Modal-spectral analysis of a gas storage tank.

Transcription

1 Modal-spectral analysis of a gas storage tank.

2 Description of the structure: A Spherical tank gas storage is formed by: 1.- S-355 steel sphere plate elements of 1.5 cm thickness. 2.- Eight vertical posts using beam" elements made of steel S-275, with an outside diameter of 30 cm and a thickness of 8 cm. 3.- Beam tank ring using "beam" elements and a reinforced concrete square section of 50 x 50 cm St. Andrew crosses using "truss" elements with a diameter of 10 cm m x 1 m concrete foundation using a shell element. 6.- Well graded soil using a "solid" element.

3 Description of the structure: Solid Beam Shell Truss The sphere has an average radius of 5.0 m and a total height of 15 m.

4 Loads and boundary conditions of the structure: Self weight (g = 9,81 m/sg 2 ) Internal radial pressure in the tank = 0.5 MPa. The structure is simply supported at the base along the "x" axis. Earthquake (EC-8 EN-1998/2004.): Elastic Spectrum Type 1 Land type A. Horizontal seismic acceleration is 1.95 m / s2. We perform a prestressed modal-analysis, preloading the structure with a gas load before applying the earthquake load. SRSS modal combination is used. The results for the following combination are obtained: (1.35 * Service weight) + (1.5 * Pressure) + (1.5 * Quake). The foundation slab is affected by a "touching" contact with μ = 0.6. Ground displacements are constrained at the sides of the solid element.

5 Mesh: Gas storage tank. _ Local mesh controls: CivilFEM facilitates the meshing process by using local mesh controls that allow choosing auxiliary curves or surfaces that modify mesh size depending on the size of this auxiliary entities. _ Merge nodes (To join nodes of different structural elements): It allows automatical merging of nodes

6 Contact model types: Gas storage tank. _ Touching contacts (For foundation-ground): CivilFEM manages to simulate friction between two surfaces. Coulomb friction model is used. _ Glue contacts: CivilFEM allows joining two different meshes with a glue contact. Contacted nodes will not separate.

7 Reinforced concrete beams: CivilFEM can generate different sections. It can also define concrete reinforcements in three different ways (amount, number of bars and diameter, and spacing and diameter). Reinforcement for bending, shear and torsion can be defined, and they can be fixed or scalable for concrete design. The model is checked with 15 cm 2 per side of bending reinforcement, 15 cm 2 /m of shear reinforcement and 5 cm 2 /m transversal, 10 cm 2 longitudinal reinforcement for torsion. Steel S-500 is used and 5 cm mechanical cover is defined.

8 Definition of elastic spectrum: A modal-spectral analysis is performed, combined with the modal SRSS method, using the type 1 horizontal elastic spectrum of the EC-8, activating the prestressing loads for the tank pressure and a 5% damping. CivilFEM spectrum calculation is performed by using two methods of modal combination (SRSS and CQC) and two methods for response combination in each direction (SRSS3 and Newmark).

9 Load Cases: Two load cases are used for the combinations. 1.- Self weight. 2.- Internal pressure gas + Seismic loads. Combination: (1,35*Self weight)+ (1,5*Pressure)+ (1,5* Seismic). CivilFEM allows the user to perform any load case combination considering different safety factors.

10 Display of loads and boundary conditions: CivilFEM allows quick visualization of loads, boundary conditions and load cases. Display of radial pressure on upper hemisphere without distribution. Display of boundary conditions on ground mesh

11 Results: Vertical displacement caused by self weight. Vertical displacement

12 Results: Von Mises stress on foundation shell. Horizontal displacements according to load combination (1,35*Self weight)+ (1,5* Seismic loads).

13 Results: X component of stress on tank Y component of stress on tank

14 Results: X component of stress on perimetral beam. Reinforced bending total criterion beam (EC-2).

15 Results: Shear-torsion total criterion (EC-2). Bending and compression buckling (EC-3).

16 Results: Axial forces in diagonal tension members. Results of the modal-spectral analysis (frequency and mobilized masses per axis). First mode frequency = 5.76 Hz.