On particles in the Arctic stratosphere

Soon after the discovery of the Antarctic ozone hole it became clear that particles in the polar stratosphere had an infl uence on the destruction of the ozone layer. Two major types of particles, sulphate aerosols and Polar Stratospheric Clouds (PSCs), provide the surfaces where fast heterogeneous chemical reactions convert inactive halogen reservoir species into potentially ozone-destroying radicals. Lidar measurements have been used to classify the PSCs. Following the Mt. Pinatubo eruption in June 1991 it was found that the Arctic stratosphere was loaded with aerosols, and that aerosols observed with lidar and ozone observed with ozone sondes displayed a layered structure, and that the aerosol and ozone contents in the layers frequently appeared to be negatively correlated. The layered structure was probably due to modulation induced by the dynamics at the edge of the polar vortex. Lidar observations of the Mt. Pinatubo aerosols were in several cases accompanied by balloon-borne backscatter soundings, whereby backscatter measurements in three different wavelengths made it possible to obtain information about the particle sizes. An investigation of the infl uence of synoptic temperature histories on the physical properties of PSC particles has shown that most of the liquid type 1b particles were observed in the process of an ongoing, relatively fast, and continuous cooling from temperatures clearly above the nitric acid trihydrate condensation temperature (TNAT). On the other hand, it appeared that a relatively long period, with a duration of at least 1-2 days, at temperatures below TNAT provide the conditions which may lead to the production of solid type 1a PSCs

A PARAMETRIC STUDY ON SEISMIC CHARACTERISTICS OF COLD-FORMED STEEL SHEAR WALLS BY FINITE ELEMENT MODELING

Shear wall panels, including cold-formed steel frames and its attached sheathing, are common lateral load resisting systems of cold-formed steel structures. In this paper, the finite element method is used to study the lateral performance of shear wall panels. The finite element model is validated against experimental results of other researchers. Using the validated model, a parametric study is described to determine strength, drift and seismic behavior of the shear wall panels. Based on the results, it is concluded that the initial stiffness and ultimate lateral strength are dramatically affected by the thickness of the frame members, type of sheathing material, edge screw spacing, height of the frame, while some parameters such as field screw spacing have a minor effect on the initial stiffness and the ultimate lateral strength. In addition, this study looks into the earthquake performance of the shear wall panels and presents the corresponding ductility factor and force reduction factor (R-factor) of shear wall panels

Post-earthquake fire performance-based behavior of unprotected moment resisting 2D steel frames

Post-Earthquake Fire (PEF) can lead to the collapse of buildings that are partially damaged in a prior earthquake that occurred immediately before the fire. The majority of standards and codes for the design of structures against earthquake ignore the possibility of PEF and thus buildings designed with those codes fail prematurely when subjected to PEF. A sequential analysis based on FEMA356 is performed here on the Immediate Occupancy (IO), corresponding to a structure designed as school occupancy, and Life Safety (LS) performance levels, corresponding to a structure designed as reside ntial occupancy, of two steel moment resisting frames. These frames are first subjected to an earthquake load with the PGA of 0.35g. This is followed by a fire analysis, using both the ISO834 model and the Natural fire model. The time it takes for the structure weakened by the earthquake to collapse under fire is then calculated. As a point of reference, fire only analyses are also performed for the undamaged structures. The results show that earthquake weakened structures are more vulnerable to fire than undamaged structures. The results also show that both fire resistance and PEF resistance of the frame designed as school are more than the frame designed as residential. Whilst the investigation is for a certain class of structures (steel moment resisting frames, 5 stories), the results confirm the need for the incorporation of PEF in the process of analysis and design and provides some quantitative measures on the level of associated effects

Finite element instability analysis of the steel joist of continuous composite beams with flexible shear connectors

Composite steel/concrete beams may buckle in hogging bending regions. As the top flange of I-beam in that arrangement is restricted from any translational deformation and twist, the web will distort during buckling presenting a phenomenon often described as restricted distortional buckling. There are limited studies available in the literature of restricted distortional buckling of composite steel/concrete I-beams subjected to negative or hogging bending. There is none however that includes the effect of partial shear interaction. In this paper, finite element models for in-plane analysis and out-of-plane buckling of continuous composite I-beams including the effects of partial shear interaction are presented

Stochastic modelling for service life prediction of underground tunnels subjected to water ingress

