Rectification of “restrained vs unrestrained”

For furnace testing of fire‐resistant floor and roof assemblies in the United States, the ASTM E 119 standard (and similarly the UL 263 standard) permits two classifications for boundary conditions: “restrained” and “unrestrained.” When incorporating tested assemblies into an actual structural system, the designer, oftentimes a fire protection or structural engineer, must judge whether a “restrained” or “unrestrained” classification is appropriate for the application. It is critical that this assumption be carefully considered and understood, as many qualified listings permit a lesser thickness of applied fire protection for steel structures (or less concrete cover for concrete structures) to achieve a certain fire resistance rating if a “restrained” classification is confirmed, as compared with an “unrestrained” classification. The emerging standardization of structural fire engineering practice in the United States will disrupt century‐long norms in the manner to which structural behavior in fire is addressed. For instance, the current edition of the ASCE/SEI 7 standard will greatly impact how designers consider restraint. Accordingly, this paper serves as an exposé of the “restrained vs unrestrained” paradigm in terms of its paradoxical nature and its controversial impact on the industry. More importantly, potential solutions toward industry rectification are provided for the first time in a contemporary study of this paradigm

The ANDROID case study; Venice and its territory : a general overview

The Work Package 7 (Research Futures & Special Interest Groups) of the ANDROID project, selected Venice and its territory as an emblematic case study of a region that could be affected by cross-border disastrous events. The paper provides a general overview on the topic, trying to organise the large amount of available scientific literature in some strategic cores, identifying undoubted milestones, open questions and future research needs, following a holistic approach to risk assessment. This case study is carried out not only as an engaging exercise, but with the purpose to provide a reference point for scientists and teachers interested to translate multifaceted knowledge into specific solutions. In fact, the paper is strongly linked as a whole to other three ones (presented at the 4th International Conference on Building resilience), which deepen respectively hazard, vulnerability/resilience, and mitigation about the site taken into consideration. Furthermore, the City of Venice takes part to the UNISDR Program “Making Cities Resilient”, and planned a robust intervention, consisting in the realisation of mobile dikes located at the openings of the lagoon (MOSE project, almost terminated), which has been strongly debated since the beginning, due to possible negative consequences on the environment. At last, the paper analyses drawbacks and benefits of the above said intervention, and suggests further proposals for the global safeguard of Venice and its lagoon

Optimum Lateral Load Distribution for Seismic Design of Nonlinear Shear-Buildings Considering Soil-Structure Interaction

The lateral load distributions specified by seismic design provisions are primarily based on elastic behaviour of fixed-base structures without considering the effects of soil-structure-interaction (SSI). Consequently, such load patterns may not be suitable for seismic design of non-linear flexible-base structures. In this paper, a practical optimization technique is introduced to obtain optimum seismic design loads for non-linear shear-buildings on soft soils based on the concept of uniform damage distribution. SSI effects are taken into account by using the cone model. Over 30,000 optimum load patterns are obtained for 21 earthquake excitations recorded on soft soils to investigate the effects of fundamental period of the structure, number of stories, ductility demand, earthquake excitation, damping ratio, damping model, structural post yield behaviour, soil flexibility and structural aspect ratio on the optimum load patterns. The results indicate that the proposed optimum load patterns can significantly improve the seismic performance of flexible-base buildings on soft soils

