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

The Role of Host Traits, Season and Group Size on Parasite Burdens in a Cooperative Mammal

The distribution of parasites among hosts is often characterised by a high degree of heterogeneity with a small number of hosts harbouring the majority of parasites. Such patterns of aggregation have been linked to variation in host exposure and susceptibility as well as parasite traits and environmental factors. Host exposure and susceptibility may differ with sexes, reproductive effort and group size. Furthermore, environmental factors may affect both the host and parasite directly and contribute to temporal heterogeneities in parasite loads. We investigated the contributions of host and parasite traits as well as season on parasite loads in highveld mole-rats (Cryptomys hottentotus pretoriae). This cooperative breeder exhibits a reproductive division of labour and animals live in colonies of varying sizes that procreate seasonally. Mole-rats were parasitised by lice, mites, cestodes and nematodes with mites (Androlaelaps sp.) and cestodes (Mathevotaenia sp.) being the dominant ecto- and endoparasites, respectively. Sex and reproductive status contributed little to the observed parasite prevalence and abundances possibly as a result of the shared burrow system. Clear seasonal patterns of parasite prevalence and abundance emerged with peaks in summer for mites and in winter for cestodes. Group size correlated negatively with mite abundance while it had no effect on cestode burdens and group membership affected infestation with both parasites. We propose that the mode of transmission as well as social factors constrain parasite propagation generating parasite patterns deviating from those commonly predicted

Multi-messenger observations of a binary neutron star merger

On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ~1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40+8-8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 Mo. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ~40 Mpc) less than 11 hours after the merger by the One- Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ~10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ~9 and ~16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta

A micro-mechanical model for the limit analysis of running bond masonry: applications and validation for in- and out-of-plane loaded structures.

In this chapter, a simple micro-mechanical model for the homogenised limit analysis of masonry is reviewed. Assuming brickwork under plane stress conditions and adopting a polynomial expansion for the stress field, a linear optimization problem is derived on the elementary cell in order to recover the homogenised failure surface of the brickwork. The implementation of the homogenised failure surfaces for in-plane and out-of-plane behaviour in a finite element limit analysis code is also addressed. Finally, relevant structural examples are treated with particular emphasis on the upper bound method and are compared with competing approaches

Experimental study of out-of-plane behavior of timber retrofitted masonry prisms

Typical unreinforced masonry (URM) walls have little strength to withstand out-ofplane loads. Under severe out-of-plane loading, URM walls failure is likely to be sudden and severe, producing devastating damages and death. Since out-of-plane failure mode has been identified as the most critical failure mode of URM walls, this study thus focuses only on investigating the out-of-plane behavior of URM wall. This paper presents a small scale testing program to evaluate the out-of-plane load capacity and deformation of masonry prism subjected to out-of-plane loading. This is the first stage of a multiphase experimental and numerical investigation into the possibility of retrofitting URM walls using timber-based panels. In this research, flexural bond strength in form of four-point bending test was obtained from nine different masonry prisms (615 x 215 x 102.5mm), three of which are tested as plain specimens. The remaining 6 specimens were retrofitted with an 18mm thick Oriented Strand Board (OSB) timber panel using two different types of connection (C1: adhesive anchor and C2: mechanical connection). Based on the results of the experimental tests, the out-plane load capacity and displacement of both plain and retrofitted specimens were assessed in order to highlight the performance of the proposed retrofit technique. It was observed that the application of OSB panel at the back of masonry greatly influences the flexural behavior of the test specimens preventing sudden failure of masonry prisms

The effect of skew angle on the mechanical behaviour of masonry arches

This paper presents the development of a three dimensional computational model, based on the Discrete Element Method (DEM), which was used to investigate the effect of the angle of skew on the load carrying capacity of twenty-eight different in geometry single span stone masonry arches. Each stone of the arch was represented as a distinct block. Mortar joints were modelled as zero thickness interfaces which can open and close depending on the magnitude and direction of the stresses applied to them. The variables investigated were the arch span, the span: rise ratio and the skew angle. At each arch, a full width vertical line load was applied incrementally to the extrados at quarter span until collapse. At each load increment, the crack development and vertical deflection profile was recorded. The results compared with similar “square” (or regular) arches. From the results analysis, it was found that an increase in the angle of skew will increase the twisting behaviour of the arch and will eventually cause failure to occur at a lower load. Also, the effect of the angle of skew on the ultimate load that the masonry arch can carry is more significant for segmental arches than circular one

A user-friendly digital tool for the structural assessment of historic domes: the case study of Saint Peter in Rome

This paper presents a digital tool for the rapid structural assessment of historic masonry domes. It is especially suited for masonry domes that present long meridian cracks, ergo each partitioned element governed by a pushing failure mode. The proposed procedure considers a Heyman's no-tension mechanical model has been implemented within a commercial user-friendly visual programming environment. The numerical approach consists of a parametric modelling of the failure mechanism and, therefore, exploring the domain of possible solutions using the theorems of the limit analysis. Hence, a heuristic search method is subsequently adopted to refine the geometry of the collapse mechanism and to compute the value of the horizontal trust. The validation of the developed approach has been achieved considering the Saint Peter's dome. As reported in the literature, the behaviour of the Saint Peter's dome gradually shifted from a rigid shell-type - stiffened by hoop stresses -, towards a pushing type of dome partitioned by long meridian cracks. The study also evaluated the structural integrity of the drum. In converse with more time-consuming and advanced methods of analysis, the present procedure allows the users to perform a structural assessment of a historic masonry dome in a fast and computationally efficient manner. The developed digital tool will be freely available from a web archive hosted by the University of Minho and, therefore, easily able to reach students, researchers and structural engineers

Correlation studies for the in-plane analysis of masonry walls based on macroscopic FE models with damage

This study explores the use of macro-modelling techniques based on smeared crack and damage-plastic constitutive laws for the cyclic in-plane analysis of masonry panels. The numerical investigation is focused on two material macromechanical models, known as Total Strain Cracking and Crack and Plasticity models. These show some limitations when analysing the behaviour of masonry structures subjected to in-plane cyclic loading. A modified version of the Drucker-Prager model including cohesive softening is introduced to overcome these shortcomings.A suite of numerical simulations is performed referring to an experimental campaign on two masonry (squat and slender) panels. A comparison of distinctive features of flexural and shear response of masonry panels is addressed. The results derived from the two FE macro-models are compared with the experimental outcomes, highlighting the effects of geometry, stiffness degradation, and post-peak energy dissipation. Furthermore, a comparison with another macromechanical model is performed

Numerical study of the out-of-plane behaviour of timber retrofitted masonry prisms

The present study addresses the retrofitting of running-bond masonry walls through the application of oriented strand board (OSB) timber panels aiming to increase the masonry flexural strength and deformation capacity under out-of-plane actions. This paper presents the numerical analysis of masonry prisms to complement the information provided by the experimental campaign developed on flexural performances of timber retrofitted masonries. The numerical model represents the masonry components (brick and mortar) as a three-dimensional volume via volumetric finite elements, i.e. hexahedral 8-node linear brick elements with reduced integration and hourglass control. The nonlinear properties of the mortar joints and the brick units have been calibrated through information that resorts from experimental characterization tests. The numerical damage pattern and load-displacement capacity curve are compared with the experimental observations. A good agreement has been found and, therefore, the calibrated model can be employed in parametric studies, to further analyse the efficiency of the proposed timber masonry retrofit technique, and to more complex structural study cases