I picked the brains of several bridge experts for our current top 10 list of the world's longest cable-stayed bridges. Ammann & Whitney's Sena Kumarasena weighed in with a lot to say, but a bit past the deadline for the article. I didn't want his wisdom to go undocumented.

New records for long cable-stayed bridges are being set frequently. Why?

There is considerable activity in building new transportation infrastructure in emerging economies of the world, much like what happened in the US and Europe a while back. In quite a few instances these new crossings provide access over very wide rivers or through marine environments which are also busy navigational corridors used by supertankers. Thus the clear span requirements are governed not only by the minimum navigational channel width that allows bi-directional navigational traffic but also to keeping main tower foundations sufficiently removed from the channel to reduce the possibility of errant vessel impact. Also other site-specific geophysical factors such as foundation feasibility play into this.

Many of these bridges would have been suspension type just a few decades in the past. However, due to the confidence gained from the record holding cable-stayed spans successfully built, the economic and practical transition point from the cable-stayed to the suspension is slowly creeping up to take full advantage of this relatively new bridge form. As a result cable-stayed bridges are slowly encroaching on the span lengths previously considered only appropriate for suspension bridges.

The bragging rights of having the longest cable-stayed span may also be a factor feeding this trend; the record holder today is replaced by tomorrows longer, sometimes only marginally so. It has largely been a process of incremental advancement, and rightfully so as in bridge engineering one does not get to make a prototype and test it before making the real thing.

What technical breakthroughs are enabling cable-stayed bridges to reach greater and greater lengths?

The key technical issues for cable stayed bridges in general are basically the same as those for these super-long spans. These include the need for reducing wind resistance on cables, towers members, improved wind stability of the roadway deck section, various constructability considerations of the towers and foundations, enhancing seismic resistance and improving durability etc.., the difference is the degree of refinement needed in these to make the overall design cost effective for very long spans. In terms of material or design technology there is no significant difference between the medium spans to super span bridges. The refinements such as aerodynamically shaped orthotropic steel cross sections, using PPWS cables to improve their compactness, incorporation of cable dampers and seismic restraints and devices, the shaping of tower members to reduce wind resistance through wind tunnel –testing etc..are not new, and has been around for some time.

The recent developments in computational fluid dynamics help long-span bridge design of all types by making it easier for designers to refine the aerodynamic design of members. This is more a convenience that helps in shaping members during early design stages before verification using actual wind-tunnel experiments. It will take a very long time before CFD could actually replace the need for physical wind tunnel tests, if it ever get there that is.. 

The challenges in super-long cable stayed spans are mostly in the planning and in effective deployment of construction techniques; building large foundations in challenging environments and constructing super tall towers etc. Again, the construction teams who build such spans typically have specialty members experienced in such things as off-shore construction, heavy lifting, tall vertical construction that they bring to the table from other segments of the heavy construction industry. 

With these super-long span cable-stayed bridges, so far, we have largely been able to basically scale-up the tower heights and cable lengths in matching proportions without having to invent new technology. As we follow this path however, the significance of the self weight and the system wind stability as critical issues will continue to intensify, and will limit or slow down how far we can practically push things. As span lengths continue to increase, it becomes increasingly harder to ensure the stability of the systems we are using today against the elements.

The continued growth of cable-stayed spans will depend on couple of things; the development of newer materials that are stronger, stiffer, lighter and proven to be practically and commercially viable in applications of this magnitude; and innovative creative systems design approaches that can exploit the enhanced physical parameters of these materials to and keep these tall slender bridges stable and keep their weights under control.

What are some of the long cable-stayed bridges you’ve worked on? What technologies were involved?

The recently completed ones where I played a major role include the Leonard P. Zakim Bunker Hill Bridge in Boston, MA and the U.S. grant Bridge in Portsmouth, Ohio. The Zakim is a very complex yet elegant cable stayed bridge borne out of the struggle to balance the demanding functional requirements against the constraints of the site. I served as the lead engineer during final design phase and as the project manager during construction. There are many first time applications and innovative design features that we were able to incorporate in to the design, too many to list here but was described in detail in a feature article on the bridge by ENR a while back. On US Grant Bridge the focus was on improving design efficiency and serviceability. I was also served as the lead engineer for the preliminary design of the new Goethals Cable-Stayed Bridge in NY prior to the start of its alternative procurement phase. I am currently in various states of involvement with couple of other cable-stayed bridge projects.

The fundamental structural systems and design methods have not seen any significant change over the last several decades except for some refinement around the edges.  However, I think there are areas where the time is ripe for us to make some changes to make some significant improvements in bridge safety and reliability, long term maintenance and exploitation of newer materials to their fullest potential using more of a systems approach to design than is employed currently in our industry.

China is building a lot of long cable-stayed bridges. Why?

I had the opportunity to visit China in November and visit some of the notable bridges both built as well a being built there and listen to the Chinese engineers who designed them and were in charge of these projects through completion of construction. They are doing an amazingly successful job in delivering these projects from concept to completion under very compressed time schedules. China has many wide rivers and marine areas that need to be crossed as it develops its infrastructure to keep in phase with the economic development. In addition to the ones already complete and being built, there are many in the planning stages as the country is expanding its national highway system and it is easy to anticipate that the major bridge building activities in China will very likely continue for considerable time into the future.