What is the Safest Rule of Thumb for Designing with a Safety Margin?

[Stephenk53]

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.

 

[gmax137]

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]

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]

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+​

Analytical models for loading and stress:​

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.

 

[trurle]

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]

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.

 

[Vanadium 50]

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? .

.

 

[mastermechanic]

Once upon a time I read something like "if you're going to sit next to it keep it at least 4" :)

 

[berkeman]

As a general rule of thumb what do you guys think is an ideal minimum safety margin when designing something?

Does the design involve life safety? Does it involve my life safety?

 

[Nik_2213]

https://failures.wikispaces.com/
The site's gone now, but held a lot teeth-sucking collection of oft-spectacular CivEng failures with careful analyses & diagnoses...

One case, IIRC, was a multi-storey SE Asian building where, for reasons unknown, the necessary 'Live Load' margin was underestimated. Possibly several more floors were built than the calculations submitted ? This margin was then eroded by subsequent changes, such as replacing some intermediate pillars on some upper floors by lintels to accommodate a more open-plan tasking. Then, IIRC, the aircon system was upgraded, and big HVAC units placed on the flat roof. Okay, there were spreaders to the pillars but, several storeys below, at least one of those pillars was perched on a lintel not rated for this additional task...

Down-side, the building structure failed, had to be demolished. Upside, it failed progressively, mitigating hazard. Given regional hazards, they were d***d lucky a middling quake hadn't collapsed the lot into the sub-basements...

 

[sysprog]

Even in matters of non-catastrophic potential consquence I try to ask myself probing questions:

What would Safety Joe do?
Would this pass UL testing?
What have I foolishly not thought of?
Am I being too optimistic?
What's the worst case scenario?
Am I crowding the specs?
What if there is a critical fault or failure:

what is the likely cost?
what consequences could be irreversible?
what can I or we or the user go back to?​

That's not nearly an exhaustive list.

 

[Nik_2213]

Given non-ideal construction situations, consider that sub-contractors may 'sell on' full-code materials such as cement and re-bar, may use el-cheapo grades rather than those you specify, and less of them, and fail to 'bond' what they do use. Then, when you see the building you designed, you may not recognise the inflated floor-count...

( Far across my extended family, a builder absconded with the project funds for modest condo. To raise the funds again, the architect, who'd been paid with the penthouse, had to re-work his plans to provide several more floors to be 'let'. That this needed a bigger elevator and stronger pillars was unfortunate...)

 

[anorlunda]

The OP offered no clarity on the context of the question.

Designing a bridge to carry traffic and landing the first man on the Moon are very different objectives with different acceptable margins. But both are engineering.

The phrase "proven technology" is often heard in engineering context. Something is proven when we have an overwhelming accumulation of experience; both successes and failures. Once proven, we can begin reducing conservatively high margins to more optimistic levels.

 

[Nik_2213]

"Once proven, we can begin reducing conservatively high margins to more optimistic levels. "

Classic example is the Forth Railway Bridge. As originally designed, it was a slim, elegant suspension design. But, just prior to the start of its construction, the Tay Railway Bridge went down in a terrible storm. The Forth's designers were horrified, as they'd used comparable factors for wind loading etc. So, rather erring on the side of safety, their now-famous, immensely strong cantilevered design was devised, to be sure, to be sure...

 

[tech99]

"Once proven, we can begin reducing conservatively high margins to more optimistic levels. "

Classic example is the Forth Railway Bridge. As originally designed, it was a slim, elegant suspension design. But, just prior to the start of its construction, the Tay Railway Bridge went down in a terrible storm. The Forth's designers were horrified, as they'd used comparable factors for wind loading etc. So, rather erring on the side of safety, their now-famous, immensely strong cantilevered design was devised, to be sure, to be sure...

On one occasion I was designing a lattice steel radio mast. Our company had standard safety factors, but in my particular case I had the luxury of testing a section of mast to destruction. So having more certainty over strength, I could reduce the safety factor.

 

[jrmichler]

Safety factors vary.

I had a job once with a safety factor barely over 1.0. It was a monorail beam to be used exactly once to install a pump. I discussed the situation with the millwrights ahead of time, making it clear that this was a marginal situation and they should watch carefully when they lifted and moved the pump. One factor considered was that the company had very strict safety rules about standing under suspended loads. The failure mode would not have been catastrophic because the beam would have yielded without buckling.

