An investigation on the behaviour of encased steel columns under load

The erection of new buildings at Leeds University provided an opportunity for examining the behaviour of the building frame under load. Stress reductions in the steel frame due to concrete encasement were demonstrated by the work done at the Building Research Station and at Leeds University. The ‘composite’ behaviour of beams, floors and walls has been well covered both theoretically and experimentally in the above mentioned works. On the other hand, the behaviour of the encased columns has been less widely covered. This paper investigates the behaviour of the steel column not only as part of the framework but also as a separate unit. The tests carried out in both the field and the laboratory show beyond doubt that the design practice of the encased column needs serious revision on account of the very low stresses observed under working loads and of the relatively high loads that an encased column can carry when tested to failure. The very low stresses in steel under working load were explained by the use of the ordinary elastic theory of reinforced concrete. The laboratory tests up to failure showed that the use of the reduction coefficients based on the buckling formula given in BS 449 (1948 and 1959) gave very conservative results. It can be shown that closer agreement between calculated and actual failing loads can be obtained by using coefficients based on the buckling formula used in reinforced concrete design.

Professor Royston Jones