Efficiency and Economy in Bridge and Building Structures

June 2016

Part 2: A Study for Structural Efficiency and Economy in Construction

This article references several detailed Tables. Unfortunately, space constraints dictate not reprinting Table 1 from the May 2016 issue of STRUCTURE.

Efficiency and economy of structures are important parts of structural engineering. Efficiency and economy are not new ideas: engineers build many remarkable bridges and buildings under strict financial constraints.

Efficiency for Bridge Structural Systems

Table 2 lists the cost and steel “efficiency coefficients” for suspension and cable-stayed bridges, including most of the longest span bridges in these two categories. In Table 3 are listed the cost and material efficiency coefficients for different structural bridge systems including representatives for each bridge system. Table 4 presents the best performance and the margins for the efficiency of the total groups (per this study) of structural systems (in establishing the average data, the highest and lowest coefficients for the group were eliminated). Table 5 lists the construction-time efficiency for bridges.

Exposed structures like bridges should be elegant: slender with simple forms and should harmonize with the surrounding environment. There is a consensus among engineers and architects that a well-designed structure, using the right structural system, usually results in an elegant and well-proportioned bridge. Also, it is very important that an aesthetically attractive bridge is also efficient and economical.

Table 3. Steel and Cost Efficiency for different Bridge Systems.

Even when the challenge was about how to build a bridge with a record span length, the cost of the structure was always an issue that could abort or postpone the project for a long period (the Messina Strait Bridge is a good example). With the progress in structural analysis and software, high-strength properties of available structural materials and improved construction methods, today it is less of a problem to obtain a longer span than ever before. Now the greatest difficulty appears to be securing the needed funding for such projects. For the same reason, engineers start paying more attention to the structural cost because using more efficient systems and technologies allows them to build “more bridge or building” (meaning more built area and longer free-of-column spans).

Table 4. Summary of Bridge Steel, Concrete and Cost Efficiency.

Different structural systems for bridges have a specific margin of efficiency coefficients for construction materials. For example – steel continuous girders have 2.55 to 3.0 kg/(L x m²); steel continuous trusses, about 1.8 kg/m³; chevron portals, 1.20 to 1.50 kg/(L x m²); cable-stayed, 0.62 to 2.46 kg/(L x m²); and suspension bridges, 0.62 to 0.98 kg/(L x m²). The Tables do not include the steel continuous trusses and chevron portals because of very limited information.

Table 5. Construction Time Efficiency of Bridges.

Two examples from the author’s experience demonstrate the possibilities provided by using the efficiency criteria:

Akashi Kaikyo Bridge, with the longest bridge span of 1991 meters (or 6,532 feet), was completed in 1998. As early as 1988, the author estimated a steel efficiency coefficient of 0.76 for the bridge, based on the limited information about this future structure at that time, using presumed similarity with other suspension bridges already completed in Japan. When, years later near the completion of the bridge, the final technical information for the bridge was made available, the steel efficiency came to 0.83, only 9% difference from the earlier estimate. The result was very close, especially considering that the Akashi Kaikyo Bridge had achieved a new world record with 1.41 times longer span than the previous record holder, the Humber Estuary Bridge. This example proves that an established criterion for bridge (or structure) efficiency can be a powerful tool for designers and developers in preliminary estimates of the material and cost required for new structures, even when new record-long spans are involved.

The replacement of the East Span of the San Francisco-Oakland Bay Bridge occurred from 2002 to 2015. During the period of review and system selection, members of the Engineering Design Advisory Panel (EDAP) for the new bridge, including the author, cautioned transportation authorities about the problems in selecting structural systems. For example, system selection for the “Skyway” and the self-anchored suspension (SAS) for the main span without providing, in advance, construction quantities and costs compared with other bridge systems would be problematic. Of special concern were the high self-weight of the concrete Skyway (resulting in higher seismic forces, heavier piers, and foundations) and the very high cost of the few self-anchored suspension systems built at the time, proposed for the suspension portion of the bridge. The authorities ignored the warnings and, as a result, the already high estimated cost escalated to levels making this otherwise elegant bridge one of the most expensive in the world. This escalation is another example that demonstrates how established criteria for efficiency may have saved billions of dollars. As engineers can learn from successfully efficient projects, it is even more important to learn from the mistakes made in large and very expensive projects.

Efficiency for Long-Span and Tall Buildings

Similar to bridges, the efficiency of single-level long-span structures for sports arenas, exhibition halls, aircraft hangars, etc., can be compared using the same efficiency coefficients; an example is given in Table 6.

Table 6. Steel Efficiency for Long-span Roof Structures.

For tall buildings or skyscrapers, a similar approach is used, replacing the span L with the height of the building H. Table 7 compares the steel efficiency for such buildings. In both Tables 6 and 7, only a few projects are listed to represent the structural systems.

Findings

Based on the best-achieved steel efficiency coefficients, suspension bridges show the best performance (E/E coefficients) with coefficient 0.62 kg/(L x m²); followed by cable-stayed, 0.62 (same as the suspension, but with higher average coefficient); steel continuous and arch, 2.48 – 2.55; concrete continuous and “extradosed”, 2.58 –2.90; and concrete arch bridges, 2.52.

