Voronoi-Like grid systems for tall buildings

In the context of innovative patterns for tall buildings, Voronoi tessellation is certainly worthy of interest. It is an irregular biomimetic pattern based on the Voronoi diagram, which derives from the direct observation of natural structures. The paper is mainly focused on the application of this nature-inspired typology to load-resisting systems for tall buildings, investigating the potential of non-regular grids on the global mechanical response of the structure. In particular, the study concentrates on the periodic and non-periodic Voronoi tessellation, describing the procedure for generating irregular patterns through parametric modeling and illustrates the homogenization-based approach proposed in the literature for dealing with unconventional patterns. To appreciate the consistency of preliminary design equations, numerical and analytical results are compared. Moreover, since the mechanical response of the building strongly depends on the parameters of the microstructure, the paper focuses on the influence of the grid arrangement on the global lateral stiffness, therefore on the displacement constraint, which is an essential requirement in the design of tall buildings. To this end, five case studies, accounting for different levels of irregularity and relative density, are generated and analyzed through static and modal analysis in the elastic field. In addition, the paper focuses on the mechanical response of a pattern with gradual rarefying density to evaluate its applicability to tall buildings. Displacement based optimizations are carried out to assess the adequate member cross sections that provide the maximum contribution in restraining deflection with the minimum material weight. The results obtained for all the models generated are compared and discussed to outline a final evaluation of the Voronoi structures. In addition to the wind loading scenario, the efficiency of the building model with varying density Voronoi pattern, is tested for seismic ground motion through a response spectrum analysis. The potential applications of Voronoi tessellation for tall buildings is demonstrated for both regions with high wind load conditions and areas of high seismicity

Comparative assessment of strut models for the modelling of in-plane seismic response of infill walls

The influence of infills on the seismic response of frame structures has long been recognised. Typically, stiffness and strength of the infill and connections between infill and frame are such that the infill affects the global seismic behaviour of the structure. Hence, the presence of infills should be considered in the analysis and design of new buildings and in the seismic assessment of existing ones. To this aim, simple models for infill walls, such as the equivalent diagonal no-tension strut model, have been developed in the last decades. The objective of the present study is to assess the validity of different strut models. To this aim, 162 experimental tests available in the literature are considered. The data set includes both reinforced concrete and steel frames, as well as confined masonry structures. The mechanical characteristics of masonry and the boundary conditions between frames and infills of the test specimens take into account a large set of situations, reflecting the great variability in the materials and in the construction techniques adopted in different countries. Moreover, the type of tests and the related results are not uniform; in some cases monotonic experiments are performed, whereas in other cases cyclic tests are carried out. As expected, the presence of different types of infill-frame systems results in a large scatter of the data. However, the comparison between experimental results and predictions show that, on the average, the infill strength can be adequately estimated by resorting to the strut model whereas major uncertainties are found for the stiffness prediction

Spectrum-to-spectrum methods for the generation of elastic floor acceleration spectra

In seismic codes, the acceleration demand of nonstructural components is commonly expressed in terms of floor response spectra and estimated by means of simple predictive equations. By using the latter, response-history analysis of the structure is not required, being floor spectra calculated directly from the peak ground acceleration expected at the site. The price for this simplicity in the method used for the estimation of floor spectra is the generally poor approximation of the obtained predictions. Codes’ equations, in fact, do not explicitly account for important factors influencing floor spectra, such as the contribution of the higher modes of vibration of the structure and the actual value of the nonstructural components’ damping ratio. Alternative spectrum-to-spectrum methods for direct generation of floor spectra have been proposed, which include these factors and improve the accuracy of the predictions. Different approaches have been used and several methods developed. Despite large research effort, however, a comparative evaluation of the currently available proposals is still lacking. The objective of this paper is to fill this gap, by reviewing selected proposals representative of practice-oriented spectrum-to-spectrum methods. A case study consisting in a six-story frame is analyzed and predictions obtained with the investigated methods are compared with exact floor spectra derived from time-history analyses of the structure, as well as spectra calculated using the Eurocode 8 equation

Topology optimization of multi-story buildings under fully non-stationary stochastic seismic ground motion

