Christchurch earthquake to develop a post-earthquake inspection duration quantification model. Firstly, the duration of the rapid assessment phase is estimated from the number of damaged buildings to be assessed, the total number of available engineers, and the median time needed for assessing each building. Secondly, the probability of a building being assigned a certain colour tag (White, Yellow or Red) is derived based on the extent of damage. Finally, both sets of information are combined to quantify the post-earthquake inspection duration. A case study is examined to demonstrate the application of the proposed model.