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【正文】 ss ?, shear stress ?, and local pressive stress ?c(or paratively large? and?) exist simultaneously at the edge of the effective web depth of build– up girders, e. g. at the intermediate support of a continuous girder or at a section where the flange changes its dimensions, the reduced stress shall be checked by the following expression 2 2 2c c 13 f? ? ?? ? ?? ? ? ? (－ 1) where ?, ?, ?c— normal stress, shear stress and local pressive stress occurring simultaneously at a same point on the edge of effective web depth. ? and ?c are calculated by Formulae () and (－ 1) respectively, while ? is determined as follows: 1nMyI?? (－ 2) ? and ?c are taken as positive while being tensile and negative while pressive。 In— moment of inertia of the beam section。 y1— distance from the calculated point to the neutral axis of the beam section。 ?1— amplification coefficient of design value of strength for reduced stress, ?1= when ? and ?c are of different signs, ?1= when ? and ?c are of the same sign or when ?c =0. Overall stability Calculation of the overall stability of the beams may not be needed when 25 one of the following situations takes place: 1. A rigid decking (reinforced concrete slab or steel plate) is securely connected to the pression flange of the beam and capable of preventing its lateral deflection。 2. The ratio of the unsupported length, l1, of the pression flange of a simply supported rolled H or uniform Isection beam to its flange width, b1, does not exceed the values given in Table . Table Maximum l1/b1 values of simply supported rolled H or uniform Isection beams to avoid checking for overall stability Steel grade Beams without intermediate lateral support, load acting at Beams with intermediate lateral support, load acting anywhere the upper flange the lower flange Q235 Q345 Q390 Q420 Note: The maximum l1/b1values of beams made of steel grade other than those shown in Table shall be that of Q235 steel multiplied byy235f. For beams devoid of lateral support within the span, l1 is the span length。 for those provided with lateral supports within the span, l1 is the distance between these supports (beam bearings are considered as supports). Except for the situations specified in Clause , members bent in their principal plane of largest rigidity shall be checked for overall stability as follows: xbxM fW? ? () where Mx— maximum bending moment about the strong axis。 Wx— gross section modulus of the beam with respect to pression fibers。 ?b— overall stability factor determined according to Appendix B. Except for the situations specified in Clause , H and Isection members bent in their two principal planes shall be checked for overall stability as follows: yxb x y yMM fWW???? () where Wx, Wy— gross section moduli about x and y axes with respect to pression fibers。 ?b— overall stability factor for members bent about the strong axis, same as in Clause . Simply supported box section beams not conforming to the first situation 26 specified in Clause shall have their cross section dimension (Fig. ) meeting the relationships h/b0≤ 6 and 1 0 y/ 95(235 / )l b f?. Simply supported box section beams fulfilling the above requirement may not be checked for overall stability. Figure Boxsection Detailing measures shall be taken to prevent twisting of the section at beam end supports. Lateral bracings for reducing the unsupported length of the pression flange of a beam shall have their axial force determined in accordance to Clause by considering the pression flange as an axially loaded pression member. Local stability Builtup girders subject to static or indirect dynamic loading should take account of web postbuckling strength with their bending and shear capacities checked in accordance with subchapter 。 crane girders and similar members subject to direct dynamic loading, or other girders not taking account of postbuckling strength, shall be provided with stiffeners in accordance with Clause . In case 0 w y/ 80 235 /h t f?, web stability shall be checked according to the requirements of clauses through . In checking the web stability of light and medium duty crane girders, the design value of crane wheel load may be multiplied by a reduction factor . Stiffeners shall be provided for webs of builtup girders in accordance with the following provisions (): 1. When 0w/ht≤y80 235/ f, transverse stiffeners shall be provided for girders with local pressive stress(?c≠ 0) in accordance with detailing requirements, but may not be provided for girders without local pressive stress(?c=0). b0 b1 b2 tw tw t 2 t 1 h 27 2. Transverse stiffeners shall be provided in case 0 w y/ 80 235 /h t f?, among which, when 0 w y/ 170 235 /h t f?(twisting of pression flange is restrained, such as connected with rigid slab, surge plate or weldedon rail) or 0 w y/ 150 235 /h t f?(twisting of pression flange not restrained), or demanded by calculation, longitudinal stiffeners shall be added in the pression zone of large flexural stress panels. For girders with considerable local pressive stress, additional short stiffeners should also be provided if necessary. h0/tw shall in no case exceed 250. In the above, h0 is the effective web depth (for monosymmetric girders, h0 shall be taken as twice the height of pression zone hc in judging whether longitudinal stiffeners are necessary), tw is the web thickness. Figure Layout of stiffeners 1 transverse stiffeners。 2 longitudinal stiffeners。 3 short stiffeners 3. Bearing stiffeners shall be pro
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