SEISMIC DESIGN

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Three modules, SE1, SE2, and DYN, are available in AxisVM for seismic analysis and design of structures: modal response spectrum analysis, pushover analysis, and time-history analysis, respectively. In modal response spectrum analysis, a standard conforming or user specified custom design spectrum can be used. The analysis takes into account torsional effects and different spectra can be selected for each global direction. With pushover analysis, the non-linear response of the structure can be obtained, taking into account geometrical and material non-linearities, non-linear finite elements, and plastic hinges. 

MODULES

The SE1 module includes several tools that facilitate the execution of Modal Response Spectrum Analysis (MRSA) in AxisVM. The tools automatically perform seismic load generation in three orthogonal directions for each vibration mode, the computation of the structural response for each vibration mode, and the combination of the modal responses into a governing seismic effect. Results from individual modes can be combined using either the Square Root of Sum of Squares (SRSS) or the Complete Quadratic Combination (CQC) method. The program calculates additional torsional moments about the vertical axis for every storey and mode shape. The magnitude of torsional moments depends on the horizontal load and eccentricity of each storey.

Requirements / recommendations

  • the SE1 module can only be used for linear analysis
  • the use of SE2 or DYN modules is recommended if non-linear analysis is required

DESIGN CODES

Eurocode 8

EN 1998-1

Swiss standard

SIA 261

Italian standard

NTC

CHARACTERISTICS

  • seismic effects can be given via code conforming or a user defined custom spectrum
  • consideration of torsional effects on each storey
  • X* and Y* directions of seismic excitation can be defined with the help of seismic excitation angle
  • arbitrary number of seismic load groups can be created in the model
  • different spectra can be used in global X, Y and Z directions
  • displacement behaviour factor
  • optimal SRSS or CQC summation rules
  • capacity design is available for concrete beams and columns

DETAILS

CUSTOM DESIGN SPECTRUM

In AxisVM, both standard conforming and user specified custom design spectra can be used to define seismic effects at the investigated site. Custom spectra are defined in the Spectral function editor after setting the Design spectrum from Parametric shape to Custom. Spectra are created in the editor by a piecewise linear approximation of the spectral shape over a list of pre-defined natural periods (T) – spectral acceleration coordinate (Sd) pairs.

TORSIONAL EFFECTS

In order to consider torsional effects in the seismic analysis, storeys of the structures and the value of accidental eccentricity need to be defined. The magnitude of torsional moments depends on the horizontal load and eccentricity of each storey. Horizontal loads are retrieved from load cases corresponding to each vibration mode and horizontal direction. Torsional moments are considered with either + or – signs, but always with the same sign at all storeys.

SUMMATION RULES

AxisVM provides control over the combination of results for individual modes and individual directions. Results from individual modes can be combined using either

  • the Square Root of Sum of Squares (SRSS) or
  • the Complete Quadratic Combination (CQC)

method.

The latter is considered more appropriate if the vibration modes of the structure are not well separated (i.e. vibration frequencies are close to each other). Results in the two horizontal and the vertical directions can be combined using either of the two commonly used combination methods, namely with SRSS or 30% rules.

Pushover analysis is a non-linear static seismic analysis method that can be used in case of structures with a dominant vibration shape. It can be used to obtain the non-linear response of a structure that is described with the so-called capacity curve. Pushover analysis is mainly used to design dissipative structures or to assess the performace of existing structures. In case of dissipative structures, plastic hinges are designed in order to ensure the dissipation of seismic energy via plastic behaviour.

In AxisVM, non-linear, plastic materials, non-linear finite elements, non-linear supports, and concentrated plastic hinges are available to characterize the non-linear structural behaviour.

Requirements / recommendations

  • at least NL1 configuration is required for non-linear analysis

DESIGN CODES

Eurocode 8

EN 1998-1

Swiss standard

SIA 261

Italian standard

NTC

CHARACTERISTICS

  • uniform and modal load distibution
  • different analysis parameters can be defined in X and Y directions
  • options and tools to model structural non-linearity
  • concentrated plastic hinges
  • effects of accidental eccentricities and the resultant torsional moments may be included
  • calculation of target displacement
  • calculation of ADRS spectrum
  • calculation of interstory drifts

DETAILS

TARGET DISPLACEMENT

Target displacement is calculated based on the elastic spectrum and capacity curve with the help of the so-called N2 method recommended in Appendix B of Eurocode 8.

The design value of seismic forces may be read from the capacity curve at target displacement level. Dissipative zones need to be checked in terms of deformations related to target displacement, while non-dissipative elements need to be checked in terms of load bearing capacity, including overstrength. Deformation capacity of the structure has to exceed the target displacement by 50%.

ADRS SPECTRUM

The Acceleration-Displacement Response Spectrum (ADRS) is calculated and displayed by AxisVM. The ductility of the structures can be seen on the ADRS spectrum.

INTERSTOREY DRIFTS

AxisVM calculates and displays absolute drifts and relative storey displacement (interstory drift) ratios. The diagram of absolute drift shows the horizontal displacement of the center of gravity of storeys relative to the ground. The interstorey drift ratio diagram shows the interstorey drift expressed as a percentage of the storey height. The latter diagram helps to check if the structure meets the drift limit requirements of the selected design code.

The time-history analysis of the DYN module can be used to evaluate the linear or non-linear dynamic structural response under seismic excitation. This type of seismic analysis is the most complex, and for this reason, it is typically used in special circumstances (high importance or seismicaly isolated structures, etc.). To perform the analysis, so-called accelerograms (ground acceleration time-histories) are required to describe the ground motion by giving ground acceleration as a function of time.

Similar to the Pushover analysis, non-linear, plastic materials, non-linear finite elements, non-linear supports, and concentrated plastic hinges are available to characterize the non-linear structural behaviour.

A detailed description of the DYN module can be found on the  Dynamic Analysis  page while this short summary covers only the capabilities with respect to seismic analysis.