Investigations On The Key Prob-Lems In The Mechanical Proper-Ties Of Cold Formed Stainless Steel Structures

Stainless steel is a high-performance green building material with super corrosion re-sistance,excellent mechanical properties,and has overwhelming advantages over regular steel material for structures,which have critical demands of corrosion resistance,high bearing ca-pacity,and beautiful architectural appearance.Stainless steel has increasing applications in building constructions but still faces a big challenge due to the high comparative expense of material relative to low carbon steel.Stainless steel has considerable strain hardening and can be easily enhanced in cold forming processes,which owns the opportunity to significantly re-duce the initial cost.However,there are few researches on the cold forming effect on the be-havior of stainless steel structures.Thus this dissertation mainly focuses on the cold forming effect on the behaviors of materials,members,and joints in cold formed stainless steel struc-tures and aims to develop a series of design methods for cold formed stainless steel structures considering cold forming effect through theoretical analysis,experimental validation and nu-merical simulation.The proposed design methods enable improvement in both accuracy and efficiency,provide reliable guidance for specification revision.Researches in this dissertation include three parts:(1)New material model and hardening rules for stainless steel were proposed,and residual stresses distribution models and material yield stress distribution models were proposed for two types of forming process:Press-braked and Cold-rolling,respectively.49 coupons,in seven different conditions were tested.A new three-stage material model together with a series of hardening rules for each model parameter were proposed.Residual stresses were measured on 160 points from eight cross sections.44 coupons from virgin materials,24 coupons from corner regions of Press-braked members,94 coupons from Cold-rolling members were tested to collect require experimental data.The residual stresses distribution models for Press-braked cross sec-tions were deduced based on the proposed material model and the plane strain pure bending assumption.Whereas residual stresses distribution models for cold-rolling cross sections were proposed based on experimental results.Moreover,material enhancement models for cold formed cross sections were developed according to equivalent strain method.Compared with relevant test data collected in literatures,prediction results of the proposed method showed good match.(2)Design methods with consideration of the cold forming effect were proposed columns and beams.23 stub columns from eight cross sections,24 long columns from four cross sections and eight beams from four cross sections were tested.Finite element models of stainless steel plates,columns and beams were developed in Abaqus with validation using the experimental data.The sensitivity study in the finite element analyses was conducted to investigate the influ-ences of cold forming effect and geometric imperfections on the behaviors of cold formed mem-bers.The column curves for columns in circular hollow sections and rectangular hollow sec-tions were proposed,respectively based on characteristics of global buckling behaviors of col-umns.Moreover,the tangent elastic modulus method for columns failure in global buckling were proposed considering the variety of material properties.The effective width method in-cluding the material strain hardening effect for rectangular hollow members failure in local buckling were developed.The design method for rectangular hollow columns failure in local-overall buckling was then built by combining the effective width method for local buckling and the column curve for global buckling.The bending capacity of rectangular hollow members considering the the interactions between plates of the cross section were improved both in the effective width method and the direct strength method,respectively.Through comparison with relevant test data in literatures,it indicates that the proposed design method provide more pre-cise and economical predictions than the design methods in current specifications.(3)Design methods considering the cold forming effect were proposed for circular and rectangular hollow section K joints.Six rectangular hollow section simple joints and five cir-cular hollow section K joints were tested.Finite element models for K joints were developed in Abaqus with validation using the experimental data.Sensitivity studies in the finite element analysis were performed to investigate the influences of the cold forming effect and the welds on the joint performance.Design method for circular hollow section K joints were modified by taking the variety of material strengths into account.Based on yield line mechanism theory,the perdition formula of bearing resistance for rectangular hollow section K joints under chord face-side wall failure was deduced.The parameter Qy accounting for the material strength variety was proposed.The design method for rectangular hollow section K joints was improved.By comparing with relevant test data in literatures,it demonstrates that the predicted results of the proposed design methods provides good accuracy.Through the above investigations,a series comprehensive tests were completed on the same batch of stainless steel plates,including material tests on stainless steel plate in annealed condition,cold forming effect of cold formed members,mechanical behaviors of cold formed members and joints.And,the parameters of the cold forming machine were also carefully rec-orded.A series of new design methods considering the cold forming effect were developed for cold formed stainless steel plates,members and joints.Compared with the current design spec-ification,the proposed methods show better accuracy in the resistance predictions and higher economical efficiency in material utilization.This work provides reference for future specifi-cation revision.