Concrete column has 2 kinds of rebar. The thick rebar parallels the column is called “main rebar” and the thin rebar surrounding main rebar is called “hoop rebar”.
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In some brochures of condominium, it is insisted that “This hoop rebar is applying Welding-closed rebar”. What is the effect of the welding-closed?
□■□ Answer ■□■
The building with Welding-closed rebar becomes strong against collapse. In the event of huge earthquake, the column gets less damaged. But it is difficult to evaluate its effect in numericaly, so that there are no good influences for structural calculation.
- The roles of rebar in column
- In the first place, what is “Welding-closed”?
- Effect of Welding-closed
- Treatment on calculation
The roles of rebar in column
The roles of column are to support weight of building and to withstand earthquake force. Earthquake force bends column and one aspect of column gets tensioned and another one gets compressed. It is the same phenomenon that your one leg uplifts and another one supports your whole weight due to sudden braking. Concrete is weak against tensile so that main rebar compensates it.
Column withstands not only bending moment but also shear force. Shear strength of concrete is not enough so hoop rebar is installed.
In the first place, what is “Welding-closed”?
I wrote that hoop rebar is surrounding main rebar, but it is originally single stick. It is bended to square before installation, how deal those edges is problem. Generally, both edges are bended to 135 degree and hanged to main rebar. In the case of “Welding-closed”, literally connect those edges by welding and closed the square shape.
Due to adding welding process to bending process, it is time consuming. So I have never seen such hoop rebar except construction sites of condominiums.
Effect of Welding-closed
In the case of shear failure of column, it is caused by concrete, not rebar. Therefore the difference of edges between 135 degree bending and Welding-closed is meaningless. The difference appears when building gets deformed largely under huge earthquake.
When buildings get deformed largely, columns have to support huge stress. The concrete outside of main rebar cannot tolerate such stress, so that it crumples and fall away from column. After that, main rebar are exposed outside and start bending outside because of compression. Then the hoop rebar hold that main rebar.
At last, hoop rebar cannot tolerate main rebar bending, column cannot support the weight of building and building collapses. At that time, margin increases as hoop rebar tolerate longer. Naturally the hoop rebar with Welding-closed can tolerate larger force than hoop rebar with 135 degree bending.
There are any other effective way for example “spiral hoop rebar” and “diagonal hoop rebar”.
Treatment on calculation
As I wrote above, the situation that Welding-closed hoop rebar works is ultimate state of the building. In the latest technology of analysis for concrete cannot reproduce the process of collapse of concrete column with high accuracy.
Concrete is mainly composed of cement, sand, gravel and water and is cast in place by putting it in the mold, so that the dispersion tends to be bigger than that of other materials. Furthermore it becomes more complicate due to combining with rebar. By arranging each parameter it is possible to adjust result of analysis to that of experiment some degree. But it is impossible to conduct such a difficult analysis for all each buildings.
It is confirmed that the effectiveness of welding-closed hoop rebar on experiments, but it is difficult to evaluate how good it is in numerically. And 135 degree bending hoop rebar also works well on the range which structural calculation deals. Therefore, whether welding-closed or not doesn’t affect the result of structural calculation.
There are some structural designs with singling parts that never appear on result of calculation due to overestimation of the result of calculation. So adopting such a devise can be highly evaluated. You don't need to stick to welding-closed hoop rebar, but it is a part that shows devise of structural engineer.