Factor of Safety

6 Key excerpts on "Factor of Safety"

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  eBook - PDF

  An Engineer's Alphabet

  Gleanings from the Softer Side of a Profession

  • Henry Petroski(Author)
  • 2011(Publication Date)
  • Cambridge University Press

    (Publisher)

  F Factor of Safety. In structural engineering, a Factor of Safety is effectively the ratio of the theoretical failure load of a beam, column, or other component to the largest actual load it is designed to carry. A Factor of Safety pro- vides assurance against such uncertainties as the design load being exceeded in service, the statistical variation in the strength of materials, and the occurrence of detrimen- tal effects during the construction and life of a structure. Factors of safety have historically varied from only slightly greater than unity, in structures where excess strength (which generally equates to excess weight) cannot be tol- erated, as in spacecraft, to as high as 6, 7, 8, or even more in civilian structures whose behavior is not completely understood or whose failure would have life-threatening or severe economic consequences, as in mid-nineteenth cen- tury railroad bridges. By extension to non-structural appli- cations, employing a Factor of Safety implies being conser- vative in design, the structure or component having reserve capability for unusual situations. The Factor of Safety is sometimes referred to as a “factor of ignorance” because it is intended to take into account unknown contingencies. Euphemisms such as “design factor” and “design margin,” which are sometimes used, mask the life-protecting impli- cations of the term Factor of Safety. Factors of safety in living organisms have been dis- cussed in an article by the physiologist and biogeogra- pher Jared Diamond. He considered the strength of bones, 101 102 Factor of Safety lungs, kidneys, and other body parts and tabulated their biological factors of safety alongside those of some con- ventional engineering structures (see “Building to Code,” Discover, May 1993, pp. 93–98).

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  eBook - ePub

  Risk Management for Geotechnical Engineering

  Hazard, Risks and Consequences

  • Duncan C. Wyllie, Duncan C. C. Wyllie(Authors)
  • 2023(Publication Date)
  • CRC Press

    (Publisher)

  Terzaghi & Peck, 1967 ).

  The Factor of Safety values listed in Table 2.1 were first proposed by Terzaghi and Peck many years ago, but it is considered that they have stood the test of time well based on extensive empirical experience, and are still accepted today. A good indication of the reliability of these values is that most geotechnical projects that have been designed and built using these factors of safety values have performed satisfactorily for many years. It should be noted that the values listed in Table 2.1 are based on ultimate limit states, referring to conditions where the structure fulfills the function for which it was designed - bearing capacity was not exceeded, for example. Limit states design is discussed further in Section 2.6 below.

  Table 2.1 Typical Factors of Safety in Geotechnical Design

  Failure type	Item	Factor of Safety
  Shearing	Earthworks	1.3–1.5
  Retaining walls	1.5–2.0
  Foundations	2.0–3.0
  Seepage	Uplift, heave	1.5–2.0
  Gradient, piping	3.0–5.0
  Ultimate pile tests	Load tests	1.5–2.0
  Dynamic formulae	3.0

  2.2 Deterministic analysis

  2.2.1 Factor of Safety

  The most common, and simplest, method of quantifying the adequacy of a design is to determine the Factor of Safety that is defined by the ratio:

  F a c t o r   o f   s a f e t y ,   F S =  

  C a p a c i t y  

  (

  r e s i s t i n g   f o r c e s

  )

  D e m a n d  

  (

  d i s p l a c i n g   f o r c e s

  )

   

  (2.1)

  The stability analysis method in which the Capacity and Demand ratio is calculated is termed limit equilibrium analysis (LEA), or working stress design (WSD). In this analysis, all the uncertainties in the parameter values, loads and the stability model are contained within the Factor of Safety. Selection of an appropriate value for each design parameter to use in the calculation of Factor of Safety can be based on a cautious, or conservative, estimate of the mean (Fenton & Griffiths, 2008

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  Time-Dependent Reliability Theory and Its Applications

  • Chun-Qing Li, Wei Yang(Authors)
  • 2022(Publication Date)
  • Woodhead Publishing

    (Publisher)

  The method ensures the safety of structure in two ways: one is that the design values of load and resistance are determined probabilistically with acceptable confidence level, i.e., probability, as shown in Eq. (1.8). The other is to apply partial factors for each load and resistance to “re-ensure” the safety of the structure to be designed. These partial factors are used to take care of: (1) possible unfavorable deviations of the characteristic values, such as extreme values; (2) possible inaccurate modeling of the characteristic values, such as the distribution of the variables; (3) possible errors in modeling load and resistance mechanically, such as wind pressure and deterioration of concrete strength; and (4) possible errors in models for structural analysis, such as rigid joint or flexible joint of beam and column. It should be noted that there are shortcomings of safety factor methods, either sin- gle factor as described in Section 1.2.1 or multiple factors in Section 1.2.2. One short- coming in safety factor methods, which is perhaps a major deficiency and may not be overcome by the method itself, is the so-called “lack of invariance.” From Section 1.2.1, it can be seen that the safety factor is the ratio of resistance R and load effect S (e.g., Eq. (1.2)) which evidently depends on the formulation of resistance and load effect used in different problems. Similarly, in Section 1.2.2, the partial factors, i.e., resistance factor φ and load factors γ w , γ D , γ L , also depend on the formulation of resistance and load effects. This phenomenon is known as “lack of invariance” in safety measures. It stems from different formulation and definitions of load and resis- tance from different assessors. Clearly, this lack of invariance can cause confusion and errors in the safety assessment of structures.

