consideration of fluid structure interaction in time history seismic analysis
DESCRIPTION
In this work seismic assessment of the 45 year old RC building is presented. The main life loads of the building are the tanks filled with water. Considering fluid as a rigid body would lead to conservative results. In order to obtain possibly accurate solution, the fluid – structure interaction (FSI), with the use of solid elements, needed to be implemented. This ensured proper water behavior during the earthquake impact. An improved method for the elaboration of precise structural models was used, which is in combination of dynamic in-situ measurements and FE-calculations. With dynamic in-situ tests under forced excitation by usage a reaction mass exciter, the dynamic behaviour of the structures were determined. That followed a model updating procedure for the established numerical finite element method and ensures to increase the reliability of the numerical investigations. Series of time – history analysis were proceeded. Some of the signals were found in Strong Motion Database, with the respect to the local earthquake and ground properties. The rest were artificially created, so that they suit the Eurocode 8 spectrum. The structure was design without consideration of the earthquake load. The main goal of this work was to asses if any retrofitting is necessary and if so, to suggest possible solutions. Table of Content: * Introduction * FE model * Fluid – structure interaction * Model updating * Seismic analysis (spectrum and transient) * Safety of the building * ConclusionsTRANSCRIPT
Consideration of fluid structure interaction in time history seismic analysis
ANSYS Conference & 6. CADFEM Austria Users' MeetingVienna 7 04 2011Vienna, 7.04.2011
Mag. Ing. Maciej Kwapisz
Outline
Introduction
FE model
Fluid structure interaction Fluid – structure interaction
Model updating
Seismic analysis (spectrum and transient)
Safety of the building
Conclusions
Introduction
The building was build in 1963 No design for earthquake impact Six water tanks with capacity of
80m³ each Masses (with life load):
tank building: 2060 t higher building: 2304 t Water: up to 400 t (5x 80t)
FE model
3D FEM Model – Ansys Elements used:
Shell63 – walls, fundaments Fluid80(solid) for water in tanks Beam4 – beams, columns
bi 14 d combin14 – ground link8 - elastic dilatation
Earthquake loadq
Fluid structure interaction Mesh of solid elements(water) Copy the mesh to shell elements(walls) Coupling of nodes – fixing direction p g g
perpendicular to the wall Problems on the funnel
Fluid structure interaction
Assures proper dynamic properties of the whole structure Changes the pressure on the walls
Fluid structure interaction
Water moves in transient and in modal analysis
Vibration measurement of the building and model - updating
Connecting the mass exciter with the building by axial chain
Sweep excitation p Obtaining eigenfrequencies Comparing with FE results
Vibration measurement of the building
Water levels during measurement
2% 62%
0,4% 0,3%, ,
0,3% 0,3%, ,
Model updating – comparison with FE results
Vergleich mit der Messung
Mode 4
Model updating – comparison with FE results
Vergleich mit der Messung
Natural frequency [Hz]Mode Natural frequency [Hz] DeviationMeasurementFE model
1 x 5.2 x2 6 3 6 2 1 6%2 6,3 6,2 1,6%3 8,7 7,6 12,6%4 11,5 11,9 3.50%
Average 5,9%
Seismic assessment
Both spectrum and transient analysis were used
Spectrum of the time- history analysis
Not enough signals were found in Strong Motion Database
Artificially created signals were used, based on spectrum from EC8
Results from both analysis were similar Results from both analysis were similar
Results of Seismic analysis Water tank building treated as alone standing Columns were determined to be the weakest part of the structure 2 Types: 40x40 cm und 100x40 cm. yp
Post processing of transient analysis - columns
Bending moments (red and violet) and Safety factoraxial force (blue)
Post processing of transient analysis - columns
Simultaneity of bending moments and normal force. Equation written to Ansys:
is the design moment around the respective axisis the moment resistance in the respective direction
a is the exponent, which depends on the axial force
The result is the highest safety factor from all columns
Conclusions
By means of experimental modal analysis and finite element calculation a very realistic computer model can be created
Fluid elements make the consideration of water motion possible, which is important for improvement of accuracy and reliability of the computations
Post- processing of transient analysis provides information about simultaneity of bending, shear and axial forces.
Similar results of time – history and spectrum analysis proved that fluid elements worked good in both of them
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