How Arches Work! (with Diagrams!): Structures 2-2

Arches are the opposite of cables. Arches work in compression while cables work in tension. Arches are rigid while cables are flexible.
And, for the same loads, the shape of the thrustline within an arch is the same, just inverted, from the shape of a cable.
So we can look at thrustlines within arches. They tell us a lot about the behavior of the compression force within the arch. If the thrustline stays within the arch then there are no concerns (here, we’re assuming the arch is made of rigid blocks that can’t carry tension between them). But, if the load is to the side we see how there can be a problem as the thrustline leaves the arch itself.
We then go through a range of other loads on an arch and how the internal thrustline changes. Each time, we compare the thrustline to the corresponding shape of a cable.
We finish with the concept of arch bridges and that of a tied arch putting tension into the deck so the end supports can be vertical-only.
Don’t forget to like and subscribe!

[@SpannerAT34]

We are seeing the power of someone who can explain a topic so well

[@PaulKassabian]

Thanks!

[@caryschainey]

Beautifully explained and demonstrated - thank you!

[@KeyWestSaltLife]

I absolutely love this video series on structures. I think you do an amazing job explaining the topics and wish all professors had your skill and passion for teaching

[@jitendrameghrajani4934]

The diagrammatic representation work is very good and easy to understand. Thanks!

[@lesterbrooker9421]

This was a wonderful video. I have pondered the 'thrust line' in an arch and have even tried to calculate it''s position from first principles. I soon realised I do not know enough on the subject to be certain of the placement of the thrust line. I was aware of this inverted chain method and its use by Gaudi and also it use in the checking of St Peter Dome but for some reason I never quite put the two together. Thank you for such a wonderful explanation. I hope that you continue your series of videos.

[@jeanettecook1088]

I'm a farmer living in New Hampshire USA. I'm fascinated by arches. I love to design small buildings, like chicken coops. I'm doing one now, with a roof that is a half arch, and having trouble finding a rigid structural material that will give the desired curve and strength, while being lightweight, streamlined and capable of receiving fasteners like nails and screws. I'm covering the roof with corrugated polycarbonate roofing panels... but I'd really like to know if one can get small arches commercially... or do I have to build my own? I'll probably have to make my own. Anyway, this was a great video, and it taught me a lot about designing with arches and partial arches. Thank you! 🎉 Edit: forgot to say, I'm a new subscriber!! 😊

[@freefrominfluence]

Wonderful, and elegant!

[@masteraccident4221]

I’m 12 and I’m here because I was confused how bridges work and it is satisfying to finally get the answer

[@rumplestilskin007]

Love your wonderful and easy to understand video tutorials! Would have loved to be a student under your tutelage

[@jorgeojeda9338]

thanks for this video, i really love the explanation

[@ronchirayil4123]

Thanks so much for your videos Paul! They are so simple and vastly helpful!

[@PaulKassabian]

Glad you like them!

[@fernandomingo3491]

Amazing explanation Paul. This video needs more publicity.

[@PaulKassabian]

Hey thanks! Doing my part to get structural engineering thinking out there...

[@kumudeshhemachandra8408]

Wow! Super nice explanation , this is so helpful to me. Looking forward to more videos like this. Thankyou so much.

[@PaulKassabian]

You're welcome and glad to hear it!

[@arizona111]

Smashing video series! You certainly have a gift of explaining structural engineering to those without training! During the segment about the bridge that you worked on in England, "this is a steel arch so this can carry tension and compression" In Video 2.1, "arch carries compression only." Assume there is something here above my KZbin structural engineering certification. Can you explain "carry tension and compression?" Many thanks!

[@colepopovic4659]

Love this!!!

[@joelhyer1929]

This is interesting! And powerful knowledge! Thank you so much!

[@PaulKassabian]

You're welcome!

[@tamarplotzker5231]

SO SO GOOD! Keep making videos to help us architecture students....

[@PaulKassabian]

That's the aim!

[@mypropertyplan]

Great video, really enjoying the series. Also thank you for the millennium bridge, beautiful bridge. I have an apartment that looks right onto it.

[@PaulKassabian]

You're welcome! Video on Gateshead coming soon!

[@syazwaniismail7171]

thank you for this video! i wish i found this sooner when i was still in engineering school

[@PaulKassabian]

You're very welcome!

[@yaminipurini8651]

Ur explanation is very clear

[@PaulKassabian]

thanks!

