Of the newly-built bridges, cable-stayed bridges are today very common worldwide for spans ranging between 200 and 900 meters. A cable stayed bridge has one or more towers (Pylons) from which the cables support the deck. This paper provides modelling, analysis and design of a prestressed harp type single pylon cable stayed bridge using MIDAS Civil. Keywords: cable stayed, box girder, prestressing, MIDAS Civil Introduction
Of the newly-built bridges, cable-stayed bridges are today very common worldwide for spans ranging between 200 and 900 meters. A cable stayed bridge has one or more towers (Pylons) from which the cables support the deck. There are two major classes of cable-stayed bridges: harp and fan.In the harp design, the cables are nearly parallel so that the height of their attachment to the tower is similar to the distance from the tower to their mounting on the deck.In the fan design, the cables all connect to or pass over the top of the towers.
The cable-stayed bridge is optimal for spans longer than cantilever bridges, and shorter than suspension bridges. This is the range where cantilever bridges would rapidly grow heavier if the span was lengthened, and suspension bridge cabling would not be more economical if the span was shortened Cable-stayed bridges may appear to be similar to suspension bridges, but in fact they are quite different in principle and in their construction. In suspension bridges, large main cables (normally 2) hang between the towers (normally 2), and are anchored at each end to the ground whereas in the cable-stayed bridge, the towers are the primary load-bearing structures which transmitt the bridge loads to the ground. A cantilever approach is often used to support the bridge deck near the towers, but lengths further from them are supported by cables running directly to the towers.
General presentation of the structure
The bridge is a single pylon cable stayed bridge having a harp-type arrangement of the cables. The total length of the cable stayed bridge is
700m with a main span of 350m. The bridge structure carries 6 road lanes divided into 2 carriageways. The deck consists of cast in place prestressed box girders
Total width of the bridge is 29.8m.
The main 350 m span will be built using the cantilever method, starting from the piers P4 & P5 simultaneously. The two cantilevers will be connected at mid span by the mean of a stitch segment. The balanced cantilevers are cast by segments of 3.5 m long, using a form traveller. The segment (n) is connected to previous segment (n-1) by tendons (internal pre-stressing). This method is used for the first ten segments from pylon. After eleventh segment, no cantilever tendon is needed as the segments will be supported by stay cables tensioned progressively with construction of segments. Hence construction cycle of segments after eleventh one includes installation and tensioning of stay cable before removal and launching of form traveler.
M50 grade concrete will be used for deck and pylons.
M50 grade concrete will be used for Piers.
Concrete properties shall be based on AASHTO LRFD Bridge Design Specifications. Young modulus as per IRC code is given in the next table (IRC:21 § 303.1.):
Shear modulus of concrete, G, is calculated using the following equation: E= Ec/2(1+Ʋ) The coefficient of thermal expansion and contraction for normal weight concrete is taken as 1.17×10-5 /°C. Density of the pre-stressed concrete is taken equal to 25 KN/m3. Steel reinforcement:
Thermo-mechanically treated reinforcement bars of grade 414 conforming to IS:1786 will be adopted. Yielding strength of passive steel reinforcement is considered equal to 414 MPa and Young modulus equal to 200 000 Mpa.
Modular ratio between concrete and steel will be taken equal to 10.
The self-weight is calculated assuming a density of 25 KN/m3 for reinforced and prestressed concrete. A density of 7.85 t/m3 is to be considered for steel. Live Loads:
Traffic live loads-
The live loads are in accordance with IRC:6-2000. The bridge has two carriageways of three lanes each and of 11 m width. Each carriageway will be loaded with three lanes of IRC class A loading.
Pedestrian live load(PLL)-
The foot path loading shall be as per clause 209 of IRC:6 with intensity of loading equal to 500 kg/m². Wind load for cable stayed bridge:
According to IS:875 (part 3)-1987
Wind loads on live loads (WL)-
The lateral wind force against moving live loads shall be considered as acting at 1.5 m above the roadway and shall be assumed equal to 300 kg/m. Creep and Shrinkage(C &S)
Creep and shrinkage effects to be considered as per CEB-FIP code for cable stayed bridge Earthquake loads
Rajasthan is located in seismic zone II. The horizontal seismic coefficient in longitudinal direction will be calculated by IS 1893:2002 Construction loads:
Normal vertical loads- A construction load of 50 kg/m2 shall be considered during cantilever erection. Weight of the traveler form is assumed to be equal to 85 tons. An impact of 10 % shall be considered for the moving construction loads. MIDAS MODEL of box:(Half span)
AASHTO LRFD Bridge Design Specifications (Third Edition, 2005 Interim Revisions); AASHTO – Guide Specifications for Seismic Isolation Design ( 2nd edition – 2000); AASHTO – Guide Specifications for Design & Construction of Segmental Concrete Bridges (1999); IRC:6-2000 Standard Specifications & Code of Practice for Road Bridges, Section II, Loads & stresses (4th edition – 2000); for definition of the live loads and earthquake loads only; IS:875 (part 3)-1987 Code of practice for design loads (other than earthquake) for buildings and structures; for wind loads only; Essentials of Bridge engineering by D.Johnson Victor
Bridge Engineering Handbook Edited by Wai-Fah Chen ,Lian Duan, CRC Press