The Theory of Critical Distances to estimate static and dynamic strength of notched plain concrete

The Theory of Critical Distances (TCD) is a well-known design method allowing the strength of notched/cracked components to be estimated accurately by directly post-processing the entire linear-elastic stress fields damaging the material in the vicinity of the stress concentrators being designed. By taking full advantage of the TCD’s unique features, in the present study this powerful theory was reformulated to make it suitable for designing notched plain concrete against static and dynamic loading. The accuracy and reliability of the proposed reformulation of the TCD was checked against a set of experimental results generated by testing, under different displacement rates, square section beams of plain concrete containing notches of different sharpness. This validation exercise has demonstrated that the proposed reformulation of the TCD is capable of accurately assessing the static and dynamic strength of notched unreinforced concrete, with the estimates falling within an error interval of ±20%. The level of accuracy that was obtained is certainly satisfactory, especially in light of the fact that static and dynamic strength was estimated without explicitly modelling the stress vs. strain dynamic behaviour of the concrete being tested

The Theory of Critical Distances to assess failure strength of notched plain concrete under static and dynamic loading

The Theory of Critical Distances (TCD) is a design method that is widely used in situation of practical interest to estimate the strength of notched/cracked components subjected to either static, dynamic, or fatigue loading. The TCD makes use of a characteristic length to post-process the linear-elastic stress fields damaging the material in the vicinity of the stress concentrators being designed. The employed length scale parameter depends on the specific microstructural features of the material under investigation. By making the most of the TCD’s unique features, the present paper summarises an attempt of reformulating this powerful theory to make it suitable for assessing static and dynamic strength of notched plain concrete. The accuracy and reliability of the proposed reformulation of the TCD is checked against a number of experimental results that were generated by testing, under different displacement rates, square section beams of plain concrete containing notches of different sharpness. This validation exercise allowed us to demonstrate that the proposed reformulation of the TCD, which is based on the use of simple power laws, is capable of accurately assessing the static and dynamic strength of the notched unreinforced concrete being tested, with the estimates falling within an error interval of ±20%. The obtained level of accuracy is certainly satisfactory, especially owing to the fact that static and dynamic strength is predicted without explicitly modelling those non-linearities characterising the stress vs. strain dynamic behaviour of concrete

Engineering reconnaissance following the August 24, 2016 M6.0 Central Italy earthquake

An earthquake with a moment magnitude reported as 6.0 from INGV (Istituto Nazionale di Geofisica e Vulcanologia); occurred at 03:36 AM (local time) on 24 August 2016 in the central part of Italy. The epicenter was located at the borders of the Lazio, Abruzzi, Marche and Umbria regions, about 2.5 km north-east of the village of Accumoli and about 100 km from Rome. The hypocentral depth was about 8 km (INGV). We summarize preliminary findings of the Italy-US GEER (Geotechnical Extreme Events Reconnaissance) team, on damage distribution, causative faults, earthquake-induced landslides and rockfalls, building and bridge performance, and ground motion characterization. Our reconnaissance team used multidisciplinary approaches, combining expertise in geology, seismology, geomatics, geotechnical engineering, and structural engineering. Our approach was to combine traditional reconnaissance activities of on-ground recording and mapping of field conditions, with advanced imaging and damage detection routines enabled by state-of-the-art geomatics technology. We anticipate that results from this study, will be useful for future post-earthquake reconnaissance efforts, and improved emergency respons

The Theory of Critical Distances to assess failure strength of notched plain concrete under static and dynamic loading

