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Old 09-15-2011, 08:47 PM   #16
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High Rise Building methods - US versus Oz - Steel versus Concrete


Choice of concrete over steel or vice versa depends more on the properties of the material in relation to construction techniques, and the nature of the design. If the building is to have say a maximum of 30-40 floors, concrete is the way to go because the building technique lends itself to concrete. Concrete is strong in compression (almost as much as steel) but weak in tension, so concrete columns, slabs and beams are reinforced. Concrete would theoretically be cheaper and easier to place, plus its far better fire rated than plain steel, in fact plain steel is so poor in fire situations, it must be covered in fire rated materials. The world trade centre buildings collapsed because after the planes set off fires, the steel columns and beams buckled with the heat. Some say, they got away with minimum fire rating materials back in the 1970's. The affected floors then compacted with the weight of the building above, and once the upper floors started dropping, the lower floors could not sustain the impacts. The buildings hardly felt the 300 ton impact of the planes directly, so this did not cause the collapse.

The choice of steel or concrete also depends on the expected flexure required. If the building is designed to sway say in earthquake zones, then steel is far better as it is flexible and can bend (within limits) witout failure. Concrete is less forgiving, and failure is sudden and catastrophic. Really the issue has all to do with circumstances and design rather than US versus OZ construction techniques. Builders will use the cheaper material, and if concrete is cheaper to build with, then it woudn't matter where the building is located, be it Dubai, US or OZ.

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Old 09-15-2011, 09:13 PM   #17
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High Rise Building methods - US versus Oz - Steel versus Concrete


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Originally Posted by JoJo-Arch View Post
Choice of concrete over steel or vice versa depends more on the properties of the material in relation to construction techniques, and the nature of the design. If the building is to have say a maximum of 30-40 floors, concrete is the way to go because the building technique lends itself to concrete. Concrete is strong in compression (almost as much as steel) but weak in tension, so concrete columns, slabs and beams are reinforced. Concrete would theoretically be cheaper and easier to place, plus its far better fire rated than plain steel, in fact plain steel is so poor in fire situations, it must be covered in fire rated materials. The world trade centre buildings collapsed because after the planes set off fires, the steel columns and beams buckled with the heat. Some say, they got away with minimum fire rating materials back in the 1970's. The affected floors then compacted with the weight of the building above, and once the upper floors started dropping, the lower floors could not sustain the impacts. The buildings hardly felt the 300 ton impact of the planes directly, so this did not cause the collapse.

The choice of steel or concrete also depends on the expected flexure required. If the building is designed to sway say in earthquake zones, then steel is far better as it is flexible and can bend (within limits) witout failure. Concrete is less forgiving, and failure is sudden and catastrophic. Really the issue has all to do with circumstances and design rather than US versus OZ construction techniques. Builders will use the cheaper material, and if concrete is cheaper to build with, then it woudn't matter where the building is located, be it Dubai, US or OZ.

Thanks for your post JoJo-Arch......

I definitely agree there are basic design considerations when the inception of a tall building is done, but my main focus of question is why such a predominant preference for North American super skyscrapers leaning towards steel, and in the southern hemisphere we have the opposite? Both North and South hemi's same design considerations, so this is not the answer as you have indicated in your closing statement.

So yes i agree, all to do with cost.....
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Old 09-15-2011, 09:47 PM   #18
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High Rise Building methods - US versus Oz - Steel versus Concrete


I beleive the tallest building or #2 is the tower in Dubai that is concrete. They built it 24/7 and there was little traffic and room around and trucks could move easily.

Most tall buildings, are built in metro areas and getting a fleet of trucks into a downtown metro area is a real problem and you cannot turn off the traffic for 16 hours of the day. Steel can be delivered in the off hours and stocked daily is a smaller area until it is erected.

The decision, to a large part, is based on the type of structure and needs it is designed for.

While only 28 stories, the Excalibur Hotel in Las Vegas was built using loadbearing concrete masonry to build the 4 buildings (it looks like 2 "L" shaped buildings now) because they needed speed, fireproofing low sound transmission and security. The floor lift slabs were poured daily in stacks around the building and there poured in 28 days starting with the top floor on the bottom and other slabs where poured in reverse order of construction. The block were made and tested (4 different strengths) in advanced and delivered when needed. The walls were built and partially grouted quickly and slabs were set when the walls were done by the rotating masonry crews. As soon as a slab was lifted in place the next slab down in the pile was loaded with materials (toilets, showers, dry wall, etc.) so it would be in place in th area needed.

Every building has different challenges. It takes an engineer working with a contract to successfully build what the architect chooses for the appearance and space usage.

Dick

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