Optimization of shoring/reshoring levels in high-rise building construction

Formwork system is a significant constituent and a basic requirement for high-rise cast-in-place reinforced concrete buildings. Usually, the builders are confronted with the decision to choose the safe, optimum number of levels of shores/reshores for a predetermined safety factor and given grade of concrete, giving due consideration to the cost of formwork system. In this study, MATLAB program is developed to calculate the load distribution between the interconnected slabs and levels of shore/reshore of a slab formwork based on a simplified method. This program is further modified by incorporating genetic algorithm for the optimization of cost of construction for high-rise building. The cost of level of shores and reshores per floor is defined as a function of cycle time which directly reflects the increase in the cost of construction. Various combinations of shore and reshore levels with several grades of concrete for various safety factors are checked to minimize the cost of construction. The optimization equation is solved using genetic algorithm considering appropriate constraints to practically ensure feasible solutions. The case of one level of shores and numerous levels of reshores is better than one level of reshores and numerous levels of shores. The result of certain combination of shore and reshore levels is not the same when the level numbers are reversed. A comparative study is carried out to check the optimum cost for various safety factors. The program is useful for the designers to decide the levels of shores and reshores with minimized cost without compromising the safety of construction

Influence of Nonlinear Fluid Viscous Dampers on Seismic Response of RC Elevated Storage Tanks

The numerical investigation on the seismic response of RC elevated liquid storage tanks installed with viscous dampers is presented. A discrete two-mass model for the liquid and multi-degree of freedom system for staging, installed with the dampers are developed for Reinforced Concrete (RC) elevated liquid storage tanks. The elevated tank is assessed for seismic response reduction when provided with Linear Viscous Damper (LVD) and Nonlinear Viscous Damper (NLVD), installed in the staging. The RC elevated liquid storage tanks are analyzed for two levels of liquid containment in the tank, 100% and 25% of the tank capacity. Three Configurations of placements of dampers viz. dampers at alternate levels (Configuration I and Configuration II) and dampers at all the panels of the staging of the tank (Configuration III) are considered. To study the effect of peak ground acceleration, eight real earthquake time histories with accelerations varying from 0.1 g to 0.93 g are considered. The nonlinearity in the viscous damper is modified by taking force proportional to various velocity exponents. It is found that the nonlinear viscous dampers with lower damping constant result in a comparable reduction in the response of RC elevated liquid storage tank, to that of linear viscous dampers with higher damping constant. A lower damping constant signifies compact the size of the damper. Doi: 10.28991/cej-2020-SP(EMCE)-09 Full Text: PD

An Abridged Review of Blast Wave Parameters

In case of blast loading on structures, analysis is carried out in two stages, first the blast loading on a particular structure is determined and second, an evaluation is made for the response of the structure to this loading. In this paper, a review of the first part is presented which includes various empirical relations available for computation of blast load in the form of pressure-time function resulting from the explosion in the air. Different empirical techniques available in the form of charts and equations are reviewed first and then the various blast wave parameters are computed using these equations. This paper is providing various blast computation equations, charts, and references in a concise form at a single place and to serve as base for researchers and designers to understand, compare, and then compute the blast wave parameters. Recommendations are presented to choose the best suitable technique from the available methods to compute the pressure-time function for obtaining structural response.Defence Science Journal, 2012, 62(5), pp.300-306, DOI:http://dx.doi.org/10.14429/dsj.62.114

Uncertainties in dynamic response of buildings with non-linear base-isolators

Dynamic response of base-isolated buildings under uni-directional sinusoidal base excitation is numerically investigated considering uncertainties in the isolation and excitation parameters. The buildings are idealized as single degree of freedom (SDOF) system and multi-degrees of freedom (MDOF) system with one lateral degree of freedom at each floor level. The isolation system is modeled using two different mathematical models such as: (i) code-recommended equivalent linear elastic-viscous damping model and (ii) bi-linear hysteretic model. The uncertain parameters of the isolator considered are time period, damping ratio, and yield displacement. Moreover, the amplitude and frequency of the sinusoidal base excitation function are considered uncertain. The uncertainty propagation is investigated using generalized polynomial chaos (gPC) expansion technique. The unknown gPC expansion coefficients are obtained by non-intrusive sparse grid collocation scheme. Efficiency of the technique is compared with the sampling method of Monte Carlo (MC) simulation. The stochastic response quantities of interest considered are bearing displacement and top floor acceleration of the building. Effects of individual uncertain parameters on the building response are quantified using sensitivity analyses. Effect of various uncertainty levels of the input parameters on the dynamic response of the building is also investigated. The peak bearing displacement and top floor acceleration are more influenced by the amplitude and frequency of the sinusoidal base excitation function. The effective time period of the isolation system also produces a considerable influence. However, in the presence of similar uncertainty level in the time period, amplitude and frequency of the sinusoidal forcing function, the effect of uncertainties in the other parameters of the isolator (e.g., damping ratio and yield displacement) is comparatively less. Interestingly, the mean values of the response quantities are found to be higher than the deterministic values in several instances, indicating the need of conducting stochastic analysis. The gPC expansion technique presented here is found to be a computationally efficient yet accurate alternative to the MC simulation for numerically modeling the uncertainty propagation in the dynamic response analyses of the base-isolated buildings

