The objective of this investigation is to study the impacts of the global response to COVID-19 on air pollution and air quality changes in major cities across the globe over the past few months. Air ...quality data (NO2, CO, PM2.5, and O3) were downloaded from the World Air Quality Index project for the January 2019–April 2020 period. Results show a significant reduction in the levels of 2020 NO2, CO, and PM2.5 compared to their levels in 2019. These reductions were as high as 63% (Wuhan, China), 61% (Lima, Peru), and 61% (Berlin, Germany), in NO2, CO, and PM2.5 levels, respectively. In contrast, 2020 O3 levels increased substantially, as high as 86% (Milan, Italy), in an apparent response to the decrease in titration by nitrogen monoxide and its derivatives. Significant differences in the weather conditions across the globe do not seem to impact this air quality improvement trend. Will this trend in the reduction in most air pollutants to unprecedented levels continue in the next few weeks or even months? The response to this and other questions will depend on the future global economic and environmental policies.
The tensile behavior of fiber-reinforced concrete is recognized as being superior to that of ordinary concrete, and since serviceability is a primary concern in existing design specifications, it is ...important to evaluate its post-cracking performance. This research experimentally identified and investigated cracks in concrete beams reinforced with synthetic fibers. Concrete beams with and without longitudinal reinforcement were studied in order to assess both single and multiple cracking scenarios. Synthetic fiber volume fractions of 0.5% and 1% were used to prepare the specimens, and the results were compared with those of concrete beams with no synthetic fibers. Six unreinforced beams and six reinforced beams with flexural behavior were tested. The digital image correlation (DIC) method was utilized to record the width, spacing, number, and locations of the cracks for all the specimens during the displacement control-type loading. The results indicated that using 1% synthetic fiber increased the failure load of the reinforced concrete beams and improved the serviceability by reducing the number of cracks and the width of the cracks. When the cracks opened significantly, the beams with 1% fiber dosage were able to carry higher loads due to the bridging action of the fibers at the crack locations.
•Development of a method to performdisplacement control analysis of distributed loads.•There is a difference betweenAASHTO and FEM results for calculating β & ε.•Strain in longitudinal rebarstrongly ...impacts the shear strength.•ACI318-14 predicted on average 59% higher than FEM results for R.C.beams.
This research resulted in the development of a new method for performing displacement control analysis of distributed loads to obtain the ultimate shear strength of structural components. A framework, consisting of several sub-frames, was designed to convert the single displacement applied at the top of the framework to equivalent uniformly-distributed forces applied to the beam. The shear capacity of beams under a concentrated load at mid-span was compared with the shear capacity of a uniform load for four different a/d ratios (3, 4, 5, and 7). The results indicated that the strain in the longitudinal rebar, which is dependent upon the loading condition, strongly impacts the shear strength of a critical section of structural components. The shear strength of the critical section of the R.C. beams studied in this research had uniformly distributed loads that were, on average, 76% greater than the shear strength of the same beam with a concentrated load at mid-span. The shear strength prediction of the AASHTO specification as well as ACI318-14 code were evaluated for beams with shear behavioral mode. A parametric study of 24 RC beams was conducted, and the results indicated that AASHTO’s prediction for strain in longitudinal rebar differs about 19%, on average, from the results of the finite element method (FEM). For prediction of the β factor, however, the difference is about 61%. The ACI318-14’s formulation for the concrete shear strength (Vc) averages 59% higher than the FEM results for the studied beams.
•β index determines the condition of columns after instantaneous column removal.•Columns should be rehabilitated for existing tall buildings subjected to sudden column removal.•Beams are more ...vulnerable for short buildings due to instant column removal.•Axial impacts for neighbor columns is increased, when the number of floors is increased.•λ index shows the gravity stiffness reduction rate.
