The restoring force characteristics of the plane steel frames with braces are clarified by a series of the cyclic loading experiment and some analyses. It is one of the Japanese side supporting tests on the full-scale seismic tests of the six-storied steel building conducted under the U.S.-Japan Co-operative Research Program Utilizing Large-Scale Testing Facilities.

The test frames were modeled to be the lower part of the high-rise building .with concentric K-type braces. So, the vertical forces, corresponding to the lateral force, were applied to the test specimens.

Results on the experiment and analyses are summarized as follows;

i)	The test frame with the composite floor slabs had a very ductile load-displacement relation. The ductility factor of the second story in Specimen No.1 was 24.4 in the positive loading direction.
ii)	The ultimate lateral load carrying capacity of the K-braced frame could be deeply concerned in the bending strength of the beam in the braced bay.
iii)	As to the braces buckled in plane, the K-factor for the buckling was equal to 0.8 to 1.07, assuming that the member length of the brace was inside measurement of gusset to gusset and the evaluation equation for buckling force was Johnson-Parabola.
iv)	In estimation of the post-buckling behavior of braces, Paris'solution could give a good approximation, assuming that the brace in the frame such as the test structure would be modified to be the pin-supported member with the length of 50 to 65 percent of the inside measurement of gusset to gusset.

*1	Research Engineer, The First Earthquake Engineering Division,International Institute of Seismology and Earthquake Engineering
*2	Head of Structure Division, Structural Engineering Department
*3	Research Engineer, Structure Division, Structural Engineering Department
*4	Structure Engineer, Toda General Constructor Co.
*5	Structure Engineer, Maeda General Constructor Co.