| A new cable-stayed bridge structure of single pylon earth-anchored PC cable-stayed bridge was researched aiming to obtain the proper system and structural parameters,and parameter interval.The structural style was studied,the topographical condition,the construction technique and the equilibrium mechanism of main beam were discussed and the proper system was determined.Based on this,the structural behavior evaluation system to ensure safety was established,taking into account economic efficiency and styling requirements.And then the research of reasonable range of key structure parameters of single pylon earth-anchored PC cable-stayed bridge was conducted.The main work and main conclusions were listed as followings.(1)According to the investigation of project case,it was found that the proper topographical condition for single pylon earth-anchored PC cable-stayed bridge was valley of rolling hills,and the proper construction technique was cantilever casting construction for main span and cantilever casting construction or bracket construction for side span.(2)The equilibrium mechanism was studied according to the calculation of internal force under dead/live loads of different cable-stayed bridge systems.The result showed that the structural characteristic of single pylon earth-anchored PC cable-stayed bridge was that the cable horizontal force difference on both sides of the pylon was larger than conventional cable-stayed bridge,so that there was a need to transmit the unbalance force to the foundation with the help of the pylon,the pile cap,the anchorage after the force was applied to the beam and there was larger bending mement on the pylon.And the axial force equilibrium mechanism of single pylon partially earth-anchored PC cable-stayed bridge was the same to that of double pylon partially earth-anchored cable-stayed bridge with stretchable joints in its mid span,both requiring friction force of anchor and lateral force of soil to balance the huge difference of cable force resulted from asymmetric structure.However,both as partially earth-anchored cable-stayed bridges,the system of double pylon triple span with separated anchor and main beam had a totally different axial force state to that of single pylon system,tension existing in the main span of the former,whose characteristic that lowering the axial force at the connection point of pylon and beam could extend the main span didn’t apply to the latter.Single pylon totally earth-anchored PC cable-stayed bridge was suitable for support system,with main beam and anchor fixed.And Single pylon totally earth-anchored PC cable-stayed bridge was suitable for fixed system without lower pylon column or with large cross section lower pylon column.(3)The styling design application was discussed.The viewpoint that the ratio of dead and live loads cable born should be considered in designing middle or small span single pylon cable-stayed bridges was proposed.The horizontal displacements of pylon top and the maximum deflection of mid span were deduced under live loads indicating the system stiffness.The system evaluation method of total cost indicating the structural economical efficiency and per unit length cost was established.And this system solved the problems relating to research method.(4)The single pylon partially earth-anchored PC cable-stayed bridge was studied using the structure performance evaluation system,the side to main span ratio,anchor to side span length ratio and side cable dip determined to be key structural parameters.The findings were showed as follows.The reasonable side to main span ratio should take a value in 0.25~0.50,larger span and higher unit price of anchor(load ratio of anchor to anchor rod)corresponding to bigger value.(1)When limited by topographical condition,only considering the span capacity of main span and the side to main span ratio was small,the dip of side cable of main span should be decreased and the back cable space of anchor should be increased.When the topographical was open,considering the span capacity of total bridge,the side to main span ratio should take a big value,the dip of side cable of main span should be bigger and the back cable space of anchor should be smaller.(2)When the topographical condition was suitable for single pylon partially earth-anchored PC cable-stayed bridge,the bigger the length ratio of anchor to side span,the higher the economic efficiency,but considering the aesthetic requirements,the value should not be bigger than 0.618.And(3)the reasonable dip of side cable of main span should take a value around 22.0°.It was indicated that single pylon partially earth-anchored PC cable-stayed bridge had a better economic efficiency in limited topographical condition considering only the span capacity of main span,comparing to common cable-stayed bridge.(5)The single pylon totally earth-anchored PC cable-stayed bridge was analyzed using the system evaluation method,the pylon dip,dip of back cable of anchor and dip of side cable of main span selected as the key parameters.The conclusions were listed as followings.(1)The reasonable pylon dip should take a value in 65°~85°,and the minimum pylon dip requirements should be satisfied.The economical pylon dip increased with the increase of span and side cable dip of main span,and decreased with the increase of anchor back cable dip and unit price of anchoring.When the dip of back cable of anchor was in the reasonable interval,every economical pylon dip was greater than the minimum dip calculated according to mechanical and styling requirements.(2)The reasonable dip of back cable of anchor should take a value about 60°,between 50° and 70°.And(3)the reasonable dip of side cable of main span should take a value about 22.5°,between 20.5° and 25.0°,getting greater with the increase of the span and unit price of anchoring.(6)The single pylon partially earth-anchored and totally earth-anchored PC cable-stayed bridges were trial designed,respectively.Based on this,the calculation formula of the horizontal displacements of pylon top,the maximum deflection of main span under live loads and the material consumption were verified. |
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