A Discussion of Power Plant Loads and Load Combinations

This paper focuses on the design of steel support structures within electrical power generating facilities (power plants) which burn fossil fuels to generate electricity. The design of power plant structures requires the determination of loads, some of which will already be familiar to any practicing structural engineer, and some of which are specific to power plant structures. This paper provides guidance to the structural engineer for developing the various loads for power plants, and for combining these loads to determine the overall design loads. The discussion will be based around the use of Minimum Design Loads for Buildings and Other Structures, ASCE 7-10. (Note: where specific text from ASCE 7-10 is quoted or cited in this paper, it is with permission from ASCE.) Many of the loads involved in the design of power plant structures are the same as those encountered in the design of any typical building or structure, such as dead load, live load, snow load, wind load, and earthquake load. However, for each of these loads there are characteristics which are specific to power plants, and these are discussed in this paper. In addition, the design of a power plant structure involves loads which are not specifically discussed in ASCE 7-10. Examples of these loads include ash load, unbalanced pressure forces, forces due to thermal expansion and contraction of high-temperature equipment and ductwork, and boiler loads. The characteristics of these loads are discussed. The basic allowable strength design (ASD) load combinations found in ASCE 7-10 are expanded to include the additional primary loads specific to power plants. Finally, considerations for developing factored load combinations with power plant loads, for use with load-and-resistance-factor design (LRFD) methods, are discussed. The First Step: Determine the Risk Category To apply the ASCE 7-10 provisions for determining flood load, wind load, snow load, ice load and earthquake load, a building or structure must be classified according to Table 1.5-1 in ASCE 7-10. The Risk Category assigned to the building or structure dictates the various importance factors to be used in determining the minimum design environmental loads. From Table 1.5-1, Risk Category III includes “Buildings and other structures not included in Risk Category IV [essential facilities], with potential to cause a substantial economic impact and/or mass disruption of day-to-day civilian life in the event of failure.” Power generating stations meet this definition and are therefore typically assigned to this Risk Category III. Although they provide power to Risk Category IV facilities such as hospitals, police stations, fire and rescue stations, those facilities typically have their own backup power provisions; for this reason power plants are not usually considered to be Risk Category IV facilities. Dead Load and Collateral Dead Load of Power Plants There are certain types of loads which are normally considered dead load, but which should be given special consideration since they may not always be present in the structure. If they are simply added in with the permanent dead load, an unsafe overestimation of the resistance to uplift forces could result. For example, the weight of fixed (permanent) equipment is normally classified as dead load. But the contents of the permanent equipment, such as the water weight in the boiler, weight of the coal in coal silos, D.F. Six