Steel bending
BRANZ has recently had a number of calls to its helpline regarding suitable bend diameters for steel reinforcing.
Incorrect bending can severely affect the performance of steel reinforcement in service. The result can be premature fracture, which will affect the capacity of the building elements to carry design loads. Of particular concern is the practice of bending reinforcing steel to too small a bend diameter.
Bending then straightening (rebending) the reinforcing on site is of even greater concern.
To avoid fracture or weakening, NZS 3109:1997 Concrete construction requires that hooks and bends are formed in accordance with the bend requirements of Table 3.1. The minimum diameter of bend is measured on the inside of the bar.
|
Grade, fy (MPa) |
Bar type |
Bar diameter, db (mm) |
Minimum diameter of bend, di (mm) |
|
|
Plain bars |
Deformed bars |
|||
|
300 or 500 |
Stirrups and ties |
6–20 |
2db |
4db |
|
24 |
3db |
6db |
||
|
All other bars |
6–20 |
5db |
5db |
|
|
24–40 |
6db |
6db |
||
Note that the above table only gives part of the requirements. For full details of standard hooks, bends, stirrups or ties, for mesh bend diameter requirements and for galvanised bar bend requirements, refer to clause 3.3 of NZS 3109:1997.
Rebending should only be carried out when unavoidable and identified at the design stage. NZS 3109:1997 and NZS 3101.1&2:2006 Concrete structures standard require that rebending is done in the specified manner and to the manufacturer’s requirements.
From BRANZ Guideline, February 2016