Journal of Rehabilitation in Civil Engineering
Volume:7 Issue: 4, Autumn 2019

Reinforced concrete structures need to be strengthened and retrofitted for various reasons, including errors during design and/or construction, so in most cases strengthening of structural elements is much more economical than rebuilding the structure. Using HPFRCC with tensile stiffening behavior has been developed to strengthen the concrete structures over the recent few years. In this paper, the usage of HPFRCC for strengthening two-way reinforced concrete slabs has been studied. A total of five two-way slabs were constructed and tested to reach their own collapse stage, one of specimen was as non-strengthened control slab, and the others were strengthened in various forms. The strengthening was carried out in two ways; by installing precast plate in the tensile area and the other by installing precast plate in both tensile and compression area at two different percentages of the fiber. The bending behavior, cracking, yielding and rupture of the experimental specimens were evaluated. The results indicated that the installation of HPFRCC pre-fabricated laminates significantly improved the bending performance of reinforced slabs, so that the ductility, energy absorption value, cracking strength, and initial hardness of the slabs was increased and the crack width was decreased. Therefore, the proposed precast HPFRCC sheets can be used to strengthening the deficient slabs.

Approximately, 50 to 70 percent of hydration products in hydrated cement paste are polymorphisms of C-S-H gel. It is highly influential in the final properties of hardened cement paste. Distinguishing C-S-H nano-structure significantly leads to determine its macro scale ensemble properties. This paper is dealt with nano-scale modeling. To achieve this, the most important C-S-H compounds, with a vast range ratios of Ca/Si from 0.5 to 3 were chosen and used in different simulations. These materials included tobermorite 9Å, tobermorite 11Å, tobermorite 14Å, clinotobermorite, jennite, afwillite, okenite, jaffeite, foshagite, and wollastonite. Furthermore, the molecular dynamics method was used to estimate important mechanical properties such as bulk modulus, shear modulus, Young's modulus and poisson ratio. Five different force fields (COMPASS, COMPASS II, ClayFF, INTERFACE and Universal) were used and compared with each other to be able to measure the mechanical properties of these compounds. Lastly, the properties of two types of C-S-H with high and low density were determined by using Mori-Tanaka method. The main aim of this paper is to distinguish the most similar natural C-S-H material to C-S-H from cement hydration and finding appropriate force filed.

Progressive collapse is a condition where local failure of a primary structural component leads to the collapse of neighboring members and the whole structure, consequently. In this paper, the progressive collapse potential of seismically designed steel dual systems with buckling restrained braces is investigated using the alternate path method, and their performances are compared with those of the conventional intermediate moment resisting frames. Static nonlinear Push-down and dynamic analyses under gravity loads specified in GSA guideline are conducted to capture the progressive collapse response of the structures due to column and adjacent BRBs removal, and their ability of absorbing the destructive effects of member loss is investigated. It was observed that, compared with the intermediate moment resisting frames, generally the dual systems with buckling restrained braces provided appropriate alternative path for redistributing the generated loads caused by member loss and the results varied more significantly depending on the variables such as location of column loss, or number of stories. Moreover, in the most column removal scenarios, steel dual systems are more capable to resist the progressive collapse loads and maintain the structural overall integrity.

Recent investigations have shown that the influences of Soil-Structure Interaction (SSI) may be detrimental to the seismic response of structure, and hence neglecting this phenomenon in analysis and design may lead to an un-conservative design. The objective of this paper is to quantify the effects of nonlinear soil-structure interaction on the seismic response of a low-rise special moment frame subjected to a family of ground motions with three hazard levels. To this end, seismic behavior of a 5-story special steel frame founded on linear and nonlinear flexible-base foundations are compared to the conventional fixed-base frame counterpart. The well-known Beam-on-nonlinear-Winkler-foundation approach is utilized to model nonlinear soil-shallow foundation. Nonlinear static and time history dynamic analyses have been conducted using the OPENSEES platform to examine the effect of modeling and ground motion parameters on their seismic performance. The results indicate some degrees of inaccuracy in the fixed-base and linear SSI assumptions when compared to its nonlinear flexible-base counterpart. It is also observed that disregarding the foundation flexibility effect may lead to over prediction of the force, drift and ductility demands of the low-rise steel structure.

