Study On The Mechanical Behavior And Design Method Of Short-Deep Plate Girders

In heavy power plants,steel plate girders are utilized to suspend the tower-type boilers.The span of the steel plate girders used in large power plants may exceed 40 m and the depth of sections can reach 8~10m.Such large scale and deep sections are not common in ordinary engineering structures.According to the shape and loading characteristics of this kind of steel beams,flexural members with span-to-depth ratio less than 10 and shear ratio higher than 0.5 are called "short-deep plate girders" in this thesis.For steel beams,the higher sections can be a more effective design of material usage,but also brought some new problems.Short-deep plate girders are subjected to the interaction of bending and shear,and at the same time their web plates are high and thin,thus influence of local instability on the overall mechanical behaviors of the members must be considered in the design process.In addition,compared with traditional beam member characters,the span-to-depth ratio of the subject girders are smaller,and so that the influence of shear deformation o n the mechanical behaviors is very significant.Aiming at these two major problems,this thesis studies on the mechanical behavior of short-deep plate girders through experimental investigation,analytical theories and optimization of design process,respectively.The specific work is as follows:At the first part of the thesis,the existing section classification procedures in all countries were summarized.Conclusions were made that the web-flange interaction and the local-global interaction are two of the significant features that influence cross-sectional behavior but have received little consideration.The existing design procedures are based on the premise that the shear ratio flexraul members are under 0.5,thus they cannot be effectively used on short-deep plate girders.A reasonable procedure is to calssify the flexural members based on member ductility.But the calculation of member ductility could be complicated,thus hard to be used in practice.In this thesis,a member classification strategy was produced based on failure mode prediction.The proposed strategy can extend the application scope of classification design procedure,and,at the same time,include the local-global interaction effects.The design and application of steel beams,as well as the stability of thin-walled steel beams,are all based on the corresponding beam theory models.Therefore,the research status of beam theory was reviewed.Timoshenko beam theory includes the effects of shear deformation,but it is limited to the flexural analysis of solid beams,just like Euler-Bernoulli beam theory.Vlasov beam theory is a classical solution to analyze the flexural and torsional behavior of thin-walled beams with open sections.However,it is only applicable for thin-walled beams whose span-height-ratios are greater than 10,due to the ignorance of the middle-surface shear deformation.So the generalized beam theory was employed herein,which can consider shear deformation and local buckling at the same time.Comprehensive effects of different deformation mode sets,and especially the influence of shear deformation,were studied.Thus,a reasonable,reliable mechanical model was built for the short-deep plate girders.After that,tests of four I-section plate girders with high shear ratio were presented and investigated,and the span-to-depth ratio of the specimens were among 3 to 6.Based on the test results,the interactive buckling types of this kind of member with high shear ratio were clarified in detail.For some of the members,shear buckling effects of the web panel were the conducting mechanical characters of member behavior.It has became apparent that the existing section classification methods based on the “individual plate rule” are not suitable for the short-deep plate girders,treatments to decouple them from each other in design process are improper.An appropriate member classification procedure should be established based on failure modes of the members.On the basis of the experimental work,studies on design properties of short-deep plate girders were conducted.A reasonable design procedure of short-deep plate girders should take into account the joint effects of the design parameters such as the width-to-thickness ratio of the constructing plates,the span-to-depth ratio of themembers and the distance between transverse stiffeners,as it is told by the work on standard beams.Taking advantage of the idea of member classification,and based on parametric study,members controlled by the shear buckling effects of the web are summarized under certain conditions.Suggestions were made to classify members controlled by bending and shear through design parameters,and to use traditional interaction formula and parameter-based emprical formula respectively.A member classification proposal with three classes,i.e.compact,non-compact and slender member,according to the load bearing capacity of the members,was provided,and the interaction effects including web-flange interaction and local-global interaction,as well as shear-buckling effects,were taken into account.And then,in order to get a more universal design approach,a plastic local buckling mechanism consists of plastic zone and the yield line was used to describe the failure modes of flexural-torsional members.The influence of plastic local mechanism on member ductility was discussed.Accordingly the calculation method of member ductility based on local plastic mechanism was produced,which could be used in plastic member design.The design method presented in this paper can comprehensively predict the failure mode,bearing capacity and deformation capacity of I-section flexural members,and is suitable for short-deep plate girders.Then,in order to establish a more reasonable mechanical analysis model,based on the traditional Generalized Beam Theory(GBT),a new formulation of the Generalized Bea m Theory(GBT)that coherently accounts for shear deformation was presented in this thesis,and the application value of which is estimated.Based on the algebraic space theory,a new GBT section analysis method was proposed to solve the problem that bending deformation and shear deformation can not be completely decoupled in traditional GBT theory.This thesis focused on the explanation of the orthogonal decomposition and physical meaning of various deformation modes in the GBT section modal space of double-axisymmetric I-section members.Much attention was posed on the mechanical interpretation of the deformation parameters in the modal space.It was shown that,in the modal space,it is possible to clearly distinguish bending deflections from deflections due to shearing strains,and to recover classical beam degrees of freedom and standard beam theories as special cases.The nonlinear GBT finite element model was established,and the corresponding program was compiled by FORTRAN language.The accuracy of the model was verified by the typical experimental results and a benchmark work.The participation factors of rigid body modes,local plate modes,and shear deformation modes were compared among four groups of examples.When the span-depth ratio of plate girders exceeds 5,participation of cross-section rigid body deformation is more than 90%,and the overall beam failure is mainly lateral-torsional buckling.When the span-depth ratio is less than 5,participation of local plate deformation and shear deformation significantly increases,and overall failure configuration characteristics with local buckling of web plate.Through the modal decomposition of the displacement field of the members with various failure modes,the mechanical connotation of the classification design procedures was explained,and the essential characteristics of the mec hanical properties of the short-deep plate girders were revealed.In practical engineering,influenced by the restrictions of transportation and construction,some short-deep plate girders are designed to be superimposed.Experimental investigations on two I-section plate girders,two homogeneous composite girders,and two hybrid composite girders,were implemented.And researches on the analytical and design methods of such members were conducted.In a composite beam,the laminated surface has the function of a longitudinal stiffener,which can improve the local stability of the member.In addition,the bolts of the laminated surface are bearing part of the shear force.Through check experiments,conclusions can be made that bearing capacity of composite girders is superior to the I-section ones,and becaused of the usage of steel plates with higher strength and indeed smaller thickness,hybrid composite plate girders can further balance the economic and mechanical efficiency of constructions.Lastly,the main conclusions were summarized,and some research points for future work were given.