Modal-spectral analysis of

a gas storage tank.

Gas storage tank. • 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 cm2.

4.- St. Andrew crosses using "truss" elements with a diameter of 10 cm.

5.- 14 m x 1 m concrete foundation using a “shell” element.

6.- Well graded soil using a "solid" element.

Gas storage tank. Description of the structure:

The sphere has an average radius of 5.0 m and a total height of 15 m.

Shell

Beam

Truss

Solid

Gas storage tank.

Loads and boundary conditions of the structure:

– Self weight (g = 9,81 m/sg2)

– 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.

Gas storage tank.

• Mesh:

_ 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

Gas storage tank.

• Contact model types:

_ “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.

Gas storage tank.

• 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 cm2 per side of bending reinforcement, 15 cm2/m of shear reinforcement and 5 cm2/m transversal, 10 cm2 longitudinal reinforcement for torsion. Steel S-500 is used and 5 cm mechanical cover is defined.

Gas storage tank.

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).

Gas storage tank.

• 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.

Gas storage tank. • Display of loads and boundary conditions:

CivilFEM allows quick visualization of loads, boundary conditions and load cases.

Display of boundary conditions on

ground mesh

Display of radial pressure on upper

hemisphere without distribution.

Gas storage tank. • Results:

Vertical displacement

caused by self weight.

Vertical displacement

Gas storage tank. • Results:

Von Mises stress on foundation shell.

Horizontal displacements according to load combination (1,35*Self weight)+

(1,5* Seismic loads).

Gas storage tank. • Results:

“X” component of stress on tank

“Y” component of stress on tank

Gas storage tank. • Results:

X” component of stress on perimetral beam.

Reinforced bending total criterion beam (EC-2).

Gas storage tank. • Results:

Shear-torsion total criterion (EC-2).

Bending and compression buckling (EC-3).

Gas storage tank. • 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.