This paper presents the effect of geogrid tensile strength by calculating pullout resistance and geogrid-soil interaction mechanism. In order to investigate this interface, a series of pullout tests have been conducted by large scale reformed direct shear test apparatus in both cohesive and granular soils. In numerical, finite difference software FLAC3D has been carried out on experimental tests and the results are compared with findings from laboratory tests and to complete investigation results. The results show tensile strength of geogrids has an important role in interface behavior. Effect of soil type also is discussed. The obtained results show that geogrids with low tensile strength have higher pullout resistance in low normal stress on the surface, this effect reversed as the normal applied stress is increased. Numerical analysis only estimates the pullout strength with good agreement in high normal stress. Obviously, in high normal stresses soils, compacted and passive resistance in front of transverse elements increases. Results also indicate that the failure mechanism of gear pullout is same as foundation bearing capacity, in clays punching shear failure occur on geogrid-soil interaction resistance and sands show hardening behavior. Furthermore, the effective particle size of soil is found to be a little more than the thickness of gear by comparing two sands with different grain size.

Enhancement of the response of reinforced concrete (RC) beams using fiber-reinforced polymer (FRP) reinforcement bars has become a popular structural technique over the past two decades due to the well-known advantages of FRP composites including their high strength-to-weight ratio and excellent corrosion resistance. Thisstudy presents numerical investigation of 20 concrete beams internally reinforced with GFRP bars without web reinforcement. The accuracy of the non-linear finite element model in ABAQUS software is first validated against experimental data from the literature. The study presents an investigation into the behaviour of FRP reinforced concrete beams including the evaluation of geometrical properties effects. In particular, the study is focused on the effects of span/depth ratio, the reinforcement ratio and the effective depth of the beam, aiming to correct deficiencies in this area in existing knowledge. It was found that the finite element model is capable of accurately simulating the flexural behaviour of FRP reinforced beams. It was able to predict, with high accuracy, the force-displacement response the beam. Results showed that FRP reinforcement is a good solution to enhance the ductility of RC beam members. Moreover, although that increasing in the span/depth ratio of the beam decreases beam’s rigidity, but, it also postpones the yielding point in the beam’s flexural response and leads to a higher level of displacement ductility for the beam.

Due to the fact that the buckling-restrained brace core yields subject to both tension and compression, it can adsorb energy and exhibit high ductility rendering it proper for tolerating earthquake loads. One of the important objectives of seismic standards is safeguarding the appropriate ductility for the structures because the structures, in case of being ductile, can dissipate a considerable amount of earthquake energy. According to the importance of the issue, the present study makes use of cumulative ductility parameter as a scale that is practically applied for describing the plasticity demand of the buckling restrained brace (BRB) member to investigate the cyclic behavior of the braces and buckling restrained braced frames (BRBF). To do so, nonlinear time history analysis was run on three steel buckling restrained braced frames in three different height rates, namely 5-storey, 10-storey and 15-storey, subject to seven earthquake records in OpenSees Software. Using the results of the analysis, hysteretic curves were delineated for the storeys and cumulative ductility demand and hysteresis energy parameters were calculated for each of the obtained curves. The results indicated that the cumulative ductility demand distributions of the storeys of the buckling restrained braced frames, designed corresponding to AISC360 guidelines are not identical and that higher ductility demands were scored for the upper storey. The storeys with more cumulative ductility demand should be considered in the design of a larger brace cross-section, although resistance does not require a larger brace cross-section.

