Klöppel and Goder 1957
The authors performed collapse load tests on hollow and concrete-filled steel tubes. They also established a formula for the design of CFTs. Three tests were examined in detail, with stresses and strains in both the steel and concrete tabulated for incremental values of concentric load. The paper was published in German, but much of the test data may be found in Knowles and Park, 1970.
Klöppel, K. and Goder, W. (1957). “Traglastversuche mit ausbetonierten Stahlrohen und Aufstellung einer Bemessungsformel,” Der Stahlbau, Berlin, Vol. 26, No. 1, January 1957.