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An energy-based approach is presented to perform the analysis of 2D and 3D no-tension masonry-like structures exploiting the API of the FEM software package Straus7. Masonry is replaced by a suitable equivalent orthotropic material with spatially varying elastic properties and negligible stiffness in case of cracking strain. A non-incremental algorithm is implemented to define the distribution and the orientation of the equivalent material, minimizing the potential energy so as to achieve a compression-only state of stress for any given compatible load. The proposed method captures collapse mechanisms predicted by limit analysis, without any a-priori hypothesis regarding the collapse mode. Applications are shown addressing vaulted masonry structures subjected to settlements and masonry walls with openings acted upon by dead loads and both in-plane and out-of-plane seismic loads. The Research Grant “Fondazione Cariplo 2017-0317” is gratefully acknowledged. |
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Maffeis Engineering proposes an innovative approach for the design of Megastructures: One Single Model. This new and challenging approach allows designer, architects and engineers to work together, sharing information, data and a 3D model between different departments with a reduction in cost, time, effort and loss of data and information. |
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One of the most difficult and delicate structural issues concerns construction cases subject to accidental actions such as fire. In these cases, the high temperatures that develop deeply modify the material behaviors and change the mechanical characteristics and the response of the structure. Variations in stiffness and resistance can carry out to complex instability phenomena. The cases of instability presented in this work refer to steel structures subject to high temperatures whose instability phenomena that may occur in the inelastic range considering the effects of initial imperfections and large displacements. This paper presents an introduction on the theoretical aspects underlying the phenomenon and further case studies with the aid of the Straus7 FEM software. |
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Generally speaking, cross passages associated with transportation tunnels, especially metro tunnels, are constructed to connect the two running-tunnels at prescribed intervals to meet safety requirements during service stage of the tunnels; and like the running tunnels themselves, the cross-passages are often located in difficult ground and under the groundwater table as well as in urban congested environment. Therefore, the design and construction of cross passage is usually a very challenging task. This paper presents a particular design and construction method devised by the authors to optimize the opening of cross passages, It is a flexible and yet practical solution, involving either ground improvement by means of jet-grouted columns executed from the ground surface, or by ground freezing executed from the running tunnels, or a combination of both techniques, depending on the access conditions, if a cross passage is to be built in a sand-dominant strata, or the dewatering technique executed from the already-excavated running tunnels if a cross passage is to be built in clayey soils . The main aims of ground improvement are to firstly create an impervious layer around the cross-passage to be built and to strengthen the mechanical properties of the surrounding ground. Such a solution is regarded as an optimization that can be readily applied in different types of soil conditions, even difficult ground conditions like the so-called mixed-face condition. Furthermore, the proposed method foresees the installation of temporary steel frames, called the “half-moon solution”, to ensure the stability of intersection between the cross passage and the running tunnel and to provide a safe work condition for the cutting of the segmental lining and opening of the cross passage. The application of the presented design-construction method is illustrated with a recent case history. For the verification of the stability and effectiveness of both the installed supports and ground surrounding a cross passage to be built by applying the proposed design-construction methodology, the commercial software Straus7 has been used. Specifically, a 3D model is developed by using Finite Elements 2D (plates) that simulate concrete segmental lining Finite Elements 3D (bricks) that simulate both upper and lower steel beams; elastic springs (compression only) for the interaction between soil and segmental lining, 1D beam elements for the steel columns and 3D brick elements for the concrete collar. Each element (be it made of concrete or steel) is defined in the calculation model with the physical and mechanical characteristics of materials used. As regards the interaction between the segmental rings, the connection has been simulated by means of dowel elements, which follows an elastic-plastic regime to model the connection behaviour. Further, the behaviour of these elements is also governed by both actual curves of shear force / displacement and pull-out force / displacements which are derived from the results of the laboratory tests provided by the supplier of the connecting dowels.The friction between the lining rings is simulated using point-contact elements.The interaction between segmental linings and concrete collar is simulated with rigid connection elements capable of transmitting only normal stress (without any shear component) and avoiding any interconnection.The 3D structural analysis, based on bedded spring beam model, is used to assess the membrane forces acting on both the temporary steel frame and the permanent RC collar. And, a 3D model is developed using plate-spring elements: this method involves the simplification of the liner soil-structure interaction. The stiffness of the ground reaction is based on a modulus of subgrade reaction (K), which can be calculated by different methods: for this analysis, the method of Galerkin (Bowles, 1982) is selected and used. The interaction between the segment and the upper and lower steel beams is modelled in Straus7 through "connection elements”, evenly distributed, able to transmit only axial and shear forces to the temporary steel frame. The entire shear force is then used for dimensioning such connections (bolts with epoxy resins or with similar characteristics). Here below is a list of the loads that can be considered in the modelling, where and when applicable:• ground loads (include all types of loads from ground and rock wedges);• water loads (considering max and min water table location);• train live loading;• equipment and superimposed loading for tunnel fit-out;• earthquake loading;• loads from interfacing structures like CP opening;• loads arising from redistribution of forces at openings;• lining self-weight;• fire load;• invert base slab;• future development loads;• creep loads;• accidental loads (machine hitting temporary structure);• blast load.During construction, before demolition of the segmental tunnel lining, a temporary steel frame should be set up at the junction of the CP with the running tunnel constructed usually by TBM. This temporary supporting steel frame provides the required stability of the junction zone on the re-distribution of the ground stress at the junction and safety on the work. This temporary steel structure should be of proper rigidity and stiffness in order to minimize ground deformation. The excavation of the cross passage shall start once the steel frame is set up. After this phase, the concrete collar shall be realized and, as soon as the concrete has hardened, the temporary steel structure will be removed. This construction sequence is followed step by step in the numerical modelling. A non-linear phase analysis is carried out to define the maximum stresses acting in the structural elements. Subsequently, the structural verification of the segmental lining, temporary steel structure (beams and columns), concrete collar and all the connecting elements (bolts and welding) are carried out. As demonstrated by the successful case history described in the paper, the proposed design-construction method is very effective and not difficult to implement. Furthermore, the experience also shows that in order to ensure a successful application of the proposed methods, it is essential to apply numerical modelling as well in-situ real time monitoring to check the design solution both prior to and during its implementation. |
