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Old September 30th, 2012, 05:09 PM   #1
metsfan
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Steel frame vs reenforced concrete.

Has anyone else noticed the huge shift from all steel to hybrid concrete/steel and all reenforced concrete construction?

Why is this?

As a resident of the northeast US, i feel 9/11 may have something to do with it. Concrete is not invincible, but is not as heat-weakness prone as pure steel structure. WTC 1 and 2 did not fail from structure being taken away, it was the remaining structure weakened by intense fire on several floors.

Steel is usually simpler to put together, and possibly cheaper, but concrete tends to create seamless "whole" structures.

Anyone else have thoughts on this?
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Old October 1st, 2012, 07:25 AM   #2
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9/11 was definitely a turning point in skyscraper construction. I'm guessing the twin towers had the odds going against them considering the structural support was made up of steel, the same steel that was weakened because of the fire and that support was overwhelmed by the concrete slabs and trusses it had to carry added to the additional damage it received.
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Old October 1st, 2012, 08:19 AM   #3
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Not to mention the Twins only had sheetrock (drywall) protecting the core columns. Good for small fires, but in a fire like the one seen on 9/11, no real protection. You didn't see many buildings with reinforced concrete cores before 9/11, but it seems like everything that's going up these days has a concrete core.
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Old October 1st, 2012, 01:45 PM   #4
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One could say that the Twins were uniquely vulnerable to a plane attack. They were steel only, which is vulnerable to fire. They had fireproofing, however that was blasted away by the explosion of the plane impact.

In steel columns they focused on quantity and not quality, which meant that the planes cut through more of the structure than they would if the Twins had less of stronger columns instead.

Last, but not least, they had rather big distances between the core and the perimeter, which means that the role of the trusses holding the floors was important in keeping the core and perimeter in one piece. The trusses were strong, but they were very vulnerable to fire. The failure of these trusses was what initiated the collapse because two structures are stronger if they are connected than they are without any connections.

I'm no engineer, so others could surely explain these things better, however, I told what I know. I'd like to add that the Twins weren't weak buildings. In fact, they were very strong buildings, able to withstand any hardship the elements would send at them, however, as I said above, they were uniquely vulnerable to a plane attack.
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Old October 14th, 2012, 07:09 AM   #5
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The construction of skyscrapers pay attention to aviation and safety , but also pay attention to the construction of service facilities , such as health care, primary school students to go to school , elderly exercise .
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Old October 14th, 2012, 10:25 AM   #6
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Quote:
Originally Posted by metsfan View Post
Has anyone else noticed the huge shift from all steel to hybrid concrete/steel and all reenforced concrete construction?

Why is this?

As a resident of the northeast US, i feel 9/11 may have something to do with it. Concrete is not invincible, but is not as heat-weakness prone as pure steel structure. WTC 1 and 2 did not fail from structure being taken away, it was the remaining structure weakened by intense fire on several floors.

Steel is usually simpler to put together, and possibly cheaper, but concrete tends to create seamless "whole" structures.

Anyone else have thoughts on this?
There's many things that influence the choice between concrete and steel frames. Investor/engineer may opt for one or the other based on local situation and their expectations. Per example , here's some good sides of steel frames:
  1. Time gain – Perhaps the most important aspect of using steel frame construction as compared to reinforced concrete is the element of time for the on-site erection of the steel frame. Reduced time presence for the Contractor on site translates to a shorter construction time for project delivery and this leads to reduced overheads during project construction. On multi storey buildings the time gain can be substantial.
  2. Quality Control – Off-site prefabrication of steel members in isolated factory conditions means better quality control of manufacturing compared to pouring concrete frames in situ whilst exposed to the weather elements and with reinforcement and formwork works subject to labour intensive construction processes that may affect quality. Furthermore, offsite prefabrication increases the overall speed of construction and facilitates enhanced safety.
  3. Design flexibility – Longer spanning structural metal frames create column free areas that offer greater flexibility and functionality of floor layouts thereby facilitating sales or lettings of the completed buildings. Pipes and ducts can run easily through steel members and can also be inspected in the future whereas no such flexibility exists with concrete members.
  4. Sustainability – Steel is 100% recyclable without any loss of quality, whereas concrete is not recyclable. In the UK, when buildings using metal frames come to an end of their useful lives, approximately 86% of the steel sections are recycled to create more steel products and 13% are reused in their existing form. Recycling rates for reinforcement bars used in reinforced concrete frames are negligible.
  5. Construction Costs – Construction costs for steel frame high rise buildings that are above 10 levels compare favorably with reinforced concrete frame high rise buildings. Although a cost premium in the order of 10-15% may be evident initially for high rise steel frame buildings, this is usually offset by the substantial reduction in the erection time of steel frame high rise buildings on site and the associated substantial overheads charges by main contractors for prolonged stay on site when slower concrete methods are used.
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Old October 14th, 2012, 11:02 PM   #7
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I don't know about the rest of the world, but in Toronto the vast majority is concrete construction. largely because most new construction is condos, and I have a feeling that concrete is used due to its soundproofing qualities. Office towers here seem to go 50/50. some are concrete construction, some aren't.
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Old October 21st, 2012, 03:03 PM   #8
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Quote:
Originally Posted by metsfan View Post
Has anyone else noticed the huge shift from all steel to hybrid concrete/steel and all reenforced concrete construction?

