Scientia Iranica
Volume:16 Issue: 5, 2009

Earthquakes in megacities such as Tehran and Los Angeles pose huge risks that could jeopardize national prosperity and social welfare. Quantifying urban seismic risk is a dicult problem because it requires detailed knowledge of the natural and the built environments, as well as an understanding of both earthquake and human behaviors. Risk assessments can be improved through international collaborations that combine the expertise of earthquake scientists and engineers. The most e ective strategies are seismic safety engineering, enforced through stringent building codes and disaster preparations informed by realistic scenarios of large earthquake cascades. These strategies rely on the ability to forecast earthquakes and their e ects and to monitor earthquake cascades in near real time. The practical problems of risk reduction are, thus, coupled to the basic problems of earthquake system science: the interseismic dynamics of fault systems and the coseismic dynamics of fault rupture and groundmotion excitation. In the United States, the Southern California Earthquake Center (SCEC) coordinates an extensive research program in earthquake system science, which includes major e orts to improve time-dependent earthquake rupture forecasts through better understanding of earthquake predictability and to develop attenuation relationships that correctly model the physics of seismic wave propagation. Earthquake system science relies on the premise that detailed studies of fault systems in di erent regions can be synthesized into a generic understanding of earthquake phenomena. Achieving such a synthesis will depend on international partnerships that facilitate the development and comparison of well-calibrated regional models, and it will require the deployment of a cyberinfrastructure that can facilitate the creation and ow of information required to predict earthquake behavior. In the not-too-distant future, we will be able to incorporate much more physics into seismic hazard and risk analysis through physics-based, system-level simulations. Keywords: Seismic risk analysis; Risk assessment; Earthquake prediction; Seismic wave propagation.

The present study attempts to investigate di erent factors a ecting the resilient modulus of hot mix asphalt. So a fractional factorial analysis of experiment was carried out considering ve factors, each at two di erent levels. These factors were the maximum nominal aggregate size, specimen diameter and thickness, the load pulse form and duration. During the course of analysis, two types of hot mix asphalts with di erent maximum aggregate sizes were taken into consideration, while Marshall compaction method was used to prepare the specimens. Furthermore, measuring the resilient modulus, sinusoidal and triangular load pulse forms were applied. Finally, our investigation examined the di erent factors interrelations which a ect the resilient modulus. Analysis of the factorial experimental design showed that the maximum nominal aggregate size was the most important factor a ecting the resilient modulus, then the load duration, the specimen geometry (thickness and diameter), and nally the interactions between the di erent factors. Keyword: Resilient modulus; Specimen''s geometry; Load factors; Factorial experimental; Asphalt mixtures.

An FE model of a pedestrian lower legform impactor has been developed and certi ed, both statically and dynamically, based on EEVC-WG17 requirements. The legform is then utilized in a series of 40 km/hr pedestrian accident analyses to assess the protection level delivered by a typical sedan vehicle. The values of maximum tibia acceleration, maximum knee bending angle, and maximum knee shearing displacement have been extracted for 25 di erent accident con gurations and compared with their admissible ranges. It has been shown that tibia acceleration is mainly in uenced by extension of the area transferring the impact load between the legform and the vehicle. However, variations of vehicle front-end structure geometry and sti ness in the vertical direction have been found to be the most decisive parameters in the level of legform knee maximum bending rotation and maximum shearing displacement. Keywords: Pedestrian accident; Lower extremity injury; Bumper design

The Endurance Time (ET) method is a new dynamic pushover procedure in which structures are subjected to gradually intensifying acceleration functions and their performance is assessed based on the length of the time interval that they can satisfy required performance objectives. In this paper, the accuracy of the Endurance Time method in estimating average deformation demands of low and medium rise steel frames using ETA20f series of ET acceleration functions has been investigated. The precision of the ET method in predicting the response of steel frames in nonlinear analysis is investigated by considering a simple set of moment-resisting frames. An elastic-perfectly-plastic material model and a bilinear material model with a post-yield sti ness equal to 3% of the initial elastic sti ness have been considered. For frames with an elastic-perfectly-plastic material model, which are P 􀀀
sensitive cases, the ET analysis for the maximum interstory drift ratio somewhat underestimates the nonlinear response history analysis results. The di erence between the results of the ET analysis and the nonlinear response history analysis for the material model with 3% post-yield sti ness is acceptable. The consistency of the base shears obtained by the two methods is also satisfactory. It is shown that, although the results of the ET analysis are not exactly consistent with the results of ground motions analysis, the ET method can clearly identify the structure with a better performance even in the case of structures with a relatively complicated nonlinear behavior. Keywords: Nonlinear response history analysis; Dynamic pushover; Endurance time method; Performance-based seismic engineering.

An ecient methodology is proposed to optimize space trusses considering geometric nonlinearity. The optimization task is performed by a continuous Particle Swarm Optimization (PSO). Design variables are cross sectional areas of the trusses and their weights are also taken as the objective function. Design constraints are de ned to restrict nodal displacements and element stresses and prevent the overall elastic instability of the structures during the optimization procedure. In order to reduce the computational e ort of the optimization process, an Adaptive Neuro Fuzzy Inference System (ANFIS) is employed to approximate the nonlinear analysis of the structures instead of performing via a time consuming Finite Element Analysis (FEA). The presented ANFIS is compared with a Back Propagation Neural Network (BPNN) and appears to produce a better performance generality for evaluating structure design values. Test example results demonstrate the computational advantages of the suggested methodology for optimum design of geometrically nonlinear space trusses. Keywords: Space truss; Geometric nonlinearity; Particle swarm optimization; Approximation concepts; Adaptive neuro fuzzy inference system.

An analytical along with numerical analysis has been carried out to investigate the behavior of concrete simply supported beams strengthened with Fiber-Reinforced Polymer (FRP) unidirectional laminates under impact loading. Concrete beams are reinforced with a minimum ratio of exural and shear steel rebars and nally retro tted with epoxy-bonded high strength carbon FRP laminates in their exure surfaces. The impact force was applied with a solid steel cylinder drop weight. Analytical results showed that composite laminates externally bonded to reinforced concrete beam substrate can signi cantly enhance the performance of theses structural members to resist impact loadings. Also, based on the obtained results, in retro tted beams the crack propagation happens in a desirable mode; an increasing of yielded rebar zones and residual beam sti ness. Retro tted beams were sti er than unretro tted ones in their rst impact response. The residual sti ness of impacted concrete beams depends on their initial sti ness and the impact energy level. The analytical method uses an idealized elastic spring-mass model and exural wave propagation theory to calculate the dynamic response of assumed beams. Analytical responses are adjusted due to an inelastic response, and nally are used for the simpli ed designation of impact resisting reinforced concrete beams retro tting laminates. Keywords: Impact loading; Reinforced concrete beam; Composite laminate; Retro tting; FEM.