The role of cloud-radiative effects and diabatic processes for the dynamics of the North Atlantic Oscillation on synoptic time-scales

Clouds shape weather and climate by regulating the latent and radiative heating in the atmosphere. Recent work demonstrated the importance of cloud-radiative effects (CRE) for the mean circulation of the extratropical atmosphere and its response to global warming. In contrast, little research has been done regarding the impact of CRE on internal variability. During the northern hemisphere winter the dominant mode of atmospheric variability over the North Atlantic and the surrounding continental areas of North America and Europe is the North Atlantic Oscillation (NAO). Here, we study how clouds and the NAO couple on synoptic time-scales during northern hemisphere winter via CRE within the atmosphere (ACRE) in observations and model simulations. A regression analysis based on 5-day-mean data from CloudSat/CALIPSO reveals a robust dipole of cloud-incidence anomalies during a positive NAO, with increased high-level clouds along the storm track (near 45°N) and the subpolar Atlantic, and decreased high-level clouds poleward and equatorward of it. Opposite changes occur for low-level cloud incidence. Satellite retrievals from CloudSat/CALIPSO, CERES and GERB as well as ERA-Interim short-term forecast data show that these cloud anomalies lead to an anomalous column-mean heating due to ACRE over the region of the Iceland low, and to a cooling over the region of the Azores high. To quantify the impact of the ACRE anomalies on the NAO, and to thereby test the hypothesis of a cloud-radiative feedback on the NAO persistence, we apply the surface pressure tendency equation (PTE) to ERA-Interim short-term forecast data. The NAO-related surface pressure tendency anomalies due to ACRE amplify the NAO-related surface pressure anomalies over the Azores high but have no area-averaged impact on the Iceland low. In contrast, surface pressure tendency anomalies due to total diabatic heating, including latent heating and clear-sky radiation, strongly amplify the NAO-related surface pressure anomalies over both the Azores high and the Iceland low, and their impact is much more spatially coherent. This suggests that while ACRE lead to an increase in NAO persistence on synoptic time-scales, their impact is relatively minor and much smaller compared to other diabatic processes. To test the robustness of our PTE-based hypothesis, numerical simulations in ICON are carried out. The PTE analysis in ICON shows results that are qualitatively consistent with the observational analysis, in particular regarding the feedback mechanisms of ACRE and total diabatic heating, which is dominated by latent heating. These PTE-based results are further tested by means of sensitivity simulations in ICON, where a NAO-related diabatic heating pattern is imposed either due to ACRE or total diabatic heating. These heating patterns are based on 5-day-mean NAO regressions of either ACRE or total diabatic heating. The sensitivity simulations confirm the observational hypothesis and show that ACRE feed back positively by up to 1–2% of 1σ NAO, while the total diabatic heating feeds back positively by up to 10% of 1σ NAO. Overall, the observational and modeling work both illustrate the substantial impact of the total diabatic heating for the NAO, while ACRE play a minor role. This highlights that diabatic processes are essential for understanding and accurately modeling the NAO short-term dynamics

Analytical framework for the substitution of steel-concrete composite columns with equivalent steel columns in structural design

The present work presents a mathematical framework to simulate steel-concrete composite columns with equivalent steel columns. A total number of three simulation methods are presented, in order to simulate circular and rectangular concrete-filled hollow sections, as well as concrete-encased I-shaped sections with steel columns of similar shape. The simulation is achieved by the satisfaction of three equations regarding their (a) axial resistance, (b) flexural stiffness about the major axis and (c) flexural stiffness about the minor axis. Solution of the aforementioned provides the dimensions of the equivalent steel sections as functions of the characteristics of the steel-concrete composite sections (a) in a closed form for all hollow sections and (b) in a high-accuracy approximate solution for I-shaped sections. The accuracy of the proposed methods and their general applicability are evaluated. The results yielded are indicative of the effectiveness of the proposed methods

Assessment of the effectiveness of cabling system configuration in retrofitting steel-concrete composite buildings

Steel cables have been extensively used in structural design. Even though their most prominent use is in the design of large span cable stayed or prestressed bridges, a variety of applications in buildings has also been realized. In structural design, cables are mainly used as components of (a) prestressed concrete or post-tensioned steel beams, in order to increase their resistance in bending moment, (b) self-centering systems as a means to restore the connected element to its initial position, or (c) bracing systems as an alternative to the typical steel sections. In the past decade, the use of cables has been proposed as a means of creating ties within a structure, in order to increase its collapse resistance. The mechanical behavior of steel cables and its numerical modeling has been extensively investigated experimentally and numerically [1-16]. However, a concise numerical investigation of their effectiveness in retrofitting steel-concrete composite buildings using three-dimensional models has not been performed. In this work, various cable system configurations are assessed with respect to their effectiveness in retrofitting steel-concrete composite buildings. The selected buildings have been found to be deficient regarding their progressive collapse resistance. Cables are installed (a) in various bays of the building, (b) parallel to its structural elements and (c) under the composite slab in order to improve their performance. The effect of post-tensioning on the efficiency of the steel cables is also evaluated. The results yielded illustrate the effectiveness of each configuration

