F, N pairs
Refers to f, the probability of life loss due to failure for each scenario studied, and N, the number of lives expected to be lost in the event of such a failure scenario. The term N can be replaced by any other quantitative measure of failure consequences, such as monetary measures.

F-N curves
Curves relating the probability per year of causing N or more fatalities (F) to N. This is the complementary cumulative distribution function. Such curves may be used to express societal risk criteria and to describe the safety levels of particular facilities.

Factor of Safety
The ratio of system resistance to the peak design loads, often calculated in accordance with established rules.

Failure
The inability of a system, or part thereof, to function as intended. In the context of structural safety (including geotechnical structures), failure is generally confined to issues of structural integrity, and in some contexts to the special case of collapse of the structure or some part of it.

Failure mechanism
A mechanism describing the physical processes and states that must occur for failure to develop.

Failure mode
A way that failure can occur, described by the means by which element or component failures must occur to cause loss of the sub-system or system function.

Fault
A planar or gently curved fracture in the earth’s upper layers across which displacement occurs. When this displacement is abrupt it gives rise to an earthquake.

Fault tree analysis
A systems engineering method for representing the logical combinations of various system states and possible causes which can contribute to a specified problematic (fault) event (called the top event).

Floods
The most frequent and costly natural disasters in terms of human hardship and economic loss. As much as 90 percent of the damage related to all natural disasters (excluding droughts) is caused by floods. Flooding occurs in known floodplains when prolonged rainfall over several days, intense rainfall over a short period of time, or an ice or debris jam causes a river or stream to overflow and flood the surrounding area. Melting snow can combine with rain in the winter and early spring; severe thunderstorms can bring heavy rain in the spring and summer; or tropical cyclones can bring intense rainfall to the coastal and inland states in the summer and fall. Several factors contribute to flooding. Two key elements are rainfall intensity and duration.

Forecast
meanings in different disciplines (see prediction). A forecast is often considered to represent the most rigorous form of prediction by stating, for the occurrence, what will occur, as well as the time and place of occurrence. Lesser forms of prediction may simply provide a probability of occurrence.

Fragility curve
Defines the probability of failure as a function of an applied load level; a particular form of the more general system response.

Frequency
A measure of likelihood expressed as the number of occurrences of an event in a given time or in a given number of trials (see also likelihood and probability).

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