柱子 (zhù zi) refers specifically to a physical pillar or post used in construction — the concrete structural element you see in a building. Its siblings 支柱 (zhī zhù), 脊梁 (jǐ liáng), and 栋梁 (dòng liáng) are more often used figuratively to mean 'mainstay, backbone' of a group or nation, though 支柱 can also be a formal synonym for a literal pillar.