2D Frame Modal Analysis Comparison

At CalcTree, we're committed to bringing you state-of-the-art tools for your daily work as a structural engineer. That's why we are implementing the OpenSeesPy Python library as part of our offer. 
This article aims to demonstrate how OpenSeesPy can be used within CalcTree as a substitute to popular FEM software such as ETABS. 
Our example problem is a 2D frame with 3-storeys and one span. We will conduct a modal analysis to find the deformed shape and periods of the three principle modes of vibration using both OpenSeesPy and ETABS and will compare the results. See figure and toggle lists below for more details about the example problem.
Technical assumptions
List of parameters 
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Proposed 2D frame for modal analysis
What is modal analysis?
Input Data﻿﻿h
:3.00m​
 
﻿﻿L
:6.00m​
  
﻿﻿m
:1,000kg​
 
﻿﻿E
:20GPa​
 
﻿﻿I_column
:675,000,000mm4​
 
﻿﻿I_beam
:67,500,000,000mm4​
 
👉For comparison between ETABS and OpenSeesPy, don't change the default input values above. 
OpenSeesPy The model was built by assigning mass to each storey at the two-column nodes, equivalent to half of that storey's mass. For the modal analysis, an elastic analysis with 1 DOF (x translation) was undertaken. 
Can’t display the image because of an internal error. Our team is looking at the issue.
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👉 See the Code source on the right-hand panel or our tutorial for more information about the OpenSeesPy model construction. 
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Results: Vibration Modes﻿﻿T1_OpenSeesPy
:0.11s​
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﻿﻿T2_OpenSeesPy
:0.05s​
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﻿﻿T3_OpenSeesPy
:0.03s​
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Can’t display the image because of an internal error. Our team is looking at the issue.
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ETABSConditions in the OpenSeesPy model were replicated in ETABS, using the default input values on this page.
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ETABS model
👉 For further details about the ETABS modelling, see toggles below.
ETABS - global settings
ETABS - beam & column settings
Results: Vibration Modes﻿﻿T1_ETABS
:0.109s​
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﻿﻿T2_ETABS
:0.048s​
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﻿﻿T3_ETABS
:0.033s​
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ETABS modal analysis 
Results and ComparisonTo compare the OpenSeesPy Python library with ETABS, we have looked at the results from the modal analysis (i.e. deformed shape and periods of the first three vibration modes), the modelling complexity and pricing. 
PeriodThe period values of the first three modes of vibration from OpenSeesPy and ETABS were very closely aligned, with only minor variations noted at the third decimal place (see below). Such negligible differences are statistically insignificant and do not impact the overall interpretation of the structural behavior.
OpenSeesPy
﻿﻿T1_OpenSeesPy
:0.107s​
  
﻿﻿T2_OpenSeesPy
:0.048s​
 
﻿﻿T3_OpenSeesPy
:0.032s​
 
ETABS
﻿﻿T1_ETABS
:0.109s​
 
﻿﻿T2_ETABS
:0.048s​
  
﻿﻿T3_ETABS
:0.033s​
 
ModellingETABS provides a user-friendly interface that allows a fluid definition of the model. 
OpenSeesPy requires a proficient knowledge of programming in Python to complete the model construction, and a third-party GUI (graphical user interface) to view results, for example CalcTree.
PricingETABS is paid software with different pricing options depending on the expected level of modelling required. Find more information here.
OpenSeesPy is open-source, free to use for non-commercial use cases (e.g. education). Find more information on their license information here.
SummaryIn conclusion, a comparison between the OpenSeesPy and ETABS modal analysis showed:
Deformed shapes were visually the same and in order of magnitude
The periods were almost identical; that is, the value varied slightly in the third decimal place, which is an insignificant difference.
Therefore, it is concluded that the choice between using OpenSeesPy or ETABS does not affect the fundamental understanding of the structure's dynamic response. Consequently, either tool can be used interchangeably without compromising the accuracy of the structure's behaviour under dynamic conditions.
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Learn More﻿
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