Buckling-restrained braces (BRBs) are composed of a steel core, which resists axial force, and a buckling-restraining system. A carefully designed clearance, often filled with unbonding material, is placed between the steel core and restraining system to avoid force transfer between the two components. A well designed BRB provides similar strength in tension and compression, and exhibit stable hysteretic behavior. BRBs are installed in a frame structure to serve as seismic response-control devices that protect the beams and columns from plastic deformation. Ideally, such a damage-controlled structure can restore its original structural performance following a severe earthquake after replacing only the BRBs. The authors have developed a new variation of BRBs that use steel-and-mortar planks for the buckling-restraining system and which allows for simple and reliable quality control. A large number of laboratory test results suggest that the BRBs using steel-and-mortar planks exhibit excellent performance on par with widely used commercialized products. Furthermore, the authors have used the experimental data to derive a design equation that predicts the compressive-to-tensile strength ratio as a function of the axial strain amplitude and slenderness ratio of the steel core. This paper reports recent findings from the research effort to develop BRBs with improved reliability and economy. The emphasis is on the buckling deformation of the flat-plate steel core and force transfer between the steel core and buckling-restraining system. Static cyclic-loading tests were performed on six BRBs. The specimens used four different slenderness ratios for the steel core, where the slenderness ratio is defined as the length of the yielding segment divided by the radius of gyration about the weak-axis of the steel core. The newly obtained data was combined with past data to refine the equation of compressive-to-tensile strength ratio to account for the distribution of axial force along the length of the steel core. The test data suggests that the buckling-mode number is proportional to the slenderness ratio of the yielding segment of the steel core. Measurement from strain gauges densely attached to the steel core, as well as deformation of the steel core observed after the test, indicate that the compressive force in the steel core is largest at the ends and smallest at a point that is fixed relative to the restraining system. The experimental observations motivated the authors to modify the equation for the compressive-to-tensile strength ratio to incorporate the location of fixed point between the core plate and restraining system. The modified equation provided better match with available test results.