[@hgblackshadowmaster]

Is there some sort of program for punching in start points, end points, and loads? That would be great!

[@ahmadsharifshajib116]

Wonderful explanation, really it is very helpful.

[@PaulKassabian]

Glad it was helpful!

[@rolandfisher]

Announcer: "Sorry, folks! You must leave. The thrust line has left the building!"

[@birendrasilwal3075]

I loved the explanation

[@PaulKassabian]

Thanks for saying that - much appreciated!

[@HHHPedigrees]

Such an amazing explanation. I wish they taught students about arches in school.

[@PaulKassabian]

Thanks for saying that!

[@muhammadsalmankhattak2352]

Hi plz make more such videos.these are really helpful for us.thank you very much

[@PaulKassabian]

Just posted the most recent and more on the way!

[@madhurjyakeot7743]

There can be many arches between two points. Am I correct? If yes, how to choose the one for my design?

[@Yogimoon]

Wonderful video

[@PaulKassabian]

Many thanks!

[@FoxRiverBridge]

I know very little so this is a very open question, but in the case of the single load off to the side, would you find the weakest point on the arch by using a differential equation to figure out at what point the area between the thrust line and the upper arch is the greatest?

[@cscsamsung9749]

ขอบคุณ สำหรับการเรียนรู้วิชา เรขาคณิตที่เรียนในระดับประถม สามารถสร้างสิ่งมหัสจรรย์ โดยอาศัยค่าสมมาตรในวิศกรรมศาสตร์.

[@rameshingale9395]

Nice.

[@iam5085]

Interesting, superb videos. Where could I build my own mini bridge, maybe in small scale first. Would I be able to calculate the max weight and size of things? Food for thoughts, if I had the time...but I find this subject refreshing among all these Ukraine this and that news (I wish Putin would give up).

[@av_naturasmr]

If arch(catenary) bridges requires forces acting on them to be balanced in order not to fall down, how does it deal with varying loads like moving vehicles on the deck of the bridge.

[@lannycrockett6571]

can you design a 3 centered arch drawn with a square, that can use a built up wooden arch truss? span say 60 feet? maybe out of 2x6's ?

[@MxdicG]

I have a question. At 4:31 say you wanted to improve the strength of the arch in the scenario, what would you have to do to prevent it from failing?

[@PaulKassabian]

Good question! Remember this plot doesn't have the self-weight of the masonry itself, so that helps. Other options for masonry arches is to add more distributed weight above so the effect of the point load is reduced, or reduce the point load itself somehow. Lastly, make the arch in a material that can take tension.

[@pawanacharya2915]

great videos!!! can you please explain mathematically, why the parabolic shape are better than semi circular arches ?

[@nicolasramirez3944]

Look up a derivation for the shape of a cable under uniform loading. The curve turns out to be parabolic. In an arch under uniform loading, the thrust line will be parabolic (anti-funicular form based on the cable). A semi circular arch is different than a parabolic curve, and that deviation will introduce bending moments!

[@PaulKassabian]

Great response...

[@thoughts7085]

Teacher you said that the arc gets destroyed if the weight gets changed. Now imagine a live load in the form a truck moving on that compression arch. How does it deal with that continuously moving load?

[@mastersamurai7683]

I'm a bricklayer and I'm wondering what the strongest arch is. I've heard segmental, catenary or triangular? What is the answer? And does a triangular arch still count as a true arch?

[@mastersamurai7683]

I was taught segmental but have also heard triangular yet Google says catenary?

[@mohammedlasmi649]

Thanks too much

[@robertingersoll5289]

I was hoping you would have discussed the buttress and flying buttresses here Maybe another video??

[@USER-jo7yz]

If the arch is an inverted catenary do you get horizontal shear at the supports?

[@greggsenne1268]

Break the force vector at the supports into two vectors, one of which is in the shear direction.

[@GeordieJimny]

Hi Paul, I have the original master betacam footage of the millennium bridge being installed. It’s been sat in my loft for maybe the last 20 years. Perhaps it would be a nice keepsake for you? I would be delighted to send it your way as a gift. Let me know if you would like it, if so you can find my email on my channel ‘about’ page. Thanks for making the series, it’s a great watch 👍

[@PaulKassabian]

Thanks and glad we connected by email!

[@gebremeskelgebrekorkos8642]

many like

[@bouzoukistudent8180]

and that is how we build wooden roofs .

[@saithadutoglu6287]

sorry Mr Paul parabola or hyperbola?