Seismic response of base-isolated structures during impact with adjacent structures

The seismic response of multi-story building supported on various base isolation systems during impact with adjacent structures is investigated. The isolated building is modeled as a shear type structure with lateral degree-of-freedom at each floor. An impact element in the form of spring and dashpot is used to model the adjacent structure (i.e. retaining wall or entry bridge). The coupled differential equations of motion for the isolated system are derived and solved in the incremental form using Newmark’s step-by-step iteration method. The variation of top floor absolute acceleration and bearing displacement for different isolation systems during impact upon the adjacent structures under different earthquakes is computed to study the behavior of the building during impact and comparative performance of various isolation systems. The impact response of isolated building is studied under the variation of important system parameters such as size of gap, stiffness of impact element, superstructure flexibility and number of story of base-isolated building. It is concluded that the response of base-isolated structures is affected when impact takes place with the adjacent structures and hence need to be avoided. The superstructure acceleration increases and the bearing displacement decreases due to impact with adjacent structure. However, even after the occurrence of impact phenomenon, the isolation remained effective as compared to the non-isolated structure. Further, it is also observed that superstructure acceleration increases with the increase of the isolation gap up to a certain value and then the acceleration decreases with further increase of gap. The effects of impact are found to be severe for the system with flexible superstructure, increased number of story and greater stiffness of the adjacent structure.© Elsevie

Influence of isolator characteristics on the response of base-isolated structures

The influence of isolator characteristics on the seismic response of multi-story base-isolated structure is investigated. The isolated building is modeled as a shear type structure with lateral degree-of-freedom at each floor. The isolators are modeled by using two different mathematical models depicted by bi-linear hysteretic and equivalent linear elastic–viscous behaviors. The coupled differential equations of motion for the isolated system are derived and solved in the incremental form using Newmark’s step-by-step method of integration. The variation of top floor absolute acceleration and bearing displacement for various bi-linear systems under different earthquakes is computed to study the effects of the shape of the isolator hysteresis loop. The influence of the shape of isolator force-deformation loop on the response of isolated structure is studied under the variation of important system parameters such as isolator yield displacement, superstructure flexibility, isolation time period and number of story of the base-isolated structure. It is observed that the code specified equivalent linear elastic–viscous damping model of a bi-linear hysteretic system overestimates the design bearing displacement and underestimates the superstructure acceleration. The response of base-isolated structure is significantly influenced by the shape of hysteresis loop of isolator. The low value of yield displacement of isolator (i.e. sliding type isolation systems) tends to increase the superstructure accelerations associated with high frequencies. Further, the superstructure acceleration also increases with the increase of the superstructure flexibility.© Elsevie

Effects of Soil-Structure Interaction on Torsionally Coupled Base Isolated Machine Foundation under Earthquake Load

In this paper, the influence of soil-structure interaction (SSI) on a torsionally coupled turbo-generator (TG) machine foundation is studied under earthquake ground motions. The beneficial effects of base isolators in the TG foundation under earthquake ground motions are also studied duly, considering the effects of SSI. A typical TG foundation is analyzed using a three-dimensional finite element (FE) model. Two superstructure eccentricity ratios are considered to represent the torsional coupling. Soft soil properties are considered to study the effects of SSI. This research concludes that the effects of torsional coupling alter the natural frequencies, if ignored, could lead to unsafe design. The deck accelerations and displacements are increased with an increase in superstructure eccentricity. On the other hand, the deck accelerations and displacements are greatly reduced with the help of base isolators, thus confirming the beneficial use of base isolators in machine foundations to protect the sensitive equipment from the strong earthquake ground motions. However, the effects of SSI reduce the natural frequencies of the TG foundation resting on soft soil conditions and activate the higher mode participation, resulting in amplifying the response

Understanding the response of reinforced concrete slabs due to contact explosion of TNT

Evaluating response of reinforced concrete (RC) structures to blast loads is now a matured field of research. The United Facilities Criteria (UFC) 3-340 design manual and similar other manuals lay out the design practice for blast resistant structures. However, most of the design methodologies are restricted to far-field (scaled distance > 1.18 m/kg 1/3 ) blast loading. The semi-empirical charts and equations presented in design manuals for far-field blast loading are not accurate in the near-field events and furthermore very little research is available on contact explosions. Contact explosions are more complex than the far-field explosion effects due to the spatially and temporally non-uniform overpressure. There are limited experimental studies available in the literature as many gauges do not survive the harsh near-field environment. Thus, most finite element models in the near-field events are validated based on post blast damage photos. This paper presents the results from field tests conducted on RC slabs with embedded piezo-electric based concrete vibration sensors (CVS). A correlation has been shown between the concrete strains and the voltage recorded by the sensors. These results have further been compared to the numerical results obtained from LS-DYNA. The contact explosion was modeled using the arbitrary-Lagrangian-Eulerian (ALE) element formulation. The study shows that contact explosion can be reliably modeled using the presented parameters. The readings obtained from CVS could capture the shock wave propagation and the strain time history in the slab at required locations