Why is this?

As a resident of the northeast US, i feel 9/11 may have something to do with it. Concrete is not invincible, but is not as heat-weakness prone as pure steel structure. WTC 1 and 2 did not fail from structure being taken away, it was the remaining structure weakened by intense fire on several floors.

Steel is usually simpler to put together, and possibly cheaper, but concrete tends to create seamless "whole" structures.

Anyone else have thoughts on this?
I thought all new WTC towers still have steel structures, combined with a concrete core.
One57 and 432 park are entirely concrete, but these towers are so slender that they probably need extra stiffness provided by concrete.
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Old October 21st, 2012, 06:41 PM   #9
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Yup, the new WTC has both steel columns and concrete, which encases those columns

Btw, I dunno but isn't steel better suited for thin buildings? Iirc it has better flexural strength. Or is the main problem not structural, but the fact that the building would sway so much that it would be a horror ride for its inhabitants?
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Old October 21st, 2012, 09:18 PM   #10
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Quote:
Originally Posted by Kanto View Post
Yup, the new WTC has both steel columns and concrete, which encases those columns

Btw, I dunno but isn't steel better suited for thin buildings? Iirc it has better flexural strength. Or is the main problem not structural, but the fact that the building would sway so much that it would be a horror ride for its inhabitants?
Earlier, the problem was that concrete was not strong enough to support huge loads of a skyscraper and in the same time avoiding large dimensions of carrying elements. Steel behaves the same when exposed to stress or compression and its consequent strength was much higher in both cases compared to concrete as this schematic diagram of a stress-strain curve shows:



So steel is good in carrying both stress and compression but is more expensive and more prone to temperature influences so whenever possible cheaper and safer concrete is being used especially with new types of concretes who can stand bigger loads. For comparison concrete can have compression strength up to 138 MPa (although the most commonly used concrete is the one with compression strength of around 28MPa) ,while the type of steel that is usually used in construction ,S235 has a yield strength of 235MPa. Note that yield strength of steel is much lower that its ultimate compression/stress strength which can go up to 510MPa in the case of S235.



Here's the same schematic curve depending on temperature influence:

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Old October 22nd, 2012, 01:34 PM   #11
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Thanks for the info. I plan on studying engineering so such info is very helpful to me
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Old October 23rd, 2012, 02:58 AM   #12
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Is it true that it is more difficult to add reinforced concrete walls to steel frame house or building? In the RC beam and column, there are protruding rebars where one could add the hollow blocks. But in steel frame beam and column, what takes the place of those protruding rebars? Just want to have an idea.
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Old October 23rd, 2012, 04:11 PM   #13
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nice job, beograd
are you civil engineer?
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Old October 24th, 2012, 07:21 PM   #14
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Quote:
Originally Posted by fatym View Post
Is it true that it is more difficult to add reinforced concrete walls to steel frame house or building? In the RC beam and column, there are protruding rebars where one could add the hollow blocks. But in steel frame beam and column, what takes the place of those protruding rebars? Just want to have an idea.
What you describe (hollow blocks) has nothing to do with RC. Reinforced concrete is made by arranging rebars in adequate way and then pouring fresh concrete in that structure having in mind minimal spacing and measures against segregation of aggregate. After 28 days and under usual conditions, it is considered that cement has reached sufficient level of hydration and that concrete is rigid enough.

In simple terms, reinforced concrete element "works" so that the concrete part carries the compression and that the steel part (rebar) carries the tension forces, but the most important thing that makes a RC is the friction between rebar and concrete that makes them "work together". Without that friction, all we have is just a useless pile of rebar and concrete. In other words:
  • rebar + concrete IS NOT reinforced concrete
  • rebar + concrete + friction IS reinforced concrete
Here's a picture of rebar structure before the pouring of concrete. In the angle of the photo you can see one part where concrete is already in place.



