Research To Methods For Seismic Reinforcement Of Existing Concrete-Masonry Mixed Structures

Nowadays, lots of existing buildings consist of mixed structure systems whichinclude concrete structures and masonry structures in the same buildingsimultaneously. They are also called Concrete-Masonry Mixed Structure systems.However, this mixed system has many disadvantages, such as the significantdifference of elastic modulus and mechanical properties, the complexity of thestructure and unclear forces transmission route, etc. As the infects of those causes,when suffering from lateral seismic forces, different components could be broken oneby one, owing to the incoordination between bearing capacities and stiffness ofvarious materials. The buildings, with the mixed structure systems, cannot meetgeneral seismic precautionary criterion which requires the building should stay goodin minor earthquakes and keeps standing during bigger ones. For the sake of safety,these buildings should be demolished according to relevant codes. However, a hugewaste and other related problems would be found from that.This thesis studied the numerical reinforcement methods of MixedConcrete-Masonry Structures. There are two different reinforcement design schemes:the quasi-concrete structure reinforcement method and the quasi-masonry structurereinforcement method. This study compares the results of finite element softwareETABS and widely used design software PKPM in both theoretical research andactual projects, confirming the availability of using PKPM to reinforce thesestructures. At last, it proposes the pervasive analysis methods and steps for generalprojects.The quasi-concrete structure reinforcement method: In analysis, the originalbearing walls have been regarded as “the splint walls” by reinforcing partial or allbearing masonry walls in mixed structure buildings. Then the splint walls areequivalent to concrete walls, so that the whole structure is equivalent to “thequasi-concrete structure”. Meanwhile, based on specific conditions of actual projects,some concrete walls are added to original concrete frames to resist extra torsion.Disregarding ring beams and constructional columns in the computing model, theoriginal frame beams and columns are checked via PKPM. If can’t achieve theseismic requirements of the codes, the original frame beams and columns would bereinforced, we can obtain the load effect design value (S) after internal force analysisto the quasi-frame-shear-wall structure. Then, resistance (R) can be obtained bychecking the formula of the concrete splints in hand. At last, ifγ0S≤R, that meanschecking computation of the reinforced structure is successful, the method can beadopted.The quasi-masonry structure reinforcement method: Bearing masonry walls areadded to concrete frames, then the structure system is close to the masonry structuresystem. To be distinguished from original one, it is called “the quasi-masonrystructure” because of existence of original frame columns and beams, not themasonry structure. The method refers to the design thinking of new masonry structure projects. First, adding new bearing masonry walls to suitable places of original framestructures, the mixed structure can be converted into a similar masonry structure.Then, after calculating bearing capacity of the walls and seismic shear of the walls byJDJG module of PKPM, the shear-bearing capacity can be obtained according toconstruction technology. Ifγ0S≤R, that means checking computation of the reinforcedstructure is successful, the method can be adopted. However, ifγ0S>R, reinforcementmeasures should be enhanced, such as increasing the strength of material of newwalls, reinforcing original masonry walls again, adding new bearing walls and so on.Then, Repeat checking computing, until meet the requirements.In addition, a preliminary study of stress lagged effects for reinforcedcomponents by using the finite element method has been conducted in this thesis. Theresearch objectives comprise a compound wall of the original frame and the new walland a brick wall of reinforcing concrete splint (i.e., the splint wall). Under theinfluence of stress lagging of the brick walls, the concrete splints bear most horizontalseismic shear force. When major seismic strength occurred, the concrete splints maybe broken before the brick walls. So the actual bearing capacity of the concrete splintsis less than the result of the quasi-concrete structure reinforcement method. Similarly,because of the effect of stress lagging of original frame columns and beams, theactual shear-carrying capacity of the compound walls is less than the result of thequasi-masonry structure reinforcement method.