Topology optimization has been mainly addressed for structures under static loads using a deterministic setting. Nonetheless, many structural systems are subjected to uncertain dynamic loads, and thus efficient approaches are required to evaluate the optimal topology in such kind of applications. Within this framework, the present paper deals with the topology optimization of multi-story buildings subjected to seismic ground motion. Because of the inherent randomness of the earthquakes, the uncertain system response is determined through a random vibration-based approach in which the seismic ground motion is described as filtered white Gaussian noise with time-varying amplitude and frequency content (i.e., fully non-stationary seismic ground motion). The paper is especially concerned with the assessment of the dynamic response sensitivity for the gradient-based numerical solution of the optimization problem. To this end, an approximated construction of the gradient is proposed in which explicit, exact derivatives with respect to the design variables are computed analytically through direct differentiation for a sub-assembly of elements (up to a single element) resulting from the discretization of the optimizable domain. The proposed strategy is first validated for the simpler case of stationary base excitation by comparing the results with those obtained using an exact approach based on the adjoint method, and its correctness is ultimately verified for the more general case of non-stationary seismic ground motion. Overall, this validation demonstrates that the proposed approach leads to accurate results at low computational effort. Further numerical investigations are finally presented to highlight to what extent the features of the non-stationary seismic ground motion influence the optimal topology

Characterisation of Site Effects by Means of Energy Spectra

The effects of subsoil conditions on surface ground motion are evaluated in terms of energy spectra. Near-field and far-field strong ground motion recorded during recent destructive earthquakes at nearby rock and soil sites characterized by a comprehensive knowledge of the geotecbnical properties are considered. The study suggests that energy spectra at soil sites are amplified with respect to those on rock sites. The maximum spectral amplification is usually well correlated to the natural periods of the sites. The most striking difference between traditional response spectra and energy spectra is the high soil amplification at longer periods, which is not apparent from the consideration of response spectra only

Performance assessment of Timber High-rise Buildings: Structural and Technological Considerations

Nowadays, a renewed momentum on the use of timber material is ensured by the development of high performing engineered wood products, which enables larger and taller structures to be built. Although the design of multi-story timber buildings is still in its early stages, the active interest shown by designers and researchers in advancing awareness and technologies in this field bodes well for the proliferation of an increasing number of tall wooden buildings

NEW APPROACH FOR THE OPTIMAL YIELD-FORCE COEFFICIENT DISTRIBUTION IN THE SEISMIC DESIGN OF BUILDINGS

One of the main objectives in seismic design of buildings is to prevent the damage concentration in certain stories. As shown by past earthquakes the damage concentration leads to severe damage and even collapse of the structure, especially for near field ground motions. Energy-based design methods address explicitly the tendency of a given story to concentrate damage, and evaluate it through a parameter that measures the deviation of the actual lateral strength of the story with respect to an optimum (yet ideal) value that would make the plastic strain energy, normalized by the yield strength and yield displacement of the story, approximately equal in all stories. The objective of this methodology is to obtain the optimum lateral strength distribution expressed in terms of yield-shear force coefficient. The yield-shear force coefficient is defined as the ratio between the i-th story yield-shear force and the total upward weight born by the story. The optimal yield-force coefficient distribution is defined as one that makes the normalized plastic strain energy equal in all stories, thus preventing damage concentration in a given story. This paper proposes a new procedure to estimate for a given building and exciting ground motion the optimal yield-force coefficient distribution. The optimal distribution is estimated by means of the Pattern Search Method by iteratively changing the shear strength distribution until a uniform normalized distribution of damage is obtained. A comparison is carried out between the seismic behaviour of two case study structures designed using the proposed procedure and alternative proposals taken from the literature. The response is estimated in terms of distribution of damage and inelastic deformation

Effect of ground-motion sequences on a unreinforced masonry wall restrained by an elasto-plastic tie-rod

This work investigates the effect of international ground-motion sequences on the out-of-plane response of an ordinary-building façade. The following assumptions are made on the wall boundary conditions: the wall is resting on a foundation, it is adjacent to transverse walls and restrained by elasto-plastic tie rods with finite elongation capacity. Four walls are considered of different aspect ratio and size; two types of masonry are assumed, and the tie is designed following a force-based procedure according to the Commentary to the Italian Building Code. The walls are modelled as rigid blocks of finite thickness and free to rotate on one side only. The rocking response of the walls, excited in the out-of-plane direction under 56 sequences of records, is evaluated. The effect of sequences is estimated by the comparison of the response experienced during the sequence and under a single record, strongest in terms of either peak ground acceleration or velocity. Finally, in order to reduce the vulnerability originated by a seismic sequence, a proposal of a reduced behaviour factor to be adopted in the design of tie rods is formulated