جستجوی مقالات مرتبط با کلیدواژه « Nonlinear time » در نشریات گروه « فنی و مهندسی »

The self-centering rocking steel braced frames are new type of seismic lateral-force resisting systems that are developed with aim to limiting structural damages, minimizing residual drifts on systems and creating easy and inexpensive reconstruction capability, after sever earthquakes. In Steel braced frames with controlled rocking system, column bases on seismic resisting frame are not attached to the foundation and the frame allowed to rock freely. The task of restoring the rotated frame to its initial location is on post-tensioned cables, which attaches top of the frame to foundation. The design of post tensioned stands and braced frame members is such that during earthquakes they remain in elastic region. Seismic energy, dissipates by plastic deformations in replaceable elements on each rock of frame. In current research work, the seismic behavior of this type of lateral resisting systems is evaluated. The research conducted on a one bay steel braced frame with controlled rocking system that is analyzed using nonlinear dynamic time history analysis (NLTHA) procedure. The frame is subjected to JMA-Kobe and Northridge ground motions records that are scaled to unit, 1.2 and 1.5 times of maximum considered earthquake (MCE) ground motion level intensity. Extracted results show that seismic behavior of this type of lateral force resisting systems are so desirable even under MCE ground motion levels. The only anxiety is about occurring fatigue in post-tensioned strands that endangers overall stability of system.

Performance-based design optimization (PBDO) is a relatively new concept in structural seismic design optimization. One of the PBDO methods which has been introduced in recent years is the optimization based on the uniform deformation theory. This method is quite different from other optimization techniques and formed based on the concept of structural performance and uniform distribution of deformation demands in the structure subjected to the seismic excitation. The aim of this method is to assign specific sections to elements such that all of the elements can reach their allowable deformation capacity during the earthquake. According to this theory، inefficient material is gradually shifted from the strong to weak areas leads to a uniform deformation (ductility) state at the end of a repetitive process. Although the base of this theory and proposed algorithm is to attain a uniform state of deformation in the whole structure، but the allowable limit of deformation values defined in PBD codes is not constant for all of structural elements. Additionally، in these codes، some actions of structural elements may be controlled by deformation and some controlled by force. Therefore، by considering the acceptance criteria of PBD codes، it is not possible to reach a uniform deformation state in the whole structure. Hence، in this paper uniform distribution of demand capacity ratio (DCR) is considered instead of uniform state of deformation. Historical review of applying this methodology shows that researchers mostly have used it to the optimum design of the structures under the earthquake records separately. Since earthquakes are random by nature، it is unlikely that the same earthquake ground motion will be repeated at some future time. This reveals that design based only one earthquake is insufficient and it is necessary to consider several earthquakes in checking the dynamic responses of a building. This paper presents an algorithm to PBDO of steel moment frames under set of ground motion records using the basic concepts of the uniform deformation theory. The proposed method consists of two phases. In the first phase of the search، to enhance the convergence rate، the search space of design variables is assumed to be continuous. Additionally in this phase of the search، only the deformation-controlled elements may vary. In the Second phase of the search، first for each structural element groups، the nearest discrete section to the imaginary section achieved in the first phase is identified and selected and then the structure is analyzed again and the DCRs are controlled. In this phase، acceptance criteria for both deformation and forced-controlled elements are supposed to be satisfied. Efficiency of the proposed algorithm is demonstrated in the optimum design of two baseline steel moment frames under a set of ground motion records. Results indicate that the proposed algorithm has a high speed to reach the optimum solution. The results are also compared with the optimum designs obtained by pushover analysis. It is shown that the optimization based on the pushover analysis results higher frame weight than time history analysis.

Due to both topography and economical conditions، the construction of skewed bridges is being more popular all around the world and thereby their design become of paramount importance among structural engineers. The most vulnerable bridge type in a transportation network is skew bridge. Because of its geometry، the response of the skew bridge during earthquake is more severe than for a straight bridge with identical mass and dimensions. The skew bridges have coupled response in global co-ordinate system and the seismic response increases with the increase in skew angle. Skewed bridges are often encountered in highway design when the geometry cannot accommodate straight bridges. These bridges are found to be quite susceptible to severe damage during past earthquakes. When the skew angle gets large it can significantly alter the response of the bridge. Investigative reports on the 1989 Loma Prieta and 1994 Northridge earthquakes in California indicated significant damage experienced by skewed highway bridges. In both earthquakes، less than five percent of the bridges exposed to ground shaking were damaged. Specifically، the damage sustained by the Pico-Lyons Bridge near Newhall، California، was noted to have been triggered by the skewed geometry of the bridge. The study showed that cases in which the angle of skew is approximately 40°، the percentage increase in stress due to the skewness effect at the end girders can be as high as 50–60%. Although the static and dynamic behavior of skew bridges has been investigated by various researchers، some phenomena are yet to be defined. This study focuses on evaluating the seismic behavior of typical single-span skewed concrete precast girder bridges by taking into account the variation of deck geometrical characteristic including length، width and the bridge skew angle. The main objective is to investigate unseating of the bridges subjected to seven sets of ground motions. The three-dimensional nonlinear finite element models of bridges using the CSI’s most renowned software، i. e. SAP2000 were developed and nonlinear time-history analyses were performed. The results indicate that by increasing the skew angle the unseating occurs due to combination of displacements in longitudinal and transverse directions، whereas for non-skewed bridges or the bridges with a small skew angle، the deck doesn’t experience significant rotation or permanent transverse displacement and only longitudinal displacement causes the deck to become unseated. It was found that for 60-degree skew bridges despite the fact that the bridges experience less longitudinal and transverse displacements، the superstructure undergoes significant rotations about the vertical axis and is permanently displaced laterally from the original location. Thus، by increasing skew angle the higher magnitude of deck rotation causes the deck to become unseated rather than the displacements and the deck rotation built up during the initial peak cycles of shaking of all input ground motions.