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Why don't we use fillets when making buildings and houses (i.e why do we leave the edges sharp?)? Does stress concentration factor not matter in making structures?

I am talking about the building like this-->

Building with sharp edges

I know this edge is not highly stress concentrated(only the atmospheric pressure is acted on these edges, which is easily bearable by this building.) but assume the time of any natural disaster, at that time these type of edges would be more likely to fail.And making the edges round doesn't affect the cost much.I think.

NOW ANOTHER EXAMPLE.

Stiars

This is also stress concentrated.As we climb the stairs there will definitely some forces will act.

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    $\begingroup$ Can you provide some specific examples of where you think fillets are routinely omitted where they would be helpful? Think about what direction the forces are acting, and what the material is. $\endgroup$ – Jonathan R Swift Dec 5 '17 at 17:34
  • $\begingroup$ I am not a civil engineer so I don't know the technical terms for this. consider a bench which is made of concrete, as the people will sit on the bench, it may increase the stress concentration at the edge which may cause the failure of the structure. $\endgroup$ – meditat Dec 5 '17 at 17:42
  • $\begingroup$ While using the fillets there may decrease the stress concentration as well give comfort to the people.is'nt it . $\endgroup$ – meditat Dec 5 '17 at 17:44
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    $\begingroup$ Says who? What materials? What country? I don't see any supersharp-edged beams around here. $\endgroup$ – Carl Witthoft Dec 5 '17 at 18:32
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    $\begingroup$ A sharp free edge isn't associated with any stress concentration. Do you see any (1) sharp cracks (2) above you in a (3) load-bearing material not otherwise reinforced that (4) can't be blunted by material deformation and (5) are propagating? If not, then you've answered your own question. If so, you found a case where the designer should have added a fillet, and you should get to safety right away $\endgroup$ – Chemomechanics Dec 7 '17 at 4:37
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The issue is simply cost. The loads in structures such as benches (which you describe in comments under the question) are very low and such structures often have huge safety margins. These margins are not intentional for such trivial structures, but are a consequence of working with reinforced concrete.

Reinforced concrete requires some concrete cover to protect the steel reinforcement from oxidation. For structures under small loads such as benches, just this concrete cover around very light steel reinforcement is already far more than sufficient to withstand the loads.

Given this, stress concentrations are no issue: worst case, a bit of concrete around the edges will be chipped off, but the structure will still be more than capable of withstanding the load.

So fillets become an added construction cost: it's cheaper to build a simple rectangular cross-section than a filleted one. Comfort is not a primary concern.

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  • $\begingroup$ Machine elements are also designed with fos(Safety margins as you say), then why we use fillets there. $\endgroup$ – meditat Dec 5 '17 at 20:36
  • $\begingroup$ Cost. Agree. @ShivamSingh Machine elements are not building elements. There's also a scale difference typically in machine elements versus construction elements. Machine elements are manufactured in a more controlled(read as: cost effective) environment and may be more optimized structurally therefore including fillets where necessary. $\endgroup$ – GisMofx Dec 6 '17 at 1:23
  • $\begingroup$ I somewhat agree with this, but any mathematical proof. $\endgroup$ – meditat Dec 6 '17 at 7:30
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    $\begingroup$ @ShivamSingh, proof of what? The reason isn't mathematical, but financial and (in the specific case of RC elements) due to material requirements. You are assuming the reason is more "formal" and "scientific" than it actually is. $\endgroup$ – Wasabi Dec 6 '17 at 13:16
  • $\begingroup$ Proof of the things that the forces on these sharp edges are less that is failure stress.And I'm sorry if it is being the unnecessary question for you. but I want to discuss it with some experts like you and other on this website. $\endgroup$ – meditat Dec 7 '17 at 8:01
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In general architecture there is a cost pressure towards standardisation and it is often cheaper to use standard parts which are easy to assemble, specify and transport.

There is also the fact that in architecture the crucial loads are often supporting the span of the roof so the big loads tend to be compression and thrust on the walls with pinned joints where fillets don't really add much.

Equally in steel framed buildings it is often cheaper to over-specify the sections used rather than optimise them.

Having said that there are certainly plenty of example of architecture where material limited (eg stone) have produced extrmely elegant structurally optimised designs :

Vaulted Ceiling

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