Torsion In Concrete Beams

Most beams are designed for bending and shear. A beam loaded off its centreline is also being twisted, and twisting has its own failure mode with its own reinforcement.

The giveaway is a crack pattern that spirals around the beam rather than running vertically up its side.

Where torsion comes from

An edge beam carrying a slab on one side. The slab load applies at an eccentricity, and the beam twists. The most common source of torsion in an ordinary building.

A spandrel beam supporting a balcony. The cantilever moment has to go somewhere, and it goes into the supporting beam as torsion. This is the mechanism behind a large share of balcony problems, and it connects to [cantilevered awnings and balconies](/blog/cantilevered-awnings-and-balconies).

A curved or kinked beam, where the load does not act in the plane of the member.

A beam supporting a column off its axis, which is a transfer condition and is covered in [transfer slabs and transfer beams](/blog/transfer-slabs-and-transfer-beams).

A beam framing into another beam, where the incoming member's end moment twists the supporting one.

Two kinds, and the difference decides the design

Equilibrium torsion, where the twisting is required for the structure to stand up. There is no alternative load path. An edge beam supporting a cantilever has to resist the torsion or the cantilever falls. This is designed for in full.

Compatibility torsion, where the twisting arises because the members are connected and would otherwise rotate independently. Here the beam can crack, soften, release the torsion, and the load redistributes elsewhere. The design can take advantage of that redistribution, provided the beam has enough reinforcement to crack in a controlled way rather than fail.

Telling the two apart is the first step, because designing compatibility torsion as if it were equilibrium torsion produces a beam with far more steel than it needs.

What resists it

Torsion is carried by a closed loop of forces around the perimeter of the section, which has three consequences for the reinforcement.

Ligatures have to be closed. An open U-shaped ligature cannot carry the loop. Closed ligatures with properly lapped or welded closures are not optional, and a ligature that was opened on site to get a bar in has removed the torsional capacity it existed to provide.

Longitudinal bars in every corner, distributed around the perimeter rather than concentrated top and bottom. Bending wants steel at the extreme fibres; torsion wants it around the outside. A beam carrying both needs both.

Anchorage at the ends is critical, because the loop has to close at the support. A torsion beam with its reinforcement stopped short at the support has no capacity there, which is exactly where the torsion is highest.

The result is a congested cage, and the clear spacing has to still let the concrete through, which is the tension described in [reinforcement detailing and congestion](/blog/reinforcement-detailing-and-congestion).

Section shape matters more than area

Torsional stiffness depends heavily on the shape.

A closed section resists torsion enormously better than an open one of the same area. A hollow box is strong in torsion; an I-section is very weak. Which means a steel beam carrying torsion is usually a hollow section, and where it cannot be, the torsion is resolved by restraint rather than by the member.

For concrete, the effective resisting section is the outer shell rather than the core, which is why a solid beam and a hollow one of the same outside dimensions have similar torsional capacity. The middle is not doing much.

The restraint alternative

Often the better answer is to remove the torsion rather than resist it.

A slab that is continuous over the edge beam, rather than simply supported on it, applies far less twist. A balcony that is supported at both ends rather than cantilevered off one beam removes the condition. A secondary beam that restrains the edge beam against rotation converts torsion into bending in a member that handles it efficiently.

Which makes torsion a layout problem before it is a reinforcement problem, and the cheapest time to solve it is on the framing plan.

How it shows up on site

Spiral cracking on the side faces of a beam, inclined at roughly forty-five degrees and running in opposite directions on opposite faces.

That pattern is specific and it is not shear cracking, which is inclined on both faces in the same sense. Recognising the difference matters, because the remedies are different, and the general diagnosis of crack patterns sits with [crack width in concrete](/blog/crack-width-in-concrete-and-when-it-matters).

A beam cracking in torsion is a beam that was detailed for bending only, and the remedy is usually external confinement or a new restraining member rather than adding bars.

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