International Journal of Civil Engineering
Volume:18 Issue: 11, Nov 2020

Soil–structure interaction (SSI) is key to the elastic and inelastic seismic responses of buildings. In this research, the ductility and strength demands of vertically irregular MDOF buildings, considering nonlinear SSI effects, are parametrically investigated. The superstructure is modeled as a nonlinear multi-story shear building, and the beam on nonlinear Winkler foundation (BNWF) concept is employed to simulate the response of shallow foundations. Specifically, combined stiffness–strength irregularities are introduced by reducing lateral properties at a specific story of the regular (reference) models. Soil–structure systems with 5, 10, and 15 stories are analyzed under three sets of earthquake records corresponding to different soil classes. A wide range of key parameters including number of stories, fundamental period, level of inelasticity, aspect ratio, and site class are scrutinized through nonlinear time history analyses. The results reveal that SSI can reduce the strength and ductility demands of the vertically irregular structures, especially those with short periods. This beneficial effect becomes even more significant for systems with low ductility ratios. It is also concluded that the median ductility demand increases in the modified story owing to the softness/weakness of the first story. Furthermore, this increase due to the code strength regularity limit reached up to 78% and 36% in fixed-base and flexible-base conditions, respectively. Finally, simplified equations are proposed to estimate the maximum ductility demands of regular and irregular structures with flexible-base conditions.

The performances of seven full-scale, fixed-supported reinforced concrete (RC) beams were investigated with the four-point bending test in this study. One of the RC beams was a reference beam (Ref) and six were strengthened beams. The strengthening of the RC beams was performed with near surface mounted (NSM), external bonded reinforcement (EBR), friction hybrid bonding (FHB), and hybrid techniques. Steel bars, CFRP bars, CFRP sheets, and mechanical fastener systems were used in the strengthening processes, according to the requirements of the applied techniques. The experimental results were evaluated for the effects of strengthening techniques and materials on the load–deflection response, ultimate load-carrying capacity, ductility, dissipated energy, failure modes, strain, and crack pattern. Strengthening applications using the NSM technique, with conventional steel, increased the load-carrying capacity of the RC beams by 22–24.9% while increasing their total energy dissipation by 40.7–68.9%. The load-carrying capacity was increased by 3.7–11.9% in RC beams strengthened by CFRP sheet and CFRP bar. However, except for the FHB technique, CFRP-applied RC beams could not perform the inelastic behavior. The FHB strengthening technique increased the load-carrying capacity and total energy dissipation of the beam by 11.6% and 21.2%, respectively. The results showed that NSM-Steel, NSM-Steel/90, and FHB-CFRP techniques quite improved the performance of the RC beams for both the elastic and plastic regions, while both of EBR-CFRP and Hybrid-CFRP techniques improved the elastic behavior of the RC beams to a great degree.

This study investigates the effects of aftershocks on the potential damage of reinforced concrete (RC) structures retrofitted by fibre-reinforced polymer (FRP). An eight-storey RC frame that was poorly confined due to deficient transverse reinforcement was selected and then retrofitted by FRP wraps to provide external confinement. Inelastic time history and damage analyses of the frame under single earthquakes and mainshock–aftershock sequences were performed. FRP retrofit did not ensure a damage-free structure, and it minimised the damage though. The results indicated different contributions of aftershocks to the damage of the retrofitted frame, and this finding illustrates the limitations of current seismic codes. The damage caused by single earthquakes is proposed to be used as a criterion for dealing with the effects of aftershocks as follows. If the mainshocks cause minor or light damage, the effect of aftershocks is negligible, whereas the effect of aftershocks needs to be considered if the mainshocks cause moderate or severe damage. The outcomes of this study can be used in making decisions about whether the effects of aftershocks should be considered.

Recently, the conditional mean spectrum (CMS) has become an important tool in ground-motion selection for seismic evaluation of structures. In the present numerical study, a 10-storey RC frame building is assumed to be situated on type-II soil and located in seismic zone V of India. The building is designed using the response spectrum analysis method of the Indian seismic code. Further, it is analysed using the nonlinear time history analysis method for three different CMS ground motions. The numerical study is performed for three cases: (1) fixed base with actual earthquake record; (2) fixed base with site-specific earthquake record considering soil amplification; and (3) flexible base considering soil–foundation flexibility and soil amplification. The results of the analysis are compared for the above-mentioned cases in terms of lateral displacement and storey drifts. It is observed that the displacement profile of the structure subjected to all the three considered ground motions is not the same, although they are matching to CMS. It is concluded that out of the two important soil effects, i.e. soil amplification and soil–foundation flexibility, soil amplification has a significant effect on the seismic response; however, the nature of response depends mainly on the ground-motion characteristics.

