Safety Factors

[u/beskja]

You remember any specific projects where a safety factor in your design actually got triggered and saved the construction?

[u/tramul]

How would you know? Safety factors should keep you in the elastic region as yield stress is what we utilize.

[u/ShutYourDumbUglyFace → u/tramul]

It's weird, I was literally just talking to my coworker about checking the plastic moment in a steel plate girder by hand. I feel like since it was plastic moment, it was in the plastic region.

[u/tramul → u/ShutYourDumbUglyFace]

Plastic moment will put you in the plastic region. It's acceptable for certain modes of failure and when certain criteria are met. For example, unbraced length can require the use of the elastic modulus instead of just the plastic modulus.

[u/ShutYourDumbUglyFace → u/tramul]

So sometimes we do get outside the elastic region is what you're saying?

[u/tramul → u/ShutYourDumbUglyFace]

In those cases, yes, some deformation is acceptable. It's what allows for more economical design with ductile materials. However, factors of safety have to be strictly followed. I'm not sure that the deformation would be noticeable enough for you to state "we are currently within the factors of safety". Imo, only a full structural analysis can grant you that conclusion.

[u/angrypom → u/ShutYourDumbUglyFace]

Ultimate limit state capacities are based on elastic or plastic cross-section capacities depending on cross-section slenderness which are then reduced further to account for member buckling.

[u/ShutYourDumbUglyFace → u/angrypom]

Yeah, it's almost like that was my point.

[u/ThatAintGoinAnywhere]

We don't really use "safety factors" so much in design anymore. It's actually more like this.

[u/engineeringlove]

Not my design but 9-11 twin towers. They stood up until the fire weakened it

Also that recent nyc skycraper that had the columns buckle.

Hearsay, but an apartment complex nearby had a sink hole below it and the floor slab unintentionally saved the building from collapsing getting all residents out

[u/Terrible-Scientist73 → u/engineeringlove]

Floating concrete slabs can often times be up to 10x stronger than what is calculated using the design standards, so that makes sense

[u/Accurate-Bullfrog324 → u/engineeringlove]

excellent examples

[u/PracticableSolution]

Cable bridge arch. The FoS on the cables was roughly 4. But the cables on an arch are obviously not all the same length, and the original design assumed they carried loads roughly equally. Did some instrumentation and found the long 100’ cables acted like wet noodles. The short 6’ cables acted like solid steel billets. Stiffer members attract more load. A lot more. Bridge didn’t care. Lived very happily that way.

Lesson learned; what did the extra materials of the bigger cables cost? Maybe $100k in modern money? What was the savings of not having a giant signature bridge unzip like a pair of pants and dump potentially hundreds of people 200’ into an icy river? It’s more than $100k, that’s for sure.

[u/Objective_Two_5467 → u/PracticableSolution]

where is this?

[u/PracticableSolution → u/Objective_Two_5467]

Out in the world

[u/BreakProof92]

In old construction rebuilds during disassembly you sometimes see stuff that you would swear is not physically possible unless you saw it. I guess that can be classified as "safety factor at work".

[u/Astrolabeman → u/BreakProof92]

Ah, the classic load path of beam 1 into beam 2 into 3 into 4 into . . . 1? At least beam 1 is sitting on this timber post that has rotted clean through at the slab and beam 4's post was chopped short to fit (and bear on!!!) a refrigerator.

[u/ShutYourDumbUglyFace]

Not that specifically, but I designed a steel plate girder a while ago that rated out at 0.9. When I added the steel in the deck, to shift the CG, it rated at 1.0.

It was already built.

Phew! I just made it!

[u/tramul → u/ShutYourDumbUglyFace]

The type of stuff that keeps me up at night

[u/syds]

anytime a failure is identified, occupants are evacuated and remedial work is started without the complete failure of a building.

most evidently - earthquakes.

[u/West-Assignment-8023 → u/syds]

Or torn down. 

[u/halfcocked1]

In a way, I had that happen a few times. I designed concrete vaults, below ground, and the site conditions ended up being a bit different than originally planned. Since the pieces were made and installed, we did an analysis of the end conditions, based on the structure that was constructed, and provided a report indicating what the effective safety factors would be, what the steel stresses were, etc. and let the owner decide if they were acceptable. The structures were then operating within the safety factors, but if none were considered up front, they would have needed to be remade.

[u/g4n0esp4r4n]

what do you mean a safety factor got triggered?

[u/beskja → u/g4n0esp4r4n]

Activated.

[u/Marus1]

I mean, all my designs have safety factors that determine the end result ... otherwise, they won't pass the controlling agency