In another case, the roof trusses were strong enough to hold a one time installation load, or a snow load, but not both. I told the millwrights to get a crew and shovel the roof if it snowed. They were lucky, it did not snow.

Structural steel has a safety factor of 1.66, but there is additional safety factor in the code specified live loads.

Small airplanes, such as the Cessna 172 that I partly own (flying club) have a safety factor of 3.8, which the airplane must meet without any damage. There is an additional safety factor of 1.5 times that (5.7) which the airplane must meet and still be able to get to a safe landing. I heard of a case where a spoiled rich kid took flying lessons, soloed, then went up by himself and tried to do a loop. The airplane survived, but with bent flap mechanisms. The student was sent back home. My father told me a story about some hotshot pilots in the South Pacific during WW II that tried to loop a PBY-5A (big twin engine amphibian) after being told that that airplane could not be looped. The book was both right and wrong. The PBY can be looped, but the entire trailing 1/3 of the wing buckled upward. They got it back, but it was a parts plane after that. He claimed that he did not take part in that, but I have always wondered...

Here is a collection of videos and photos showing various airplane wings under maximum loads: https://www.popularmechanics.com/flight/g2428/7-airplane-wing-stress-tests/. Something to keep in mind the next time you fly commercial, and see the wing tips moving a few inches in turbulence.

 

[Nik_2213]

Long, long ago, I was flying out to Greece to meet up with family who'd driven/camped overland. Plane was a weary 707, like a well-used 'tube train', laden with package-holiday 'Old Dears'. As one of the very few 'young adults' aboard, I was seat-switched to beside starboard wing's emergency exit. It had rather more leg-room, for which I was very, very grateful, but I had to promise that, 'in extremis', I'd set an example by using the deployed chute 'toot sweet'. And, if necessary, opening the exit and deploying the chute myself...

So, off we went. I'd never flown before, so was initially alarmed by sight of 'my' wing's jet engines wriggling. A scientist of sorts, I worked through the engineering logic that appropriately flexible wings and engine mounts were actually better than stiff, which would be far too heavy...

Around came the drinks trolley. As a designated 'chute diver', I was one of the very few who took a 'soda', while the 'Old Dears' totally pillaged the spirits. Then, social niceties sorted, folk began to look around the plane. Lady immediately in front of me sorted through her spectacles, peered out the window, did a splendid double-take.

You remember that Classic Twilight Zone episode with the gremlin on the wing ?
Season 5 | Episode 3, https://en.wikipedia.org/wiki/Nightmare_at_20,000_Feet

"The wing's waving ! The engine's wobbling ! The engines are wobbling ! They're going to fall off ! We're DOOMED !"
"Nah..."
"You look ! See ??"
"Egads ! The wing IS waving ! The engine IS wobbling ! The engines ARE wobbling ! They're going to fall off ! We're DOOMED !"
Thinking very quickly, I leaned forwards, tapped the lady on the shoulder and said, "Don't worry, Dear ! The plane won't drop its engines ! See the logo ? They're made by 'Rolls-Royce' !"
"Ooh ! So they are ! Thank you, young man !"

That message of "...whisper-whisper 'Rolls-Royce' whisper-whisper..." duly proceeded up my side of the cabin and down the other...

We flew into a huge thunderstorm over Italy and the Adriatic. Seat-belt light came on as we crossed the Alps, stayed on. Air got bumpy, then seriously bumpy. As we descended towards Athens, aircraft was being tossed about like a toy. You could see the cabin flexing. The cabin crew fought a running battle against overhead lockers popping open. Those pretty blue strobe-lights outside were sheet lightning. The cabin lights kept tripping and being reset. I was totally terrified. An agnostic, I was chanting the exit and chute deployment steps...

Then, from the seat behind me, an old lady taps me on the shoulder and says, "Don't worry, Dear ! The plane won't drop its engines ! See the logo ? They're made by 'Rolls-Royce' !"

I was still giggling helplessly when the pilot got down on the third or fourth bounce, aquaplaned to the very, very end of the runway...