Why Pre-Stress Concrete?
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- Опубліковано 13 тра 2024
- Pre-stressed concrete technology has come a long way since some of the first patents only about 100 years ago. In this video we cover the basics of pre-stressing in contrast to traditionally reinforced concrete.
00:05 - plain concrete
00:30 - traditionally reinforced concrete
00:50 - tension zones
01:10 - pre-tensioned concrete
02:00 - pre-stress calibration
02:15 - shrinkage
02:50 - high strength materials
03:15 - post-tensioned concrete
04:00 - benefits and costs
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Content provided for entertainment purposes only. - Наука та технологія
Post tensioned concrete is why you never cut into a concrete slab without first KNOWING that it is not post-tensioned, or without having it scanned so you know where the rebar or post tension rods are. If you cut into one of the rods it can explode out of the slab and cut through anything in its path, finishes, furniture, or you.
Thanks, that is extremely valuable advice!
It's true, I retired from bridge maintenance and this statement holds a lot of truth. Bridges that have been damaged by high loads or earthquakes were repaired by my team. However, I always felt uneasy about pre-post tensioned bridges.
don't worry, my magical enchanted coring drill rig always *always* seems to find electrical conduits, rebar, and stress members. as long as I'm not on the job, everything will be fine 😐
They are building many flyovers using post tensioning in the holes of girder slabs. It looks dangerous to me.
Damn, good to know; thanks for sharing.
I’m a mechanical engineer. I graduated with my degree in 1986. I’ve always been amazed at the things that civil engineers design. Bridges and buildings are built to last a very long time under very heavy loads. Thanks for a quick peek behind the scenes.
I'm a civil engineer and am blown away by the megaliths built around the world. The transport, hoisting and precision would push modern methods to their absolute limits.
I worked on a project where we grouted the cables after post tensioning.
The grouting reduced the risk that a cut cable or a cable's locking wedges slip and the cable launching itself horizontally from the end of the slab into the neighbour's property. 😮😊
No one comments on my comments. 😁😃
Your comment is out of context
@indrajeetbanerjee2159
Hahaha Hahaha 😆
Really enjoyed your presentation. As an ironworker I installed a lot of post tension cable on-site. Always thought it was interesting.
Cheers! Thanks for the comment
I've only ever designed rebar reinforced concrete so this video was a great introduction into some pre and post tensioned design considerations.
I had the oppurtunity to build as high as engineering allowed in many post tension elevated concrete decks all along the western seaboard of the U.S.
Great introduction to the complexity of the subject matter.
My dad was a civil engineer for Wells Concrete Products and I spent my college summers working as a laborer there. It always amazed me to see 60 or 70 foot double tees bend when the pre-stressed cables were cut loose from the form. The center would pop up 3 or 4 inches above the form. You look at concrete and think "That can't bend". Oh, yes it can!
No, it can't. Concrete doesn't bend, it develops micro-cracks, not visible to a naked eye which allow for deformation that you've witnessed, so you might think it was bending
@@MrRightNowRebar also don't bend then, bc they also develop micro-cracks.
I was working on a 12 story reinforced concrete building downtown SF when the '89 earthquake hit. I looked up at the slab above and it had waves in it. I looked out at the skyline and saw the Transamerica and other high rises swaying back and forth as designed. Wild stuff.
Could you make a full 101 video on traditionally reinforced concrete?
Really interesting overview of pre-stressing--thanks!
Possibly yes, if many people are interested!
@@TheEngineeringHub yes please make it.
@@TheEngineeringHub I'd be interested!
@@TheEngineeringHub yes please!
@@TheEngineeringHub Yes please. More deep dives please.
Great Video. As requested, future discussion on the cable stretching engineering theory, along with the the equipment and methods for stretching PT cable would be insightful to watch. Also a video about post tension cable repair would be useful to.
Informative, thank you 🤝
Interesting and easy to understand. Cheers.
An educational video. Thanks for your time.
I'm also interested to see a video on floor diaphragms and how they interact with stair cores, shear walls. Anything to do with lateral forces in general 👍
Noted--thank you! We have been contemplating some lateral system topics. Highest on the list was a video about wind forces on structures.
