Eng. practice of allowing 5% overstress

When reviewing engineer work would you allow 5% margin, similar to the 5% overstress?

How do people feel about the 5% overstress?

Is this typical in your location?

Should this be reviewed with better analysis platforms like FEM and FEA?

My experience is at secondhand, in automotive. We typically use a factor of safety on fatigue life of 8 cycles, which is around 140% on stresses. As such 5% is no biggy.

For me, the background of the situation has a lot of impact with this, and it is definitely in the realm of “engineering judgement”.

For example, if reviewing an existing structure, perhaps one designed using a different analysis method, and which has a history of satisfactory performance, not susceptible to buckling or non-linear effects, I’d consider allowing it. My own building code has provisions for this type of situation, with reduced load factors for some situations.

I have allowed yielding in new design once, and in that instance, I used non-linear FEM to keep plastic strain to 2%. Eurocode for plate structures (EN1993-1-5 C.8 Note 1) recommends 5% plastic strain as a limit state for FEM, provided that the structure is not susceptible to buckling. I used 2% due to a number of load reversals anticipated. Note that the percentage of plastic strain and linearly-determined overstress beyond yield are not the same.

With the means and methods we have now, I am very reticent to go over 1.00.

Less so 30 to 40 years ago when I started doing engineering. We all used slide rules then…

In aero, we look for a margin of safety >0 (ie. safety factor = 1) in most cases, but many joints and primary structural connections are not ruled by static strength, but rather by fatigue or fracture mechanics. Other things are only passed by test but preparation for the test involves design work to prevent failure, and many tests require deflection to be measured as an assurance that the functions are preserved.

Generally what’s accepted in one discipline is moulded by the environment of that discipline. By environment you can include the weather, air, earth and the ocean, but also the sensitivity to weight, impact, vibration, heat/cold, radiation and a lot of other factors. They also form an “environment” which are very different when considering a building foundation, a jet engine, or a microprocessor.

Personally, I think we experienced a 100% over stress in 2020… :-1:

To compensate, I am sitting by my warm wood stove having a cup of coffee before I tackle two small projects today. Destressing is good, even for materials.

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Then… you are a-neeling by your stove to relieve your stress??
But… is coffee the right choice to quench your thirst?
Sorry… this might precipitate more puns.
I may have crossed a grain boundary.

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Here’s what I have done in the past. Typically on existing projects, if there is a minor change of use or change of load, then there is a “trigger” point of overstress that would cause a required retrofit. I think it’s usually a bit more than 5%. But, I use whatever the trigger point is for that job or that jurisdiction.

For new construction, I don’t generally allow a 5% overstress. However, on occasion, I have done so. But, I always provide a justification… Where I cite a number of conservative assumptions I have made in my loading or analysis that should more than offset the the 5% overstress that I am allowing. Essentially, I’m just pointing out the fact that I kept my calculation reasonable and short, but that anyone with decent engineering judgment would easily see that the 5% isn’t a true overstress and doesn’t represent a genuine design problem.