A legal nonconformity (i.e., parcel, structure, use) may be maintained and continued with customary maintenance and repair without any expansion or enlargement of area, space, or volume. A legal nonconformity may be maintained; provided, that no structural alterations shall be made to structural supporting members (e.g., bearing wall, column, beam, girder, rafter, joist, etc.), except as required for health and safety as determined by the chief building official.