Cheers,
That's a broad topic is there something more specific you want to know? Also, are you talking about concrete floors or framed/plywood? As their behaviors differ quite a bit (rigid vs flexible). Are you familiar with shear walls and collector elements (draglines and chords)? If not you'll want to investigate those for diaphragms to make sense and how they're simplified for application. In a nutshell, diaphragms transfer load horizontally to the collector elements, and the collector elements drag/push that load to a vertical resisting element such as a shear wall or frame. If a stairwell core is being used as shear walls, then the diaphragm will transfer load to the collector (which is in line with the wall), and the collector will be anchored to the shear wall. If the stairwell is not being used for shear walls, then the diaphragm will use the collector elements to transfer shear loads around the opening. In concrete, the collector element is usually just rebar, in wood construction collectors are generally the top plates but can also be rims, beams, joists, trusses, or blocking with metals straps connecting them all to the shear walls in one way or another.
Thank you so much, The Subject was very helpful .
This was interesting. Thankyou!
Great videos as always
Thanks, I used it for my Engineering students!
keep these kind of videos coming thanks
fortunate i found your video. thannks
Thanks for sharing your knowledge and time.
Thanks for the comment!
You learn something everyday. Thanks
railway sleepers are also made either with reinforced concrete (a simple steel bar mixed in concrete) or with prestressing (a steel bar stretched, during the drying time of the concrete.
Top content keep it up! A Video about Tunnels keeping dry and withstand sheering forces in the mountain would be Interesting.
Thanks!
I heard that in some cases prebuilt pretension concrete stuff was actually cheaper since you could control the conditions and just build a bunch of them as needed
Hi Robert,
Yes, you are right, there are cases when precast pretension concrete structures are cheaper, particularly in terms of labour and materials for construction. Fabricating off site can facilitate quick construction and high quality control. In my observation, this is very common with concrete structures in Europe. I've noticed in North America there can sometimes be a larger pool of skilled concrete labourers though, leading to cast-in-place being cheaper. Another consideration would be maintenance over the life of the structure--another topic that should have it's own video!
Another great video..nice work
Thanks!
I love this clarity
Great video folks!
Very interesting information
Thank you Sir
Great Video
Thanks!
Thank you 🎈
I would love a video on foundations in permafrost areas (siberia, alaska, anartica). People have tested non-pier, raft-like foundations in alaska (for houses) as a way to provide better insulation.
Noted! That is an interesting topic we should look into further. In our professional practice, we have seen some very high compressive strength insulation products that are designed for loading on or under slabs.
If anyone wants to see the most incredible, INCREDIBLE example of pre-stressed concrete, look-up how they built the super-skinny CN Tower in Toronto way back in the 70s. The entire tripod bottom of the tower is basically three 1000' skinny pre-stressed beams. The combination of step-forming and running highly stressed cable throughout the construction process is still mind blowing all these years later. Then there's the fact that the entire 1800' tower is built on a foundation that is only 16' deep, sitting on bedrock.
It really is an amazing structure. I've visited that one with my family growing up
The experience used in its construction are still used today in the construction of large concrete slip-formed structures such as oil platforms and nuclear reactor containment buildings.
Well explained as always. I'm curious how much longer as a % would a pre-tensioned beam will span compared to a typical reinforced concrete beam.
Co pay I worked for gave a 100 year warranty for the pre stress beams
Nice
Good video, i would go in detail on graphs and how the sections are optimised for a technical video.
Noted--thank you for the comment!
con que software se realiza esas animaciones? por cierto muy interesante el contenido
I'm not into engineering but thanks to UA-cam algorithm I learned how concrete are reinforced with the help of steel
Thanks for the comment !
Cheers
Very good presentation kid.
I wish I had more in depth info, but, I just recently heard, they found a way to make 'self repairing/healing concrete,' by putting yeast into the mix.
Have you heard of such a thing? If so, does it actually work?
i learnt something today
Would stirrups reduce shrinkage?
How does cured concrete camber without cracking?
Please do videos about retention system
The statement at 2:00 is somewhat incorrect. The ultimate flexural strength of the section is not affected by changing conventional reinforcing steel for high strength prestressed steel.
However many other aspects of concrete behavior are significantly affected. Consider many of the negative behaviors of concrete- beams crack under bending loads and deflect more, beams crack under bending loads and have lower shear capacity, beams and slabs crack under bending loads and allow water infiltration that deteriorates the reinforcing steel. If the concrete is precompressed then the negative aspects of these behaviors is greatly reduced. This is how longer spans/shallower members are allowed. This is how concrete parking garages exposed to the weather can have greater service life.
