How do you find the neutral axis of an I beam? ?

Question

How do you find the neutral axis of an I beam? ?

Answer 1

If i remember, the neutral axis is just a centre lilne of the beam - the line that does not do any work or carry any load.

You will see some large lorries, and they will have I beams in their structure. Sometimes you will lsee large diamond shapes but out of the I beams in order to save on weight. This is because the centre line does not carry any load.

Answer 2

When you say I beam, you infer a symmetrical cross section and therefor the neutral axis is simply the centerline between the two flanges.

To find (calculate) the neutral axis you use the neutral axis theorem.

Answer 3

Assuming that the I-beam is symmetric, the neutral axis will be situated at the midsection of the beam. The neutral axis is defined as the point in a beam where there is neither tension nor compression forces. So if the beam is loaded uniformly from above, any point above the neutral axis will be in compression, whereas any point below it will be in tension.

However, if the beam is NOT symmetric, then you will have to use the following methodology to calculate the position of the neutral axis:

1. Calculate the total cross-sectional area of the beam (we shall call this A). Let x denote the position of the neutral axis from the topmost edge of the top flange of the beam.

2. Divide the I-beam into rectangles and find the area of these rectangles (we shall denote these areas as A1, A2, and A3 for the top flange, web and bottom flange respectively). Additionally, find the distance from the edge of the top flange to the midsection of these 3 rectangles (these distances will be denoted as x1, x2 and x3).

3. Now, to find the position of the neutral axis, the following general formula must be used:

A*x = A1*x1 + A2*x2 + A3*x3

We know all the variables in the above formula, except for x (the position of the neutral axis from the top edge of the top flange). So it is just a case of rearranging the formula to find x.

In general, when finding the neutral axis we always work in mm and mm squared. Also remember to draw diagrams.

I hope this helps.