Long span, suspension and cable-supported bridges can be very light weight, can be very flexible, and consequently, can be sensitive to loads from strong winds.  Included in this paper are highlights of recent aerodynamic studies for six long-span, steel, suspension cable-supported bridges.   Four are suspension bridges, and two are cable-stayed bridges.  In all cases, there was a specific aerodynamic related problem that had to be solved.  Those problems are identified, and the solutions are presented.

1. SAN FRANCISCO/OAKLAND BAY BRIDGE (NEW EAST SPAN)
   San Francisco/Oakland, California, USA
   A streamlined, twin-box, single tower, self-anchored suspension bridge has been designed to replace the existing cantilevered truss bridge.  The twin boxes will be connected with large cross box beams so the entire bridge deck will act as a single unit.  The shape is extremely stable in very strong winds, but the section is sensitive to vortex induced motions.  Shed vortices from the windward box drives the leeward box.  A solution that eliminated the formation of vortices from the windward box was designed.
   
   
2. I70 MISSISSIPPI RIVER CROSSING
   St. Louis, Missouri/St. Clair County, Illinois, USA
   The design of the deck for this large cable-stayed bridge also consists of two portions, connected with cross beams, with a gap in the middle.  Again this twin-deck section is extremely stable in extreme winds, but it too is sensitive to vortex induced motions.  The solution used for the San Francisco/Oakland Bridge could not be used on this bridge, so another had to be found.  The solution designed reduced the formation of vortices from the windward deck, and inhibited vertical, oscillatory flow in the gap.
   
   
3. LIONS GATE BRIDGE RECONSTRUCTION
   Vancouver, British Columbia, Canada
   This project was particularly unusual because the deck and hangers on this existing suspension bridge were completely replaced, while the bridge was open to traffic (except for 6 hours every night).  During construction, for one construction stage, the critical flutter wind speed was significantly lower than the design criterion flutter wind speed in construction.  Aerodynamic dampers, in the form of covers to the walkways, were added that significantly increased the critical flutter wind speed for this particular construction stage.
   
   
4. WILLIAM PRESTON LANE BRIDGE
   Chesapeake Bay, Maryland, USA
   It has been proposed to replace the existing open traffic barriers on this suspension bridge with solid, "Jersey" barriers and to fill the grilled vents in the roadway.  The proposed modifications would degrade the performance of this bridge in strong winds (the critical flutter wind speed would drop below the specified threshold).  Additional modifications were proposed to increase the critical flutter wind speed back above that threshold.
   
   
5. KAP SHUI MUN BRIDGE
   Hong Kong, China
   The Kap Shui Mun Bridge is a cable-stayed bridge with a main span length of 430 m.  The deck section consists of a deep box with automobile traffic on the upper deck, and rail traffic within the box.  The box section as originally designed had sharp corners and had a critical flutter wind speed that was high, but still did not meet the design criterion.  By beveling the top corners of the deck slightly, the design criterion was met easily.
   
   
6. THIRD CARQUINEZ STRAITS BRIDGE
   San Francisco Bay, California, USA
   The third Carquinez Straits Bridge is a new suspension bridge with a main span length of 728 m.  The deck section is a streamlined box section that experienced a sensitivity to the solid ratio of a tall bicycle railing.  The critical flutter wind speed for this bridge with the originally designed bicycle railing did not meet the design criterion.  By decreasing the railing solid ratio to 13%, the critical flutter wind speed design criterion was easily met.

The six case studies discussed demonstrate how sensitive long span suspension and cable-stayed bridges can be in strong winds to very small details in the bridge deck geometry, and how correspondingly small changes in those details can improve that performance in strong winds significantly.