Talk:Factor of safety

Stress analysis

please mention the formula for factor of safety in stress analysys. —Preceding unsigned comment added by 59.95.41.226 (talk) 04:51, 12 September 2007 (UTC)

Not exactly sure where this should go, but the following wording makes the article a bit unclear: The safety factor, or yield stress, is how much the designed part actually will be able to withstand (first "use" from above). The design factor, or working stress, is what the item is required to be able to withstand (second "use"). "The safety factor, or yield stress" seems to say that safety factor = yield stress, but safety factor is unitless while yield stress is force/area. A similar argument exists for "The design factor, or working stress". This is made even more unclear by the equation which states safety factor = yield stress/working stress. Perhaps the parenthetical clauses "or yield stress" and "or working stress" can be removed, as well as the 'first "use" from above' and 'second "use" '. Flexpicker (talk) 19:21, 20 January 2021 (UTC)

Doses

I don;t have time to look it up, but the discussion at the bottom about meds can be extended to LSD vs Heroin. With the first it's virtually impossible to overdose, while the margin for heroin is extremely small, and decreases when the body gets used to it, so many addicted persons will keep increasing the dose until lethal overdose in search of the "high".192.88.166.35 (talk) 10:57, 6 May 2008 (UTC)

Working load limit

Should probably define and describe the commonly-used term "working load limit", or WLL. —Preceding unsigned comment added by 72.146.123.231 (talk) 03:03, 25 August 2008 (UTC)

Math Formatting

Some math formatting may be well used in this article.

${\text{Design Factor}}={\frac {\text{Failure Load}}{\text{Design Load}}}$

Blue Leopard (talk) 13:42, 11 September 2008 (UTC)

Margin of Safety

Margin of Safety was given in the article as follows:

Margin of Safety = [Failure Load /(Design Load*FoS)] - 1

I don't think this is correct because FoS seems to be the same as Design Factor in this article and would cancel out the other terms in the equation. Overall, I find the distinction between FoS and Design Factor confusing in this article. Blue Leopard (talk) 13:42, 11 September 2008 (UTC)

Me too. In the first paragraph,

Factor of safety (FoS) can mean either the fraction of structural capability over that required, or a multiplier applied to the maximum expected load (force, torque, bending moment or a combination) to which a component or assembly will be subjected. The... first is a measure of the reliability of a particular design, while the second is a requirement imposed by law.... The first sense [is] a factor of safety, ... and the second sense [is] a design factor.... The Factor of Safety is given to the engineer as a requirement. The Design Factor is calculated by the engineer.

appears to be self-contradictory.—Dah31 (talk) 03:37, 5 December 2008 (UTC)

As a Chartered Civil Engineer I find the whole article less than satisfactory.

The term "Margin of Safety" is one that I've not come across, though the formula quoted looks to be a reasonable candidate if such a quantity is in use. However, the term "Factor of Safety" (FoS) is not an easy one to describe. Gone are the days when engineers worked with just one Factor of Safety equated to the load that the structure is designed to carry divided by the load that it is expected will need to be carried. In other words if an engineer calculated that 1 Tonne needed to be carried and designed an appropriate structure to carry 1.25 Tonnes, then the FoS would have been 1.25.

In modern structural design in the UK, we use Limit State Design and there is no overall Factor of Safety - rather each Limit State will have its own FoS.

To quote from clause 2.2.1 of British Standard 8110-1:1997

"Well-detailed and properly-erected structures designed by the limit state method will have acceptable probabilities that they will not reach a limit state, i.e., will not become unfit for their purpose by collapse, overturning, buckling (ultimate limit states), deformation, cracking, vibration, etc. (serviceability limit states) and that the structure will not deteriorate unduly under the action of the environment over the design life, i.e., will be durable. The usual approach is to design on the most critical limit state and then to check that the remaining limit states will not be reached"

The engineer's skill lies in predicting which of the limit states will be "the most critical", and since it's not always obvious there are many instances in which the design process becomes highly iterative.

Hence Engineers cannot simply calculate a load to be carried and design a structure to carry it. Instead they must ask what loading scenarios are likely to be encountered during the structure's lifetime. Two broad categories of Limit State are considered - the Service Limit State and the Ultimate Limit State. The first reflects the expected, maximum, everyday (what used to be called "working") loading scenario, whilst the second covers the state of the structure at the point of collapse.

Firstly, the loads to be carried by a structure are multiplied by a range of partial safety factors dependent upon the type of load, the loading scenario (or combination) and the Limit State being considered. Hence, self weight attracts a lesser factor than wind load since we can be more certain about density of materials. In essence therefore, these partial load factors reflect our uncertainty concerning the variability of load throughout a structure's life.

Secondly the strengths of the materials used (determined by laboratory test) are divided by partial material safety factors. Here again the actual values depend upon the material and the Limit State being considered. For example, the ultimate strength of steel is easier to predict and attracts a figure of between 1.0 and 1.15 (normally) whilst concrete, being more variable is given a partial safety factor of around 1.5.

Finally, add to this recipe the ingredients of incomplete understanding of structural behaviour and human error (both at design stage and during construction) and the ability to define one single, simple factor of safety becomes an exercise in futility.

That said most Civil Engineers recognise that the "ball park" figure for the FoS they work to is somewhere between 1.5 and 2.0. This compares very favourably with the FoS in use within the airline industry where (so I'm told) values of 1.1 are in evidence from time to time!