In recent decades, there have been many reports of buildings collapsing due to incidents such as gas explosions, terrorist attacks, and vehicle accidents. The goal of this paper is to present the findings of a study of the how the number of stories of a reinforced concrete (R.C.) structure impacts the building when a first-story column is removed instantaneously. Three R.C. buildings were studied, all of the same plan, but with varying numbers of floors (3, 5, and 7 floors). The criteria established for comparing the behavior of the buildings when sudden column removal occurs are (1) performance level of the beams, (2) gravity stiffness of the buildings, and (3) variations of applied axial forces to the columns. The Column Sensitivity Index (β) was utilized to compare the condition of neighboring columns before and after the column removal. The results of the study showed that the β increases when the number of floors increases, which means that the neighboring columns are more vulnerable to damage. On the other hand, when the number of floors increases, the performance level of the beams improves, and the number of created moment plastic hinges in the beams decreases. The Displacement Sensitivity Index (λ) was generated to compare the gravity stiffness of the structures at the top node of the removed column before and after column removal. The results showed that the reduction rate of the gravity stiffness decreased when the number of floors increased.
•Rubber replacement plays a key role in strength reduction of geopolymer concrete.•Appropriate rubber amount may be replaced without significant strength reduction.•The fly ash type and Na2SiO3/NaOH ...also leads to the strength reduction.•The regression models are used to identify critical parameters and interactions.
This study presents the feasibility of geopolymer concrete to which crumb rubber from recycled tires has been added. Geopolymer concrete utilizes industrial by-products like fly ash. Therefore, the use of rubberized geopolymer as a binder in concrete production not only reduces the emission of carbon dioxide, because of the elimination of cement, but also utilizes an industrial disposal of recycled tires to produce a sustainable construction material. In this research, fly ash, an alkaline liquid mix of sodium hydroxide and sodium silicate, and crumb rubber were used as the basic constituents of the geopolymer. Various factors that influence the compressive strength were studied, such as molarity of sodium hydroxide, size of aggregates, amount of rubber, and types of fly ash. An appropriate amount of rubber may be replaced with an equal volume of fine aggregates in rubberized geopolymer concrete. The analysis of variance (ANOVA) indicates that fine aggregates can be replaced with an equal volume of crumb rubber, up to 5% in three types of fly ash-based geopolymer concrete at the 95% confidence level. The regression model indicates that the correlation between rubber replacement and other parameters are not statistically significant.
AbstractThis study focuses on the concrete shear strength (Vc) of the top slab of reinforced concrete (RC) box culverts deeply buried under uniformly distributed loads. Experimental data was used to ...calibrate the finite-element models of the RC box culverts and several parameters that impact the shear strength, such as span, rise, and the top slab’s thickness, were investigated using numerical models. The results of 210 case studies assessed AASHTO’s methodology for determining the shear strength of the top slab of a box culvert. A comparison of the results of the analysis with the equations in the specification indicated that AASHTO’s approach results in an underestimation of the prediction of the concrete shear strength (the β factor); however, the difference is reasonably acceptable in predicting the strain value of the longitudinal rebar. The comparison revealed, on average, an 89% difference in the prediction of the β factor and a 19% difference in the prediction of the strain in the longitudinal rebar in the shear critical section, which is at a distance d from the haunch. The regression analysis proposed multivariable formulations to predict the shear strength of the concrete in the shear critical section of the top slab of RC box culverts.
An increase in the fill height of buried box culverts leads to an increase in the thickness of the slab and wall, as well as in the number or size of longitudinal slab reinforcements required to ...resist flexure. This geometrical configuration imposes a shear behavioral mode. This study focuses on determining the shear strength of reinforced concrete (RC) box culverts with uniformly distributed load at the top slab. A framework, consisting of several subframes, was designed to convert the single displacement applied at the top of the framework to the equivalent uniformly distributed forces at the top slab of the culvert, allowing a displacement control analysis algorithm to be performed. To validate the loading mechanism, using the proposed framework, the load was applied on the top of an RC beam in the laboratory, and numerical studies were conducted. After validation, two sizes of RC box culverts were experimentally and numerically investigated. The results from the experimental program and verified numerical models differed from AC1 318-14 formulation for the shear strength of top slabs of RC box culverts. Keywords: box culverts; displacement control; framework; shear behavior; uniformly distributed load.
