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Chapter 3
Loads and Load Combinations

Excerpt

As mentioned earlier, development of reliability-based load and resistance factor design for piping requires the consideration of the following three components: (1) loads, (2) structural strength, and (3) methods of reliability analysis. Obviously, the load and load combinations are of primary importance for such a development. Load and their combinations are to be defined in order to develop the necessary partial safety factor values (PSF's) for use in an LRFD design equation such as Eq. 2-2. These factors are determined probabilistically so that they correspond to a prescribed level of reliability or safety. LRFD format uses different safety factors for each type of load. This allows for considering uncertainties in loads and scaling their characteristic values accordingly in the design equation. Historically, in the ASD (sometimes called working stress) format, individual load cases such as dead load, internal pressure, and earthquake load were simply added together and compared to an allowable stress value that is reduced by means of a factor of safety (FS). The ASD method cannot properly account for uncertainties in loads and load combinations because it uses only one safety factor.

Typical loads that are used individually or in combination with each other include dead load, internal pressure, and seismic loading. Provisions for these loads and their combinations are found in almost all structural and building codes such as AISC, ACI, API, ASSHTO, ASCE-7, UBC, BOCA, and IBC. For comparison purposes, load combinations used in various classification agencies are listed and discussed in Section 3.2 of this chapter.

This chapter also provides a definition for the primary loads that are of interest for this study. That is, the weight of the piping, the internal pressure and the seismic loads are presented as described in ASME Boiler and Pressure Vessel (BPV) Code, Section III, Division I (1992). Moreover, it gives the load combinations in an LRFD form that other major organizations utilize for the design of elements. Finally, combinations of primary loads for the design of nuclear piping are recommended.

  • 3.1. Primary Loads
  • 3.1.1. Dead Weight
  • 3.1.2. Internal Pressure
  • 3.1.3. Seismic Loading
  • 3.1.4. Nonreversing Dynamic Loads
  • 3.2. Load Combinations in Non-ASME Structural Codes
  • 3.2.1. American Institute for Steel Construction Code
  • 3.2.2. American Society of Civil Engineers Code
  • 3.2.3. American Petroleum Institute Code
  • 3.2.4. American Association of State Highway and Transportation Officials Bridge Design Specifications
  • 3.2.5. Eurocode 1990
  • 3.3. Load Combinations for Components of Nuclear Plant
  • 3.4. Recommended Load Combinations for Piping
Topics: Stress

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