Evaluation of optimal lateral resisting systems for tall buildings subject to horizontal loads

The tendency of modern designs towards optimal structures often leads to the lightest and best performing choice among a large set of design alternatives. In a similar scenario, the introduction of automated tools to further guide designers in achieving efficient solutions has been a recurrent topic for mechanical and structural engineers, over the past decades. Nowadays, topology optimization is considered a powerful preliminary design tool to determine the optimal material distribution in a design domain, i.e. the most effective configuration that satisfies a given set of prescribed constraints while reducing the consumption of structural material. Among different applications in the field of Civil Engineering, this work focuses on the definition of optimal layouts of lateral resisting systems for multi-storey steel building frameworks subject to lateral loads using topology optimization techniques. The objective of the research is to illustrate the benefits deriving from the introduction of automated routines within the preliminary design stage and establish reliable guidelines for performing accurate and objective optimization procedures. Since the optimal material distribution follows the load flow within the structure, optimal topologies are especially sensitive to the alteration of support and loading conditions: different loading scenarios naturally lead to distinct optimal layouts. In order to avoid the loss of objectivity and preserve the optimality of the results, the effects that preliminary modelling and loading assumptions produce on final layouts are investigated. Numerical applications to high-rise building models are presented and discussed

Quantification of energy-related parameters for near-fault pulse-like seismic ground motions

An energy-based approach facilitates the explicit consideration of the damage associated with both maximum displacements and cumulative plastic deformations under earthquakes. For structural systems that can undergo pulse-like seismic ground motions close to causative faults, an energy-based approach is deemed especially appropriate with respect to well-established force-or displacement-based strategies. In such a case, in fact, most of the damage is attributable to the dominant pulse-like component, which usually occurs into the velocity time history of the seismic ground motion, thus implying high energy levels imparted to a structural system. In order to enable the implementation of an energy-based approach in the analysis and design of structures under near-fault pulse-like seismic ground motions, this study presents a comprehensive numerical investigation about the influence of seismological parameters and hysteretic behavior on the spectra of the following energy-related parameters: inelastic absolute and relative input energy; input energy reduction factor; hysteretic energy dissipation demand; hysteretic energy reduction factor; dimensionless cumulative plastic deformation ratio. Closed-form approximations are proposed for these spectra, and the numerical values of the corresponding parameters have been also calibrated (with reference to both mean and standard deviation values) as functions of earthquake magnitude, type of hysteretic behavior (i.e., non-degrading or degrading) and ductility level. The outcomes of this study are meant to support the derivation of design spectra for the energy-based seismic design of structures under near-fault pulse-like seismic ground motions

Design of Shake Table Tests of Multi-Leaf Masonry Walls Before and After Retrofitting

A significant proportion of the built heritage in historic centres is constituted by rubble stone masonry structures. Collapses by leaf separation and disaggregation observed after earthquakes highlight their dramatic vulnerability, especially under out-of-plane loads. Nevertheless, their dynamic response still needs to be fully investigated and their capacity may be overestimated by assessment approaches based on rigid-body mechanics. Effective retrofitting solutions are also needed to protect human lives and safeguard the built heritage, while ensuring the conservation of its architectural value. This paper describes the design of a shake table investigation on stone masonry walls, whose materials and arrangement reproduce those surveyed in the villages of central Italy struck by the 2016-2017 earthquake sequence. The test setup was conceived to induce out-of-plane vertical bending under earthquake base motion and investigate the dynamic response of multi-leaf rubble stone masonry and the gain in seismic capacity that can be achieved with mortar-based composite reinforcements, designed to prevent the leaf separation and disaggregation of the wall without compromising its fair face

Investigation of Rubble-Masonry Wall Construction Practice in Latium, Central Italy

The 2016-2017 Central Italy seismic sequence severely affected existing unreinforced-masonry constructions in four regions. Those in Latium region proved the most prone to fragmentation because of an unfortunate combination of undressed natural stone units and very low lime content in mortar. Within the framework of a research project funded by the regional government, shake table tests are planned to investigate masonry disintegration as well as possible intervention techniques, as described in a companion paper. All specimens will have natural stone units retrieved from the debris in Collespada, a settlement of the municipality of Accumoli, one of the most affected by the seismic sequence. To push further the representativeness of the specimens with respect to field conditions, wall geometry, masonry fabric and mortar recipe are carefully designed. The wall thickness will be approximately equal to 0.5 m, close to average thickness surveyed in the area. Following the survey of several vertical sections of actual masonry walls, the specimens will present unconnected external leaves with a limited nucleus. Based on tests on mortar sampled from collapsed buildings, mortars will be prepared by a part of natural lime every nine parts of sand. Shear tests on sampled mortar delivered apparent cohesion and friction coefficient that are used as preliminary values of a finite-discrete element model, which can account for masonry fragmentation in dynamic non-linear analyses. The numerical model was tested under the envisioned sequence of records, belonging to the Amatrice station and related to the East component, approximately fault normal, of the two main seismic events, 24 August and 30 October, 2016

Shake-table testing of a stone masonry building aggregate: overview of blind prediction study

