Seismic Load - Basic Guide

Seismic Load - Basic Guide

General

  1. You will learn how to incorporate the basic seismic loading into your model.
  2. You will know how to view & check the seismic mode shapes
It is very important that the user to refer the relevant earthquake code for detail explanations of the seismic analysis and design. 
This article uses EuroCode 8 for illustration. The procedure will be similar to other earthquake / seismic codes.
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You may also refer to below design guide and video to understand about seismic isolation.
  1. Base Isolation
  2. Introduction to Seismic Isolation (Video)

Procedures

Select a Design Code

Select an Earthquake Code:  Building Setout tab >  Settings Center >  Codes tab > Choose an Earthquake code from the list.



Define Load Cases and Combinations

Generate the Seismic Loading cases:  Loading Combinations >  Loading Generator > Horizontal Load Combinations > Check the  Seismic Loading > select  Equivalent Static load  or Modal Response Spectrum.


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Guidance with reference to EuroCode :
  1. "Equivalent Static Load" or "Lateral Force Analysis Method"  is only applicable for buildings with fundamental periods smaller than 2.0s & buildings that are regular in elevation (may differ country specific code or National Annex).
  2. "Modal Response Spectrum" is applicable to all structure types & is a more accurate assessment of seismic forces. As such we strongly recommend user use this method for all buildings.

Update the Seismic Design Parameters

You can define the seismic parameters by going through all tabs in Seismic Parameters.   Seismic Parameters >  Parameters Tab, Analysis, Structural Irregularities or Settings.



You can adjust the number of vibration modes:  Seismic Parameters  >  Analysis Tab


Notes
Increase the number of vibration mode to achieve cumulative mass participation of 90% (as required by most seismic code). Recommended value is 15. 
If the criteria is not satisfied, there will be appropriate warning prompt & recorded in the  Post Analysis Checks Report.

User-defined Periods in Equivalent Static Load (EQS) Analysis

To determine the natural modes of vibration, ProtaStructure automatically performs a 3D eigenvalue analysis. After the vibration modes are detected, they are sorted with respect to their mass participation in three degrees of freedom, namely, Translation X, Translation Y and Rotation Z. The dominant modes in X and Y direction are selected as candidate modes to be used in equivalent static earthquake load method.

However, the period of vibration from these modes can not be directly used in EQS method. Seismic codes always put an upper limit to these values by empirical formulas. The most conditions are: 
1. Teigen ≤ Rayleigh formula
2. Teigen ≤ Cu.Ct.hnx
​3. Teigen ≤ 0.1N
ProtaStructure automatically checks these conditions and their variation depending on the selected seismic code.

Users are allowed to directly impose user-defined periods to be used EQS analysis. To enter user-defined periods:
1. Navigate to “Seismic Parameters > Analysis” tab.
2. Check “Use User-Defined Periods in Equivalent Static Analysis” option.
3. Enter period values in X and Y direction.
4. Exit the window by clicking OK button.
Notes
The user-defined periods will be directly used in equivalent static base shear calculation without applying any upper limit restrictions.

Define the Live Load Participation Factor

You can assign the live load participation factor and number of rigid basements in  Building Setout  >  Edit Storey.


Live load participation factor, n affects seismic weight calculation of each floor :
Seismic Weight, W = [Dead load] + n.[Live Load]

Define No. of Rigid Basement

If basements are also modelled, it can be excluded from the mass irregularity checks. The number of basements can be specified in the “Edit Storey” dialog.



Building Analysis

Go to Building Analysis > Model Options > Material Effective Stiffness Factors



Review & change the stiffness factors in accordance to the code of practice.  Seismic load cases will always consider cracked section properties. 
Start the Analysis. A summary report will appear upon completion of analysis.  Details can be found in the  Post Analysis Checks Report. 


Click OK to continue. 

Post-analysis : Analytical Model

The Analytical Model view can be created & opened, upon completion of Building Analysis. 
Go to Analysis tab >  Analytical Model > Building Analysis Model >  Select a modal shape or load cases  from the right panel.


  1. Ensure to click " Animation" to clearly visualize the vibration mode of the building.
  2. Click " + Increase Scale " or "Auto Scale" if the vibration is not visually significant.
Figure below shows an example animated vibration mode 3.



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Some guidelines & notes :
  1. The first 3 modes should be "X-Direction" or "Y-Direction" or "Torsion" for a stable model.
  2. If you find the first few modes of vibration not as above, there is likely local instability (e.g. due to wrong modelling, too many hinges applied or isolated member not restrained properly) 
  3. Observe carefully the animation of the building to determine the location of instability and rectify the model to suit.
  4. For guidance analytical model verification, watch this e-learning video : 15 Post Analysis, Analytical Model

Reports

The results of the seismic analysis can be found in  Building Analysis dialog > Reports tab
  1. Post Analysis Checks Report : contain details of seismic analysis results & clause compliance
  2. Strong Column Checks : only be created for medium or high ductility analysis
  3. Joint Shear Checks : only created for medium or high ductility analysis
  4. Eigenvalue results report :  Contains information of natural frequency & period
Alternatively, all reports can be accessed from Drawings & Reports tab > Reports drop down list. 





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