When the steel rod is heated up to 100 ° C , due to thermal expansion it’s length will increase with temperature. Since it will be expanding freely , there won’t be anything to oppose the expansion. Longitudinal Strain develops when there exists an opposing force to the expansion of the length.
Hence, there will be zero longitudinal strain as it does not have opposing stress .
If the rod was clamped at its ends or restricted by some object at it’s ends then longitudinal strain would come in the picture