How Prestressing Works! (Structures 6-4)

What if we could plan ahead for expected loads on a structure? Well we can with prestressing! Using tension to “precompress” a beam or a slab allows us to span further or have shallower structures…we just have to understand how it works and how to shape the prestressing forces to our advantage. Enjoy!
I'm Paul Kassabian. I'm a structural engineer and a Principal at SGH in Boston, MA. I taught graduate students at MIT for nine years and currently teach on/off at Harvard's Graduate School of Design (GSD). These are videos based on my years of teaching structures to students.

[@amirjabbarzadehkhoei7116]

One of the best vids ive seen as a structural engineer.

[@gnidnoeled786]

This video demonstration of prestressed beam is BRILLIANT. I mean, way way way BRILLIANT.

[@mikeschneider901]

In a guitar neck we call that a "truss rod" and it counters the effect of the string tension. Great vids BTW.

[@PaulKassabian]

Thanks for the info!

[@geckauss]

Thank you Paul, this Structures series was space bending and mind blowing! We can only be greatful for your clearness knowledge and passion. Cheers!

[@PaulKassabian]

Great to hear it - Thanks again!

[@getnettamrat9537]

i've came across different videos explaining Prestressing concrete and this is remarkable way of explaining things. that's an art and wisdom many don't have.

[@joaquinfabrega]

Complex principles explained so easy and clear.

[@FoxRiverBridge]

I've watched this video multiple times to get a better understanding of it. Thank you for what you do!

[@PaulKassabian]

Glad it was helpful!

[@VetorDigital]

I've just watched all your videos, they are excellent, you know how to teach and this is very rare actually. Thanks for all the knowledge, please do more. 😀

[@PaulKassabian]

Thanks and will do!

[@welham539]

Feedback: props are great. A pack of card and them tied or pretensioned is great. For a beam try using a foam and for tendons use an elastic band. Pass a tendon through a foam beam.

[@taliksiram5522]

Thanks a lots Paul, after 20 years I am learning again with new zeal and enthusiasm as if doing my engineering again. Kudos to your absorbing vedios.

[@PaulKassabian]

Great to hear!

[@SoloRenegade]

the deck of cards was an excellent example. I'm going to have to remember that one.

[@onlyeyeno]

In my youth I had a "Temp job" where I worked with a company that was (one of many) contracted to work on the "annual refurbishment" of a "car factory". [All refurbishment had to be done during the 4 (summer) weeks when the factory was "standing still"] . And the factory buildings were "enormous", with 150" long "Ceiling girders" of pre-stressed concrete". One day I and one of regular employees of the "refurbishing company" were tasked to relocate the placement of rails and power outlets for welding robots, and when doing so we ran into problems with getting the repositioned power cables to reach the "assigned power ladders". A problem which my ""colleague"" chose to ""solve"" by drilling into one of the ceiling girders to attach an "extension"... When this was discovered (naturally) all hell broke loose, and my "colleague" was more or less interrogated about "what had happened" when he drilled.... But they didn't do anything "practical" about it for as long as I was there. And I don't think the ceiling caved in afterwards either.... And my "colleague" wasn't even reprimanded, so he went on to (unwittingly) destroy a vital part of the "100-ton scrap metal compactor" that was used to compress ALL "off cuts and stamp outs" that were produced in the whole factory... That man(child) was a "certified tool"... Oh happy days ;/

[@PaulKassabian]

wow...

[@klondike9716]

Just one word for you Sir, excellent!

[@PaulKassabian]

Thank you kindly!

[@Dagonius.]

Hi Paul! I was looking for a video on bridges and you popped up. I watched a video, then another, then all of them and just had to subscribe. Brilliant work, love your explanations and can´t wait for more! Thanks and take care!

[@PaulKassabian]

Great to hear and thanks for letting me know!

[@dilpreetsc]

Wonderful informational coming from a very smart man 👏👏👏... I feel I should take a responsibility to make your channel super successful. It's going to be a norm from today in my organization that everyone has to watch and react to your videos... perhaps next I can get them to ask sharing with everyone they know

[@PaulKassabian]

Go for it!

[@sterlingbachan4760]

i love this guy .

[@kath2297]

wow, i'm amazed! you're a brilliant tutor, hoping a lot others know about this channel too

[@PaulKassabian]

Thanks - well do send it round to people you know that would be interested!

[@gachokijackson4979]

Well explained and understandable

[@karunanayakem.p.k.w8085]

That's a great explanation. Thank you

[@humbertonicolasdimitrius1935]

excellent video! i'm very much enjoying all of them! this one reminded me of the shape of a truck's set of leaf spring, specifically the way its curved.

[@sanjaygowda7765]

thank you for making these videos! your videos helped me.

[@olson.pamela]

Love these, thank you!

[@RaselMiah-po8mu]

It was a very nice lecture, and helpful to have a clear concept.

[@PaulKassabian]

Glad you liked it

[@muhammadasyraf3638]

its very good explanation. thank you very much

[@PaulKassabian]

You are welcome

[@jameskennedy7093]

A thing that doesn’t quite make sense to me yet is why when we have a bending moment in the opposite direction (up) we don’t cause the concrete the fail in that direction while we’re waiting for the load to be added. Is there a danger of a structure like this failing if you remove load? Or if you are building it and haven’t added a load yet?

