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The 13th controlling criterion is structural capacity. This refers only to the load-carrying capacity of the bridge. Because it is not strictly an element of geometric design, structural capacity will not be covered in detail in this guide. Designers should be aware, however, that the inability to design for the designated structural capacity requires a design exception. There is also information in the Green Book on conditions under which existing bridges may remain in place.
Bridge rail that is structurally sound and meets current crash test standards is an important safety consideration, and updating substandard barrier is an important safety improvement on 3R and other projects. However, the type or condition of bridge rail is not considered to be one of the 13 controlling criteria that require a formal design exception.
Each of the 13 controlling design criteria is established to reflect a desired operational and/or safety benefit. Designer understanding of the nature of the benefits and the design sensitivities will lead to good decisions regarding design exceptions.
Based on the topics discussed in this chapter, designers should appreciate that the inability to meet a minimum threshold criterion should not be made lightly, and that the expected performance for a lesser design may be based on many conditions. Designers should expect that to some extent adverse operational and/or safety effects may occur with a design exception. The next chapters of this guide discuss how designers can mitigate potential adverse effects and deliver a design with acceptable performance.