Hey Reader

In my last YT video, I showed you how a mono stringer steel stair can fail if you don’t design it properly.

Torsion isn't something we deal with every day in structural engineering, particularly in buildings.

In fact, we often try to design structures to avoid torsion rather than having to deal with it. Because why not?

But every now and then, we come across situations where torsion is unavoidable.

Whenever a load is applied eccentrically to a structural member, causing it to twist, we need to be a bit more careful.

When dealing with torsion, I generally look at two options:

1. Use a closed section. Tubular sections like SHS, RHS and CHS are generally much better at resisting torsion than open sections (have a look at the table below for RHS sections. Not bad.)

2. Design out the torsion. If I'm using an open section, I'll look at whether I can introduce cross members or other restraints to provide a load path for the torsional forces and limit the twisting.

One thing you'll notice is that torsion often becomes a stiffness problem rather than a strength problem.

The member might be strong enough to carry the loads without failing, but that doesn't necessarily mean it will perform well.

Excessive rotation or deflection can make a structure feel unstable or uncomfortable to use, even when its strength is adequate.

That's exactly what happened with the mono-stringer staircase in my video.

I do have some rules I use to limit deflection and rotation, but at the end of the day there’s still a bit of judgement involved.

Bounciness is bounciness. Everyone notices when a stair feels like a trampoline.

But the difference between very stiff and stiff enough can be a bit more subjective. Different people will notice movement differently.

So, to the extent possible, I’d rather design for stiff enough to avoid client unsatisfaction.

Anyway, I think I’m getting a bit too philosophical about stairs now.

Cheers,
Gabe

PS: We go through plenty of interesting real-life structural cases like this inside BengLab. You can join here​