Distribution of tensile load in relation to temperature and sag of steel-cored aluminium conductors

The main objects of the investigation described in this paper may be summarized thus:¿ (i) To determine the effects of temperature and tension upon the sharing of the tensile load between the steel and aluminium components of a steel-cored aluminium (s.c.a.) conductor. (ii) To ascertain whether variations in the load distribution seriously affect the normal sag and safety of the conductor. (iii) To propose measures for ensuring that the normal sag remains unaffected after the stringing of the conductor. Tensions in the steel and aluminium at various points along typical s.c.a. conductors were measured by a magnetic method; a single length of steel-cored copper (s.c.c.) conductor was also included in the tests. The effects of load-distribution changes were also estimated by noting the corresponding variation in the elastic modulus Ec of the conductor. Adhesion between the steel and aluminium was also examined. The results indicated that the final load distribution is determined by the rate of creep in the aluminium wires, and is independent of the stringing temperature. Although more data concerning creep are required, the tests suggested that it will be some years before creep has any marked effect. It was found that the inelastic stretch of a conductor can cause large increases in sag which far outweigh those that might result from creep. Most of the inelastic stretch was removed when the conductor was tensioned up to the maximum design load, and pre-stressing of the conductors before final stringing is therefore strongly recommended. The steel core appears to carry slightly more than its share of the total load, but since its safety and that of the conductor are not threatened thereby, no change in the B.S. Specification governing the strength of s.c.a. conductors is advocated.