30M long Span make possible?
how does it affect longevity compared to reinforced or simple concrete, especially in terms of minimally or poorly maintained structures?
no
wait can i change my answer
@@Jack-he8jv no
I feel like you didn't explain well enough how stretching the reinforcement makes the beam stronger. The part was a little short for me. Doesn't stretching the reinforcement make the reinforcement already stretched so that it can handle less load since the added load will stretch it more? Will a pre-stretched reinforced beam not bend until the load is greater than the pre-tensioned bottom so that it goes from compression to tension?
Hello!
Fair enough--it is a complicated mechanism that could have been described further. Here is an elaboration:
The pre tensioning of reinforcing puts the tendons in tension, but once released the tendons contract towards a state of equilibrium. This creates the compression zone in the bottom at 1:32. Now you can imagine the tendons in a sort of neutral state, and then the applied load begins causing tension in the tendons.
So the pretension and tension due to applied load are not additive on the tendons.
Hope that helps!
And thank you for the comment.
Cheers,
The point is to keep the concrete in compression for as long as possible as it is usually the limiting factor of the beam/slab. So when you release the stretched cable, it wants to shrink back to its original length, which puts a constant compression force in the beam. A simple analogy is to think about lifting a stack of books lined up vertical on a shelf. Can you just grab the book on each end and lift up the whole stack? No, you have to first squeeze all the books together so they are compressed and can't slip relative to each other when you lift them all up. The more compression force you apply, the more books you can lift while still maintaining sufficient compression throughout the entire row. On a side note, the cables are never stretched anywhere near their capacity so they would generally never be the limiting factor in design strength, even as more load is added. In fact the cables are often all tensioned with an industry standard force which is well below their ultimate strength, so if more compression force is needed, we don't stretch the cables more we simply add more of them.
The prestress reinforcing is a very high strength steel, almost 4 times the strength of normal reinforcing steel. It gets elongated farther but it stays in its elastic range so added load can stretch it more and it still regains its strength. This is the behavior under normal day to day loading. If the beam has a severe overloading then it will deflect farther and behave more like a conventionally reinforced beam but with much stronger reinforcing. This is the check for maximum bending strength, or ultimate strength.
shrinkage is real! but seriously, great video
Thanks!
Now I have a very clear picture about you
Pre-stressing actually doesn't increase the strength of the beam. Its main benefit is to reduced deflection and allow for shallower sections. You can use pre-stressing steel in a beam without pre-stressing the section and it will have the same bending strength as the pre-stressed section. The higher strength of the steel and concrete increases the strength but the actual pre-stressing does not increase the bending strength. However, the prestress will reduce the deflection at the same load level.
Creep in the concrete along with shrinkage (and to some degree temperature) reduce the pre-stressing. The creep is generally the major factor. Relaxation of the steel also reduces the effective long-term pre-stressing.
@@hafeeznoormohamed1259 The load carrying capacity is proportional to the bending strength (assuming shear does not govern). The stiffness and strength are two separate checks. One is a life safety issues and the other is a serviceability issue. Pre-stressing improves the serviceability but not the life safety issue (with the exception that shear strength is improved but bending usually governs over shear). Pre-stressing decreases the deflection of a slender section but does not increase the load carrying capacity.
There are essentially two reasons to use pre-stressing steel. One is to reduce deflections of a shallower section and the other is if a section is too congested with mild reinforcing. Pre-stressing can also increase the shear capacity of a section but that is not a primary reason to use pre-stressing. There are other means to increase the shear capacity.
The reason pre-stressing needs a high strength is to increase the pre-stressing strain. The more strain you can put on the steel, the fewer losses you will have from shrinkage, creep, cable relaxation, and cooling temperatures.
@@hafeeznoormohamed1259 Bending strength is unchanged. Bending stiffness is affected because precompressed concrete doesn't crack in bending and remains stiffer. So for longer spans that might be controlled by deflections, you can use a shallower beam if it is precompressed. The deeper beam can still have greater strength.
@@billj5645
@Bill J,
Agreed that the bending strength is unchanged, but what about the bending demand?
The assertion in the video is that the 'prestress beam could carry more load, all else equal'. This is based on a bending strength-governed beam where the same load will cause a lower peak flexural demand in a prestress beam vs non-prestressed.
As you allude to, a huge advantage of prestressed beams is higher stiffness and creating larger open spaces which are deflection governed; the scenario here is only a hypothetical scenario for comparison purposes.
Thanks for the discussion!
Maybe not on topic I’ve tried repeatedly to wrap my head around a roll maneuver on a non-equatorial launch and the reasoning for it.
I took structural engineering classes in 1978/79. Unfortunately we did not have computers back then, just books full of data and tables.