GordonSmy (talk) 12:13, 23 December 2008 (UTC)GordonSmy

I would like to reiterate this request for clarification on Margin of Safety. This is something that is still occasionally used in certain fields but no longer taught in most engineering curricula. The provided method and equation of solving for the margin, as pointed out by the first post, would seem to always result is a margin of 0. As, by definition, the design load times the factor of safety is the failure load.

128.158.1.166 (talk) 17:50, 28 July 2009 (UTC)

There is nothing unclear about the difference between factor of safety and design factor in this article, as read by a Colorado and Texas licensed professional engineer with three engineering degrees. "Design Factor" is a minimum requirement for excessive strength over the maximum stress expected, as required for legal and professional due diligence in accounting for expected uncertainty in material properties under certain types of load and environmental conditions. "Factor of Safety" is what excessive strength you actually achieved in the design because the catalog bolts, fasteners, or material thickness available must be picked from a catalog such that the "Design Factor" is not violated. For example, a 1/2" rivet might give a Design Factor of 1.1. It might hold the actual stress initially, while being in contractural or legal volation of a required Design Factor of 2.0. Imagine that the next bigger rivet size is 5/8". To get over the DF of 2.0, you must go to the next size up, but that next size up might give a FoS of 2.8. That doesn't mean that the engineered overdisigned the product. I just means that that is what resulted from the smallest available rivet size to satisify the DF of 2.0. It would be outrageously expensive to custom fabricate rivets to force the FoS to exactly match the DF. Robert Patlovany 15:25, 8 June 2010 (UTC) —Preceding unsigned comment added by 192.91.173.36 (talk)

Initial Description

The initial description should be broad enough to include other fields of discipline that use Factors of Safety. The following is a generalized example

Factor of Safety is the ratio of an ultimate limit state over an anticipated condition where:
ultimate limit state - The point after which conditions are unsafe
anticipated condition - a quantified value often dictated by codes, literature and standards of practice

This broader definition can be applied to the engineering fields as well as to the medicinal field per the example at the bottom DerivePI (talk) 21:14, 18 February 2009 (UTC)

Yes. We should also discuss that these are not "points" but distributions with a range of values. See: Statistical interference. Rlsheehan (talk) 22:59, 18 February 2009 (UTC)

Reliability=

I believe that the use of the term"reliability" is misleading in this context. Higher factors of safety do not necessarily improve reliability, which is essentially a measure of how likely something is to fail during a fixed period of operation due to inherent or induced defects. The FoS must refer to an expected magin of strength above the the expected level. Reliability is normally associated with randonm processes, thereas here it is addressing the adequacy of a design to meet the conditions in which it is placed. I suggest that the use of reliability is struck.

gstaranto--Gstaranto (talk) 07:50, 3 March 2009 (UTC)

The wording relating to reliability (and other sections) is certainly awkward and needs improvement. In many uses, however, reliability is a closely related subject. Reliability is a probability of performing for a certain time period under specified contitions. Often a higher design factor of safety will result in higher reliability. FoS, also can be looked on from a probabilistic standpoint. Rlsheehan (talk) 16:13, 3 March 2009 (UTC)

Red link hatnote

The following was copied over from my talk page. Wizard191 (talk) 14:50, 19 December 2010 (UTC)

Red links are good for the wiki. Leave it in. Maury Markowitz (talk) 02:16, 19 December 2010 (UTC)

I have no problem with red links in general, but I do for hatnotes. If you are working on an article, just add the link once the article exists. That seems pretty easy to me, so I don't get why this is a big deal. Wizard191 (talk) 14:46, 19 December 2010 (UTC)

I requested a third opinion. Wizard191 (talk) 14:56, 19 December 2010 (UTC)

Comment from an uninvolved editor: Normally, I like redlinks - they encourage wikipedia to grow - but I think a redlink in a hatnote is inappropriate. It's at the very top of the article. (There also seems to be consensus against redlinks in "See also" sections).

I would suggest saying the same thing without a hatnote - by moving the redlink into the body of the article, with a heading and a couple of sentences to outline the concept of a safety factor in plasma physics, until somebody actually writes a separate article on the subject. Would that be acceptable? bobrayner (talk) 15:49, 19 December 2010 (UTC)

Calculations

There should be some examples given for how to calculate FoS based on the literature. It is bizarre that on the page about FoS there is no explanation of how to calculate it! Jackhodari (talk) 00:16, 3 June 2020 (UTC)

Title change (Factor and margin of safety)

I put in a request to have the title reverted back to "Factor of safety" because I think the original title is better than "Factor and margin of safety". My understanding is that a margin of safety is a sub-topic of factor of safety, rather than an equivalent term. To better address margin of safety in the article, I would suggest adding some summarized information about the margin of safety to the lead and also changing the Margin of safety link to Factor of safety § Margin of safety. However, @NearlyMad and anyone else interested in discussing this, please let me know your thoughts so we can reach consensus about the title. Thanks, Inverted Hourglass (talk) 19:53, 11 September 2021 (UTC)

Title Change

This issue with the title of this article is that Factor of Safety is rarely used in Structural Analysis. Margin of Safety is the final answer in almost all structural analyses. If it is best to have both the Factor of Safety and Margin of Safety definitions in the same article, either both should be mentioned in the title or the title should be changed to Margin of Safety and the order of the definitions switched. I suggest this for usability and clarity. Joseph L. Moore (talk) 19:36, 12 September 2021 (UTC)