1,720,986 research outputs found
Residual displacements for non-degrading bilinear oscillators under seismic actions
No full textThis work aims to provide a relationship to assess the residual displacements for one-storey buildings standing in seismic-prone areas. The maximum residual displacement that a structure can possibly sustain is clearly identified by means of its structural characteristics alone. It is demonstrated that the upper bound is independent of the peak displacement. The proposed procedure can help in controlling the structural residual deformations, which is an issue that is gaining momentum into the seismic community: a large body of research has been dealing with post-seismic damage. However, no general rule or theoretical mechanic model with general validity are reported in the literature. The solely valuable approach, though widely accepted, for predicting seismic-induced residual displacements is a statistical one. In order to frame the given relationship into the established probabilistic framework, an extensive numerical campaign has been carried out by means of nonlinear time-history analyses. Artificial spectrum-compatible accelerograms are used as proxies to account for seismic events. As a final result, residual displacement response spectra are proposed
An optimal seismic force pattern for uniform drift distribution
No full textThe force distribution proposed by codes, which in many cases is framed in the equivalent static force procedure, likely leads to design structures with non-uniform drift distribution in terms of inter-storey drift and ductility demands. This can lead to an unbalanced drift demand at certain storeys. This phenomenon may also amass cyclic damage to the dissipative elements at this very storey, therefore increasing the probability of premature failure for low-cycle fatigue. This work proposes a new force design distribution that accounts for higher mode effects and limits the displacement concentration at any storey thus improving the dissipative capacity of the whole structures. The main advantage of the proposed method stands in its formulation, which allows to spare any previous set up with structural analyses. The proposed force distribution has been applied to multi-degree-of-freedom systems to check its effectiveness, and the results have been compared with other proposals. In addition, in order to obtain a further validation of the proposed force distribution, the results obtained by using a genetic algorithm have been evaluated and compared. Additionally, the results provided in this work validate the proposed procedure to develop a more efficient lateral load pattern
Residual drift control strategies: extension and application to MDOF systems
No full textThe paper proposes a new methodology for the control of residual displacements in multi-storey steel structure frames. The work starts from the validation and extension of a mechanical model based on single degree of freedom systems, already proposed in previous work, following with analyses performed on a set of natural accelerograms. The same model is applied to multi-degree of freedom (MDOF) systems through an anlaytical design procedure. The procedure is used to dimension study cases composed of concentrically braced frames that are analysed through nonlinear dynamic analyses with earthquake records excerpted from the previous dataset. The results confirm the effectiveness and suitability of the proposed procedure for MDOF systems
Seismic design-assisted-by-testing approach for racks with dissipative baseplates in the cross-aisle direction
No full textA design-assisted-by-testing strategy is proposed for the upright frames of adjustable pallet racks. This methodology defines a procedure that aims to locate marked post-elastic behavior at the floor-to-upright connections of lateral resisting frames in the cross-aisle direction. Such connections are intended to enhance the performance of the cross-aisle frames when subjected to dynamic forces in their plane. The chosen baseplate connection is experimentally tested using monotonic and cyclic protocols at the Steel Structures Laboratory of the National Technical University of Athens (Greece). Then, a numerical model of the baseplate is calibrated using literature parameter definitions and hence validated using physical data. Additionally, the dynamic response of two upright frames is numerically investigated: one with traditional hinge connections and another with dissipative ones. This study indicates that the proposed procedure successfully enhances ductility, resulting in a significant reduction (up to 50%) in upright axial forces compared to the conventional configuration. Finally, the open-source code for ground motion selection, together with its database, is released
Shake table testing and numerical modelling of a steel pallet racking structure with a seismic isolation system
No full textThis study aims to evaluate the performance of a steel storage pallet racking system equipped with a novel seismic isolation system, which is effective along the cross-aisle direction alone. The isolation system consists of two IsolGOODS® isolators, which are curved surface slider bearings as framed into the European Standards for anti-seismic devices. The efficiency of the proposed device in reducing the seismic effects on pallet racks is investigated. Firstly, uniaxial real-time shake table tests are performed, showing a great reduction of floor accelerations. With the help of a herein validated numerical model, advanced knowledge is obtained. Six accelerograms, two normal-fault records, two pulse-like, and two spectrum-compatible series are selected for drawing generally valid statements. As proposed in this work, the outcomes confirm that the isolation system provides substantial reduction in terms of floor accelerations. In contrast, no up-lift of the base-connections is recorded: the uprights are always engaged in compression. On the whole, the most severe condition comes from the pulse-like record (Emilia 2012), in which displacement demand must be carefully considered while defining the isolators to be put in place
