Question regarding Ultimate Limit State Design 12

[fracture_point]

I was doing some additional background reading on ULS design, and found the following quote on wikipedia:

The ULS condition is computationally checked at a certain point along the behavior function of the structural scheme, located at the upper part of its elastic zone at approximately 15% lower than the elastic limit. That means that the ULS is a purely elastic condition, located on the behavior function far below the real Ultimate point, which is located deep within the plastic zone.

I can't find a reference for the approximation of 15%. We perform sectional analysis of members based on the plastic condition, and these loads are based on partial factors of safety to materials and loads. But I can't find anywhere that provides details about it remaining elastic.

 

[r13]

The steel yield "stress vs strain" curve below answers your question. Note that point B is the true yield stress. The yield stress we use for design is point A, which is at the end of linear range. At true yield, the member won't be fully restored to it's original shape after the load is removed.

 

[IDS]

If that was intended to be a general definition, it is ridiculously over specific.

The wording is also unnecessarily obscure.

The ULS capacity is an approximation of the applied action that will cause section failure, factored down either by a single factor, or by partial factors applied to different materials. How the stress and strain conditions relate to the actual behaviour of the material depends on the material and the design code.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[r13]

The ULS condition is computationally checked at a certain point along the behavior function of the structural scheme, located at the upper part of its elastic zone at approximately 15% lower than the elastic limit. That means that the ULS is a purely elastic condition, located on the behavior function far below the real Ultimate point, which is located deep within the plastic zone.

What are the base for Fy and E in ULS - same as ASD, within linear elastic range.

 

[IDS]

On reflection, I think the Wikipedia statement is just wrong, as well as being over-specific and unnecessarily obscure.

The definition of the ULS for bending is strain based, with a strain well in excess of the elastic limit. In the great majority of cases sections will remain elastic under the maximum load, because the design load is factored up and the design strength is factored down, but in the rare cases where the ULS is actually approached or reached, because the section is over-loaded and under-strength, it will be in the plastic strain range, which it should be able to accommodate without total failure.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Tomfh]

I agree ULS is often beyond the pure elastic limit. Eg it is common to use plastic modulus and effective section modulus to calculate ULS which by definition assumes the section is past the pure elastic limit.

As for the 15% margin, it depends upon the material. I agree this figure is far too specific

 

[Blackstar123]

Agreed with the above 2 posts.. I've read the statement more than 5 times and i still don't get what the statement is actually trying to convey. Or may be i should say that i get what it's trying to state but I don't agree with 90% of the definition.

Ultimate limit states are defined for designing purpose which state the maximum load/stress or strain a material can bear, before it starts to deform plastically (in case of ductile material) or fail (in case of brittle material) theoretically.

In actual failure may occur after alot more load or alot more deformation than the defined ultimate limit states.


Euphoria is when you learn something new.

 

[BridgeSmith]

Bottom line - don't attempt to use wikipedia as a technical resource.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[r13]

Note that we design the material to be "at yield" condition, not "have yield", doesn't it means the material remains in the elastic range? The OP's quote simply points out that the word "ultimate" in ULS leads to the misconception that it is associated with the ultimate stage in the plastic region, which is further down the property curve from the yield point (To be in the plastic range, the stress must be greater than Fy, even slightly). I guess the 15% lower is meant, that even in the ULS design, the stress in the material remains below the yield, again, in the elastic range.

 

[r13]

A graph is better than thousand words.

 

[Tomfh]

One of us should delete or modify it.

 

[IDS]

Bottom line - don't attempt to use wikipedia as a technical resource.

I disagree, Wikipedia is a valuable technical resource, you should just not rely on anything you read there without confirmation from an independent reliable source, but that applies to any technical resource.

retired13 - a graph with a few words of explanation might be worth a thousand words, but I'm not sure what your added red squiggly lines represent.

I think your previous post is a good interpretation of what the Wikipedia article is saying, but the Wikipedia article is wrong. To take reinforced concrete as an example, because the strain limits are explicitly stated, at the Ultimate Limit State the concrete will have a maximum strain about 5 times greater than the elastic limit and about 3 times the strain at which it goes plastic. The tension steel in an under-reinforced section will be in a similar state. Most concrete sections will never reach anything like this state because of the design load factors and strength reduction factors, but the whole basis of designing for the ULS is that if a section does reach this state it should allow sufficient plastic strain before failure for the load to be distributed elsewhere. It is not a requirement of ULS design that all real structural members remain in the elastic range under all actual load conditions.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[IDS]

One of us should delete or modify it.

Good luck with that.

If anyone knows someone with an established academic reputation in structural design it might be worth getting them to have a look at it.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[BridgeSmith]

I disagree, Wikipedia is a valuable technical resource,

Maybe it's ok for materials and product references, but as demonstrated in this case, it's not a reliable resource for design or analysis.

...you should just not rely on anything you read there without confirmation from an independent reliable source,

If you have to take that extra step, why not just refer to a reliable source from the outset?

...but that applies to any technical resource.

I disagree. There are plenty of reliable technical resources that don't require a secondary source.


Rod Smith, P.E., The artist formerly known as HotRod10

 

[IDS]

If you have to take that extra step, why not just refer to a reliable source from the outset?

The same reason you should check calculations, even if they come from a reliable source.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[BridgeSmith]

Reliable sources, e.g. published articles, research studies, design guides, etc. from recognized entities, such as PCI, AISC, TRB, PTI, and ACI, can confidently be assumed to have been checked by at least several experts in the field. I would not, and do not, have any hesitation about using them without independent verification. I don't put wikipedia on anything close to the same level. For anything related to engineering analysis or design, I won't bother to look at wikipedia, since I'd want an authoritative source for my information, anyway, so I would consider it a waste of my time.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Tomfh]

I deleted the offending sentences. The whole section deserves a rewrite.

 

[human909]

I agree. Wikipedia is a fantastic source of information, in fact it is largely clearer and more accurate than the average post from eng-tips.com.

However that doesn't mean that you should you as a professional engineer use it as an authoritative source. Beside EVEN "authoritative" sources have had and STILL have their own issues. Generally these are edge case scenarios but still highlights how even 'authoritative' sources get it wrong or in some circumstances.

Exercising you intelligent judgment is rather than simply following a recipe is what make you an ENGINEER and not a sheep. (The phrasing and the blog is not mine, but another eng-tips contributor)

 

[r13]

at the Ultimate Limit State the concrete will have a maximum strain about 5 times greater than the elastic limit and about 3 times the strain at which it goes plastic.

Very difficult to connect concrete with "plastic". I know the magic number is 0.003 (ultimate strain), after reaching that point, if not crushing outright, the stress will decrease rapidly. So what is the benefit of allowing larger strain rate?

Please do not give up on fundamental to make an argument that will not stand test.

 

[IDS]

So what is the benefit of allowing larger strain rate?

I didn't say there was a benefit in allowing a larger strain.

I said that at the ULS strain the concrete was already well past the elastic strain, as is shown in the graph you posted.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/