Wind design loads for temporary structures

Determining appropriate wind speeds and loads for temporary structures for construction projects has perplexed owners, engineers, and contractors alike. In addressing wind loads for temporary structures such as shoring, personnel, and work space structures, two avenues appear to have been taken. For commercial construction projects, less conservative methods have been practiced, including the lack of design altogether. An entirely different approach would be that taken for government construction projects, in particular the United States Department of Energy (DOE). The DOE does not differentiate between temporary, short-lived structures and permanent structures whose normal design lives approach 50 years. Thus, the same wind design load criteria are applied to both classes of structures. It is estimated that, on average, the cost of temporary structures to resist wind loads for commercial construction is low, relative to the overall cost of construction. Given the disproportionate number of structural failures that occur during construction, as compared to failures during service life, this low cost at times becomes apparent. A large number of these failures occur due to lateral loading, primarily wind. On the other hand, the DOE seems to advocate a "hell for stout" approach to the design of temporary structures. This conservative approach is resulting in increased construction costs and schedule slippages. This is especially evident for DOE decontamination and decommissioning (D&D) construction projects where construction change orders are resulting from a difference in design philosophy between contractors and the DOE pertaining to temporary structures. Typically, when a contractor prepares a bid for a D&D construction project, a unit cost for the design and construction of temporary structures will be included. This cost will normally be based on the contractor's historical design and construction practice for temporary structures and will tend to be less than that required for the structure to resist the wind design loads for permanent structures. With the anticipated total cost of all DOE D&D construction projects approaching $50 billion (Decontamination and Decommissioning Integrated Demonstration Strategy, 1991), a significant cost savings may be realized if a more refined method for determining wind loads for temporary structures is determined. In this study, the rationale for applying the same criteria for wind design for permanent structures and temporary structures for DOE D&D construction projects is explored. Related codes, guides, and publications used for determining wind design loads for temporary structures in other industries and countries are also explored and evaluated. For the purpose of this study, temporary structures are defined as those structures with design lives less than five years. Permanent structures are defined as those structures with design lives greater than this. In addition, the scope of this study is limited to those structures classified as PC-1 or PC-2 (DOE 5480.28, 1993), or General Use or Important/Low Hazard per criteria as defined in the now superseded document, UCRL 15910 (1988). These classifications are consistent with the type of operations primarily conducted within temporary facilities for DOE construction projects and can be related to commercial structures by comparing the appropriate importance factors to governing basic building codes. Structural reliability methods are used to determine design wind speeds for DOE PC-1 and PC-2 type SSCs with design lives from 2 to 5 years. These are obtained by setting the reliability indices and probabilities of failure for temporary structures equal to those obtained for permanent structures. Wind speeds are obtained with regard to the nature of the wind loading as well as the structural resistance of the SSC.