Design Method And Engineering Application Of Prestressed Concrete Transfer Truss

In recent years,the transfer floor has been used widely to extend the structural function,and it has already become a trend.Compared to other types of transfer structure,the transfer floor helps to make use of the building space more sufficiently.Moreover,with its inherent advantage of clear force-bearing state and minor stiffness mutation,the truss girder usually suffers less earthquake effects.For those long-span girder transfer floor with large vertical loads,it could be converted to pre-stressed concrete truss by applying pre-stress to the tensional chord members,which makes it significantly important to study the characteristics of pre-stressed concrete truss beam.In this article,based on a practical project containing pre-stressed concrete truss beams,we conduct a specific research on the design method of transfer truss,especially on the structural response due to earthquakes.Firstly,we use the general finite element software SAP2000 to determine the structural force under the vertical loads and horizontal seismic loads according to the updated Chinese building code,and calculate the pre-stressed and normal reinforcement of the bottom chord members;Secondly,by using finite element software SAP2000 and ANSYS and mode-superposition response spectrum method and time-history analysis simultaneously,we analyze the structural dynamic performance of this pre-stressed concrete transfer truss under different earthquakes,some conclusions are summarized:(1)The pre-stressed reinforcement of the bottom chord members should be calculated with the formula?sc-?pc??ctftk,?lc-?pc?0.5tk,and the normal reinforcement should be determined by the requirements of strength and ductility.Our analysis shows that the tension of pre-stress is generally dominated by loads' long-term effects combination.(2)On the basis of summarizing the previous engineering practice,it's recommended that the method 'staging and batching stretch-draw' should better be used for applying pre-stress on transfer truss.Namely,the pre-stress is applied by stages and batches on proper constructing floors,which can decrease the effects of difference between construction and service phases.(3)According to the Chinese seismic code,the vertical seismic loads are not needed for most structures located in the intensity-seven region,however,our finite element analysis on the transfer truss beam shows that the horizontal earthquake load has very small influence on the structural force,but the vertical earthquake load could have a significant effect on the members of transfer truss,which reminds us that we should pay attention on the vertical seismic load during the design of such kind of structures.