City centres of Europe are often composed of unreinforced masonry structural aggregates, whose seismic response is challenging to predict. To advance the state of the art on the seismic response of these aggregates, the Adjacent Interacting Masonry Structures (AIMS) subproject from Horizon 2020 project Seismology and Earthquake Engineering Research Infrastructure Alliance for Europe (SERA) provides shake-table test data of a two-unit, double-leaf stone masonry aggregate subjected to two horizontal components of dynamic excitation. A blind prediction was organized with participants from academia and industry to test modelling approaches and assumptions and to learn about the extent of uncertainty in modelling for such masonry aggregates. The participants were provided with the full set of material and geometrical data, construction details and original seismic input and asked to predict prior to the test the expected seismic response in terms of damage mechanisms, base-shear forces, and roof displacements. The modelling approaches used differ significantly in the level of detail and the modelling assumptions. This paper provides an overview of the adopted modelling approaches and their subsequent predictions. It further discusses the range of assumptions made when modelling masonry walls, floors and connections, and aims at discovering how the common solutions regarding modelling masonry in general, and masonry aggregates in particular, affect the results. The results are evaluated both in terms of damage mechanisms, base shear forces, displacements and interface openings in both directions, and then compared with the experimental results. The modelling approaches featuring Discrete Element Method (DEM) led to the best predictions in terms of displacements, while a submission using rigid block limit analysis led to the best prediction in terms of damage mechanisms. Large coefficients of variation of predicted displacements and general underestimation of displacements in comparison with experimental results, except for DEM models, highlight the need for further consensus building on suitable modelling assumptions for such masonry aggregates

On the application of the yield-line method to masonry infills subjected to combined in-plane and out-of-plane loads

The influence of infills on the seismic response of frame structures has been long recognised. On the one hand, the presence of infills may be beneficial, due to their contribution to dissipate energy. On the other hand, irregular infill distributions in plan or elevation can lead to concentration of the displacement demand in localised parts of the building. It is noted that the lack of one or more panels may depend on the original building layout or may be generated by the infill collapse during a seismic event. It is therefore of interest the assessment of their capacity to resist out-of-plane loads. In this paper, the use of the yield-line theory for the estimation of the out-of-plane infill strength is investigated. The method is described in detail and an example of derivation of the related equations is presented. Afterward, a modification of such equations is suggested to account for different contact conditions at the infill-frame interface. Moreover, specific attention is paid to the assessment of the masonry flexural strength, which is a basic parameter for the application of the method. Finally, a reduction factor is calibrated to extend the method to those cases in which previous in-plane damage is present. Experimental tests available in the literature are used to verify and calibrate the proposed equations and coefficients

Observations of out-of-plane rocking in the oratory of san giuseppe dei minimi during the 2009 L'Aquila earthquake

Contrary to other structural types, unreinforced masonry buildings display local collapses during strong earthquakes, especially in the case of large-hall constructions. This is also what has happened in the Oratory of San Giuseppe dei Minimi, located in the historical centre of L'Aquila (central Italy) and affected by the 2009 earthquake. The church suffered the out-of-plane response of its façade and parapet belfry. These mechanisms are studied recurring to non-linear dynamic models, calibrated with experimental data. The outcome of such analyses is in reasonable agreement with observed damages. Hence, it is possible to recommend this approach for the seismic assessment of similar buildings

Experimental investigation of energy damping in masonry mechanisms

Severe earthquakes throughout the world show that existing masonry buildings are prone to suffer out-of-plane local-collapse mechanisms. In such mechanisms energy dissipation is mainly due to impacts against the remainder of the structure. Using data from an experimental campaign on free-rocking walls, estimation are suggested for the coefficient of restitution in both two-sided (façade resting on a foundation) and one-sided (façade adjacent to transverse walls) rocking. Both brick and tuff elements are considered, and the relevance of test repetition and wall height-to-thickness ratio is addressed

Seismic demand of the 2016–2017 Central Italy earthquakes

The seismic sequence which started on August 24th, 2016, caused hundreds of casualties, damage and collapses in four regions of Central Italy (Lazio, Umbria, Abruzzo and Marche). The strongest event, which occurred on October 30th (Mw6.5), was forerun by four earthquakes with magnitude between 5.4 and 6.0. So far, a total of nine events with magnitude greater than or equal to 5.0 have taken place in the affected area. The earthquakes were caused by normal faults, all of them having NW–SE or NNW–SSE strike, approximately along the spine of the Apennine Mountains. The hypocentres of the events were at a shallow depth, between 8 and 10 km. The building stock in the affected area is mainly characterised by unreinforced masonry and reinforced concrete ordinary buildings, churches and historical constructions. Different municipalities, severely damaged, were not classified as seismic prone until 1981, or were originally attributed to a seismic zone with lower seismicity compared to the current one. This circumstance can explain, to a certain extent, the observed seismic response. In this study, aimed at interpreting the observed damage, the assessment of the damage potential of the ground motions recorded during the strongest events is performed by means of conventional and unconventional parameters. Specifically, elastic spectral demands, in terms of pseudo-accelerations, energies (equivalent velocity), displacements and rocking rotations are estimated, discussed and, whenever appropriate, compared with those of Italian seismic codes. Finally, different parameters related to the ground motion records destructiveness are calculated and compared with those obtained for other Italian earthquakes, highlighting how severe the 2016–2017 seismic sequence was