[@debbysandoval7196]

Thank you!

[@PaulKassabian]

You're welcome!

[@WilliamDengLongar]

Structural engineering is very nice.

[@williamcheong2725]

I'm new to this channel, and I'm really enjoying it so far. Thank you for providing such great content. I have a small request, though. Would it be possible for you to create a video discussing poststressing reinforcement? pleaseeeee It would be greatly appreciated. thank you

[@vasiliospapastamoulis1086]

Excellent video! 👏👌

[@brianmugabi7516]

Hullo Paul. I like your videos. You're articulate and explain complex principles with illustrations easily. I would like you to be my teacher in structures. Plus I request to make a video on the design considerations in depth. Thanks

[@ArthurSabbatiniBuoro]

Obrigado pela aula, Professor!

[@hudi42]

these videos are excellent. Thank you!

[@PaulKassabian]

Glad you like them!

[@stephaniewomacks8827]

Watching the video got me thinking! I am considering how to build my shop, a 30 x 30 post and beam. I intend to make it two story for living space upstairs. In order to make the most of shop space I don't want any interior posts in the middle. I watch your videos to learn. Could I fabricate a beam and insert rebar so I can span 30' and carry appropriate load requirements?

[@bouzoukistudent8180]

i remember one time when they were unloading those prestressing beams for a bridge , and they manage to place it upside down .only for them it shatered to pieces .thet foreman fired up right away . (not his fault but for the media someone had to take the blame )

[@babueratte5962]

Very informative

[@PaulKassabian]

Glad you think so!

[@philipripper1522]

How much more pressure is the tendon under in your no bending moment example? Is it all tensile for the tendon? Wouldn't it be magnified or focused on that tendon? So you'd need a material that's quite strong. Is this correct?

[@HeJurm]

Also, is the compression in the beam increasing simultaneously as the tension in the tendon would increase in the no-bending-moment example? So you'd need a material really strong in tension for the tendon and a material really strong in compression for the beam?

[@isaacm6312]

Prestressing strands are made of very strong steel. Much stronger than conventional reinforcing steel. And how much pressure any part of the girder is under is part of the design of the girder.

[@SoloRenegade]

Florida bridge failure is a perfect example of "cutting" a tension member in a prestressed beam.

[@neilgarrad4931]

Thanks

[@CorporateZombi]

9:22 Do not install these upside-down! Lol.

[@HeJurm]

Hi Paul. Thanks for the great videos! I build wooden arch bridges (mainly for hiking trails), and have been thinking if it would be advantageous to use steel cables, that would be following the arch shape, inside the arch to prestress them. The ends of the arch, against which the cable would be tensioned, would be either cut vertically or radially, if it makes a difference. My intuition is wavering: tensioning the cable could force it to straighten, which would be counterproductive for my use, or tensioning the cable could force the cable to a tigther/smaller radius because the ends of the cable are below the centroid of the arch, like in 4:07, which would be useful for prestressing my bridges. Am I on the right track at all? :D The goal is to make sturdier and thinner long single-span bridges. A tied arch does help, and I think could be pre-tensioned as well so that the arch would carry more load. But many of my bridge sites do not allow this for aesthetical reasons or because thick ice sheets forming on the rivers that the bridges span across.

[@dymitrsmith485]

I was wondering if prestressed technique increase bearing capacity of beams or it is only about elimination of cracks?

[@isaacm6312]

Pre-stressing greatly increases the capacity to carry bridge loads for a certain girder cross-section. Concrete bridges that pre-date prestressing tend to get very massive as the bridge gets longer (so you can add a lot of internal reinforcing steel). BTW, this type of bridge girder goes back to the 1960s in the US (to my knowledge). The girder shown in the video is from Washington state, which is a premium precast girder area for the whole world.

[@luftstolle]

So how much concrete and/or steel can you save by this technique? My gut feeling is that you are just replacing concrete with steel, but I guess I'm wrong since it doesn't make sense economically.

[@isaacm6312]

I think of it more like a marriage between steel and concrete that takes advantage of the strengths of both and minimizes the weaknesses.

[@undefined.infinity3106]

which software did you use

[@srdjansimanic2404]

comment for the algo), great stuff!

[@ptinnovativedesignxdesignc1330]

❤

[@jm5143]

Hello, I sent an e-mail with a question but I didn’t receive any answer : please, did you receive it ?

[@user-ed2nd2fs3u]

How do you put a tension in a wire that is sloping in a curved position, in the beam, can anybody explain that to me please thanks

[@PaulKassabian]

There is a curved "conduit" in the beam so, when the cable is tensioned it stays in that curved form and applies equal/opposite reactions to teh restraining beam material.

[@user-ed2nd2fs3u]

Thank you Paul for your answer

[@afzalali-tn2wv]

How to contact you sir

[@gopalkrishnakhairawadagi9668]

How cantilevered prestress beam works

[@milfordjohnson2289]

haha... my old industry definetly needs some regulation then 🤣 i mean.. personally i knew, but i didnt have to sit a test 🤣

[@STOP_RIGHT_THERE_CRIMINAL_SCUM]

hey pointdexter