This paper presents a methodology in which both the temporal and spatial variation of water seepage in tunnel lining can be considered. Temporal variation is modelled using the Gamma process, while copulas are used to model spatial correlation in the seepage process. Simulation based on the Monte Carlo technique is employed to find the extent of water seepage over the tunnel. An illustrative example is used to show applicability of the proposed method. The results showed that the likelihood and extent of seepage-induced damage rapidly increase with time. Based on an acceptable probability of failure of 0.20, service life of tunnel for 50% extent of damage is more than three times that of 0.05 of extent of damage. Output of this research highlights the need for more research on statistical analysis of water seepage in concrete

Performance of light-gauge cold-formed steel strap-braced stud walls subjected to cyclic loading

The performance of cold-formed steel (CFS) strap-braced walls is evaluated by experimental tests on full-scale 2.4m x 2.4m specimens, and techniques to improve their behavior are presented. Different strap arrangements have been introduced, and their performance investigated by means of cyclic loading of a total of twenty full-scale walls. Several factors affecting the performance of cold-formed steel frame shear wall have been considered for each arrangement. This paper presents the failure modes of each system and the main factors contributing to the ductile response of the CFS walls to ensure that the diagonal straps yield and respond plastically with a significant drift and without any risk of brittle failure, such as connection failure or stud failure. Discussion of the advantages and disadvantages of including the non-structural gypsum board on lateral performance of the walls is also presented

Comparative study on reliability of ductility and strength limit states in design of reinforced concrete beams according to fib Model Code

Ductility is an important limit state for the design of reinforced concrete beams. For reinforced concrete beams designed based on the fib Model Code 2010, reliability of this limit state is investigated using reliability-based assessment with ductility defined by strain ratio and it is then compared with that of traditional strength limit states. The modelling uncertainty for the ductility limit state typically is much greater than that for structural strength limit state. Results show that the ductility ratio generally follows a right-skewed distribution, and due to variability in the material properties and model error, there is high variability in the ductility. This is reflected in the corresponding reliability indices of limit state defined for ductility. Some of the resulted reliability indices could be considered unacceptably low

Buckling analysis of thin-walled cold-formed steel structural members using complex finite strip method

In this paper, a generalised complex finite strip method is proposed for buckling analysis of thin-walled cold-formed steel structures. The main advantage of this method over the ordinary finite strip method is that it can handle the shear effects due to the use of complex functions. In addition, distortional buckling as well as all other buckling modes of cold-formed steel sections like local and global modes can be investigated by the suggested complex finite strip method. A combination of general loading including bending, compression, shear and transverse compression forces is considered in the analytical model. For validation purposes, the results are compared with those obtained by the Generalized Beam Theory analysis. In order to illustrate the capabilities of complex finite strip method in modelling the buckling behavior of cold-formed steel structures, a number of case studies with different applications are presented. The studies are on both stiffened and unstiffened cold-formed steel members

An experimental investigation on the lateral behavior of knee-braced cold-formed steel shear walls

Experimental investigations were conducted to evaluate the lateral seismic characteristics of light-weight knee-braced cold-formed steel structures. In all, four full-scale 2.4×2.4 m 2 specimens with different configurations were tested under a standard cyclic loading regime. This paper focuses on the specimens maximum lateral load capacity and deformation behavior and provides a rational estimate of the seismic response modification factor, R, of knee-braced walls. The study also looks at the failure modes of the system and investigates the main factors contributing to the ductile response of CFS walls. That is in order to suggest improvements so that the shear steel walls respond plastically with a significant drift and without any risk of brittle failure, such as connection failure or stud buckling. A discussion on the calculated response factors in comparison to those suggested in the relevant codes of practice is also presented

Investigation of the ductility of RC beams : based on AS3600 design

Reinforced concrete beams are likely to experience high bending moment when subjected to ultimate loading. Curvature ductility is a well accepted index of expressing the ability of the structure to sustain high load intensities. The member ductility – rotational ductility in case of bending moment - arises from section ductility e.g. curvature ductility. Issuing of the new version of AS3600 and emergence of 500N rebar steel have introduced new possibilities in RC member design in Australia. Concerns on 500N rebar steel ductility due to its lower elongation strain compared to other ductile steel require a more careful focus on the design process. This study is an attempt to quantify the section ductility of RC beams designed based on the latest Australian code using 500N rebar steel. Different section dimensions as well as various concrete compressive strengths were used in the study. The nonlinear moment-curvature analysis carried out for designed cases. As expected heavily reinforced sections exhibit lower ductility comparing to those reinforced lightly. In addition increasing the section height and the concrete compressive strength have positive effects on curvature ductility. It is found that the curvature ductility for considered cases ranges between 2.0 and 12.0. It seems that for small sections, the curvature ductility is low and is less than predicted. Generally speaking, more research is necessary in order to verify the ability of 500N rebar steel in providing reasonable ductility required for load redistribution and other requirements in limit state design