Steel frame vs reenforced concrete.

[Jerry666]

Originally Posted by Kanto

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?

The fabrication and the erection of a structure is between 2 to 5 times the cost of the material, 2 if it is a simple structure to 5 if it is highly complex.
The finishings + cladding are the main component of the price.

[Jerry666]

Originally Posted by dfiler

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.

You are right ,theroretically, the best structural choice is a tube. It gives you the highest moment of inertia with the minimum amount of material as you put the steel far away from the neutral axis.
Steel tubes are usually more expensive per ton than rolled sections. Connections between tubes and beams are more complex to achieve, and you are transporting a lot of air during shipment.

In the US, they are using more rolled bared H sections as columns fire protected with spray (vermiculite) or encased H columns into concrete to get a composite effect with automatic fire protection. The net area is much greater than with concrete, the fabrication costs are minimal and the erection speed is the highest.
In China, they are using today more built-up sections made of plates (a lot of "poor" fabrication) also encased into concrete or they are using tubes filled-in with concrete for medium size buildings.
After a huge earthquake, it will be very difficult for them to check/repair the concrete inside the tubes.

[Kanto]

Thanks for the info

[Highcliff]

can anyone answer my question?
in brazil the most of buildings are made in concrete, not in steel

why this?

[China Hand]

What is inside those columns?

Originally Posted by Jerry666

You are right ,theroretically, the best structural choice is a tube. It gives you the highest moment of inertia with the minimum amount of material as you put the steel far away from the neutral axis.

This would explain the current super-column trend. They look to be square tubes, curved steel for this not yet feasible for 700 m structures.

What about the combined materials? Do they have greater strength/stress handling/deformation tolerance compared to only steel or only concrete?

[Jerry666]

Mega columns in high-rise are made of steel surrounded by concrete. Steel is 5 to 7 times stiffer than concrete. The columns are optimized to limit their overall cost.

In Bazil, they are using more concrete than steel because there are many rebars producers. Small sections are available but large sections needs to be imported. It is possible to import them but it takes a few months. So for small fast track projects, concrete is the fastest solution. For high-rises, steel will become the best solution to reduce the sizes of the columns