<p> Many collapse damages of suspended ceilings during recent severe earthquakes were observed. This ceiling damage has come to draw a lot of attention. In order to assure the safety of facility users, high level aseismic design is required for suspended ceiling. For this purpose, braces were installed between ceiling level and top of hanging bolt to transmit lateral load of ceiling surface to structural component such as upper floor slab. Also, high level aseismic parts were developed. In this paper, connecting parts between brace top and hanging bolt is focused. Newly developed high spec connecting parts behaves as rigid body, which has one degree of freedom of angular displacement which is yawing angle around hanging bolt (Fig. 4) by Euler buckling of brace. This yawing angle arise torsional angle around axis of brace because brace has oblique angle (α₀) from hanging bolt (Eq. 1).</p><p> </p><p> Static lateral loading tests for unit models were conducted for various braces member and various length of hanging bolts (Table 1). Their ultimate damage modes are classified into following three modes (Table 3),</p><p> (1) Flexural-torsional buckling of brace</p><p> (2) Slip of connecting parts between brace and hanging bolt along hanging bolt downwards</p><p> (3) Top of board screws penetrate board and joists separate from boards. As the results, ceiling surface cannot hold lateral load</p><p> In this paper, ultimate damage mode (1) is focused, and evaluation formula of flexural-torsional buckling capacity is developed based on equilibrium between external supplied energy and internal strain energy (Eq. 27). From the formula, flexural-torsional buckling capacity is characterized by Euler buckling amplitude which is defined as a_c in Eq. 29.</p><p> </p><p> After Euler buckling occurred in brace and buckling amplitude was increasing according external energy was being supplied, following two phenomena could be observed, respectively.</p><p> (1) Yielding of brace by bending moment of Euler buckling mode at Euler buckling amplitude a_Ey which is defined in Eq. 33</p><p> (2) Flexural-torsional buckling of brace at Euler buckling amplitude a_c</p><p> (a) In the case of a_Ey < a_c, flexural-torsional buckling does not occur because bending yield occur before flexural-torsional buckling occur.</p><p> (b) In the case of a_Ey > a_c, flexural-torsional buckling occur.</p><p> </p><p> Following three numerical examples are shown in 4.3, Fig. 8-Fig. 10</p><p> Case (a) Brace member C-25×19×5×1.0 L = 2000 in which a_Ey > a_c and flexural-torsional buckling occur.</p><p> Case (b) Brace member C-40×20×1.6 L = 2000 in which a_Ey < a_c and flexural-torsional buckling does not occur.</p><p> Case (c) Brace member C-40×20×1.6 L = 3400 in which a_Ey > a_c and flexural-torsional buckling occur.</p><p> Finally, critical brace length (L_min) which satisfy a_Ey = a_c are also given in Table 4 for typical brace members. When brace length L is shorter than L_min, flexural-torsional buckling does not occur.</p><p> </p><p> In reference 10), it was clarified that Euler buckling did not lose but maintain aseismic capacity for further load. But flexural-torsional buckling treated in this paper lost aseismic capacity for further load as shown in Fig.s 8 and 10 Considering these situation, special attention must be paid for flexural-torsional buckling phenomena in the aseismic design for ceiling.</p>