Abstract

This paper presents an optimal design of three dimensional multi-story reinforced concrete structures using recently developed meta-heuristic algorithms, namely; the charged system search and the enhanced charged system search. The design is based on the ACI 318-05 code and loadings are based on ASCE7-05. Analysis of the structures is performed by the standard stiffness method. All members are subjected to biaxial moments and axial loads. Pre-determined sections are assumed for beams and columns, and the corresponding interaction curves are utilized to check whether the selected section for each member is acceptable. The objective function is taken as the weight of the structure, and constraints consist of the slenderness of compression members, the maximum allowable drift of the structure and the natural frequency of the structure. It should be mentioned that second order effects are also considered and that the end moments of the columns are magnified when needed. First, a 7-story frame with 3 spans is considered and optimized. Then, a sensitivity analysis is performed by optimal design of nine frames having 3 stories and 2 spans. In each story, different span lengths and loading conditions are assumed, and the results are compared.

Abstract

Deployment and retrieval of drilling risers are among the most highly stressed marine operations processes, and allowable imperfections or missed-inspection cracks are likely to expand into limited defects, making crack identification essential during the deployment and retrieval of risers. This study’s main objective is to investigate the performances of natural frequencies and modal shapes on crack identification in a drilling riser during installation and retrieval. First, a computation code is programmed to extract the natural frequencies and modal shapes of the intact and cracked risers, and its validity is verified by two numerical methods, i.e., precise integration method and differential transformation method in the literature. Second, the crack-identification ability of natural frequencies versus crack depth and riser-suspension length is studied in detail. An approach using the normalized fourth derivative of modal shapes is developed for crack identification compared with the traditional modal-curvature and modal-curvature difference approaches. Finally, the cracks near the middle span and both ends of the risers are identified, respectively, during the deployment. From the research on crack identification during installation, several conclusions and suggestions are drawn to provide some valuable references for the safety of drilling and production operations.

Abstract

The analytical and experimental seismic response of La Mission Bridge, and the site sub-surface characterization on its neighborhood is presented in this paper. SAP 2000 (Structural Analysis Program) was used to model and characterize numerically the expected response of the bridge in terms of their vibration natural frequencies and maximum displacement. It was also generated the scenario of the bridge response upon strong motion. In both cases, ambient vibration measurements were used for the experimental studies. The stiffness matrix method was used for the analytical study (forward modeling) of wave propagation in layered media. The relevant characteristics of the results are next succinctly described: For the bridge: the theoretical fundamental frequency of vibration was 3.10 Hz., and the estimated experimental fundamental vibration frequency was 3.30 Hz. For the site characterization, our results are described starting at the north end of the bridge going through its south end: (i) The site S1_2, which is the site where the soil was artificially compacted, has a dominant frequency of 2.5 Hz and an average shear wave velocity of 195 m/s. (ii) For sites S3_1 and S2_4, the dominant frequency was 1.5 Hz and the average shear wave velocity was 115 m/s and 123 m/s, respectively. (iii) Site S2_2, the dominant frequency was 3.5 Hz and an average shear wave velocity 275 m/s. (iv) Site S2_5, in this site two clear peaks at frequencies of 1.5 and 3.75 Hz were observed, regarding to the average shear wave velocity it was 128 m/s. (v) Site S2_3, the dominant frequency was 4.0 Hz and the average shear wave velocity was 346 m/s. All the experimental sites were in soils at natural conditions, but the site S1_2, which was artificially compacted. The soil characteristics along the 163 m length of the bridge, clearly shows a rigidity change of the soil sub-surface conditions. That condition is an indication that different amplifications of the ground response will probably be experienced, being in the range of the 4.0 Hz at the south-end side of the bridge, and 1.5 Hz on the portion of the north-end of the bridge. In no case the fundamental vibration frequency of the bridge matches the fundamental vibration frequency of the ground surface. However, they are somehow no too far apart each other.

Abstract

Elastic deformation and dynamics response of the linear structures due to fluid loads are studied to understand the Fluid Structure Interaction (FSI). A modal coupling solver is developed by solving dynamic equation of motion with external loads, using the mode superposition method with the help of relevant mode shapes and natural frequencies associated with the structure. Natural frequencies and mode shapes have been pre-calculated and provided as input for the simulation. Modal coupling is integrated into the Lagrangian Differencing Dynamics (LDD) method, utilizes finite differences within the framework of Lagrangian context, and strong and implicit formulation of Navier Stokes equations to model the incompressible free-surface fluid. Elastic deformation of the structure due to fluid force obtained from the flow solver is calculated in the modal coupling algorithm using direct numerical integration. Then the elastic deformation is imposed in the flow solver to account for change of the geometry and obtain new flow pressure and velocity fields. The two-way coupling of fluid and structure is successfully validated by simulating dam-break through an elastic gate. Since the LDD method works directly on surface meshes, the simulation is quickly setup and direct coupling of structural deformation eliminated the usual step of mapping of fluid results on the structural mesh and vice-versa