Optimized design of steel buildings against earthquake and progressive collapse using cables

Progressive collapse is a procedure in which local failure of a structural component can cause failure of the overall structure or a smaller part of it. This phenomenon is the subject of intensive investigation by researchers the last decade. This work presents a design of structures against earthquake and progressive collapse. Cables are used as means to achieve the desired structural performance when the buildings are subjected to (a) seismic excitations, (b) accidents which result in failure of structural members. The design strategy is based on the use of cables located in suitable locations in the structure. The element sizes and cable topology are attained by an automatic optimization procedure in an effort to achieve the most effective use of structural materials. The effect of various design constraints is evaluated in the performance of the optimized buildings. The analysis results indicate the promising potential of cables as a means to increase the building’s progressive collapse resistance, as well as a promising alternative to typical bracing sections used in practice

The Cost of Retrofitting Steel-Concrete Composite Buildings Against Progressive Collapse with Steel Cables

Steel cables are an attractive means of retrofit with various engineering applications. They have been extensively used to strengthen deficient buildings against gravitational or earthquake-induced loads. This work investigates the use of steel cables as a means of retrofitting steel-concrete composite buildings against progressive collapse. The effect of the building’s characteristics on the total retrofit cost is studied. A fair assessment of designs defined for different requirements is achieved by definition of the most cost-effective solution for each scenario. This is achieved by an optimization algorithm, i.e. the Evolution Strategies, which is employed to define the solution with the desired performance and, at the same time, the minimum cost. For this purpose, a total number of 144 optimizations have been performed. The results yielded reveal the different properties of each retrofit scenario

Seismic design optimization of multi–storey steel–concrete composite buildings

This work presents a structural optimization framework for the seismic design of multi–storey composite buildings, which have steel HEB-columns fully encased in concrete, steel IPE-beams and steel L-bracings. The objective function minimized is the total cost of materials (steel, concrete) used in the structure. Based on Eurocodes 3 and 4, capacity checks are specified for individual members. Seismic system behavior is controlled through lateral deflection and fundamental period constraints, which are evaluated using nonlinear pushover and eigenvalue analyses. The optimization problem is solved with a discrete Evolution Strategies algorithm, which delivers cost-effective solutions and reveals attributes of optimal structural designs

Earthquake-resistant buildings with steel or composite columns: Comparative assessment using structural optimization

This work investigates and compares the cost-effectiveness of seismically designed buildings having either pure steel or steel-concrete composite columns. In order to ensure an objective comparison of these two design approaches, the assessed building designs are obtained by a structural optimization procedure. Thus, any bias that would result from a particular designer's capabilities, experience, and subjectivity is avoided. Hence, a discrete Evolution Strategies optimization algorithm is employed to minimize the total cost of materials (steel and concrete) used in a structure subject to constraints associated with: (a) Eurocode 4 provisions for safety of composite column-members, (b) Eurocode 3 provisions for safety of structural steel members, and (c) seismic system behaviour and resistance. Extensive assessments and comparisons are performed for a variety of seismic intensities, for a number of building heights and plan configurations, etc. Results obtained by conducting 154 structural design optimization runs provide insight into potential advantages attained by partially substituting steel (as a main structural material) with concrete when designing the columns of earthquake-resistant buildings

The effect of rotational component of earthquake excitation on the response of steel structures

This work is on the influence of the rotational component of earthquake excitations to the response of steel structures. In most studies, seismic input is being modeled only using the translational component of the ground acceleration, while the rotational one is ignored. This was due to the observation that the rotational component had minimal effect on low-rise buildings. Hence, the accelerometers used would not measure it, leading to a lack of records. Nowadays, technology provides such instruments and relative records are made available. Indicative of that is that elastic design response spectra for rotational components are introduced to the design codes. In this paper, the results on structural response and internal forces due to the rotational component of a seismic excitation on the steel structures are examined. Dynamic time history analysis and response spectrum analysis of different steel structures are performed (a) considering the rotational component of the excitation and (b) without it. From the numerical results it is shown that the impact of rotational component in structural response and internal forces of the steel structures is significant and should not be ignored during structural design