Alex Sixie Cao


Last Name

Cao

First Name

Alex Sixie

ORCID

0000-0002-8455-5892

Organisational unit

08809 - Frangi, Andrea (Tit.-Prof.)

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2021 - 202631
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Publications1 - 10 of 31
  • Lateral stiffening systems for tall timber buildings – tube-in-tube systems
    Item type: Journal Article
    Binck, Charles; Cao, Alex Sixie; Frangi, Andrea (2022)
    Wood Material Science & Engineering
    In this paper, an adaptable and architecturally flexible lateral stiffening system for tall timber buildings between 50 and 147 m is developed and investigated. The system is based on a tube-in-tube concept. The internal tube consists of a braced timber core, and the external tube consists of a frame structure with semi-rigid beam-column joints in the facade. Based on a finite element framework, more than 500 000 simulations with different configurations are carried out to assess the performance of the lateral stiffening system subjected to wind loading. The resulting data is used to assess the feasibility of the tube-in-tube system and stiffness requirements for the beam-column joints.
  • Progressive Collapse of a Timber Frame Building Subjected to Single- and Multi- Column Damage Scenarios
    Item type: Conference Paper
    Cao, Alex Sixie; Frangi, Andrea (2025)
    Lecture Notes in Civil Engineering ~ Safeguarding Structural Resilience Under Extreme Events: Proceedings of PROTECT 2024
    As part of the transition to a more sustainable future, more and larger multi-storey timber buildings are being realised. Simultaneously, the frequency and magnitude of potential hazards have been increasing. Therefore, the resistance of timber buildings to disproportionate collapse is becoming increasingly relevant. However, current robustness guidelines to limit disproportionate collapse were developed for reinforced concrete and steel buildings and are inappropriate for timber buildings. Consequently, there is an urgent need to bridge the knowledge gap on the robustness of timber buildings. To investigate the robustness of buildings, it is common to assess single-column damage scenarios on the ground-floor. However, recent research has shown that relevant damage scenarios are not limited to ground-floor damage. Various hazards, such as vehicle impact and far-field blast loading, may entail damage to multiple columns. To address these knowledge gaps, this paper presents a case study of a timber frame building subjected to single-and multi-column damage scenarios. A damage assessment and sensitivity analyses were conducted, where the societal and material consequences were quantified based on the life quality index. The analyses are conducted using a recently developed framework for modelling progressive collapse of inelastic building structures using nonlinear dynamic analysis.
  • A nonlinear dynamic model for collapse investigations in tall timber buildings
    Item type: Conference Paper
    Cao, Alex Sixie; Esser, Lukas; Glarner, Ben; et al. (2023)
    World Conference on Timber Engineering WCTE 2023
    Structural robustness of timber buildings is becoming increasingly relevant because of the increasing use of timber in the built environment. An important tool for assessing the robustness of any structure is an efficient numerical model capable of simulating progressive collapse. With such a model, physical tests can be limited to a few carefully selected validation tests and the robustness of a wide range of building typologies and geometries can be investigated efficiently. In this paper, a parametric nonlinear dynamic model for simulating progressive collapse of timber buildings is presented. Because of the parametric capabilities of the model, a vast range of buildings can be modelled. Moreover, the entirety of a collapse can be simulated with the recently developed mixed element method and the implementation of stress- and energy-based failure criteria for normal loading and impact loading. To demonstrate the capabilities of the model, a case study is presented on a symmetrical three-dimensional frame building with varying cross-sectional member dimensions. The model is an indispensable tool for investigating the robustness of timber buildings.
  • A unified constitutive model for pressure sensitive shear flow transitions in moderate dense granular materials
    Item type: Journal Article
    Cheng, Xiaohui; Xiao, Shize; Cao, Alex Sixie; et al. (2021)
    Scientific Reports
    Granular shear flows exhibit complex transitional regimes that are dramatically affected by the pressure level and shear stress state. New advances in granular shear tests at low pressure have enlightened the understanding of the two granular shear flow transitions: between quasi-static and moderate shear flows, and between steady-state and transient shear flows. However, a unified constitutive model to describe these two transitions is yet to develop. In this work, a simplified and unified model is proposed based on innovative triaxial shear flow tests, using two dimensionless physical variables. Model results validated against experimental data suggest that the shear flow transition between a quasi-static to a moderate Isotach type flow state is highly pressure-dependent. At extremely low pressure, the granular viscosity becomes the primary mechanism, suppressing the quasi-static mechanism even under “quasi-static” shear rates. In transient to steady state granular flow transitions, a mobilized shear stress ratio or mobilized friction coefficient between zero and the critical state ratio for consolidated granular packings is taken into consideration. This is coupled with the mechanism of granular viscosity. These findings have not been discussed before and are of great relevance to granular mechanics as well as space and earthquake engineering.
  • On the quantification of robustness and its thresholds
    Item type: Journal Article
    Cao, Alex Sixie; Beck, André Teófilo (2026)
    Structural Safety