The Theory of Critical Distances (TCD) is a design method that is widely used in situation of practical interest to estimate the strength of notched/cracked components subjected to either static, dynamic, or fatigue loading. The TCD makes use of a characteristic length to post-process the linear-elastic stress fields damaging the material in the vicinity of the stress concentrators being designed. The employed length scale parameter depends on the specific microstructural features of the material under investigation. By making the most of the TCD's unique features, the present paper summarises an attempt of reformulating this powerful theory to make it suitable for assessing static and dynamic strength of notched plain concrete. The accuracy and reliability of the proposed reformulation of the TCD is checked against a number of experimental results that were generated by testing, under different displacement rates, square section beams of plain concrete containing notches of different sharpness. This validation exercise allowed us to demonstrate that the proposed reformulation of the TCD, which is based on the use of simple power laws, is capable of accurately assessing the static and dynamic strength of the notched un-reinforced concrete being tested, with the estimates falling within an error interval of ± 20%. The obtained level of accuracy is certainly satisfactory, especially owing to the fact that static and dynamic strength is predicted without explicitly modelling those non-linearities characterising the stress vs. strain dynamic behaviour of concrete

Centrifuge investigation comparing the rocking response of two soil-structure systems

Seismic protection of structures by means of rocking isolation is becoming increasingly popular, because allowing uplift is an inexpensive way to reduce structural demand. However, understanding the role of soil–structure interaction in the response of rocking systems is important to define what type of rocking system might be most effective. To address this challenge, a campaign based on centrifuge modelling and testing is currently ongoing. The primary objective is to assess the force demand that rocking systems experience during their motion. Flexible structures that rock while stepping on discrete footings (structural rocking) and flexible structures with discrete footings rocking on soil (foundation rocking) are both considered. Following this distinction, two building models were designed with the only difference being the connectivity of the columns to the footings. For structural rocking, columns were designed to detach and step on their footings, while for foundation rocking the footing-column connection was designed to be rigid. The two building models were tested side-by-side in a centrifuge. A second test was also conducted, where thin steel “fuses” were installed in the interface of structural rocking, to further study the allocation of energy dissipation between structural elements and fuses, and soil medium. The building models were placed on the surface of dense sand and then tested using sinusoidal ground motions which caused a combination of sliding and rocking. The global response of the models in terms of overturning moment and storey shear was investigated and back validated by obtaining directly the internal loads, which were found capped regardless of the extent of rotation. More-over, the base isolation effect was evident during large amplitude resonant excitations, whereas during a low frequency low amplitude excitation there was no clear benefit of rocking. Finally, no significant effect was observed in limiting the base shear demand by using the steel fuses

Soil behaviour beneath buildings with structural and foundation rocking

Structural or foundation rocking are effective ways to prevent the development of large force demands in building systems experiencing ground shaking. In practice, structural rocking may result in large displacements and therefore is often accompanied with dampers, while residual and differential settlements are sometimes a barrier for utilisation of foundation rocking. This paper presents a novel approach to test simultaneously in a centrifuge the performances of a building model rocking above its foundation level (structural rocking) and of a dynamically equivalent building rocking below its foundation level (foundation rocking). These tests are used to explore the role of the foundation and soil response when considering the relative trade-offs between the two building types. In the experiments, dry sand was used in both dense and loose states and a series of earthquake excitations were applied. Results demonstrate that the rocking motion of the buildings is evident in the soil response beneath the structures, and foundation rocking causes larger dynamic differential settlements than structural rocking for a given rocking amplitude. However, these differential settlements may still be small enough to be tolerated from a serviceability limit state point of view

Seismic performance of buildings with structural and foundation rocking in centrifuge testing

Rocking motion, established in either the superstructure in the form of a 2-point stepping mechanism (structural rocking) or resulting from rotational motion of the foundation on the soil (foundation rocking), is considered an effective, low-cost base isolation technique. This paper unifies for the first time the 2 types of rocking motion under a common experimental campaign, so that on the one hand, structural rocking can be examined under the influence of soil and on the other, foundation rocking can be examined under the influence of a linear elastic superstructure. Two building models, designed to rock above or below their foundation level so that they can reproduce structural and foundation rocking respectively, were tested side by side in a centrifuge. The models were placed on a dry sandbed and subjected to a sequence of earthquake motions. The range of rocking amplitude that is required for base isolation was quantified. Overall, it is shown that the relative density of sand does not influence structural rocking, while for foundation rocking, the change from dense to loose sand can affect the time-frequency response significantly and lead to a more predictable behaviour