EN 1992-1-1 §8.3: Permissible Mandrel Diameter for Bent Bars

About this Permissible Mandrel Diameter for Bent Bars Calculator

This calculator finds the smallest mandrel a reinforcing bar may be bent around to EN 1992-1-1 Clause 8.3. It takes the larger of two limits: the one that avoids damage to the bar, from Table 8.1N, and the one that avoids crushing or splitting of the concrete inside the bend, from Expression (8.1).

• Detailer set bend diameters for highly stressed bars, such as the hooked ends of beam bars at a corner or the bends in a corbel, where the standard bar limit may not be enough.
• Reinforcement scheduler confirm that a shape code's standard former suits the bar, its spacing and the concrete strength.
• Checking engineer test whether a tight bend near a member face risks splitting the cover.

The governing diameter is reported both in millimetres and as a multiple of the bar diameter, and a sketch shows the bar around the mandrel. It is an engineering-grade calculation on CalcTree that you can duplicate and reuse.

More info on Mandrel Diameter for Bent Bars

Limit to avoid damage to the bar

Table 8.1N sets the minimum mandrel diameter for bends, hooks and loops as a multiple of the bar diameter, with a larger multiple for bigger bars. This limit depends only on the bar diameter.

Limit to avoid failure of the concrete

Expression (8.1) limits the bearing stress on the concrete inside the bend. The bar force F_bt comes from the design stress at the start of the bend, which can be the design yield strength or a stress from ultimate loads that you enter. The distance a_b is half the centre-to-centre spacing of the bars perpendicular to the plane of the bend, or the cover plus half the bar diameter for a bar next to the face of the member. The design concrete strength in the expression is not taken greater than that of class C55/67.

Governing diameter

The mandrel must satisfy both limits, so the larger governs. Clause 8.3(3) lets you skip the concrete check in some cases, for example where the stress at the end of the bend is below f_yd; the page reports it in every case so you can see the margin.

Common Calculation Errors to Avoid

• Checking only the bar limit Table 8.1N protects the steel, not the concrete; a heavily stressed bar at close spacing or small cover can need a larger mandrel.
• Using the full spacing for a_b a_b is half the centre-to-centre spacing for interior bars, not the full spacing.
• Treating an edge bar like an interior bar a bar adjacent to the face takes a_b as cover plus half the bar diameter, which is often the smaller value.
• Using a high-strength concrete at full value the design strength in Expression (8.1) is capped at the C55/67 value.
• Applying the rules to welded mesh bars and mesh bent after welding, and bundled bars, follow other rules not covered here.

FAQs

Why are there two limits on the mandrel diameter?

A bend that is too tight can crack or weaken the bar itself, which Table 8.1N guards against. A bend under high force can also crush or split the concrete inside it, which Expression (8.1) guards against. The larger of the two governs.

Which stress should I use at the start of the bend?

Taking the design yield strength is conservative. Where the bar is less highly stressed at the bend, enter the design stress from ultimate loads to get a smaller concrete limit.

When can the concrete check be skipped?

Clause 8.3(3) does not require it where the anchorage needs no more than a short length past the end of the bend, where the bar is not at the surface and has a cross bar inside the bend, or where the stress at the end of the bend is less than f_yd. The page still reports it.

What is a_b for a bar at the edge of a member?

For a bar adjacent to the face of the member, a_b is the cover plus half the bar diameter, measured perpendicular to the plane of the bend. For interior bars it is half the centre-to-centre spacing.