Seismic analysis in AxisVM10 can be performed according to three different methods.

Pushover analysis

Response Spectrum Analysis Method

Analysis from acceleration diagram

Seismic Analysis with Pushover Analysis method (SE2 module)

When doing pushover analysis, displacement of a specific node (control node) in the model is increased in several steps. At each step the intensity of a parametric, typically horizontal load with pre-defined distribution needs to be evaluated. This parametric load shall be in equilibrium with the reaction forces and an also pre-defined, typically vertical constant load. The latter takes gravitational quasi-static loads into account, which have considerable influence on P-Δ effects. Horizontal loading represents the effects of seismic excitation.

Pushover analysis is highly recommended for dissipative structures, because one of the results is the level of ductility for the given structure. Using the proper ductility level can reduce design earthquake loads significantly.

The horizontal so called pushover load distribution is either based on the dominant mode of vibration or on the structural mass distribution in accordance with Eurocode 8 provisions. These load distribution types can be generated in two perpendicular horizontal directions resulting in a maximum of four pushover loads for each structural model.

Any node of the model can be selected as control node for the analysis. Apart from the control node, the direction and maximum level of displacement and the constant and parametric load types also have to be specified. General parameters of nonlinear static analyses are also available (e.g. number of increments, convergence criteria etc.). Results for each displacement step are available in the same format as for other nonlinear analysis types.

An important result of pushover analysis is the capacity curve, which is shown in a separate window. Using the capacity curve the target displacement for a given pushover load case and elastic response spectrum is evaluated using the so called N2 method. Response spectrum parameters are identical to the ones used in elastic modal response spectrum analysis (Seismic load case). The elastic and inelastic acceleration-displacement response spectrum curve is also displayed based on the aforementioned data. Besides the N2 method recommended by the Eruocode 8 (EN 1998-1:2004) standard, the module also supports a different method for evaluation of target displacement specified in the Italian OPCM 3274 standard.

seismic analysis with Response Spectrum Analysis Method (se1 MODULE)

Calculate the first n vibration mode shapes and frequencies.

AxisVM generates equivalent static loads (for each vibration mode shape) which are then applied to the model in a static analysis. internal force results obtained for each mode shape are summed using the method described in design code specifications.

Supported Design Codes

• Eurocode 8 Code (EN 1998-1:2004)  
• German Code (DIN 4149:2005-04)
• Hungarian Code (MI-04.133-81)
• Italian Code (OPCM 3274)
• Romanian Code (P 100-92)
• Swiss Code (EN 1998-1:2004)

Defining parameters

• After creating a seismic load case additional load cases are calculated to represent seismic forces in X and Y direction
  (and in Z direction if a vertical spectrum is defined) with + and – signs. Two additional load cases represent the sum of the + and – effects.

• Design spectrum and other parameters required by the design code can be specified in a separate dialog. 
• Common parameters are:
  • Horizontal design spectrum
  • Vertical design spectrum
  • Design ground acceleration
  • Horizontal behaviour factor
  • Consideration of torsional effects
  • Combination method of modal responses

  • Combination method of spatial components

Results

Results of seismic load cases can be evaluated the same way as those of regular load cases. A seismic load group appears automatically so seismic loads contribute to the critical combination as well.

seismic analysis on the basis of acceleration diagram ( dyn MODULE)

Acceleration functions can also be used for seismic analysis. In this case it is recommended to obtain proper seismic accelerograms and assign these functions to support nodes to analyse the effects of the earthquake. This method provides more exact results than the response spectrum analysis and can be used even if nonlinear elements are defined in the model (nonlinear supports, tension-only trusses, etc.). Its disadvantage is that it cannot be combined with other load types automatically.