Infrastructure resilience under multiple hazards necessitates the development of new materials, devices as well as digital fabrication concepts that could maximize a system’s robustness over its life-cycle. Scale, rate as well as long-term effects are among the challenges to be addressed before the aforementioned concepts can be further integrated into the civil infrastructure design. The development of computational models of various complexities informed by sensing capabilities is imperative to further our understanding on the physical mechanisms that cause deterioration of materials components and systems under complex loading conditions. RESSLab at EPFL effectively utilizes the GIS’s unique experimental facilities to conduct a broad range of physical experiments across-scales to address the aforementioned challenges. In particular, universal and Gleeble testing machines are employed to properly characterize and model the behaviour of conventional and high-performance steel materials under rate-dependent thermomechanical multi-axis cyclic loading through fracture. RESSLab researchers effectively utilize a newly developed multi-purpose test setup (Cage) to develop smart fuses that minimize the impact of random vibrations on built infrastructure and promote the material reuse after deconstruction for energy efficiency. The ‘Cage’ leverages a state-of-the-art controller (Inova EU 3000), high-precision instrumentation (wireless tracking, digital image correlation) with up to eight high-capacity servo-hydraulic actuators simultaneously. It allows for hybrid simulations in which a system is split into numerical and experimental substructures that are tested simultaneously based on coupled finite element simulation carried out in a computer. This offers the opportunity to comprehend the behaviour of an entire system under complex loading conditions without having to physically test the entire structure. Data-driven simulation tools and risk-based metrics are then benchmarked and utilized in a holistic framework to measure resilience of structures after natural and man-made disasters.
Deterioration mechanisms in composite steel moment resisting frames under cyclic loading
Collapse assessment of structures necessitates the development of numerical models to further comprehend the physical mechanisms causing deterioration under cyclic loading. In the case of composite steel moment-resisting frames the frame continuity could result into much larger plastic deformation capacities than typically expected from conventional beam-to-column tests. This project will provide landmark experimental data from a full-frame test (shown above) conducted in the multi-purpose test setup (Cage). The experimental data will facilitate the development of high-fidelity models to further our understanding on what causes strength and stiffness deterioration in composite-steel structures and how the framing action increases the plastic deformation capacity of steel structures.
Innovative column base connections for mitigating steel column nonlinear geometric instabilities under cyclic loading
Steel wide-flange columns experience nonlinear geometric instabilities that could significantly compromise a steel building’s seismic performance leading potentially to structural collapse. Boundary effects could significantly alter the column performance to a more beneficial one depending on the interaction of the steel column with the concrete footing. The scope of this project is to develop a fundamental understanding on the seismic behaviour of steel columns interacting with concrete footings by means of experimental testing and corroborating continuum finite element studies. The final goal of the project is to develop innovative column base connections that minimize structural damage in first story steel columns of moment-resisting frames in the aftermath of earthquakes.
Hydrocarbon storage reservoirs
When unanchored steel liquid storage tanks are subjected to strong ground motions, the contained liquid may cause uplift of the tank base resulting in compressive buckling of the tank shell and or ultra low-cycle fatigue (ULCF) damage to the shell-to-base welded connections. The phenomenon of ultra-low cycle fatigue (ULCF) and damage of welded joints under multi-axial and variable amplitude strains is complex and not completely understood. This phenomenon has particular significance for safety and environmentally relevant structures (above-ground petroleum tanks, masts for wind turbines, pressure vessels and tubular bridge piers) subjected to extreme loading such as seismic or wind gust loads. In the case of above ground tanks, severe seismic loading conditions can cause the tank bottom to uplift. This behavior causes extensive plastic deformation in the region near the welded connection between the tank wall and the bottom plate, which occur with highly variable and stochastically distributed amplitudes. Very little information exists concerning the ULCF response of welded connections subjected to multiaxial loading. Confirmed knowledge and new engineering design procedures would allow for an increase in safety under load scenarios causing extraordinary local cyclic strains in short times.
Results from our studies led to a better understanding of the Ultra Low Cycle Fatigue of welded steel joints under multiaxial loading. It was shown that significantly higher connection rotation capacity is available than what is specified in the current Eurocode standard (set at 0.2 rad); actually, another study on four existing tank geometries (on broad and slender tanks) subjected to moderate seismic excitation highlighted that the reservoirs are vulnerable to shell buckling and not damage to the shell-to-base welded connections.
The main outcome are : 1) improved assessment of the safety of hydrocarbon storage tanks against cyclic loadings, spillage risks under control, savings from costly reinforcements, 2) Under ULCF, i.e. where material ductile properties are used to their maximum, it was shown based on a large testing database generated that a design procedure using local strains in finite element modeling, with a Manson-Coffin type of approach, is valid.
Ultra Low Cycle Fatigue of welded joints under variable multi-axial strains. SNF-DACH grant and CARBURA. Collaboration with KIT, Prof. Th. Ummenhofer, and Graz Univ. Prof. A. Taras. PhD student Albano Castro e Sousa.
Pure shear monotonic test on double notched specimen in steel S770QL
Multiaxial high-cycle fatigue
Full-scale transversal welded attachments as well as tube-to-plate welded joints under multiaxial loadings are tested and modelled. Both mild strength S235JR steel and high strength S690QL steel are investigated. The experimental program includes series of uniaxial and multiaxial fatigue tests and was partially carried out on a new multiaxial setup that allows proportional and non-proportional tests in a typical welded detail. The fatigue lives are then compared with estimations obtained from local approaches with the help of 3D finite element models. The multiaxial fatigue life assessment with some of the well-known local approaches is shown to be suited to the analysis under multiaxial stress states. The accuracy of each models and approaches is compared to the experimental values considering all the previously cited parameters. Furthermore, a novel strain-based approach recently proposed by Remes for the fatigue strength modelling of welded steel joints is extended here to consider multiaxial loading conditions and three dimensions finite element models.
Multiaxial fatigue of welded steel joints (effects of residual stresses; application of high-strength steel). SNF grant project and collaboration with AALTO Univ. (Fin), Prof. H. Remes. PhD student Martin Garcia.