| The prestressing technology in our country has rapidly developed in several fields such as material,construction craftwork,design theory and technical standard since the mid 80 s of last century,which has reached the international advanced level generally.However,the application of prestressed structure in seismic area is generally more cautious and should take reliable measures.In order to ensure the seismic performance of prestressed concrete structure,most foreign codes use a single index such as reinforced index of longitudinal steels to design the section of frame beam,which is similar to the method of limiting the relative height of compression zone in Chinese code.The actual damage of prestressed concrete structure after earthquake accounts for a relatively small proportion,which is mainly caused by the support structure and node damage etc.Additionally,the partial prestressing ratio(PPR)is set up in Chinese Code.The code is so stringent that the amount of rebar is increased a lot,which is uneconomical and reinforced intensively in joint area.Accordingly,it is difficult to control quality of construction and adhere to the basic design principle of strong column and weak beam.Based on the summary of previous research results,the influence of stringent PPR on the seismic performance of prestressed concrete frame structures is studied in this dissertation.The main research contents and results are as follows:First of all,the formula of the ductility coefficient of section is established through the theoretical analysis.The main factors affecting the ductility are analyzed,which include concrete strength,ultimate compressive strain of concrete,steel yield strength,relative height of compression zone and PPR.It is proved that PPR has little effect on the ductility of section,which provides a theoretical basis for relaxing the PPR limit.Secondly,in order to verify the feasibility of relaxing the PPR limit,two partially prestressed concrete frames whose PPR are 0.85 and 0.95 are tested under static cyclic loads.Based on the frame test results,the failure mode is grasped,and the seismic performance is analyzed such as ducitility and energy dissipation capacity.The test results show that:(1)the pretressed concrete frame structures with high PPR exhibit good seismic performance.The failure modes are ductile fracture,which are similar to concrete frame structures.the hysteretic curve is like bow and full.(2)Although the PPR in test exceeds the upper limit of 0.8,the pretressed concrete frame structures with high PPR exhibit good seismic performance of deformation and energy dissipation.The ductility coefficient is greater than 6.0.The energy dissipation coefficient is close to 2.0.With the increasement of PPR,the performance of deformation and energy dissipation decreased slightly.Finally,according to the results of theoretical analysis and experimental research,the design for prestressed concrete frame structure with high PPR are discussed.Besides the basic principles of seismic design,designer should give priority to control the relative height of compression zone and take steps on the plastic hinge.PPR should be adjusted lastly.In conclusion,recommendation for design of prestressed concrete frame structure is put forward: The upper limit of PPR can be relaxed in Code for Design of Concrete,Specification for Seismic Design of Prestressed Concrate Structures.For second or third grade seismic design,PPR can be relaxed to 0.85,or even be canceled. |
