Demand Spectrum

In order to apply the design response spectrum to prepare the demand spectrum, define a design spectrum specified in a design standard.

Demand Spectra at Damping Ratios (%)

Using the specified damping ratio, produce the demand spectrum.

Constant Period Lines at Period (sec)

Produce the corresponding period lines for the specified periods.

Evaluation of Performance Point

Using the effective damping ratios suggested in ATC-40, evaluate the inelastic demand spectrum and compare it with the capacity spectrum to find the performance point.

Procedure-A: The effective damping ratio for inelastic behavior of a structure is evaluated first, and a corresponding demand spectrum is created. The performance point is found through a repetitive process.

Procedure-B: Assuming the ductility demand curve and using the corresponding effective period and effective damping ratio, the path of the performance point is evaluated and the intersection with the capacity spectrum is found.

Damping Parameters

Enter the parameters determining the effective damping ratio.

Inherent + Additional Damping (%)

Specify the total damping ratio (%) reflecting the inherent damping ratio and additional damping resulting from physical devices installed in the structure.

 

Structural Behavior Type

Select A, B or C to obtain a reduction factor applicable for effective damping.

 

Note

Structural Behavior Type is classified in the table below.

 

 

 

For example, if 5% is specified for the damping of the structure, the effective damping becomes,

Refer to ATC40 for the definitions of the variables.

 

Performance Point

 

It provides the information on the performance point on the spectrum and the performance point transformed into a multi-degree of freedom system. 

 

V, D

 

Maximum base shear and maximum displacement of the inelastic structure by the performance point 

 

Sa, Sd

 

Spectral Acceleration (Sa) and Spectral Displacement (Sd) at the performance point   

 

Teff, Deff

 

Inelastic effective period and effective damping at the performance point

 

 

Demand Spectrum

Design Spectrum: Select the design spectrum (i.e.= The elastic response spectrum associated with a reference return period of 475 years) to be applied to calculate the target displacement for the Limit State of Significant Damage (SD).

Eurocode-8(2004)

User Defined: Predefined user-defined response spectrum function can be selected.

Target Displacement

 

The target displacement of a structure is determined through the transformation to an equivalent single degree of freedom system. For the detailed formula, refer to ANNEX B DETERMINATION OF THE TARGET DISPLACEMENT FOR NONLINEAR STATIC (PUSHOVER) ANALYSIS, EN 1998-1:2004. The target displacement, which is obtained from the above, corresponds to the seismic demand of the Limit State of Significant Damage (SD). Target displacement of the Limit State of Near Collapse (NC) is taken equal to that of SD multiplied by 1.5. Target displacement of the Limit State of Damage Limitation (DL) is taken equal to that of SD divided by 2.5.

 

Note

 

When transforming MDOF system to SDOF system, midas applies different modal participation factor to each direction for the 3 dimensional structure. The direction of each mode is determined based on the maximum Modal Participation Mass. For example, if Modal Participation Mass is as follows, Modal Participation Factor for mode 1 and 2 is applied to X-direction and Y-direction, respectively.

 

 

Demand: Roof displacement corresponding to the target displacement for the seismic action is considered. Note that this is the target displacement for the MDOF system, while the vertical lines on the pushover curve represent the target displacements for the equivalent SDOF system.

 

Capacity: Global capacity of the masonry structure in terms of roof displacement (Master node). Only applicable to Masonry material models.

 

Global capacity of the Limit State of Significant Damage (SD) is taken equal to the roof displacement at which total lateral resistance (base shear) has dropped below 80% of the peak resistance of the structure, due to progressive damage and failure of lateral resisting elements.

 

Global capacity of the Limit State of Damage Limitation (DL) is taken as the minimum value between a) displacement corresponding to the maximum base shear in the pushover curve and b) displacement corresponding to the story drift of 3/1000.  

 

Step: The nearest increment step to the target displacement

 

Remark: Assessment of the result in terms of the global response for the masonry structure. Only applicable to Masonry material models.

 

Limit State of SD: The assessment is OK if the global capacity of the Limit State of SD is greater than the target displacement of the Limit State of SD   and   is less than 3. is the ratio between the acceleration in the structure with unlimited elastic behavior  and in the structure with limited strength .

 

Limit State of DL: The assessment is OK if the global capacity of the Limit State of DL is greater than the target displacement of the Limit State of DL (i.e.= Target Displacement of the Limit State of SD divided by 2.5.

 

 

 

Load Case : The pushover load case defined in Pushover Curve is displayed.

 

Define Additional Step : Select the method to define additional steps.

 

Monitored Displacement : Define additional steps based on the displacement.

 

Resultant Base Shear : Define additional steps based on the base shear.

 

User Input : Define the additional step name, Reference Step and Distance Ratio directly.

 

Note Check pushover analysis results for additional steps  Additional Step

Entered additional step can be checked in Deformed Shape and Story Drift.

 

Calculate Reference Step and Distance Ratio : Click  button in order to define an additional step by entering Distance Ratio from the Reference Step to the additional step.

 

Additional Step Data : Determine the displacement of additional step

 

Load Case : Load case name for which an additional step is added.

 

Reference : Reference pushover step to determine the position of an additional step

 

Distance Ratio : Displacement ratio of the reference step to the additional step