# Emergency Thermal Expansion question(Mech. Materials)

## Main Question or Discussion Point

If the change in temperature is negative:

1st of all, is it possible? and second, if it's a bar between two rigid supports, does the reaction would have to be in tension instead of compression to compensate for the bars trying to contract due to temperature? Also what if it has an axial force as well?

Related General Engineering News on Phys.org
minger
Of course there can be a negative temperature change. The bar is 100°, and now it's 70, that's a -30° temperature change.

As for the bar, you can use the principle of superposition to not only think about the reactions, but the second load as well. First, consider the bar with only one support. As the bar cools down, it shrinks. Superposition says that in order to achieve the state that the bar is in, the second support must "stretch" the bar until it's at its original position.

This means that the bar will be in tension. As for the supports, typically we look at the beam alone, for detailed connection analysis, you would need more information about how the beam is actually connected.

As for the separate axial load, use the principle again. Find the solution for thermal effects alone; find the solution for the axial force alone. Add them.

I was able to solve the problem without using super position and doing them by sections. I assumed the reactions going against the wall instead of away and it worked out perfectly.