Rebars bending

Reinforcement bars are being bent either in order to follow the stresses developed in the member or to get properly anchored.
The minimum diameter Øm (mandrel diameter) to which a bar is bent shall be such to avoid bending cracks to the bar and ensure the integriy of concrete inside the bent of the bar where large forces appear. The smaller the mandrel diameter is, the larger these forces are and consequently the concrete may fail thus leading to the disintegration of the cross-section.

Practically, the minimum bend diameters are:

Bending Pin Øm=4Ø για Ø≤16 mm Bending Pin Øm=7Ø για Ø>16 mm

In wired meshes:
(a) The anchor should be at least 5Ø (Thus for a wired mesh with Ø10 wires, at least 50 mm)

(b) The bending pin should be at least 5Ø (Thus, in the example is Øm=50 mm) for all cases except when the transverse reinforcement wire is external and in distance ≤3Ø (thus, in the example 30 mm), where the pin should be 20Ø! (Thus, in the example Øm=200 mm!!!)

When the bar anchorage for slabs or beams is referred to as ‘simple hook anchorage’ then metal pins with a diameter ≥Ø should be placed in the interior of the bending curves.

In columns that have a small dimension in the anchorage direction, (see the table below) adequate beam rebar anchoring is ensured by bending the rebars using a large diameter drum.
The minimum diameter of the drum is inversely proportional to the strength of the concrete and proportional to the steel strength.

For grade C30/37 concrete and B500 steel, for various bar diameters, the minimum bending drum diameters and the minimum column dimensions are displayed in the following table

Rebar anchorage in a small sized column using a large diameter drum.