So I hope it is now more clear why your description of "rebar in hollow blocks" is wrong.

Quote:
Originally Posted by Highcliff View Post
nice job, beograd
are you civil engineer?
Not yet, but hopefully in couple of weeks I will be.

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Old November 5th, 2012, 09:12 AM   #15
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This skyscraper construction is very good, if the services do well , the better !
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Old November 5th, 2012, 07:43 PM   #16
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Just reinforced concrete is cheaper. Don't mind those analyses.
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Old November 6th, 2012, 03:29 PM   #17
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Quote:
Originally Posted by metsfan View Post
Has anyone else noticed the huge shift from all steel to hybrid concrete/steel and all reenforced concrete construction?

Why is this?

As a resident of the northeast US, i feel 9/11 may have something to do with it. Concrete is not invincible, but is not as heat-weakness prone as pure steel structure. WTC 1 and 2 did not fail from structure being taken away, it was the remaining structure weakened by intense fire on several floors.

Steel is usually simpler to put together, and possibly cheaper, but concrete tends to create seamless "whole" structures.

Anyone else have thoughts on this?
Concrete as a material in general has improvemed a lot lately. There are several modern types of concrete that allow for being pumped higher and allow for better and faster hardening etc. That and improved construction technologies and machinery allow for cheaper and faster and higher and thus more competitive constructions.

The material steal has of course also seen modern improvements, but hardly at all in the field of those rather cheap steal types that are used for construction of buildings.

Besides design aspects basically very much a matter of cost effectiveness. and the prices for steel have gone up a lot in the past years.
On the other side concrete has seen various technical improvements and still does see various modernisations.
Fiber reinforced concrete (especially glass fiber enforced) instead of the traditional steel reinforced concrete is for example beginning to become more and more popular for example.

So naturally concrete as a material in general is becoming more and more competitive relative to construction steel or other materials.

Another aspect, very generally speaking not only in the field of civil engineering or architecture, it is becoming more and more popular and necessary to use optimized material mixtures in all kinds of constructions, be it automobiles, lightweight aircrafts or also buildings, skyscrapers.
The use of the reasonably best (or cheapest) available material in each part of a modern construction is becoming more and more a requirement to allow for improved performance, for higher - bigger - better - faster-whatever... or for lower cost and more competitiveness on the market.

For very long times actually is has allready been standard to use different materials in different places of a building. For example concrete foundations, steel frames, concrete floors, aluminum /glass faccades etc... This very classic principle which basically even dates back into ancient times, is naturally being more and more perfectionized everywhere and also in modern skyscrapers and the higher they get the faster they have to be constructed to be profitable and the more modern or improved materials are available, the more the focus will naturally be placed on chosing and using materials smartly.
And the more worldwide competition arises from countries like China and India and others, the more designers and engineers in western countries are forced to be open for using modern high tech materials instead of hanging on to long known traditional ways of construction in order to gain or maintain an advantage over worldwide competitors in performance or quality or lowering construction costs.

This increasing worldwide competition is a very big motor in terms of motivation or pressure to research and experiment for and with modern hightech materials, modern types of concretes. And that has certainly also lead to concrete being still or becoming more and more popular.
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Old November 9th, 2012, 01:15 PM   #18
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The resistance of concrete is from 30 to 100 MPa.
The resistance of steel sections is from 235 to 485 MPa.
It means that steel is about 5 to 10 times stronger than concrete.
Let's take an example.
For a 40 floors concrete building, the size of the column at the bottom is about 1.35 x 1.35 meters = 1.82 square meter
For the same building designed in steel, the size of steel column is about 0.42 x 0.42 = 0.18 square meter.
1.6 square meter can be saved per column, but steel typically costs more.
When you buy an apartment or an office, you always have to ask the gross and the net area. Real estate companies are displaying only the gross area which is not correct. But the today’s trend is changing. The have to display both values, the gross and the net area.
In the example, the 1.6 square meter can be rented for the next 100 years. When you consider the high rental fee per square meter of an office or an appartment in a big city, steel becomes cheaper than concrete after a few years.

