The finite-difference time-domain method is one of the most widely used numerical electromagnetic computation techniques, and it has become an effective tool for analyzing electromagnetic transient phenomena in three-dimensional structures and grounding systems. However, the available approaches for representing thin wires apply only to bare conductors such as overhead power lines and earth electrodes. In this work, we demonstrate the applicability of thin-wire representation techniques to wires surrounded by both lossy and lossless media, such as reinforcing bars inside lossy concrete. The validated technique is then illustrated through the study of a reinforced-concrete building struck by lightning to evaluate the effect of concrete on the penetration of lightning electromagnetic pulses inside the building.