Based on cost (economy) coefficients, the suspension bridges are again the best with coefficient $6.51/(L x m²); followed by cable-stayed, $7.45; steel continuous, $9.19; concrete continuous, $12.20; steel arch bridges, $19.27; and concrete arch and “extradosed”, $20.57–21.82. Note that the lack of representation of pedestrian bridges in this article’s tables is indicative of their cost, as they are often significantly more expensive. For example, Calatrava’s Sundial Bridge at Redding, CA, has a cost of $15,670/m². Juan Sobrino provides information for costs per meter square of several pedestrian bridges with spans of 150–200 meters ranging between $16,300/m² and $57,400/m². These costs are significantly higher than costs per m² of suspension and cable-stayed bridges with spans exceeding 500–1000 meters (Tables 2 and 3).

The results above are from Table 3, where the suspension and the cable-stayed bridges are without competition for the first and second position of most efficient structures. The highest efficiencies for suspension and cable-stayed bridges are valid only for the “classic” types of these structures. Self-anchored suspension bridges and suspended ribbon-decks are not as efficient (Tables 2 and 3).

Based on self-weight of the total structure, again the steel suspension, cable-stayed, continuous and arch bridges are more efficient than the remaining systems.

Based on construction time coefficients, the suspension and cable-stayed bridges are built faster than the remaining systems.

Notes

Suspension and cable-stayed bridges are mostly steel structures, but they are often combined with concrete towers (pylons), composite steel-concrete decks, or both, thus making their rating more difficult.
The reinforcing and tensioning steel efficiency coefficients (kg per square meter times the average span) used for concrete continuous and “extradosed” bridges (with 40- to 250-meter, or 131- to 820-foot, spans) are more than the steel for longer cable-stayed spans (230 to 890 meters, or 754 to 2919 feet). The coefficients are closer to the steel used for suspension bridges with significantly longer spans (from 720 to 1990 meters, or 2362 to 6528 feet).
Some recently built bridges in the country, highly acclaimed for their innovativeness and “efficiency”, actually exhibit poor performance in cost, materials and construction time efficiency.
Some “signature” bridges tend to be between the least efficient and least economical; such bridge designs should be used very carefully unless donations from individuals or companies cover the costs and it is not a burden on state or federal budgets.

Conclusions and Recommendations

The data presented in this article can be used as a start for building a larger “Database” for efficiency of structures and structural systems. A more extensive Efficiency Database will provide very useful information and guidance for total efficiency and its elements: material, cost, construction time, weight for bridges, bridge systems, and other structures. By using the Database, engineers will be able to find the most efficient structures within the system groups and will learn how to further improve their projects based on the specific solutions for these best performance examples. At the same time, engineers will be able to avoid using systems that are significantly less efficient.
A developed Database will allow engineers to select the most appropriate concept for the overall project, the most important and challenging part of engineering. Economy depends mainly on the design concept.
The expansion of this Database should be developed with the help of the State Departments of Transportation, design and construction companies, academia and the professional publications for engineers, architects and builders.
The goal should be to have the Database become a reliable guide for use by professional designers and builders, structural manufacturers, construction managers and owners of bridges and other structures. The use of such a Database can save billions of dollars as early in the process as the selection of the structural system, thus saving necessary funding for construction and renovation of other structures.
All design and construction of bridges and other larger structure projects with a cost above $30 million, when funded by state or federal budgets (public taxpayer’s money), should be awarded only by design (or design-build) competitions. No such project should be approved if the material and cost efficiency coefficients for the project span exceed the typical E/E coefficients for such structures by 20 percent or more.
In the author’s opinion, engineers should monitor their own projects’ efficiency, comparing them to industry efficiency averages. This comparison will help engineers discover, at an early stage, whether their project requires adjustments and corrections to remain competitive and provide motivation for further improvement. Being efficient and economical in the design will result in more economical constructions, reduced costs, materials and carbon footprint.

Structural efficiency has become a globally important issue as, in general, efficient constructions with their reduced “carbon footprint” help protect the environment. Concrete, steel, and other materials have significant carbon dioxide emissions released during their production, manufacture and construction. There is no better way for reducing the “carbon footprint” of the construction industry than reducing the quantity of structural materials used in construction.

Given the inherently competitive nature of structural engineering, we may slightly modify the Olympic Games motto, Citius, Altius, Fortius, as Faster, Higher, Stronger, Longer and Lighter. Thus, to the established competition criteria for higher, longer-span and stronger structures, we can also add those for faster and lighter (less consuming) structures.▪

References
(Combined for Parts I and II):

Vitruvius. The Ten Books on Architecture, Book I, Chapter II.

Billington, D. The Tower and the Bridge, Basic Books, Inc, New York, 1983.

Sobrino, J. A Bridge is More Than a Bridge: Aesthetics, Cost and Ethics in Bridge Design, SEI, 3/2013

Middlebrook, R. and Mladjov, R. San Francisco – Oakland Bay Bridge. STRUCTURE magazine, USA, 2/2014.

Mladjov, R. The Steel Structures, Sitius, Altius, Fortius, pp. 59 – 62; 113. Technica, Sofia, 1979.

Mladjov, R. The Super-Long Spans in Bridge Engineering. Construction, Sofia, 3/1988.

Mladjov, R. The Most Expensive Bridge in the World. Modern Steel Construction, USA, 9/2004.

Mladjov, R. Long-Span Bridges and the Art of American Bridge Engineering. SEAOC Conference, San Diego, CA, USA, 2009.

Yanev, B. Bridge Management. John Wiley & Sons, Hoboken, NJ, USA, 2007.

National Bridge Inventory, U.S. Department of Transportation, December 31, 2014.