Finding Neutral Axis of an Unequal I-Section
In this first article, we will estimate the neutral axis of an unequal I-section.
The original problem, sourced from an online lecture [video link here, please note that you will be leaving our website. We are not responsible for the content or safety of external sites.] on finding the centroid of an I-section by Manas Patnaik, is modelled using GeoProp to compare the neutral axis of an unequal I-section.
Problem Definitions
The unequal I-section is illustrated in the sketch below.
Sketch
Unequal I-Section
Numerical Procedure and Results
(Detailed modelling steps are provided at the end of this article.)
Following the analysis using GeoProp, the neutral axis is evaluated on the Results Page of the app:
Neutral Axis: x = 15 cm and y = 10.96 cm.
The analytical solution from the online lecture gives a neutral axis of x = 15 cm and y = 10.96 cm, which perfectly aligns with the neutral axis estimated from GeoProp.
Note that the Results Page also contains other section properties typically used for design purposes.
GeoProp Results Page
Section Properties
The .txt input file for the GeoProp model is available for download here.
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Step-by-Step Modelling Procedure
Please note that while the step-by-step procedure may look tedious at first, the modelling process will become quick and efficient once you get used to the app workflow.
Our first step is to create the I-section.
Open the GeoProp app and tap on the Section Page. Tap <Solid> to ensure that the Solid Element is selected. We will treat the I-section as the Solid Element as this is the base building element for the app.
Section Page
Tap <Solid>
[Tips & Tricks: To save time when creating a section, utilise the built-in Section and Shape Templates and adjust the creation to suit your section.]
Next tap on the Section Template button to access the Section Template Page.
Section Page
Tap the Section Template Button
In the Section Template Page, tap the Universal Section button to bring up the template for an I-section.
Section Template
Tap the Universal Section button
Under Input Unit, select cm from the drop down list to change the unit. Then enter the following dimensions:
bf = 30 in cm
df = 25 in cm
tf = 5 in cm
tw = 5 in cm
Leave r0 empty.
Ensure that Element is set to Solid.
Universal Section
Change unit and enter your dimensions
Tap on the Scribble button to create the I-section.
Back in the Section Page, we see that the I-section created has two equal flanges. We will modify the top flange to match the unequal I-section.
Section Page
View the section
Toggle on the Modify toggle button and select Point ID 8. The current coordinates for x-Coord and y-Coord of 0.0 in cm and 25 in cm respectively will be shown.
Section Page
Turn on the Modify feature
As we are just changing the width of the top flange, under x-Coord enter 5 in cm.
Section Page
Modify the section
Next, tap the Eraser button next to the Point ID.
Section Page
Apply the changes
To complete changes, repeat for the following points each time tapping the Eraser icon to modify the points,
Point ID 9 (x-Coord: 0 -> 5 in cm)
Point ID 7 (x-Coord: 30 -> 25 in cm)
Point ID 6 (x-Coord: 30 -> 25 in cm)
Finally, tap on the Results Page button to access the Results Page to view the results.
Section Page
Tap the Results Page Button
The Results Page summarises the section properties for the section.
Results Page
View the Section Properties
