INTERNATIONAL JOURNAL OF CIVIL ENGINEERING
Year:2011 | Volume:9 | Issue:3|Papers Count:11

The objective of this paper is to determine the drift demand hazard curves of steel moment-resisting frames with different number of stories in territory of Tehran, this is done through the combination of the results obtained from probabilistic seismic hazard analysis and the demand estimated through the best probabilistic seismic demand models. To select the best demand model, in this paper, a Bayesian regression has been used for the statistical analysis of the results obtained from incremental dynamic analysis in order to estimate the unknown parameters of model and to select the best Intensity Measure (IM) parameter, also the probability of overall collapse of structures has been computed. Considering the efficiency and sufficiency of the models, the results indicate that the accuracy of models with one single IM is a function of the number of stories, consequently the current widely used model with spectral acceleration in first period as IM is not suitable for all structural heights. Furthermore, regarding the fact that it is difficult to prepare a seismic hazard curve for a combined IM, it seems that the best model can be found among models with two single IMs. In other words, the best model to cover all structural heights is the one with linear combination of spectral acceleration of the first and the second period. Furthermore, using different models to calculate the curves shows that regardless of the number of IMs, estimated demands strongly depend on the standard deviation of model.

An energy based damage index based on a new nonlinear Finite element (FE) approach applicable to RC structures subjected to cyclic, earthquake or monotonic loading is proposed. The proposed method is based on the evaluation of nonlinear local degradation of materials and taking into account of the pseudo-plastic hinge produced in the critical sections of the structure. A computer program is developed, considering local behavior of confined and unconfined concretes and steel elements and also global behavior and damage of reinforced concrete structures under cyclic loading. The segments located between the pseudo plastic hinges at critical sections and the inflection points are selected as base-models through simulation by the proposed FE method. The proposed damage index is based on an energy analysis method considering the primary half-cycles energy absorbed by the structure during loading. The total primary half-cycles absorbed energy to failure is used as normalizing factor. By using the proposed nonlinear analytical approach, the structure's force-displacement data are determined. The damage index is then calculated and is compared with the allowable value. This damage index is an efficient means for deciding whether to repair or demolish structures after an earthquake. It is also useful in the design of new structures as a design parameter for an acceptable limit of damage defined by building codes. The proposed approach and damage index are validated by results of tests carried out on reinforced concrete columns subjected to cyclic biaxial bending with axial force.

An evolutionary structural optimization (ESO) method is used for plastic design of frames. Based on safe theorems some criteria are derived and made an effort to satisfy them during the optimization process. In this regard, equilibrium is checked and yield condition is gradually satisfied during the optimization process. In this method, the amount of used material and the stiffness for each element are improved, simultaneously, to impose upper bound of moment in the element. Frame analysis and optimization algorithm are implemented as PLADOF (PLAstic Design of Frames) computer code. Four examples are presented to illustrate the performance of the algorithm.

In this research, a novel numerical algorithm is introduced for computation of temperature-induced before crack steady strain stress field in plane-stress problem. For this purpose, two dimensional heat transfer equation and force equilibrium equations are sequentially solved using Galerkin Finite Volume method on identical unstructured triangular meshes when proper convergence for each field is achieved. In this model, a proper thermal boundary condition that is suitable for unstructured triangular meshes is introduced for analysis. Two test cases are used to assess accuracy of thermal and structural modules of the developed solver and the computed results are compared with theirs analytical solution. First, thermal analysis is performed for a rectangular plate which is connected to a supporting body with constant temperature and expose to surrounding liquid at three edges. Second, structural analysis is performed for a plate under distributed loads in two directions. Having obtained acceptable results from thermal and structural modules, thermal stress analysis is performed for a plate with fixed-end condition at one of edges, due to a uniform temperature field and the computational principle stress contours are compared with the Finite Element method results which have been reported in the literatures.

Corrosion of reinforcement due to frequently applied deicing salts is the major source of deterioration of concrete bridge decks, e.g. severe cracking and spalling of the concrete cover. Since crack width is easily recordable in routine visual inspections there is a motivation to use it as an appropriate indicator of condition of RC bridge elements in decision making process of bridge management. While few existing research in literature dealing with spatial variation of corrosion-induced cracking of RC structures is based on empirical models, in this paper the extent and likelihood of severe cracking of a hypothetical bridge deck during its lifetime is calculated based on a recently proposed analytical model for corrosion-induced crack width. Random field theory has been utilized to account for spatial variations of surface chloride concentration, as environmental parameter, and concrete compressive strength and cover depth as design parameters. This analysis enables to track evolution of cracking process, spatially and temporally, and predict the time for the first repair of bridge deck based on acceptable extent of cracked area. Furthermore based on a sensitivity analysis it is concluded that increasing cover depth has a very promising effect in delaying corrosion phenomenon and extension of the service life of bridge decks.