Recent development on the seismic devices for steel storage structures
No full textGoods and products are stored in framed systems, such as pallet racks, used for industrial and commercial activities. In the last years, pallet rack code provisions for seismic loads have been significantly improved, but there are still relevant aspects that need attention for guaranteeing a safer structural design. For example, in the current European and American standards, no indications are given about the seismic isolation systems applied to these structures. Only two ways to enhance the performance of racks in seismic zones are reported: rack netting and structural strengthening. Both methodologies present logistic and technical problems. For this reason, researchers are investigating more efficient solutions, like the base isolation systems. An accurate isolation system can bring benefits in terms of reduction of the structural damage and improving the safety of the stored items. Since the cost of the structural frame is often negligible, with respect to the cost of the stored products, avoiding the overturning of merchandise is an important challenge. Moreover, falling pallets can bring to the overall global collapse due to an impact given on beams or columns. In the paper, a critical overview of base isolation systems developed for different steel storage rack typologies is presented and discussed, highlighting the main characteristics and the advantages associated with their use in practical cases. Furthermore, four different applications of energy dissipation devices are briefly discussed, comparing these systems with the previously introduced devices
Dam Break-Induced Urban Flood Propagation Modelling with DualSPHysics: A Validation Case Study
No full textSustainable, safe and healthy growth of urban areas is currently achieved by a series of measures and initiatives, including the planning, management and mitigation of natural and anthropic possible hazards. Among potential menaces, extreme precipitation events of short duration, inducing severe urban flooding, may pose a significant threat for residents, especially if no counter-measures are planned to tackle with. Urban development, if not integrated with flooding mitigation strategies, generally goes with an increase of the impermeable surface, yielding a decreasing of infiltration and water evaporation as a result, and ultimately an increase of runoff peaks and a decrease of concentration times. These kinds of phenomena tend to worsen due to the ongoing climate change. In this framework, the sudden release of water through a simplified scaled urban configuration, following an abrupt dam breaching, is here numerically investigated. The open-source Lagrangian-based DualSPHysics solver was used for this aim. The aligned square city layout of 5 x 5 buildings case of Soares-Frazão and Zech (2008) was adopted as a validation case study. DualSPHysics post-processed free surface vertical profiles were compared with data recorded at 17 water-level gauges and numerical results of a finite-volume shallow water scheme, showing a reasonable agreement
Seismic devices for steel storage structures
No full textThe importance of the logistics sector has increased even more in the last year, due to the Pandemic event which lead to an increasing of the online purchases. Goods and products are generally located in steel frames known as steel racks or simply racks. Consequently also the safe design of these structures, i.e. preserve their fully functionality and avoid their collapse, is becoming of paramount importance, especially when they are located in strong and moderate seismic zones. Despite pallet rack provisions for seismic loads have been significantly improved in the recent years, only two classic ways to enhance the seismic performance are considered: rack netting and structural strengthening. Both of these suggested solutions are not fully effective to preserve the integrity of the stored products that, when subjected to strong accelerations, can topple and fall down. The only reliable and effective systems seem to be the introduction of seismic devices, such as base-isolation and energy dissipation systems. Since no indications about these techniques are reported in the standards, many researches worldwide are trying to fill this gap. Unfortunately, up to now, just theoretical studies and very limited applications are available in literature. In the present paper a short overview on the base isolation systems available on the market for different steel storage rack typologies is presented highlighting main advantages and defects of each solution
Seismic design lateral force distributions based on elastic analysis of structures
No full textIt has been often observed, as outcome of non-linear dynamic analyses that structures designed according to the force distribution proposed by Eurocode8, i.e. proportional to first mode shape, show a displacement demand at the top storey higher than those occurring at other intermediate storeys, thus increasing the probability of premature failure at this very storey. The scope of this work is to propose a new force design distribution that allows for the higher mode effects and therefore further structural dynamic characteristics
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