Reliability-based Criteria for Bridge Design and Evaluation

Confederation BridgeUnique load and resistance factors were necessary for the Confederation Bridge, shown in the figure above, to achieve the stringent target reliability levels and design life required.. Dr. Bartlett, supervised by Dr. J. G. MacGregor, derived the mathematical formulation for the reliability calculations, and calculated the load and resistance factors for all ultimate load combinations except those involving earthquake, wave, or ice.

New wind load factors were necessary for the new Canadian Highway Bridge Design Code (CHBDC), particularly to accommodate advances in the state of long span bridge engineering practice. These were computed by the Dr. Bartlett, with the assistance of Mr. J. P. C. King of the Boundary Layer Wind Tunnel Laboratory, resulting in the first formal calibration of wind load factors in any Canadian or American bridge code. The new wind load factors are markedly greater than those in previous codes, providing a necessary extra margin of safety. New code provisions were also prepared for the computation of project-specific wind load factors for wind-sensitive structures investigated by wind tunnel testing. These unique criteria allow leading-edge structures that are assessed using state-of-the-art techniques to be designed using the CHBDC, and so are an example of code provisions that accommodate innovation by the designer.

While an employee of Buckland and Taylor, Dr. Bartlett was Project Engineer for the research and code writing effort that is the basis of criteria currently used to evaluate existing highway bridges in Canada that are also the basis of the evaluation criteria in the new CHBDC. He has since prepared provisions for the CHBDC which address the conversion of actual measured in-place material properties of concrete, structural steel, and reinforcing steel to equivalent specified strengths for use in conventional resistance equations with customary resistance factors. He also collaborated with Gagnon of Buckland and Taylor Ltd. to derive the load factors for existing bridge evaluation in the new CHBDC, that depend on the target reliability index of the component being evaluated.

bridge code pages graph

The graph right shows the growth of the number of pages in Canadian bridge codes since 1930.