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This paper reports about the seismic risk computation of a representative liquefied natural gas (LNG) tank isolated at its base level with two types of seismic isolation systems: Lead-Core Elastomeric Bearings and sliding bearings.The fragility analysis of the LNG tank is then performed by consolidated analytical and numerical tools. The seismic risk is finally computed by considering an hazard function related to a medium seismicity site with two different extended PDFs (truncated and not truncated).The problem is solved numerically by means of a detailed finite element model, taking into account fluid-structure interaction effects.The Finite Element Modeling (FEM) strategy which was used to simulate dynamic response of the liquid tank system was described and the FEM was validated using a set of manual calculation which is used in available design guidelines. The isolation systems are modeled as non-linear spring-dashpot elements with properties calculated on the basis of data obtained from the literature.The seismic excitation considered is an artificial accelerogram compatible with the Italian code provisions (NTC2018).Results concerning base shear force, sloshing vertical displacement and deflection of the container are presented. In order to measure the effectiveness of the isolation systems, percentage reductions of the peak response of all mentioned quantities are calculated using the non-isolated tank as reference.Finally, the reliability-oriented cost-benefit optimization for structural base isolation components in the so-called one-level approach was investigated. Acceptibly safe structures are determined by the LQI-approach (Life quality Index ). Optimization is performed from the public’s and the owner’s point of view. |
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Self-supporting automated warehouses, used in industrial facilities to optimize storage spaces, is a goal of structural engineering, which combines the structural efficiency of metal construction with handling systems. The main construction feature is to produce thin, lightweight metal profiles, which allows a limitation of weight, cost and assembly time, but it makes them vulnerable to the action of fire due to loss of stability. Therefore, these metal profiles are also difficult to protect with the traditional passive fire protection systems because of their unfavourable critical section factors and critical collapse temperatures. National regulations and international standards for fire safety generally require a minimum fire resistance performance for structural elements and a verification of collapse procedures to prevent damage to adjacent structures. It is optional for the client to equip these warehouses with active fire protection systems (sprinkler etc.), in correspondence with the activity carried out. The presence of an active fire protection system inside the warehouse strongly limits any structural damage. In this article, two case studies are done for fire behaviour analysis of self-supporting automatic warehouses with presence and absence of active fire protection systems. After defining the possible fire scenarios, a series of fluid dynamic analyses are conducted with CFD models (by FDS software) to evaluate the dynamics of the fire and the temperatures on the structural elements. Then, the heat response of the structure is investigated through non-linear analyses with FEM models (by Straus7® software) for the evaluation of the possible collapse mechanisms of warehouse structure. |
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The study of the seismic behavior of buildings with a mixed frame-wall structure puts the designer in front of important conceptual problems, such as the verification of the connections between the different construction elements, as well as the distribution of horizontal actions among the resisting walls and the torsional deformability due to the eccentricity between center of stiffness and center of mass. The proposed solution consists in implementing the suite of calculation (Straus7) with an Add-on that executes non-linear static analysis (Pushover analysis) for reinforced concrete buildings, with the following features: • Calculation of non-linear reinforced concrete slabs and walls modeled with Plate/Shell elements according to the Modified Compression Field Theory (MCFT) for slab elements and the Disturbed Stress Field Model (DSFM) for wall elements (models developed by Prof. Frank Vecchio, University of Toronto). • Possibility to perform non-linear analysis of mixed wall-frame structures, in which it is possible to combine, through the sub-modeling technique, the nonlinearity of Straus7 with the secant stiffness formulation of the smeared crack models implemented for walls and slab elements. • Possibility to perform multimodal pushover analysis, whose load distribution is continuously updated during the analysis, to reflect the progressive stiffness degradation of the structure. This solution offers both computational support in dealing with non-linear analyses of buildings, and a great versatility, integrating into the environment of a "general purpose" computer program such as Straus7. The report will illustrate in more detail the theoretical-practical aspects associated with the functionalities listed above, accompanied by examples of two-dimensional and three-dimensional structures. |
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The theme of the wooden structures, and in particular of the wood panel structures (cross-lam, platform, etc.) for their massive use for the realization of "green" structures, in both public (school, gymnasiums) and private buildings, has been increasingly relevant in the last few years. The paper aims to outline the path that leads to a correct modelling of this structural typology through the tools that FEM Straus7 code makes available. The various aspects of the design of wood panel structures will then be illustrated, which can be listed below: - Schematization of the structural problem: the wooden structure as a box element; - Aspects related to the ductility of the constructive system; design in the hierarchy of the resistances; choice of the behaviour factor; - Modelling of the wall panel and of the connections using the equivalent stiffness method; - modelling of floors and wall connections; - Verification of panels and connections. In the end, some tips will be provided for the automation of the modelling and verification processes through the use of the Straus7 API. |
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