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  Creative Design of Mechanical Devices

  • Nelson Bolívar(Author)
  • 2019(Publication Date)
  • Arcler Press

    (Publisher)

  Major concerns of the designer include properties of materials, the validity of mathematical models, load variability as well as fabrication fidelity. Several mathematical methods exist to address uncertainties. The primary techniques for addressing uncertainties’ are stochastic and deterministic ones. The deterministic approach sets up a design factor on the basis of absolute uncertainties of a loss-of-function characteristic or parameter and a maximum acceptable parameter. In this case, the parameter can be stress, load, or deflection, etc. Therefore, we define the design factor n d as follows (Dieter, and Schmidt, 2013; Dym et al., 2005) In the case of the parameter being load, the maximum acceptable load can be established from 1.8. DESIGN FACTOR AND SAFETY FACTOR A common design approach to the problem of acceptable load vs. loss-of-function load is the deterministic design factor technique, and occasionally known as the classical method of design. The basic equation is written as above, where n d is known as the design factor. Analysis of all loss-of-function modes must be carried out. The mode which results in the smallest design factor is selected. After the completion of design, the actual design factor might get change due to changes like rounding up to a typical size for a cross-section or making use of off-the-shelf components who have higher ratings in place of using what has been calculated through the design factor. Thus, the factor is afterward known as the Factor of Safety, n. The definition of a Factor of Safety is the same as the definition of the design factor; however, it normally differs numerically (Holman and Gajda, 2001). As stress might not fluctuate linearly with load, still employing load as the loss-of-function parameter might not be suitable. The more common Creative Design of Mechanical Devices 18 practice is to express the design factor with regard to a stress and associated strength (Sapuan, 2001).

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  Soil Strength and Slope Stability

  • J. Michael Duncan, Stephen G. Wright, Thomas L. Brandon(Authors)
  • 2014(Publication Date)
  • Wiley

    (Publisher)

  Although and are equally logical measures of stability, there is less experience with their use, and therefore less guidance regarding acceptable values. Another consideration regarding use of reliability and probability of failure is that it is sometimes easier to explain the concepts of reliability or probability of failure to people who do not have technical backgrounds and experience. However, some find it disturbing that a slope has a probability of failure that is not zero, and may not be comfortable hearing that there is some chance that a slope might fail. Factors of safety and reliability complement each other, and each has its own advantages and disadvantages. Knowing the values of both Factor of Safety and probability of failure is more useful than knowing either one alone. 13.1 Definitions of Factor of Safety The most widely used and most generally useful definition of Factor of Safety for slope stability is 13.2 Uncertainty about shear strength is often the largest uncertainty involved in slope stability analyses, and for this reason it is logical that the Factor of Safety—called by George Sowers the factor of ignorance —should be related directly to shear strength. One way of judging whether a value of provides a sufficient margin of safety is by considering the question: What is the lowest conceivable value of shear strength? A value of for a slope indicates that the slope should be stable even if the shear strength was 33 percent lower than anticipated (if all the other factors were the same as anticipated). When shear strength is represented in terms of and, or and, the same value of is applied to both of these components of shear strength. It can be said that this definition of Factor of Safety computed using limit equilibrium procedures is based on the assumption that is the same for every point along the slip surface

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  What Every Engineer Should Know About Risk Engineering and Management

  • John X. Wang, Marvin L. Roush(Authors)
  • 2000(Publication Date)
  • CRC Press

    (Publisher)

  That is why skyscrapers continue to stand. Figure 5.5 Building swaying in the wind. 5.3 FROM SAFETY FACTOR TO SAFETY INDEX Whenever an item is subjected to varying stresses or loads, it will continue to function properly as long as the item has adequate strength. Failure will occur if the load exceeds the strength. Load apd strength are considered here in the broad-est sense. Load may refer to mechanical stress, an electrical voltage, or thermal stress such as temperature. Strength may refer to any resisting physical property, such as hardness, strength of material, fatigue strength, melting point or adhesion. To ensure that a system will not fail, it is standard practice to design all parts to have greater strength than the load that is anticipated. It is common to charac-terize this over-design in terms of a safety factor which is calculated by dividing the load required to cause failure by the maximum load expected to act on an en-gineering product. Risk Acceptability 117 Load Causing Failure Safety Factor = SF = Max. Anticipated Load s (5.1a) = L The critical failure mode in a column is likely to be buckling. In the case of the pyramids where their shape makes buckling virtually impossible, compression is the critical failure mode upon which the safety factor should be based. A col-umn of marble, concrete, or limestone could be as high as 12,000 feet before col-lapsing in compression under its own weight. Stronger stone, like granite, could reach 18,000 feet. But the tallest stone buildings ever erected, the limestone pyra-mids of Egypt, are less than 500 feet. Prudent ancient engineers incorporated a Factor of Safety of at least 24 in their pyramid designs. Due to the uncertainties shown in Table 5.2, neither load nor strength are fixed quantities. For example, consider a rope with a rating of 6,000 pounds being used in a hoist which is rated to lift 1,000 pounds at a time.

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