Is the term "ultimate load" the same as "design ultimate load"? 1

[CamJPete]

I'm a little embarrassed that I haven't figured this out thus far into my career, but alas, here I am.

Let me start by stating (in simple terms) what I think is true on this topic:

1. The limit load LL is the maximum expected applied load that could occur during service.
2. The design limit load DLL is the limit load multiplied by required factors of safety, especially yield. Used to design against yield failure.

(Update: I just asked my supervisor about this, and said that DLL and LL are understood to be the same. Maybe it just depends on who you ask.)​

3. In the aircraft industry, ultimate load UL is by definition 1.5 X limit load. Used to design against ultimate failure?
4. In the space industry (my line of work), I've seen the term design ultimate load DUL, which I've seen defined as the limit load multiplied by required factors of safety, especially ultimate (I've seen 1.4, 1.5, and 2.0).

My question: Is the following true?
LL is not the same as DLL, where the latter includes a multiplying yield factor of safety (at minimum).
UL is the same as DUL, where UL already implies "design"(?)

I want to make sure that I shouldn't be multiplying the UL by safety factors to get DUL, like I do the LL to get DLL.

 

[SWComposites]

LL = DLL (there is no factor applied to DL to get to DLL).
UL = DUL
However, there are different factors of safety to get from LL (or DLL) to UL (or DUL). Aircraft is mostly, but not always, FoS = 1.5 (there are some failure cases for which FoS = 1.0, 1.25, etc. For spacecraft, FoS can be 1.4 to 2.0, depending on customer, load condition, etc.

 

[CamJPete]

I assumed you mean "there is no factor applied to LL to get to DLL"? If so...

That is interesting. I'm doing a bolted joint analysis and a NASA Standard (NASA-STD-5020) gives these definitions:

Limit Load: The maximum expected applied load, including load transferred across joints as a result of thermally induced loading of the structure; it does not include preload or changes in preload occurring as a result of temperature change.
Yield Design Load: The product of the yield factor of safety, the fitting factor, and the limit load. Also referred to as “design yield load.”
Ultimate Design Load: The product of the ultimate factor of safety, the fitting factor, and the limit load. Also referred to as “design ultimate load.”

Okay, so I remembered the definition incorrectly where it says "design yield load", not design limit load. If you hear the term "design yield load", does your brain think "design limit load", or does it think "design limit load X yield factor of safety"? If not, what term do you use when you want to communicate that the load is LL * yield safety factor?

Also, maybe you meant to say DL. Do you use design load, design limit load, and limit load interchangeably?

I feel satisfied that UL and DUL are the same.

Thank you very much for your post. This has been on my mind for some time.

 

[rb1957]

well, what do you call Fitting Factor ?

yes, limit load is the maximum load expected in service. Ultimate load is limit load* ultimate FoS (typically 1.5)

but what about Fitting Factor, Casting Factor, etc ? Maybe DLL is LL*design factors ? it's all "just" words.
I think these factors are applied on top of LL or UL due to design considerations.

what about composite factors ?

If you want to call Limit load * design factors "Design Limit Load" I don't think anyone will put you up against a wall !

Ok, noticed your reply ... if the NASA doc define things, then that's what they mean.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[SWComposites]

Do you use design load, design limit load, and limit load interchangeably? > Yes.

Yield Design Load: The product of the yield factor of safety, the fitting factor, and the limit load. Also referred to as “design yield load.” > this is a lousy terminology. the yield factor of safety and the fitting factor should go into the margin of safety equation as explicit values. the product of those times "limit load" should not be called a "design load".

Ultimate Design Load: The product of the ultimate factor of safety, the fitting factor, and the limit load. Also referred to as “design ultimate load.” > this also bad. the fitting factor should not be multiplied by the factored limit load (ultimate load) to get a "design load"; the fitting factor belongs in the margin calc explicitly.

 

[dik]

Because of the confusion it's best to ask who specified the loading... I generally use service DL and LL and factored DL and LL, so there's no confusion.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[CamJPete]

this is a lousy terminology > You can see part of the source of my confusion. I thought it was understood industry-wide that when you multiply by safety factors, only then does it become "design" loads.

I generally use service DL and LL and factored DL and LL > This is a pretty clean way of describing it. I may adopt this as well.

I worked on launch vehicles for a while, where the term Maximum Expected Operating Pressure (MEOP) was used. I'm assuming that was in a sense that was the limit load. I also work now on spacecraft where we vibration test our payloads, and the term Maximum Predicted Environment (MPE) is used. I'm assuming this is also the limit load. It seems the term limit load is a broad term that can apply to forces (say 1000 lbf), or accelerations (say 35g), or pressure (1400 psi), or acceleration spectral densities (say 7 g-rms). It seems there is a subtle but important meaning between the terms "load" and "force".

