Probability-Based Load Criteria for Structural Design

Structural codes and standards provide the foundation of good engineering practice and a framework for addressing safety and serviceability issues in structural design. They identify natural and man-made forces that must be considered, define magnitudes of these forces for design, and prescribe methods for determining structural resistance to these forces. The framers of these documents on which the structural engineer places so much reliance must address the question: “How safe is safe enough?” on behalf of society as a whole. Code development is a grave responsibility and, for the most part, has clearly been done well since failures of constructed facilities are rare. On the other hand, such failures, when they do occur, are highly visible and their consequences are severe in human and economic terms for all involved. This publication, Development of a Probability-based Load Criterion for American National Standard A58 [1], marked a major advance in the approach to formulating such codes. At the root of the structural safety problem is the uncertain nature of the man-made and environmental forces that act on structures, of material strengths, and of structural analysis procedures that, even in this computer age, are no more than models of reality. The natural consequence of uncertainty is risk. Structural engineering, as applied to civil construction and in contrast to other engineering fields, relies heavily on analysis and computation rather than on testing because of the scale and uniqueness of typical civil projects in both public and private sectors. Structural codes are linked to computational methods of safety assessment, and their primary purpose is to manage risk and maintain safety of buildings, bridges and other facilities at socially acceptable levels. Until the 1960s, the safety criteria in structural codes were based on allowable stress principles. The structural system being designed was analyzed under the assumption that it behaved elastically (the fact that structures seldom behave elastically to failure was disregarded). Uncertainties were addressed by requiring that the computed stresses did not exceed a limiting stress (at yielding, rupture, instability) divided by a factor of safety. These factors of safety were selected subjectively; one might, for example, identify the load acting on a structure and then design the structure so that the elastic stresses due to that load remain below 60 % of the stress at yield (implying a factor of safety of 5/3). Of course, no one knew what the risk of failure was for such a structure. The factor of safety of 5/3 simply represented a value judgment on the part of the standard-writers, based on past experience. During the past century, with the advent of formal structural calculations, the trend in the factor of safety generally has been downward. This judgmental approach to safety works well as long as the technology being dealt with is stable or evolves slowly and there is opportunity to learn from experience in the standard development process. Occasionally, of course, engineers become overconfident, ignorance catches up, or construction practice overreaches the state of the art; then failures occur. More than in most other engineering disciplines, the profession of structural engineering seems to have progressed by learning from its mistakes. To the discomfort of many structural engineers, this learning process usually takes place in the public arena. During the late 1960s and 1970s, a number of natural disasters occurred worldwide that caused extensive loss of life and property damage and focused the attention of the structural engineering community and the public on the need to advance building practices for disaster mitigation. Professional staff from the Structures Division in the Center for Building Technology (CBT) of the National Bureau of Standards were involved in a number of the damage surveys and failure investigations that followed these disasters. Among the more notable of these were the structural failure investigations that followed the San Fernando, California, Earthquake of 1971, the Managua, Nicaragua, Earthquake of 1972, and the Miyagi-ken-oki Earthquake of 1978; the investigation of snow and rain load conditions prior to the collapse of the Hartford Civic Arena roof in 1978; and the evaluations of wind loads, wind load effects, and building performance following Hurricane Camille on the Gulf Coast (1969) and Cyclone Tracy in Darwin, Australia (1974). These and other investigations of building performance revealed a number of deficiencies in the provisions for structural safety appearing in the codes of practice of the time, and emphasized the need for improvements in design for natural hazards.