Mechanical Analysis Of Temporary External Cables In The Construction Phase Of Outer-inclined Low-pylon Cable-stayed Bridge

The low-pylon cable-stayed bridge is a composite-bridge structure with the main pylon,main beam and stay cables as the chief bearing components.It has the outstanding characteristics of low pylon,rigid beams and concentrated cables.The outward-inclined low-pylon cable-stayed bridge is the shape of the main pylon leaning outwards,and the bending moment generated by the weight of the inclined tower column at its root is used to offset the influence of the stay cable’s horizontal component on the root of the upper pylon column.However,before the stay cable is tensioned,the concrete at the root of the upper pylon column is prone to tensile stress,which is unfavorable to the force of the whole bridge.In addition,the main girder of the low-tower cable-stayed bridge is relatively rigid and equipped with cable stays.Based on this feature,a cable-stayed construction technology that can shorten the construction period of the cable section and create parallel construction space is proposed.Due to the characteristics of the anchorage position of the longitudinal prestressed steel strands on the top plate of the main girder in the cable zone,tensile stress is likely to appear on the upper edge of the main girder during cantilever pouring,so further mechanical analysis of the construction phase is required for this process.This paper takes the Chengxiang River Bridge(87+160+87,outward-inclined low-pylon cable-stayed bridge)as the background project,and uses Midas civil Finite Element Software to establish the overall model of the construction phase.Meanwhile,This paper will perform mechanical analysis of the construction process in allusion to the upper pylon column and the main beam owning the stay cable.The main research contents are as follows:First,the upper pylon column of the Chengxiang River Bridge is in the state of a single pylon leg,and the stress distribution of the concrete section at its root is analyzed.It is further proposed to set temporary transverse cables between the pylon legs,and the theoretical formula is used to calculate the root section stress when the upper pylon column is poured in sections.It will calculate the optimal anchoring position and pre-tension value of the temporary transverse cable by trial calculation,and finally use the finite element software to establish the upper pylon column construction stage model,and compared and analyzed with the theoretical calculation results and the measured value,in order to determine the pretension value and the installation position of the temporary transverse cable between the tower limbs.In addition,a number of finite element overall construction models have been established to compare and determine the optimal time for the removal of temporary transverse cables,which is in order to ensure that the overall-bridge structure will not be affected by the temporary transverse cables during the subsequent construction and completion of the bridge.The lagging construction process of stay cables means that when the main girder owning the stay cable is suspended and poured,the hanging basket is moved forward in advance,and then the stay cable is stretched and the next beam section is tied to form a parallel construction mode,thus shortening the construction period.According to the layout characteristics of the longitudinal prestress tendon of the Chengxiang River Bridge,the four kinds of cable-stayed hysteresis technology(the no-measure hysteresis-tension technology,partial hysteresis-tension technology,temporary external tendon hysteresis-tension technology,longitudinal prestressed tendon tensioning in sections hysteresis-tension technology)is proposed.It will use finite element software to establish a model of cable-stayed lagging construction stage,and carry out mechanical analysis and comparison of the construction stage with traditional construction,so as to determine the optimal plan and perform the local mechanical analysis and economic analysis.According to the above research content,the following conclusions can be drawn:According to the theoretical calculation formula,it is necessary to arrange two temporary lateral cables during the pouring of the upper tower column,each of which is stretched twice for a total of 120 t.The calculation results are compared with the model calculation results and the the actual measured values,and they are basically the same.And the root concrete tensile stress value meets the requirements during the construction phase,which provides a theoretical basis for the subsequent construction of similar bridge towers;In addition,the detailed calculation of the finite element analysis software shows that the bridge is in After S5 stay cable is tensioned,the effect of temporary horizontal stay cable on the structure is minimal.According to the characteristics of anchoring the longitudinal beam of the main beam in the beam section of the cable area,in order to shorten the construction period and increase the parallel working space,four kinds of lagging construction techniques for stay cables are proposed.Excessive stress loss is not applicable.The other three lagging techniques have no adverse effects on the subsequent construction and the main bridge after the completion of the bridge.After comparative analysis,when the temporary external cable construction technique is adopted,it can shorten the construction period and ensure having no tensile stress on the upper edge of the main girder during the cantilever casting process,so this scheme is determined to be the best scheme;The local mechanical analysis of the anchorage area of the main girder roof with temporary external cables is carried out,and the analysis shows that the maximum compressive stress appears around the roof anchor hole,but there is no tensile stress;And the use of stay cable lagging construction technology saves equipment rental costs,which has good economic efficiency.