Network Arch Bridges. Presenter: Robert Salca technical support engineer, Midas UK

Network Arch Bridges Presenter: Robert Salca technical support engineer, Midas UK

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Contents Introduction to Network Arch Bridges Advantages of Network Arch Bridges Network Arch vs. Arch with Vertical Hangers Components - Arch Components - Hangers Components - Tie Erection Process Static Analysis Construction Stage Analysis Time History Analysis Sutong Bridge

Introduction to Network Arch Bridges Network arches are arch bridges with inclined hanger where some hangers cross other hangers at least twice; The structural behaviour is similar to that of a truss. The arch works in compression and the tie in tension; Most of the shear force is taken by the hangers and little is transmitted to the arch and tie; There is little bending in the arch and tie; The network arch idea was developed by the Norwegian engineer Per Tveit at the end of the 1950s. The first constructed network arch bridge designed by Per Tveit was the bridge at Steinkjer, Norway, built 1963-1964. Schematic of Steinkjer Bridge

Advantages of Network Arch Bridges Very attractive slender bridges, blending in very well and not hiding the landscape behind; Very stiff structures, making them suitable for railway bridges; When designed properly saves up to 50% on materials can be achieved; Very suitable for spans between 60 and 300m; Not sensitive to uneven loading or uneven settlement; Good resistance to earthquake loads due to high resistance and low weight; A high percentage of the total cost goes into wages ( increases employment and decreases impact on environment); Steinkjer Bridge, Norway

Network Arch vs. Arch with Vertical Hangers

Network Arch vs. Arch with Vertical Hangers Deflection under uneven loading:

Network Arch vs. Arch with Vertical Hangers Bending moment diagram under uneven loading:

Network Arch vs. Arch with Vertical Hangers Bending moment diagram under uneven loading Damage to tie:

Network Arch vs. Arch with Vertical Hangers Steel weight comparison:

Components Arch Can be made out of concrete or steel (usually steel); Can have parabolic or circular shape (usually circular); The bending moments can be smoothened if the radius of the arch is decreased near the ends; Axial force dominates (compression); Forces in the arch increase only a little as we go down from the top of the arch;

Components Arch The recommended section is a universal column section with the week axis in the plane of the arch; Circular sections may be used and for larger spans tube sections may be more suitable (but these increase costs significantly); The arch works in compression, with little bending => no need for a large moment of inertia; Universal columns make use of less welding, smaller dimensions and simpler details;

Components Arch Connection along the arch: Connection with tie: Connection with wind braces:

Components Hangers Can be wires or rods; High strength steel; Small diameter usually between 40 to 60mm; Do not take up compression, relaxing instead. The relaxation in the hangers depends of their steepness (steeper => more relaxation); The hangers should be connected along the arch as equidistantly as possible; Crossing cables are connected with plastic tubes (usually), that allow relative rotation but prevent them from banging against each other; The optimal hanger arrangement should be considered; one of the most efficient is the radial arrangement.

Components Tie Can be made out of concrete or steel (for narrow bridges concrete decks are preferred); Axial force dominates (tension); In many cases the transverse bending is larger than longitudinal bending; The weight of concrete decks helps with tensioning the hangers; Concrete ties are longitudinally prestressed; Transverse prestressing of the deck improves the durability of the concrete; During construction temporary steel ties can be used;

Erection Process On Site Construction

Erection Process Off Site Construction Placing in Final Position

Erection Process Off Site Construction Incremental Launching

Static Analysis Correct geometry of the bridge; Linear analysis ( cables considered as truss elements); Initial prestress in cables (Unknown Load Factor); Cable Force Tuning;

Static Analysis Unknown Load Factor Optimizes tension forces in cables at the initial equilibrium position of a cable structure; Calculate the initial cable force by inputting restrictions (displacement, moment, etc.) and satisfying the constraints; Does not include the change in stiffness of the cable due to the change in pretension => truss elements;

Static Analysis Unknown Load Factor Object Function type: Select the method of forming an object function consisted of unknown load factors: Linear: The sum of the absolute values of Load factor x scale factor; Square: The linear sum of the squares of Load factor x scale factor; Max Abs: The maximum of the absolute values of Load factor x scale factor; Sign of Unknowns: Assign the sign of the unknown load factors to be calculated: Negative: Limit the range of the calculated values to the negative (-) field; Both: Do not limit the range of the calculated values; Positive: Limit the range of the calculated values to the positive (+) field; Simultaneous Equations Method: Using linear algebraic equations, the equality conditions are solved. If the numbers of the unknown loads and equations are equal, the solution can be readily obtained from the matrix or the linear algebra method;

Static Analysis Cable Force Tuning Reduce the repetitive computation process to obtain the optimum cable pretension; Calculates the effects of the cable pretension (or load factor) on the displacements/ member forces/ stresses through influence matrix and updates the results graph in real time;

Construction Stage Analysis Nonlinear behaviour of cable elements; Construction sequence (temporary supports);

Construction Stage Analysis When is nonlinear analysis required? Final Stage Analysis -> Initial cable forces. If the cable force is above 70% of the yield force the cables will behave similarly to truss elements => No need for large deformations analysis; For very large span bridges ( >600m) nonlinear analysis is required; No clear criteria stating whether nonlinear analysis is required for all cable structure; The safer approach is running both linear and nonlinear analyses and compare the results;

Time History Analysis Simulate snap of hanger; Check behaviour of structure after snap; Apply dynamic load as time varying static load case;

Time History Analysis

Q&A Thank you! en.midasuser.com For any questions send an e-mail at: uksupport@midasuser.com