Ngarai Sianok Bridge consists of 3 main spans, each is 170 m. It also has 2 side spans with 87.5 m long. The two center pylons are 173.6 m tall while the height of other 2 pylons is 91 m. Deck is hybrid type that consists of prestressed box girder part, longitudinal steel stringer, cantilever steel cross girder and reinforced concrete deck. Joints between stringers and cross girders are bolted.
At the first stage of design, three types of deck were studied: single steel box girder; steel box girder with longitudinal steel stringers and cantilever steel cross girders; prestressed concrete ox girder with longitudinal steel stringers and cantilever steel cross girders. Heavy deck such as concrete box girder was avoided, since its great weight would produce large moment at pylon base during earthquake. Construction of such deck would also a big problem, since heavy lifting for precast segment could not be applied in this bridge due to topography concern. Cast in-situ construction method would require large and heavy form traveler. The first option, although has favorable aerodynamic properties was dropped because of its complex construction method. Launching method for steel box girder was considered not possible due to lack of experiences and equipments within local contractors. The second option also dropped, since its small size steel box provides small stiffness and resistance against large torsional vibration and moment in deck. Therefore, the third option was used in the design.
Aerodynamic forces were measured by indirect measurement using pressure taps and pressure scanner. There were 44 pressure taps along the deck model. Test were performed under wind speed of 5, 10 and 20 m/s at various angle of attack. Full bridge model with scale 1:200 was tested for two configuration: with and without terrain. Tests were carried out for skew angle 0°, 15°, 30° and 45° for without terrain configuration.
Wind tunnel tests were carried out at Aero-Gas Dynamic and Vibration Laboratory, or known as LAGG (Laboratorium Aero-Gas dan Getaran) in Serpong-Tangerang Selatan, Banten Province. This facility is owned by Indonesian government under special body for doing research and appplication of technology named BPPT- PUSPIPTEK. Series of wind tunnel test were performed, incuding static sectional test and full model test without terrain. Considering the unique surrounding of the site, full model wind tunnel with terrain also performed to make simulation as close as possible to actual condition. This was the first experience for LAGG to do full model bridge test with terrain.