# Safety margin rule of thumb

## Main Question or Discussion Point

As a general rule of thumb what do you guys think is an ideal minimum safety margin when designing something? I know that this is a very broad question and could vary depending on the situation (ie something on a static load may need a smaller margin than dynamic) and since I do not have a specific situation in mind, feel free to specify a situation in which your rule of thumb applies. As a general rule I think the maximum safe loading capacity should be at least double the maximum expected load.

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Design to the applicable code - ASME, AISC, etc. whatever is applicable to the thing being designed. If none of the codes are applicable maybe you can use them anyway. And, don't forget that sometimes there is margin in the properties to be assumed, not just in the results.

tech99
Gold Member
I remember that before the days of everything being standardised, we used consultants to advise on safety factor for lifting men on radio masts.
They considered the uncertainty in the loads, both static and shock loading, any slight misuse of the equipment (geometry of the lifting rig etc), likely wear and tear, inspection intervals, consequences of a failure, uncertainty in the strength of materials and equipment and whether a back-up was used in case of failure.
So whereas the codes for structural steel incorporated a SF of perhaps 2:1, ropes for goods lifting were rated at 6:1` and those for man lifting at 12:1.

jack action
Gold Member
From a design book I had years ago, you had to choose the highest from $N_1$, $N_2$ and $N_3$:

Material property available from tests:

Quality of information$N_1$
The actual material used was tested​
1.3​
Representative material test data are available​
2​
Fairly representative material test data are available​
3​
Poorly representative material test data are available​
5+​

Environmental conditions in which it will be used:

Quality of information$N_2$
Are identical to material test conditions​
1.3​
Essentially room-ambient environment​
2​
Moderately challenging environment​
3​
Extremely challenging environment​
5+​

Quality of information$N_3$
Models have been tested against experiments​
1.3​
Models accurately represents system​
2​
Models approximately represents system​
3​
Models are crude approximations​
5+​

It does overlap with the general recommendations found on engineeringtoolbox.com.

As a general rule of thumb what do you guys think is an ideal minimum safety margin when designing something? I know that this is a very broad question and could vary depending on the situation (ie something on a static load may need a smaller margin than dynamic) and since I do not have a specific situation in mind, feel free to specify a situation in which your rule of thumb applies. As a general rule I think the maximum safe loading capacity should be at least double the maximum expected load.
x2: General applications (i.e. gadgets, enclosures, toys)
x3: Static loads for injury-if-failed applications (i.e. transport, furniture)
x5: Structural members of buildings, bridges etc.
x8: Shock loads (i.e. mountain climbing equipment)

Reducing design margins below indicated above is possible, but require a thorough qualification by test and FEM simulations.

DEvens
Gold Member
This is entirely too broad a question to give a specific answer.

It depends on very many things. Here are just a few.
- Length of expected service
- Available monitoring and maintenance during service
- Potential failure mechanisms and harm from them
- Potential accident or upset conditions and potential harm from them
- Cost of providing margin
- Constraints on operation
- Environmental sensitivity

Say you are building a bridge. You might like to build that bridge to have a huge safety margin. But you can't just make the bridge arbitrarily strong because extra strength costs money. And uses more space, where you might need to leave room for things like traffic under the bridge. And there may be many other constraints. Building the bridge out of stronger materials might interfere with, for example, seismic requirements. Or it may need to have expansion joints to accommodate temperature changes. Or it may have to accommodate whacky things like salt on the travel surface in the winter. A lift bridge has to be capable of lifting, so it can't be heavier than the lift mechanism will support.

Say you are building an aircraft. A heavier aircraft might be stronger, but might not be able to fly. Or might not be able to fly with a fuel use rate that could be supported by an airline selling tickets.

There are also questions of a thing being safe in one direction for one consideration, but less safe for another consideration. A trivial example: A wooden structure with more wood is presumably stronger, and so safer with respect to structural failure. But more wood is more fuel in the case of a fire. So a wooden structure that must exist in the presence of potential ignition sources might be safer with less wood and more material to protect from the ignition sources.

Staff Emeritus
A heavier aircraft might be stronger, but might not be able to fly.
But it won't crash!

I agree - "it depends" is the answer. That's why one needs to hire engineers and not just buy calculators.

OCR
but might not be able to fly.
But it won't crash!

OK. . . ?

Then, I'll define crash in terms of. . . altitude above you, and runway behind you. .

Now what? .

.