This article presents the application of two algorithms: heuristic big bang-big crunch (HBB-BC) and a heuristic particle swarm ant colony optimization (HPSACO) to discrete optimization of reinforced concrete planar frames subject to combinations of gravity and lateral loads based on ACI 318-08 code. The objective function is the total cost of the frame which includes the cost of concrete, formwork and reinforcing steel for all members of the frame. The heuristic big bang-big crunch (HBB-BC) is based on BB-BC and a harmony search (HS) scheme to deal with the variable constraints. The HPSACO algorithm is a combination of particle swarm with passive congregation (PSOPC), ant colony optimization (ACO), and harmony search scheme (HS) algorithms. In this paper, by using the capacity of BB-BC in ACO stage of HPSACO, its performance is improved. Some design examples are tested using these methods and the results are compared.

An experimental-analytical investigation was conducted to study the behavior of high-strength RC deep beams, a total of sixteen reinforced concrete deep beams with compressive strength in range of 59 MPa£f'c£65 MPa were tested under two-point top loading. The shear span-to-effective depth ratio a/d was 1.10, all the specimens were simply supported and reinforced by vertical, horizontal and orthogonal steel bars in various arrangements. The test specimens were composed of five series based on their arrangement of shear reinforcing. The general behavior of tested beams was investigated. Observations were made on mid-span and loading point deflections, cracks form, failure modes and shear strengths. The test results indicated that both vertical and horizontal web reinforcement are efficient in shear capacity of deep beams, also the orthogonal shear reinforcement was the most efficient when placed perpendicular to major axis of diagonal crack. Concentrating of shear reinforcement within middle region of shear span can improve the ultimate shear strength of deep beam. The test results were then compared with the predicted ultimate strengths using the ACI 318-08 provisions, ACI code tended to either unsafe or scattered results. The performed investigations deduced that the ACI code provisions need to be revised.

Self Compacting Concrete (SCC) specimens with limestone (L) and quartz (Q) powders were formulated. The influence of the type of the powder on the properties of fresh and hardened concrete was evaluated. Dense packing theories were used for mix design of samples. The equation of Fuller and Thompson for particle size distribution (PSD) of aggregates was modified with considering fine particles and a proper PSD curve was obtained for SCC. Experimental results showed that this method needs use of less powder content and results in higher strength/cement ratio compared to traditional mixing methods. No significant difference was observed between the compressive strengths of specimens containing limestone (L-specimens) and quartz (Q-specimens) powders, with similar proportions of materials. The residual compressive strength of specimens was examined at 500oC and contradictory behaviors were observed. One Q-specimen suffered from explosive spalling, while no spalling was occurred for L-specimens. On the other hand, the residual strength of remained Q-specimens showed considerable increase compared to L-specimens. The results show the necessity for more detailed investigations considering different effective parameters.

In this study compressive strength of carbon nanotube (CNT)/cement composite is computed by analytical method. For this purpose representative elementary volume (REV) as an indicator element of composite is chosen and analyzed by elasticity relationships and Von mises' criterion applied to it. It is assumed that carbon nanotubes are distributed uniformly in the cement and there is perfect bonding in the interface of cement and nanotube. At first for simplicity of computations, carbon nanotubes (CNTs) are assumed to have unidirectional orientation in the cement matrix. In following, the relations are generalized to consider random distribution of nanotubes in cement, and a new factor suggested for random orientation of fibers in the CNT/cement composite. The results of analytical method are compared with experimental results.

Considering normal concrete (NC) the type of concrete need to be vibrated after placing in the formwork, Lightweight concretes have been successfully applied in the building constructions for decades because of their low specific weight in connection with a high strength, a high capacity of thermal insulation and a high durability. The development leading to a self compacting light weight concrete (SCLWC) represents an important innovative step in the recent years. This concrete combines the favorable properties of a lightweight concrete with those of a self compacting concrete (i.e., the type of concrete need no vibration after placing in the formwork). Research work is aimed on development of (SCLWC) with the use of light weight aggregates "Light expand clay aggregate (Leca)". In this investigation, by trial and error procedure, different mix design of SCLWC were caste and tested to reach a so called standard self compacting concrete in fresh matrix phase such as, values of slump flow, L-box, V-funnel and in hardened phase, the 28 day compressive strength. Based on the results obtained, for two best so-called standard mix design of SCLWC the stress-strain diagrams are drawn and discussed. Also by three different methods, the modulus of elasticity of SCLWC are obtained and discussed here. It was found that a brittle mode of failure is governed in SCLWC.

In recent years, different damage indexes have been introduced in engineering literature. The most prominent one among other counterparts is the 1985 Park and Ang's damage index (DIPA), which demonstrates well calibration against experimental results. Hence, it has traditionally had broad application in the field of structural engineering. Commonly, in DIPA relevant parameters are assessed based on plastic-hinge approach, which is not well suited to consider the coupled response between stress resultants (axial force and flexural moment) especially in grossly nonlinear domain. The reason is that named approach is utilized constant shape plastic moment-curvature curve, which is not capable of varying the shape throughout loading history. Another drawback of plastic-hinge method is the difficulty of representing precisely partial yielding of the cross-section. To remedy the situation, the fiber discretization technique is used in this paper. Based on the fiber discretization strategy, not only have the stiffness and strength degradation been characterized more accurately, but also the distribution of plasticity along the plastic zone has been considered. Besides, the multi-directional effect of axial force and flexural moment is considered to assess DI parameters. Additionally, this strategy directly incorporates the effect of transverse confinement into cross sectional constitutive behaviour.