S.C. Code § 57-13-70
If it shall be necessary to erect or repair any bridge over waters of this State which shall constitute a boundary line between counties, the governing bodies of such counties shall cause such bridge to be erected or repaired in the manner provided in Section 57-13-60, each county bearing an equal share of the expense so incurred. And when any such bridge already exists or shall hereafter be built, such governing bodies shall divide the same, by measurement from the center to the end of each approach, and each county shall be responsible for the good condition of the half next adjoining such county. And when it becomes necessary to build a new bridge or to replace entirely an old one which has been carried away or destroyed, the governing bodies of the two counties shall do so, as aforesaid.
S.C. Code § 57-13-70