Elastic-Plastic Resistances Of Steel Brace, Moment-Resisting Frame And Their Cooperation In Dual System

The dual system consisted of the concentrically braced frames and moment-resisting frames is one of the most common lateral force resisting systems used in the steel constructions in seismic zones. To prevent the premature buckling and unexpected plastic deformation undergoing severe seismic action, the braces in this system were required to be very strong by introducing an amplification factor to the design force and a reduction factor to their compression strengths in the design stage. While the former adjustment to the design of these braces, i.e. the amplification of design force, has been canceled in the latest version of Chinese code GB50011-2010. The braces of large slenderness are considered to have the best performance under seismic load in the some widely accepted codes (e.g. The United States Code, the European Code and Japanese Code), and thus a small design force can be adopted for these braces under the some conditions. The design of the beam in the bays containing chevron braces always becomes difficult, as a very large unbalance force in the vertical direction may form in the cross point of these two braces due to the buckling of the compression brace. To entirely understand the involved issues, a systematic study on the behaviors of the concentric braces and steel frames as well as the composed dual system under severe seismic load becomes necessary. The cooperation of the two components of the dual system, the brace and steel frame, in the elastic-plastic stage is also concerned in this thesis.The beams of the steel frames of the bays containing chevron braces may be strengthened to consider the vertical unbalance force mentioned previously. The lateral force resistances of three typical brace pairs, i.e. the chevron brace pair with strengthened beam, the chevron brace pair with unstrengthened beam and the cross brace pair, are firstly concerned in this thesis when the material nonlinearity is considered. The main concerns include:(1) The load carrying capacities and overstrengths of brace pairs. It is found that the performance of the brace pairs depends largely on the slenderness of the braces. The performance of the braces of large slenderness are the best followed by those of small slenderness, and the braces of medium slenderness have the worst capacity of resisting seismic load. (2) The controlling condition of these brace pairs under lateral loads. The buckling of the compressive brace and yield of tensile brace are the two conditions governing the lateral strength of the chevron brace pair with strengthened beam and the cross brace pair, while this controlling conditions for the chevron brace pair with unstrengthened beam become the buckling of compressive brace and the formation of plastic hinges at the middle sections of the beam, as the yield would not happen to the tensile brace. The drift ratios of the frames containing these brace pairs under these controlling conditions and the relationship between the post-buckling out-of-plane displacements of the mid-span section of the compressive brace and the drift ratio are also given in this thesis. (3) A new strength reduction factor for the compressive brace is proposed, which provides a convenient way to precisely calculate the vertical unbalance force for the steel frames containing the chevron brace pairs in design of beam, instead of the current method in Chinese code.The analyses conducted in this thesis revealed that the capacity of the tensile brace in the chevron brace pairs to resist lateral force is mainly dominated by the strength of the beam. Therefore, the strengthening the beam may lead to the strength of chevron brace pairs may be beyond the load corresponding to the buckling of compressive brace, due to the reduction of the strength degradation of the compressive brace after buckling and the increase of contribution from the tensile brace because of its stronger upper support at the mid-span section of the beam. This lateral strength of the chevron brace pairs may be very close to that of cross brace pairs as long as the beam is so strong to be capable for supporting the yield load of tensile brace. While the lateral load carrying capacity of the chevron pairs with unstrengthened beam decreases significantly as long as the buckling of compressive brace occurs, as the tensile brace cannot reach the yield load in this case, leading to poor performance under seismic load.The lateral load carrying capacity of the moment-resisting frames is then concerned. The relationship between the lateral load and displacement at the top of column for single storey steel frames are analyzed theoretically, where the beams and columns are of I-sections and the elastic-perfectly plastic material model is adopted for steel. The effects of vertical loads including the uniformly distributed loads and a mid-span point load at the beam on the behavior of the moment-resisting frames in supporting lateral loads are studied, and simple analysis models are presented to consider these effects at the different loading stages.Based on the studies on the three brace pairs (chevron brace pair with strengthened beam, chevron brace pair with unstrengthened beam and cross brace pair) and moment-resisting steel frames, their respective cooperation in three dual systems consisted of these two components under severe seismic load are then concerned, in which the material nonlinearity of steel is included. The four interesting issues as follows can be then found:(1) The peak values of the lateral loads of the two components, i.e. the brace pairs and the moment-resisting frames, do not reach simultaneously, which means the capacity of the dual systems cannot be given by simply adding those of the two components. The drill ratios corresponding to the peak values of the brace pairs and moment-resisting frames are about 0.08%-0.28% (buckling of compressive brace) or 0.32%-0.75%(yield of tensile brace) and 0.53%-2.05% (formation of plastic hinges). (2) The overstrength of the dual system, besides the overstrength of material, is mainly due to the potential contributions of the moment-resisting frame and tensile brace after the buckling of compressive brace. The overstrength is more significant for a dual system with a larger contribution of moment-resisting frame to its total lateral load carrying capacity and a smaller slenderness of braces. (3) The minimal value of vertical unbalance force for the beams to support is proposed for the chevron pairs with unstrengthened beam in terms of ensuring basic capacity of supporting seismic load. (4) 25% is an appropriate ratio of the contribution of the moment-resisting frames over the total lateral force in a dual system:almost no descending branch can be found in the lateral load-displacement curves of a dual system with the cross brace pairs; the hardening stage can be seen in the curves of a dual system with the chevron brace pairs of medium slenderness, the worst case, right after a slight decrease of lateral load (about 20%).Studies are also conducted on the effects of the gravity load to the performance of steel frames with chevron brace pairs under lateral loads, from which it can be seen that the gravity load may advance the buckling of compressive brace and postpone the tensile brace, while to the overall lateral strength of the dual system its effects are very limited.The hysteretic behavior of the dual systems containing the chevron brace pairs with strengthened beam is analyzed. Dramatic decreases on the lateral load of brace pairs can be seen from their skeleton curves after the buckling of the compressive braces, which indicates the slip-type hysteretic properties of the chevron brace pairs.