Research On Mechanical Properties And Design Methods Of Concrete Beam With Encased Steel Box

Concrete beam with encased steel box (CBESB) refers to the beam composed of concerete, encased steel box and reinforcement cage, in which steel box is made up of channel or steel plate welded into box and reinforcement cage is binded around the steel box, at last, concrete is cast. CBESB can be either used in the ordinary new building or used for main beam or secondary beam in the first top floor of new-added outer-jacketing structure for adding stories around the existing buildings; also it can be used for frame beam or secondary beam in mega story of mega structure. It can greatly decrease self-weight of structure when used in new building. The floor is selt-supporting during construction when used in outer-jacketing structure or mega structure because the bottom formwork can be hung from the steel box, on which side formwork can be installed, and the steel box can carry the fluid concrete weight and construction load. The self-supporting floor can ensure the safety of the existing building and decrease the expense of shore in construction of mega frame structure building and shorten the construction period.Compared with concrete beam with encased H-steel, CBESB has its own characteristics, and at the same time, there is no systematic report on plastic properties of indeterminate CBESB. In consideration of these problems, experiments on 5 simple CBESB and 4 continuous CBESB are performed, and the valuable test data are got.Compared with concrete beam with encased H-steel, the ratio of touching area of steel box in unit length and concrete to the cross section area of steel box usually is relatively smaller. On the other hand, the encased steel box can not restrain the concrete beside the web effectively as that of encased H-steel. Considering these characteristics, the reduction factor of flexural bearing capacity is introduced into the calculation formula of flexural bearing capacity which is based on the plane section assumption. This reduction factor can reflect the characteristic that the flexural bearing capacity of CBESB is relatively small. The value of the reduction factor is given by comparing and analyzing test results and calculated results. In this way, the calculation formula of flexural bearing capacity of CBESB is perfected.In view that the touching area of steel box in unit length and concrete is relatively smaller and the bond property between steel box and concrete is weak, the calculation formula of the average crack spacing is amended by using actual touching area and the reduction factor of bond property in the calculation formula; the value of influence factor of elongation of concrete to crack width between cracks is calculated through analyzing test data, so the calculation formula of the average crack width of CBESB is obtained; the magnification factor showing the ununiformity of crack width is got by fitting crack distribution frequency diagram. At last, calculation formula of crack width of CBESB is brought forward.Considering the fact that steel box is hollow, and it can not restraint the concrete beside web effectively and the slippage between steel box and concrete is relatively bigger, the idea for calculating stiffness of CBESB is introduced, that is: the total stiffness is composed of stiffness of steel box to neutral axis of cracked transformed section and stiffness of concrete beam around the steel box. Then, the calculation formula of stiffness of CBESB is given.The frame beams and continuous secondary beams in floor are allowed to be designed in plastic method in ordinary building. Usually, the sufficient redistribution of plastic internal force can occur in indeterminate steel beams, and it can be designed on condition that all the fibers in section enter into plastic stage. A lot of scholars have made researches on plastic design of concrete continuous beams and slabs, and the moment modification coefficient has been listed in table. Based on plastic design of steel beams and concrete beams and slabs, the object can be modified is put forward, which is the residual of removing flexural moment of steel box in ultimate limit state on the plane section assumption from elastic calculated moment of crictical section at intermediate support. Based on test results, and making yield of tensile flange of steel box and yield of tensile of longitudinal reinforcement as occurrence sign of plastic hinge respectively, the value of the length of equivalent plastic hinge zone and the calculation formula of moment modification coefficient with the variable of plastic rotation are presented. Morever, the calculation formula of moment modification coefficient with the variable of relative depth of concrete compression zone is presented.Design propositions are presented for CBESB including general principles, selection of materials and sections, calculation of resistance, control and check of crack, control and check of deflection, construction detail requirements and so on.