For super tall buildings, in most cases, the design is not governed by the strength of the steel and the concrete, but more by the stiffness of the material which is : E (Young Modulus) multiplied by I (moment of inertia)
The E of steel is fixed and cannot be changed: 210000 MPa
The E of concrete is changing with its strength: from 30000 MPa to 40000 MPa.
The moment of Inertia is just linked to the area. Two identical area of steel or concrete have the same moment of inertia.
So steel is 5.25 times to 7 times stiffer than concrete.
It means that for the super mega columns of super tall buildings, 1 square cm of steel can replace 5.25 to 7 square cm of concrete. So huge increased of net area can be achieved by using steel instead of concrete.
Same story. If the investor is selling/renting gross area, he prefers to use concrete. If the investor is selling/renting net area , he will be more in favor of steel.
The optimal combination is of course composite design to optimize the size of the columns and to fire proof the steel. Concrete also needs to be fire proof but most people are ignoring that. Achieving 2 to 3 hours fire rating imposed to put the rebars more inside the concrete slabs. Slabs are getting heavy. So once again, composite design of floors is the optimized solution.

Most people don't know that the main structure of a super tall building costs only 3 to max. 5% of the total cost of the structure.
If steel is 10% more expensive than concrete, it is 10% of 3% = 0.3% cost difference which is peanuts.
On top, the marble you put on the columns costs more than the steel. So smaller columns means less marble.

Steel is fully recyclable and for ever.
Concrete is not fully recyclable (we can make roads, reclamation area,…) and it is, after water, the most used commodity on earth. One day, the earth will be cover by roads.
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Old November 10th, 2012, 04:17 PM   #19
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So many things I didn't know, thanks for your post

Btw I have a small question, you said the structure of a skyscraper costs 3-5% of the total cost, what elements of a skyscraper cost the most?
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Old November 10th, 2012, 07:46 PM   #20
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Quote:
Originally Posted by Jerry666 View Post
The resistance of concrete is from 30 to 100 MPa.
The resistance of steel sections is from 235 to 485 MPa.
It means that steel is about 5 to 10 times stronger than concrete.
Let's take an example.
For a 40 floors concrete building, the size of the column at the bottom is about 1.35 x 1.35 meters = 1.82 square meter
For the same building designed in steel, the size of steel column is about 0.42 x 0.42 = 0.18 square meter.
1.6 square meter can be saved per column, but steel typically costs more.
When you buy an apartment or an office, you always have to ask the gross and the net area. Real estate companies are displaying only the gross area which is not correct. But the today’s trend is changing. The have to display both values, the gross and the net area.
In the example, the 1.6 square meter can be rented for the next 100 years. When you consider the high rental fee per square meter of an office or an appartment in a big city, steel becomes cheaper than concrete after a few years.

For super tall buildings, in most cases, the design is not governed by the strength of the steel and the concrete, but more by the stiffness of the material which is : E (Young Modulus) multiplied by I (moment of inertia)
The E of steel is fixed and cannot be changed: 210000 MPa
The E of concrete is changing with its strength: from 30000 MPa to 40000 MPa.
The moment of Inertia is just linked to the area. Two identical area of steel or concrete have the same moment of inertia.
So steel is 5.25 times to 7 times stiffer than concrete.
It means that for the super mega columns of super tall buildings, 1 square cm of steel can replace 5.25 to 7 square cm of concrete. So huge increased of net area can be achieved by using steel instead of concrete.
Same story. If the investor is selling/renting gross area, he prefers to use concrete. If the investor is selling/renting net area , he will be more in favor of steel.
The optimal combination is of course composite design to optimize the size of the columns and to fire proof the steel. Concrete also needs to be fire proof but most people are ignoring that. Achieving 2 to 3 hours fire rating imposed to put the rebars more inside the concrete slabs. Slabs are getting heavy. So once again, composite design of floors is the optimized solution.

Most people don't know that the main structure of a super tall building costs only 3 to max. 5% of the total cost of the structure.
If steel is 10% more expensive than concrete, it is 10% of 3% = 0.3% cost difference which is peanuts.
On top, the marble you put on the columns costs more than the steel. So smaller columns means less marble.

Steel is fully recyclable and for ever.
Concrete is not fully recyclable (we can make roads, reclamation area,…) and it is, after water, the most used commodity on earth. One day, the earth will be cover by roads.
Does this analysis take into account the shape of steel and concrete columns?

The reason I ask is that steel columns are not solid cross sections. This means that while steel is stronger, there is a smaller difference in tems of area required for a column. A steel column with a hollow square or H shaped cross section will occupy more space than indicated in a direct strength comparison.

Either way, thanks for the write-up! It is a fascinating read.
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