Braced frames, besides other structural systems, such as moment resisting frames or shear walls, have been an effective and valuable method to enhance structures against lateral loads. In wind or seismic excitations, inclined elements react as truss web elements which would bear compression or tension stresses. This axial reaction results in less moments and therefore smaller sizes in beam and column sections with respect to members in similar moment resisting frame. However, low tensile strength of concrete material, made it a challenge to use in inclined members. In practice, there have been various methods to consider this defect such as disengagement of brace elements in tension or utilization of prestressed braces.The purpose of this article is to study the nonlinear response of reinforced concrete frames which contain reinforced concrete braces as the major structural elements against earthquake loads. The advantages of nonlinear behavior in reinforced concrete elements and their adequate energy absorption in cyclic loading are taken into account. Also stiffness and strength degradation of structural members under cyclic loading is considered.Two different braced frames with K and X braces are analyzed numerically for four, eight and twelve story buildings. This study focuses on evaluation of strength, stiffness, ductility and energy absorption of reinforced concrete braced frames and comparison with similar moment resisting frames and frames with shear wall. Results are plotted in diagrams and discussed extensively. According to this study it is concluded that besides effective lateral stiffness rising in reinforced concrete braced frames, there is a considerable amount of energy dissipation during earthquake loading.

Due to several reasons as the low resistance of constructed concrete and also change in codes or application of structures, some concrete frames need to be retrofitted. By adding the steel prop and curb to the reinforced concrete, many parameters are changed such as ductility, resistance, and stiffness. This study investigates numerically the impact of adding the prop and curb, slit damper, gusset plate and also prop with a ductile ring on stiffness, resistance, energy dissipation and ductility of RC frames. For this purpose, the effect of the aforementioned methods on the linear and nonlinear moment frame behavior of reinforced concrete under monotonic loads have been numerically investigated using the ABAQUS software. In the present study 12 samples of reinforced frames with one story and one span retrofitted by different methods. The novelty of the paper was using such props and slit damper in RC frames. The results obtained from the modeling showed the retrofitted frame with a ring, slit damper and gusset plate also showed a better behavior in terms of resistance and stiffness compared to the RC frame and the sample with slit damper and prop with a ductile ring as well as compared to the sample with the prop and curb showed more ductility and energy dissipation.

Directly-connected internal steel bracing of RC frames has received some attention in recent years, both as a retrofitting measure to increase the shear capacity of the existing RC buildings and as a shear resisting element in the seismic design of new buildings. Although its successful use to upgrade the lateral load capacity of existing Reinforced Concrete (RC) frames has been the subject of a number of studies, guidelines for its use in newly constructed RC frames need to be further developed. An important consideration in the design of steel-braced RC frames is the level of interaction between the strength capacities of the RC frame and the bracing system. In this paper, results of experimental investigations aimed at evaluating the seismic response of brace-frame system and the level of interaction between the bracing system and the RC frame are discussed. For these investigations, cyclic loading tests are conducted on scaled moment resisting frames with and without bracing. Test results confirm the ability of the bracing system to enhance the strength capacity of the RC frame while maintaining adequate ductility. They also provide an insight into the causes and the levels of interaction between the strength capacities of the bracing system and the RC frame.

This paper reviews the results of some recent works conducted by the author on new methods of retrofitting the RC frames. On the local retrofit of RC members, it includes the work on the application of a new high performance fibre-reinforced cementitious composite material. The composite can be applied either as a wet mix to the desired thickness or attached as precast sheets or strips to the face of the member using a suitable epoxy adhesive. The suitability of this technique of member retrofit to enhance the strength and ductility of the retrofitted member compared with other methods of local retrofit, such as steel plates and FRP laminates, is discussed. Other works reviewed in this paper include those carried out recently on global retrofit of RC frames using direct internal steel bracing. Results of inelastic pushover tests on scaled models of ductile RC frames, directly braced by steel X and knee braces are presented which indicate that such bracings can increase the yield and strength capacities and reduce the global displacements of the frames to the desired levels. Also, the results of direct tensile tests on three full scale models of three different types of brace/RC frame connections are presented. Finally, the values of seismic behaviour factor, R, for this type of brace/frame system, evaluated from inelastic pushover analysis of dual systems of different heights and configurations are presented.      

