Confusions about Structural Engineering 3

[ssj123]

Hello everyone!
I am new member on this forum and this is my first post here.I am very passionate about structural engineering,there is something which has always bothered me. We see engineers normally talk a lot about stresses in materials, from my understanding if we design a simple member in tension and apply tension force at its ends and stretch it, the stresses inside the member will resist and act opposite to the external force in order to keep the member intact .So when we design this member , we make sure that these stresses do not exceed the strength of the material of the member like steel for example, strength is the maximum stress in the member after which it fails , this is the principle for design right?
Please clarify my doubt.

 

[SWComposites]

In a very simplistic sense, yes.

 

[GregLocock]

"Structural Engineering is the Art of molding materials we do not wholly understand into shapes we cannot precisely analyze, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance."

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[rb1957]

I'm confused about what the confusion is ?

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

 

[ssj123]

Actually i presented how i think structural design is like, i just want you guys to tell me is what i wrote correct?

 

[Eng16080]

That's a nice quote above.

Your understanding is roughly correct, except that for most structures (buildings/bridges), you wouldn't be designing to the stress at which the material fails. You would be designing to a stress which is much lower, accounting for a factor of safety.

Things can start to get a bit convoluted (like with LRFD design) where you're using load combinations which have the effect of amplifying the applied loads, then comparing those amplified loads to material strengths which are reduced. Overall, I think this roughly boils down to a factor of safety somewhere around 2 or 3 for most structural materials.

 

[rb1957]

so rephrase "we make sure that these stresses do not exceed the strength of the material" to "we make sure that these stresses do not exceed the allowable for the material/structural_element"

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

 

[milkshakelake]

Yes, you're correct. But...

I don't think it's really the strength of the material that we're most concerned about. The material is generally the same for different members, but different members do different things. Therefore, they have different safety factors. For example, a column will have a different safety factor than a beam. You might even have some members where ultimate stress is used instead of yield stress. Also, some member design will count for combined stresses, and some won't. Also, Euler buckling uses "stress" in a very different way than tension, to the point where it's totally different.

But it is true that all member checks are based somehow on the material strength. It's just to different degrees. If we're only talking about non-slender tension, then yes, it's directly making sure that the stresses in the member don't exceed what the material is capable of. Axial tension is the most pure, simple form of a stress check.

 

[lexpatrie]

I'm intrigued, non-slender tension? Is that some allusion to like the weight of the element creating bending or something?

 

[human909]

Is this ChatGPT new approach? Ask questions on forums to build up its reference database.

 

[rb1957]

then we're ok ... GIGO.

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

 

[civeng80]

GregLocock

Excellent quote.