    Structural systems need to be safe enough against foreseeable loads, but they also need to be robust enough against unforeseeable or abnormal loading. In this paper, a novel entropy-based robustness index for arbitrary perturbations is derived for coherent path-dependent systems, which is consistent with information-theoretic and thermodynamic principles. Using a reliability-based robustness index and the entropy-based robustness index, quantitative robustness thresholds are derived that enable the explicit classification of low, medium, and high robustness based on the sensitivity of the system to arbitrary perturbations. Furthermore, relations between the entropy- and reliability- and risk-based robustness indices are explored, where thresholds for the risk-based robustness indices are provided based on the novel entropy-based robustness index. The use of the various robustness indices and the thresholds are exemplified in three case studies, involving a redundant system subjected to various degrees of damage, damage propagation in frame structures, and a network. For the first time, quantitative thresholds for the robustness of coherent path-dependent systems are provided, which can be applied to structures, networks, and more. This paves the way for providing quantitative guidance on acceptable degrees of robustness in such systems, which may lead to more economic and rational systems with an appropriate degree of robustness.
  • Column removal analyses of timber structures - Framework to assess dynamic amplification factors for simplified structural design methods
    Item type: Conference Paper
    Cao, Alex Sixie; Palma, Pedro; Frangi, Andrea (2021)
    World Conference on Timber Engineering (WCTE 2021)
    Structural robustness in timber structures is a topic that is becoming increasingly relevant as taller timber buildings are built. To increase the robustness of a building and by virtue preventing disproportionate and progressive collapse, an appropriate design concept tailored for the properties of timber and its connections must be chosen. The most common way to assess the building’s resistance to disproportionate and progressive collapse is done through column removal analyses. However, the majority of such analyses are carried out by linear elastic quasi-static means. For other materials, literature reports significant dynamic effects for sudden removals of structural members. The dynamic effects increase for lower plasticity systems. Timber is a brittle material in shear and tension with limited plasticity. It is therefore important to quantify the dynamic effects for an adequate robustness assessment. In this paper, 216 nonlinear dynamic analyses are carried out on small planar timber skeleton frames in the finite element framework OpenSees to investigate the relation between the dynamic amplification factor and different structural parameters. The most important parameters are the damping ratio and the connection stiffnesses. The resulting dynamic amplification factors are between 1.9 and 2.0.
  • Pendulum Impact Hammer Tests on Spruce Glued Laminated Timber – Preliminary Results
    Item type: Conference Paper
    Cao, Alex Sixie; Lolli, Marco; Frangi, Andrea (2022)
    Civil-Comp Conferences ~ Proceedings of the Fourteenth International Conference on Computational Structures Technology
    In this paper, preliminary results are presented on pendulum impact hammer tests on full-scale glued laminated timber specimens with dimensions 200x200x3600 mm for the first time. For the tests, a pendulum with an effective mass of 3500 kg and length 4700 mm was built. The pendulum was instrumented with a high-speed camera, angular encoder, optical position-tracking system, and accelerometers. By considering potential energies and the damping in the system, the material toughness was determined to be between 327 and 534 kJ/m2. The maximum impact forces were determined from the deceleration of the pendulum during impact and were between 3565 and 4106 kN. The failure time was determined qualitatively from high-speed imagery to be between 10.18 and 11.58 ms. The dissipated energy is roughly equivalent to the kinetic energy of a 500 kg projectile traveling at 30 km/h.
  • A pressure-sensitive rheological origin of high friction angles of granular matter observed in NASA-MGM project
    Item type: Journal Article
    Cheng, Xiaohui; Xiao, Shize; Yang, Sen; et al. (2024)
    Chinese Physics B
    An abnormally high peak friction angle of Ottawa sand was observed in (National Aeronautics and Space Administration) NASA-(Mechanics of Granular Materials) MGM tests in microgravity conditions on the space shuttle. Previous investigations have been unsuccessful in providing a constitutive insight into this behavior of granular materials under extremely low effective stress conditions. Here, a recently proposed unified constitutive model for transient rheological behavior of sand and other granular materials is adopted for the analytical assessment of high peak friction angles. For the first time, this long-eluded behavior of sand is attributed to a hidden rheological transition mechanism, that is not only rate-sensitive, but also pressure-sensitive. The NASA-MGM microgravity conditions show that shear-tests of sand can be performed under abnormally low confining stress conditions. The pressure-sensitive behavior of granular shearing that is previously ignored is studied based on the μ(I) rheology and its variations. Comparisons between the model and the NASA microgravity tests demonstrate a high degree of agreement. The research is highly valid for pressure-sensitive and rate-dependent problems that occur during earthquakes, landslides, and space exploration.
  • Dynamic strength increase of glued laminated timber beams subjected to impact loading
    Item type: Conference Paper
    Cao, Alex Sixie; Frangi, Andrea (2023)
    INTER Proceedings - Meeting 56
  • Case Study of a Tall Timber Building Subjected to Ground-Floor Column-Loss Scenarios
    Item type: Conference Paper
    Cao, Alex Sixie; Ullmann, Silvan; Frangi, Andrea (2024)
    Forensic Engineering 2024: Finding Answers to the What, Why, Who, and How of Preventing Failures
    Extensive research was conducted on the fields of robustness and progressive collapse of buildings comprising steel and reinforced concrete. However, there is a knowledge gap for timber buildings. To assess the robustness of a building, it is common to investigate column-loss scenarios. Here, a case study is presented on a 10-story tall timber building with a trussed core and connections comprising slotted-in steel plates and laterally loaded dowels. The building was designed for the ultimate and serviceability limit states for wind and gravity loads. Ground-floor column losses were assessed using a modeling framework for progressive collapse. The results show that the building is robust. However, there are still uncertainties regarding the connection behavior.
Publications1 - 10 of 31