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Study On The Direct Displacement-based Seismic Design Method For Multi-story Self-centering Braced Steel Frame

Posted on:2020-11-06Degree:MasterType:Thesis
Country:ChinaCandidate:X F YangFull Text:PDF
GTID:2392330575995018Subject:Civil engineering
Abstract/Summary:PDF Full Text Request
Braced frame is a commonly used form of lateral force resisting system.This kind of structure usually exhibit large residual displacement after the earthquake due to the dissipation of seismic energy through its own elasto-plastic deformation during the earthquake,which increases the repair cost.To improve the seismic performance of ordinary braced frame,reduce the repair cost,enhance the functional recovery,a new type of structure form—self-centering braced frame has been widely investigated.Performance-based seismic design has become a hot research direction in the field of seismic engineering,which requires structures to achieve specified performance targets at the expected seismic level.Among many methods to realize the structural performance design,direct displacement-based seismic design method is considered to be an effective approach.In this thesis,the pre-pressed spring self-centering energy dissipation braced steel frame is taken as the research object.Through theoretical analysis and research,the direct displacement-based seismic design method for self-centering braced steel frame structure is established.The main study contents are as follows:(1)Based on the principle that the elastic system and the elastoplastic system are equal in energy dissipation under the same external excitation,the general form of the equivalent damping ratio formula of the equivalent elastic system is derived.The equivalent damping ratios of concrete frame,concrete shear wall,steel frame and ordinary steel braced frame are compared and analyzed under the same ductility coefficient,the result shows that hysterestic characteristics of the structure have great effect on the equivalent damping ratio.By considering bilinear hysteretic energy dissipation of steel frame and the friction energy dissipation of brace,the expression of equivalent damping ratio of self-centering friction energy dissipation steel braced frame is proposed.Stiffness ratio interval of brace,which has impacts on the expression,is proposed.Based on the principle of equivalent linearization,the equivalent damping ratio expression is verified.Results show that the one story and multi-story self-centering braced frame designed with direct displacement-based design method can meet the requirement of performance targets and the brace stiffness demands can be reduced when considering the equivalent damping ratio calculated with the proposed expression.The design value of brace stiffness is larger and conservative when the equivalent damping ratio is not considered.(2)The influence of the lateral displacement curve on the equivalent parameters,the design story stiffness and the design base shear force of multi-story self-centering braced structure is analyzed.The results show that the equivalent displacement,the equivalent mass and the design drift ratios of the structure calculated by different lateral displacement curves are different.If the story shear forces are not significantly different,the stiffness of the bottom story obtained by using the bending-type lateral displacement curve is greater than that obtained by using the linear-type lateral displacement curve.And the stiffness of the upper story obtained from the linear-type lateral displacement curve is smaller than that obtained from the shear-type lateral displacement curve.The lateral displacement curve has little effect on the design base shear force of the low-story self-centering braced structure,but has significant influence on the design base shear force of the multi-story self-centering braced structure.(3)Considering the number of stories and structural nonlinearity,the effects of three kinds of lateral displacement curves of linear-type,convex-type and concave-type on the actual lateral displacement profile,story drift ratio distribution and seismic performance of multi-story self-centering friction energy dissipation braced steel frame are studied.The results show that the design lateral displacement curve has no significant effect on the actual lateral displacement profile of the structure.The actual lateral displacement profile of the three-story braced frames under the major earthquake is nearly linear-type,while the actual lateral displacement profile of the six-story braced frames exhibit the characteristic of bending-type,despite the fact that these braced frames are designed based on different lateral displacement curves.The trend of design story drift ratio's distribution depends on the design lateral displacement curve.Under major earthquakes,although the maximum drift ratio of 3-story and 6-story braced structures designed based on different lateral displacement curves can meet the limit requirements,the upper drift ratio of the three-story self-centering friction energy dissipation braced frame structure is nearly equal,which conforms to the distribution feature of the linear-type lateral displacement curve whose drift ratios are same at each story,and does not conform to the distribution feature of the concave-type lateral displacement curve whose drift ratios gradually decrease from the bottom to the top story.So it is suggested to adopt the linear-type lateral displacement curve to design the low story self-centering braced steel frame about 3-story.The drift ratios distribution of the multi-story self-centering friction energy dissipation braced frame about 6-story has the feature of gradually increasing from bottom to top story,which is consistent with the characteristics of the bending-type lateral displacement curve,and it is advised to adopt the bending-type curve to design.(4)The flow chart of direct displacement-based seismic design method for self-centering braced steel frame is proposed and the differences in seismic performance,such as story drift ratios,residual drift,and base shear of three-story braced structures designed by force-based design method and direct displacement-based design method under minor,moderate and major earthquakes,and the effects of the braces designed with the two method on the internal forces of beam and column are compared and analyzed.The results show that the structure's brace stiffness obtained by the force-based design method is greater,and its seismic performance under minor and moderate earthquakes is better than that of the structure designed using direct displacement-based design method.However,the seismic performance of the two structures is almost the same under major earthquakes,which indicates that the force-based design method is conservative.Direct displacement-based design method can not only achieve the performance target of the structure under major earthquakes,but also make the design more economical.When the activation displacement of brace is the same,the greater the stiffness of brace,the greater the internal force demand of columns connected to the brace.When the braces are arranged in a chevron configuration,the brace parameters have little influence on the shear force and bending moment of the beam,but the structure with large brace stiffness still has large demand on beam axial force.
Keywords/Search Tags:self-centering energy dissipation braced steel frame, direct displacement-based seismic design, performance-based seismic design, equivalent damping ratio, designed displacement profile, seismic performance analysis
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