Alkali Pozzolan cement (APC) comprises a high volume of fly ash, lime, ordinary Portland cement (OPC), and a dry Na2SO4 activator. APC can be stored in dry form and only requires water to create a cementitious binder. Mechanical and microstructural properties of APC were investigated to determine the compressive strength and hydration products of different APC pastes. Air-cured APC pastes (w/b = 0.3) were found to gain 65–77% of the strength of similar OPC pastes at 28 days, while being 75% greater than that of corresponding high-volume fly ash ones. Microstructural properties studied using X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and thermal analysis were able to explain the observed mechanical properties. The attacking of the smooth fly ash spheres (observed via SEM) by Ca(OH)2, evidenced by a reduction in the latter (observed through thermogravimetry) over a period of 112 days, could be linked to the strength gain of APC over that period. This strength gain could also be associated with the formation of ‘broad humps’, at the relevant 2θ values (around 29° and 32°) in XRD curves, which became more pronounced with time. Strength could also be correlated to the percentage of hydrate bound water, with the differing amounts of lime consumption by fly ash partly responsible for scatter.

The objective of this study is to evaluate the effects of adding rock wool fibres on the improvement in the dynamic properties of the hot mix asphalt. To this aim, the effects of 0.2, 0.4, 0.6, and 0.8 of percent rock wool fibres by weight of mixture on the dynamic properties of asphalt mixtures were evaluated. Accordingly, indirect tensile stiffness modulus, indirect tensile fatigue and repeated load axial tests were performed to measure the stiffness modulus, fatigue life and rutting resistance of asphalt mixtures. The fatigue life of modified mixtures with different concentrations of rock wool increased 4%, 32%, 35% and 65% at 25 °C with regard to control, respectively. Furthermore, adding 0.8% of Rockwool resulted in 84% and 130% increased in resistance to permanent deformation at the stress of 150 kPa and 300 kPa, respectively. Consequently, incorporation of rock wool in asphalt mixture can be beneficial as it enhances the performance of road pavements against distresses such as rutting and fatigue cracking.

This study presents a mathematical approach to distribute portable excess speed detectors in urban transportation networks. This type of sensor is studied to be located in a network in order to separate most of the demand node pairs in the system resembling the well-known traffic sensor surveillance problem. However, newly, the locations are permitted to be changed introducing the dynamic form of the sensor location problem. The problem is formulated mathematically into three different location problems, namely SLP1, SLP2, and SLP3. The aim is to find the optimal number of sensors to intercept most of the daily traffic for each model objective. The proposed formulations are proven to be an NP-hard problem, and then heuristics are called for the solution. The methodology is applied to AL Riyadh city as a real case study network with 240 demand node pairs and 124 two-way streets. In the SLP1, all the demand node pairs are covered by 19% of the network’s roads, whereas SLP2 model shows the best locations for each assumed budget of sensors to purchase. The SLP2 solutions range from 24 sensors with 100% paths coverage to 1 sensor with nearly 20% of paths coverage. The SLP3 model manages to redistribute the sensors in the network while maintaining its traffic coverage efficiency. Four locations structures manage to cover all the network streets with coverage ranges between 100% and 60%. The results show the capability of providing satisfactory solutions with reasonable computing burden.

The influence of model assumptions on the dynamic impedance functions of shallow foundations is investigated using finite elements in two studies. The first investigates the effects of model assumptions in different combinations including embedment of the foundation, variation of modulus with depth, and permanent load acting on the foundation. The second study is a parametric analysis investigating the effects of permanent load at varying soil depths and with different soil modulus coefficients. Shallow foundations in strata of frictional soil on top of bedrock are considered. Small-strain modulus and modulus reduction relationships are used in an iterative process to update the modulus due to the permanent load. The results show that model assumptions can have a large influence on impedance functions. The static stiffness coefficients differ, in some instances by more than 100%. The impedance functions, normalized with the static stiffness coefficients match each other well in the pre-resonance frequency range. However, in the frequency range above the fundamental frequency, the normalized impedance functions show a large variation. Further, the results show that the influence of the permanent load is largest in the case of shallow and stiff soil strata, both regarding normalized impedance functions as well as the static stiffness coefficient, which can be increased up to 67%. The change in fundamental frequency was however minimal.