Now that I type that out, I think for most of my career I've always thought that a "load" must have the same units as "force" (and the internet generally agrees), but I think load is used more broadly by engineers to mean generally "anything that acts on a system", even something like a voltage or acceleration which clearly does not have units of "force". Do you agree?

I appreciate your insights.

 

[SWComposites]

Yes, "Limit Load" can include thermal effects, accelerations, etc, etc. Its kind of a generic term. So yes, your MEOP and MPE are essentially "limit load" conditions.

But every customer/regulator (NASA, USAF, USNavy, FAA, EASA, etc) have different terminology and approaches which can make things confusing.

I was taught many years ago by Stress leads in a big aerospace company that its bad practice to bury "factors" into "loads" (other than the "ultimate" safety factor applied to limit loads to get ultimate loads), as it adds confusion to documentation, makes it hard to sort out the analysis later, makes it hard to remove factors when necessary for MRB, etc.

And as an aside, in ASTM D30 committee we have a very long "debate" regarding whether to use "load" or "force" as applied to a test specimen, and in the end decided to use "force".

 

[CamJPete]

Yes, that is clear now how burying factors into the loads can be problematic.

This has been insightful. Thanks all for the time you've taken to help me. It has really cleared things up in my brain.

 

[Sparweb]

Woah. Yield is a structural response to load, not a load. Mixing those up is sloppy.

You guys are making this way too complicated.

Limit load needs to include all operating load factors and special factors if they apply (fitting factor, environmental factor, knock-down factor, you name-it factors, etc.). Show the special factors in your work, as SWC recommends.

Ultimate load is Limit Load * 1.5 (when 1.5 is the typical factor of safety, but other FOS may apply to other structures).

OK it's not always that simple:
Some special factors are defined as only applicable to ultimate load case, so you wouldn't include them in the Limit Load (for example, the seat fitting factor per FAR 25.785) but others do belong in the Limit Load (such as environmental condition factors). So you have to know what the special factor means to apply it correctly.

 

[CamJPete]

Thanks Sparweb.

It sounds like there are differing approaches to what should be called limit load. Some say don't include factors, and other say do include them. One of the confusions that can come with including a yield safety factor in the limit load, is that you then have to divide it out before you multiply by 1.5 to get ultimate. It seems to me much cleaner to make limit load the unfactored maximum expected load, and then operate on that with the SF's to get yield and ultimate margins. It also seems consistent with how the space industry I'm in describes limit load, besides this seemingly funky definition of "design yield load".

 

[CamJPete]

I might also add that I found this definition today in NASA-STD-5002:

Limit Load: The maximum anticipated load experienced by a structure during a loading event, load regime, or mission. The factors of safety are not included in the limit load.

Update: and found this a couple hours later: Design Loads: The product of the factor of safety and the limit load.

So, looks like it is important for us to clarify what we mean by limit load or design load when we say them so that we're on the same page.

Cam

 

[rb1957]

yes, we should know what we mean when we say something ! (smile)

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[Null Full]

It sounds like you have a good understanding of the various load terms and how they are used in different industries. Here is a summary of what you have described:

Limit Load (LL): The maximum expected applied load that could occur during service.
Design Limit Load (DLL): The limit load multiplied by required factors of safety, especially yield. Used to design against yield failure. In some cases, LL and DLL may be used interchangeably to refer to the same thing.
Ultimate Load (UL): The maximum load that a structure can withstand before failure. In the aircraft industry, UL is defined as 1.5 times the limit load. Used to design against ultimate failure.
Design Ultimate Load (DUL): The ultimate load multiplied by required factors of safety, especially ultimate (e.g. 1.4, 1.5, or 2.0).
To answer your question, it is generally true that LL is not the same as DLL, and that UL is the same as DUL. In other words, DLL and DUL are calculated by multiplying LL and UL, respectively, by appropriate safety factors. It is important to note that these safety factors may vary depending on the industry and the specific requirements of the design.

 

[rb1957]

yeah, well, that's not typical practice.

First, ultimate load is not the maximum load a structure can withstand before failure. It is the minimum load a structure must withstand before failure.

Second, limit load must to reacted without excessive yielding (often simplified to "without yielding").

Now the OP has I think a specialised situation, outside of typical commercial airplanes, where the certifying agency has defined a limit load FoS, and should have defined the terms in their usage.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[SWComposites]

Concur with rb.

Null Full - you have multiple things wrong. UL is max required load, not max load the structure can withstand. DUL is not UL times the ultimate factor. Please, it’s confusing enough without incorrect info added.