e-mosty 1 2017 Queensferry Crossing. Forth Road and Railway Bridges. March 2017 | Page 18

closures due to high winds . The purpose of the wind shields is to reduce wind speeds across the top of the deck to an acceptable level .
Since high-sided vehicles are more susceptible to problems , it was found to be more efficient to have greater shielding at a higher level . Making the wind shield more open at a lower level also reduces visual blockage . However , it was found that one rail is required at low level and this was carefully positioned to be behind the second parapet rail to minimise visual obstruction .
4.2 Earthquake
UK application of Eurocode 8 states that there is generally no need to consider seismic loading except for ‘ certain types of structure , [ which ] by reason of their function , location or form , may warrant an explicit consideration of seismic actions ’. For the Queensferry Crossing , the scale of the structure and the potential consequences of failure warrant such consideration . An assessment of the site concluded that ( a ) there were no known active faults at or near the site ( b ) the levels of ground motion would be insufficient to cause liquefaction ( c ) the hazard from tsunami is sufficiently negligible to be excluded from consideration .
Based on this assessment , the specimen design was verified with a response spectrum analysis following the provisions of the Eurocode . Two performance levels were established , with the higher level corresponding to a 2475-year event under which the bridge may undergo limited ductile behaviour but should be open to emergency vehicles immediately after the event .
4.3 Ship impact
Each of the main spans crosses a navigable channel . The southern span crosses the Forth deepwater channel , which is the main shipping route in the Forth estuary ,
Figure 6 : Wind Tunnel Testing : Deck section and balanced cantilever from Central Tower
1 / 2017
providing access to the ports of Grangemouth and Crombie . The northern span crosses the approach channel to the port of Rosyth immediately upstream of the bridge . Vessels of up to 40 000 DWT ( deadweight ) typically pass under the bridge and ship impact is an important design consideration that required thorough investigation .
The Forth Ports vessel traffic service ( VTS ) provided a rich data resource giving details of all vessels entering or leaving the upstream ports , recording the name of ship , time of movement , type and size of vessel , cargo and draft . A database of these records was established and analysed . The VTS system also incorporates radar tracking that records the timing , position , speed and heading of all vessels within the surveillance area . Routes were identified from the raw radar data using geographic information systems ( GIS ) software .
The VTS radar paths showed that , with 650m main spans , the proposed tower locations were well clear of the existing vessel transit paths and that ships would not have to modify their navigation routes once the bridge is built . This was confirmed by navigation simulations carried out at South Tyneside College where local pilots independently reported that ‘ the completed bridge will have little impact on the ability of ships to navigate in the River Forth ’ ( Michel and Walker , 2009 ).
A quantitative marine collision risk assessment , based primarily on Eurocode 1 ( BSI , 2006 ), was carried out to assess the design impact forces for each of the foundations . Risk acceptance criteria were established considering the as low as reasonably practical ( ALARP ) principle . Dynamic analysis was carried out to assess the foundation capacities using large-displacement finite element models and considering energy absorption in plastic hinges in the piles . This ductile design approach as expected showed significant additional capacity compared with an elastic design .