Uncertainties in dynamic response of buildings with non-linear base-isolators

Dynamic response of base-isolated buildings under uni-directional sinusoidal base excitation is numerically investigated considering uncertainties in the isolation and excitation parameters. The buildings are idealized as single degree of freedom (SDOF) system and multi-degrees of freedom (MDOF) system with one lateral degree of freedom at each floor level. The isolation system is modeled using two different mathematical models such as: (i) code-recommended equivalent linear elastic-viscous damping model and (ii) bi-linear hysteretic model. The uncertain parameters of the isolator considered are time period, damping ratio, and yield displacement. Moreover, the amplitude and frequency of the sinusoidal base excitation function are considered uncertain. The uncertainty propagation is investigated using generalized polynomial chaos (gPC) expansion technique. The unknown gPC expansion coefficients are obtained by non-intrusive sparse grid collocation scheme. Efficiency of the technique is compared with the sampling method of Monte Carlo (MC) simulation. The stochastic response quantities of interest considered are bearing displacement and top floor acceleration of the building. Effects of individual uncertain parameters on the building response are quantified using sensitivity analyses. Effect of various uncertainty levels of the input parameters on the dynamic response of the building is also investigated. The peak bearing displacement and top floor acceleration are more influenced by the amplitude and frequency of the sinusoidal base excitation function. The effective time period of the isolation system also produces a considerable influence. However, in the presence of similar uncertainty level in the time period, amplitude and frequency of the sinusoidal forcing function, the effect of uncertainties in the other parameters of the isolator (e.g., damping ratio and yield displacement) is comparatively less. Interestingly, the mean values of the response quantities are found to be higher than the deterministic values in several instances, indicating the need of conducting stochastic analysis. The gPC expansion technique presented here is found to be a computationally efficient yet accurate alternative to the MC simulation for numerically modeling the uncertainty propagation in the dynamic response analyses of the base-isolated buildings

Seismic performance evaluation of deficient steel moment-resisting frames retrofitted by vertical link elements

In many earthquake prone regions in developing countries, substandard steel moment resisting frame (SMRF) systems pose a profound danger to people and economy in the case of a strong seismic event. Eccentric bracing systems with replaceable vertical links can be utilized as an efficient and practical seismic retrofitting technique to reduce future earthquake damages to such structures. This paper aims, for the first time, to demonstrate the efficiency of eccentric bracing systems with vertical links as a seismic retrofitting technique for the SMRF structures with WCSB and to develop fragility curves for such structures. To achieve this aim, first, the effect of the vertical links on the behaviour of 3, 5 and 7-storey frames are studied through conducting the Nonlinear Static Analyses (NSA) as well as Nonlinear Time History Analyses (NTHA) using the artificial accelerograms compatible with the target design spectrum. The analysis results indicate that, as aimed in the design stage, the seismic damage is only concentrated at the replaceable vertical links and remaining structural members work mainly in the elastic range. In addition, the proposed retrofitting technique considerably improves the performance of the deficient SMRF systems by effectively restricting the displacement response and damage distribution in such structures. Following the NTHA, Incremental Dynamic Analyses (IDA) are performed to develop the seismic fragility curves for the retrofitted SMRF systems. The results indicate that the proposed retrofitting technique significantly reduces the fragility of such systems, and therefore, can provide a simple and efficient method to improve the seismic performance of deficient steel moment resisting frames in seismic regions

Seismic reliability analysis and estimation of multilevel response modification factor for steel diagrid structural systems

Diagrid systems are emerging as one of the structurally efficient and architecturally aesthetic solutions for tall buildings. Despite the fact that such systems are increasingly used in modern construction, current literature lacks detailed information regarding their structural behaviour and seismic design parameters to ensure satisfactory performance under different earthquake intensity levels. This study aims to assess the seismic reliability of diagrid structural systems and develop more efficient performance-based design methodologies. Demand and supply response modification factors are calculated for 16, 24 and 32-storey buildings with diagrid structural systems using 65° diagrid angle and designed in compliance with current standards under a set of 12 spectrum compatible earthquakes. The results are then used to develop a novel multi-level response modification factor (R-Factor) for diagrid structural systems as a function of site seismicity and acceptable damage level. Subsequently, comprehensive seismic reliability analyses are conducted to assess the seismic performance of the selected structures under intensity levels corresponding to DBE and MCE hazard levels (earthquake scenarios with return periods of 475 and 2475 years, respectively). In general, results of this study demonstrate acceptable seismic performance and reliability of steel diagrid systems. It is shown that even using an R-Factor equal to 4 in the seismic design process could ensure that diagrid structures remain in a performance level higher than Life Safety (LS) for both DBE and MCE hazard levels. Multi-level response modification factors proposed in this study can be directly used in performance-based design of diagrid structures to satisfy different performance targets under any seismic hazard level