I'm seeking an explanation in reference to the photo below. I'm unsure how the solution within the red box calculates the max tension of the Aluminium beam and the max compression off the Aluminium beam.

How are you able to differentiate whether the Aluminium is in tension or compression within this scenario?

As well as how they are able to make the statement that "the steel is entirely compressive bending stress and the max bending stress is at the top of the flange" enter image description here



1 Answer 1


It has to deal with the location of the Neutral Axis (NA). That whole calculation for Ῡ =37 mm is key here. Bending occurs about the NA. That big yellow arrow represents the moment in the beam, or in other words which direction it is bending. Based on the moment arrow everything above the NA will be compression, and everything below it will be tension.

The calculations show the NA at a distance of 37 mm from the bottom and from the cross section dimensions we see the aluminum section is 40 mm deep. This means all the steel is above the NA which in turns means all the steel for this sections is in compression. The portion of the aluminum between the NA and the steel is also in compression. The maximum stress occurs at the furthest distance from the neutral axis.

  • $\begingroup$ So if the arrow was flipped in the opposite direction, does that mean that the top half would be in tension and the bottom would be in compression? $\endgroup$ Jun 22, 2019 at 0:16
  • 1
    $\begingroup$ Correct! And the steel would be in pure tension instead. $\endgroup$
    – Forward Ed
    Jun 22, 2019 at 0:19

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