AbstractThis paper presents experimental and numerical investigations of the shear strength of synthetic fiber-reinforced concrete (SYN-FRC) box culverts designed for fill height of 610 mm (2 ft) or ...less and subjected to the American Association of State Highway and Transportation Officials (AASHTO) HL-93 wheel load. Shear and flexure material tests associated with numerical analysis were conducted to obtain the material properties of concrete with compressive strength of 34 MPa (5,000 psi) and synthetic volume fraction of 0.52%. The material tests showed that the shear and flexure strength of SYN-FRC were greater than those of plain concrete (without adding fiber). The flexure material test showed that SYN-FRC can carry load even after concrete cracking, unlike the plain concrete, which collapses immediately after cracking. In addition, four full-scale SYN-FRC box culverts were tested in the laboratory, and numerical models were calibrated, using experimental data. The load was applied through a rigid steel plate at distance d from the tip of the haunch to satisfy the AASHTO HL-93 wheel load requirements. The selected load location was a critical section for the shear and imposed shear failure mode. A comparison of box culverts with and without synthetic fibers revealed that adding a synthetic fiber volume fraction of 0.52% increased the shear capacity and the ductility. All specimens failed in the shear failure mode associated with an inclined shear crack from the tip of the haunch to the middle of the loading plate. The results demonstrated that synthetic fibers can be a viable alternative to shear transverse reinforcements. Additionally, numerical verification of SYN-FRC box culverts validates the use of a concrete, brittle, cracking material model for simulating SYN-FRC in the finite-element method (FEM).
API650-2008 is one of the prominent codes consisting of seismic specifications to design steel storage tanks for earthquakes resistance. In spite of the code's broad application, there are some ...failure modes such as slide bottom, elephant-foot buckling, sloshing and uplift needing more evaluation. In this paper, 161 existing tanks in an oil refinery complex have been classified into 24 groups and investigated using both API650-2008 rules and numerical FEM models. Failure modes and dynamic characteristics of studied models have been calculated by numerical FEM analysis and compared with code requirements. The results demonstrate that, in some cases, there are some imperfections in the code requirements that require further investigation.
► Seismic performance of liquid storage tanks in an oil refinery complex investigated. ► Failure modes of studied steel storage tanks calculated by FEM analysis. ► Numerical FEM analysis results compared with API650-2008 code requirements. ► We report some imperfections of API650-2008 for seismic design requirements of tanks.
Progressive collapse is defined as the spread of an initial damage from one member to another, leading to extensive partial or total collapse of the structure. In this research, the potential of ...progressive collapse due to a sudden removal of vertical load-bearing elements in reinforced concrete buildings structures with different floor plans such as geometrical regular and irregular floor plans as well as floor plans with and without torsional irregularity were assessed. The buildings were designed according to ACI 318-14 provisions and Iranian seismic code. The progressive collapse potential of the structures was assessed following of a sudden column or shear wall removal in different locations in their first floor using nonlinear dynamic analysis (NDA). Displacement sensitivity and column sensitivity indexes were utilized to compare different cases of load-bearing element removal in each model. Results indicated that in all geometrical regular floor plan, floor plan with reentrant corner and floor plan with torsional irregularity, the most critical case of column removal was removing columns located in outer corners of the plan. In addition, removing external columns was more critical than internal columns. In buildings with shear walls, removing shear walls led to much more critical scenarios than removing columns. Furthermore, results revealed that buildings with torsional irregularity floor plan, designed according to Iranian seismic code, had a lower potential of progressive collapse rather than those buildings with no irregularity.
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