The shear capacity of masonry-infilled reinforced concrete (RC) frames is determined experimentally by testing of five 2/3 scale, one story, one bay infilled RC frame specimens with varying degrees of separation between upper and lower portions of the masonry infill panel. The main premise of this experimental study was to find the contribution of masonry panel in global resistance of infilled RC frames versus shear force. Based on the experimental results, a new method is proposed for the determination of the shear resistance and the contribution of the masonry infill panel is derived. Comparisons of experimental and analytical results show that the proposed method for evaluating the shear resistance of the masonry panels offers a promising approach for the design of infilled RC frames. Furthermore, the results of this study indicate that the shear resistance of masonry panel in infilled RC frames is strongly influenced by the stiffness and lateral resistance of the RC frame.

One of the methods for the seismic strengthening in structural engineering is using FRP composites. These composite has some advantages such as increase in ductility, stiffness and lateral strength, the ability to adapt with the architecture, and also the minimum weight added to the structure. Uncertainty in the structure is due to reasons such as the lack of prediction of additional loads over the lifetime of the structure, the inadequate knowledge of the mechanical properties of the materials, the existence of human errors and the simplifications in analytical relations for modeling, and makes reliability analysis of structural inevitable. The First-Order Reliability Method (FORM) and Monte Carlo Simulation (MCS) are the most common and accurate methods of reliability analysis. Structural reliability analysis leads to the construction of an acceptable safety grade structure. In this paper, an optimal pattern for reinforcing RC frame with FRP layers is presented using reliability analysis. Carbon fiber reinforced polymers (CFRP) are used to increase the shear strength of existing RC frame. The beams and columns are wrapped by the CFRP layers at the ends, and in the reinforcing patterns, the reinforced beams are assumed to be constant and the difference is in length of the reinforcement of the column. After verifying and ensuring the results of modeling, the seismic behavior of the 8-story RC frame was assessed by nonlinear time history analysis (NTHA) with finite element program OpenSees under three far-field records earthquake from fault TABAS, Borah Peak and Imperial-Valley. Four random variables represented the variation in compressive strength of concrete, yield strength of steel, live load, and elasticity modulus of CFRP materials are defined and the limit state function defined to perform reliability analysis based on the maximum drift ratio inter-story. The reliability analysis of RC frame under three earthquake records and five reinforcement patterns was first determined using the Importance Sampling Method (ISM), and then the accuracy of the method was measured using MCS. Based on the results of the reliability analysis, the optimal length value corresponding to the maximum value of the reliability index (,) for each earthquake record is determined. Survey results show that increasing the length of the reinforcement does not lead to an increase in the reliability index and even decreases with the inappropriate reinforcing length. The results of reliability analysis show that the number of layers of CFRP is not considered safe for Borah Peak record and requires more layers to reinforce. The optimum lengths of reinforcement in TABAS and Borah Peak earthquakes are 20% of the length of the column and in the Imperial-Valley record is 30% of the length of the column, while with a change of 5% of the length strengthening, the reliability index is significantly reduced. The most accurate method for analyzing reliability and calculating the probability of structural failure is MCS, but this method requires a large number of simulation samples to perform calculations. Which significantly reduces the number of simulation samples and the time to perform calculations by selecting the ISM method and the appropriate amount of random variables to begin the analysis.

In this paper, the seismic behavior of three intermediate moment-resisting concrete space frames with unsymmetrical plan in five, seven and ten stories are evaluated by using pushover analysis. In each of these frames, both projections of the structure beyond a reentrant corner are greater than 33 percent of the plan dimension of the structure in the given direction. The performance of these buildings has been investigated using the pushover analysis. Results have been compared with those obtained from non-linear dynamic analysis.