Noice Vid
Do you happen to know why industrial concrete doesn't contain recycled glass?
👍
What about fiberglas reinforcing rods and fibers?
This is an interesting topic. Have you worked with these kinds of reinforcing in the field?
@@TheEngineeringHub I have worked with fiberglass rod additive concrete for industrial floor slabs. The concrete is certainly less workable with this additive, and I was not aware of the full mix design of that concrete at that time (20 years ago). But I'm very intrigued by the newer nano-fibers on the marked and if they may be useful for controlling shrinkage cracking in long life structures, like marine exposed bridges. I'd be skeptical of using them to reduce steel reinforcing, but I know that is exactly what is sometimes done for on grade slabs in the building industry.
Each one teach one thanks
Makes it strong, give's it flexibility.
If you didn't get an A for making this, we need to take the professor out back.
why the armature here is curved? i thought it always must be straight
But you fail to mention the problems with pretentioned concrete. It has a limited life span. Many buildings have had to be demolished due to the steel weakening over time as it is under constant stress thus leading to metal fatigue and comprising its strength, resulting in a load down grade over time.
I'm not aware of this ever happening in the US. I've been designing and building reinforced concrete and prestressed concrete buildings for over 47 years.
Yes this happened to a high rise biult in the 1960's in Adelaide Australia. They found that because the structural integrity of the building was compromised as the tensioned rebars had fatigued. This may have been an engineer error as the members loose some load capacity over time, they may have speced it too close tothe safe limit.
Look at all those electrical conduits snaking through the rebar. God that drives me crazy
Hah! Agreed. Thanks for the comment!
To make it feel like how we feel doing a thankless task....
can never have enough topicals on concerets since it is the most used material ever.
All ok but water does not evaporate from the concrete. It becomes the concrete due to a chemical reactions.
Lots of water escapes too, and that is the main reason why concrete shrinks while curing. Much of the water is there for workability of the concrete mixture during construction, and not for hydration of the concrete.
Don't do this, it creates the possibility of sudden failiure, see fiu bridge collapse, to mitigate this the materials have to be so good that you could buy more then twice the amount of cheap steel instead.
If you don't trust prestressing you probably shouldn't drive on any bridge in the US. Anything in the 20-150ish foot range of span is likely to be prestressed concrete in the last 50-60 years. I don't know much about the FIU pedestrian bridge that collapsed, but there were design errors that led to that collapse. As traditionally reinforced concrete gets into longer and longer spans the concrete has to get progressively thicker (and heavier) to hold all that steel, and becomes prohibitively expensive compared to other options. Bridge engineers essentially don't use traditionally (mild steel) reinforced bridge superstructures anymore, except for special case structures like arch bridges.
@@isaacm6312 I mean use steel without concrete like the ones built by eiffel, or the fourth in scotland, you can span a gap for less weight with just steel, use concrete for pillars as it is good under compression, use steel for spans as it is good under tension.
Good video, but the voice of presenter could be mo reengaging and energetic.
Hi Saeed,
Thanks for the feedback! I know I struggle with this.. I'll try to drink more coffee or work on audio recording when I'm most energetic!
Cheers,
And UA-cam is good, ye ye ye . . .
What isn't explained is how it's actually done
Ha! Good observation. Would you like to see some future videos on practical methods from say a contacting perspective?
@@TheEngineeringHub Sure, would be interesting
You get the 2 strongest construction workers you can find on the jobsite and tell them to pull very hard on the wires. Each 1/2" diameter wire gets pulled to 33,000 pounds to start with.
Almost all of the photographs in the video were actually of post tensioned concrete which is done differently. "pre" vs. "post" describes it. In "pre" you typically do this in a factory and you stretch the reinforcing steel within the formwork then pour the concrete in and let it cure. Then you cut the steel loose and it transfers stress to the concrete. In "post" you do this at the jobsite- cast the concrete first then use small hydraulic jacks to stretch the strands. A high strength strand can stretch 10" or more during this process. A video showing how both methods are done would be interesting.
all the guys who put the buildings up will not go back in once built ..because they are not safe
That's a good test. I've been on construction sites where the contractor would not step out on some parts of the structure because they didn't think it would work. But prestressed concrete has been proven in testing and experience to be very strong and resilient. The building code used to have special provisions to insure that a post tensioned building didn't completely collapse if there was a fire or other damage at one end of the building. Through testing and real world experience it was shown that post tensioned buildings did not collapse under extreme conditions even without those special provisions so they were removed from the building code.