Buckling restrained braces (BRBs) have a similar behavior under compression and tension loadings. Therefore, they can be applied as a favorable lateral load resisting system for structures. In the performance-based earthquake engineering (PBEE) framework, an intermediate variable called intensity measure (IM) links the seismic hazard analysis with the structural response analyses. An optimal IM has desirable features including efficiency, sufficiency and predictability. In this research, the efficiency and sufficiency of some traditional, cumulative-based, and advanced scalar IMs to predict maximum interstory drift ratio (MIDR) demand on low- to mid-rise steel structures with BRBs, under near-fault ground motion records having forward directivity, are investigated. The results show that most of the IMs considered are not sufficient with respect to source-to-site distance (R), for predicting MIDR. It is also shown that decreasing the strain hardening ratio decreases the efficiency of the IMs. In addition, IMM(λ=0.5) and Saavg are more efficient and also sufficient with respect to pulse period (Tp), for predicting MIDR demand on the low-rise steel BRB frames under near-fault ground motions, when compared with the other IMs. In the case of mid-rise structures, PGV and IMM(λ=0.33) are selected as optimal IMs. Due to the higher efficiency and sufficiency of the selected optimal IMs, the obtained fragility curves calculated using these IMs, are more reliable in comparison with the fragility curves calculated using other IMs.

Enhancement of strength and ductility is the main reason for the extensive use of FRP (fiber reinforced polymer) jackets to provide external confinement to reinforced concrete columns especially in seismic areas. Therefore, numerous researches have been carried out in order to provide a better description of the behavior of FRP confined concrete for practical design purposes. This study presents a new approach to obtain strength enhancement of FRP confined rectangular concrete columns by applying artificial neural networks (ANNs). The proposed ANN model is based on experimental results collected from literature. The results of training, validation and testing sets of the model are compared with experimental results. All of the results show that ANN model is fairly promising approach for the prediction of compressive strength of FRP confined rectangular concrete columns. The performance of the ANN model is also compared with different proposed formulas available in the literature. It was found that the ANN model provides the most accurate results in calculating the compressive strength of FRP confined rectangular concrete columns among existing compressive strength formulas. Finally, a sensitivity analysis using Garson’s algorithm has been also developed to determine the importance of each input parameters.

In this research in-plane shear Behavior of composite steel-concrete shear walls was investigated considering the Effects of steel plates thickness, spaces between shear studs, shape and type of shear studs and existing of minimum reinforcement in wall section. Several finite element models were analyzed and Numerical results of two models were taken under verification with existent experimental results.numerical results coincide experimental result with an acceptable accuracy.numerical models analysis results show that increasing steel plate thickness, yield shear strength and ultimate shear strength become higher. Increasing shear stud spaces causes shear resistance decrease to some extent. Models have iron angles in compare with models have shear studs of the same stud spaces, have higher yield shear resistance and ultimate shear resistance. Angles connection way to steel plates are effective in increasing or decreasing ultimate shear strength.Wall with minimum reinforcement in comparison with the wall with no rebar has greater ductility and shear strength.

Although several studies have investigated the effect of degradation on the behavior of structures, investigations on collapse margin ratios are rare in the literature. In this study, the effect of strength and stiffness degradation on collapse capacity of steel moment frames is investigated. The aim is to determine margin of safety against collapse using a probabilistic approach. For this reason, 14 moment frames are designed including 4 long period and 3 short period models with 5 and 8m bay length. These buildings are representative of common office and residential buildings built in cities. Buildings are designed according to ASCE7-05 specifications. In the first stage, effective seismic parameters are calculated using a pushover analysis. In the second stage, collapse performance levels are determined using incremental dynamic analysis by considering seismic excitation uncertainties. Results show that the overstrength factor that is recommended by ASCE code is not always conservative. Overall, structures designed with common building codes show acceptable margin of safety against collapse.

Seismic rehabilitation provides existing buildings with more resistance to seismic activity, ground motion, or geotechnical failure due to earthquakes. Performance-based rehabilitation is a general concept through which the retrofitting criteria are defined regarding to performance objectives when the structural and nonstructural members are subject to different levels of earthquake hazards. In this study several moment resistant steel frames with different numbers of stories were initially designed as vulnerable models. The models were retrofitted based on the current seismic rehabilitation standards and codes criteria. Three models of shear walls were used to retrofitting the vulnerable structures. In the first model, the wall surrounds column perimeter as boundary elements. In the second model, wall is connected to the column and in the 3rd model, wall is placed with a small gap from the column, and there is no contact between them. The nonlinear behavior of buildings is evaluated using adaptive modal pushover and incremental dynamic analysis before and after rehabilitation.