Designing Steel Connections for Membrane Structures

Designing Steel Connections for Membrane Structures – Lessons from the Weird Side of Steel

If there is one thing membrane structures will very quickly teach you, it’s humility.

After years of designing “normal” steel structures — warehouses, frames, halls full of repeating logic — you suddenly find yourself staring at a mast tilted in space, cables pulling from directions your intuition doesn’t immediately like, and connections that look more like mechanical sculptures than structural details. And that’s exactly what happened during a recent webinar focused on membrane structures and their steel connections.

At first glance, it felt familiar. A global model, a couple of load cases, a clean analytical picture. But as soon as we zoomed into the nodes, things got interesting.

Global analysis is the easy part

The workflow started the same way most of ours do. A membrane structure was analysed globally, load cases and combinations prepared, deformations checked, everything neat and tidy. At that stage, the structure looked elegant — masts, cables, membranes all doing their thing.

Then came the connections.

Unlike standard frames where nodes repeat themselves like wallpaper patterns, here every important joint was its own animal. Mast heads with cables coming in at strange angles, anchoring nodes interacting with concrete, pins here, welds there, eccentricities everywhere. None of them were “typical”, yet all of them were critical.

That’s the moment when exporting the nodes into IDEA StatiCa stopped being an option and became a necessity.

Suddenly, geometry matters. A lot.

Once the nodes were imported, the difference between membrane connections and “normal” steel details became obvious immediately.

Everything lives in 3D. Members are not orthogonal. Plates are rarely rectangular. Forces are mostly axial — until they aren’t. The smallest positional tweak introduces secondary moments you didn’t ask for but absolutely have to deal with.

Instead of selecting a connection template and tweaking bolt numbers, the design became a sequence of operations: adding plates, cutting members, welding parts together, importing shaped gusset plates from DXF files. It felt much closer to a real fabrication process than to classical connection “design”.

And that’s actually a good thing. With membrane structures, abstraction is your enemy.

Cables are not beams (and software will gladly let you forget that)

One of the most important moments in the whole session was also one of the most subtle.

By default, connection models happily assume that members transfer all internal force components. Fine for beams and columns. Completely wrong for cables and pinned elements.

In membrane structures, cables and rods are there to carry tension. That’s it. No bending moments. No rotational stiffness. Forget to change this, and you can end up with a connection that “works” numerically while being physically nonsense.

Switching cable members to normal force and shear only is not a software trick. It’s structural reality catching up with the model.

The bearing member problem you only notice when things go wrong

Another thing that rarely matters in simple structures but becomes crucial here is the bearing member.

Every connection model needs a fixed reference to keep equilibrium. In a tangled membrane node with multiple cables, the automatically selected bearing member is very often the wrong one. And if the structure has small eccentricities — which it always does — those unbalanced forces will end up somewhere.

Putting them into a cable or a pin connection is a great way to get misleading utilization results. Assigning the mast or main column as the bearing member, on the other hand, makes the whole system behave like it does in reality: the stiff member quietly absorbs the mess.

It’s a small setting with very non‑small consequences.

This is where FEM finally earns its keep

Watching the results appear was probably my favourite part.

Instead of green/red pass‑fail tables, the model showed stress fields flowing through oddly shaped plates, concentration zones around pin holes, deformation shapes that immediately matched (or challenged) engineering intuition. You could clearly see which plate was really working and which one was just along for the ride.

For membrane structures, this is invaluable. These are details where “it passes the equation” is simply not enough. You need to see what is happening.

And yes — sometimes you discover that the governing component is not the one you expected at all.

Anchors, concrete, and knowing where to stop

The anchoring nodes brought a healthy reminder of another boundary engineers often blur.

In the connection model, concrete is represented in a simplified but efficient way. It’s enough to check load transfer, compression, anchor forces and interaction with steel. Once uplift, cracking, or reinforcement become dominant, it’s time to jump to a dedicated concrete detail model.

Trying to do everything in one place usually leads to doing nothing properly. The separation actually helps you focus.

Templates still help — just not in the obvious way

At first glance, membrane projects don’t look like good candidates for templates. Everything feels custom.

But once a few anchoring nodes or cable connections are solved properly, saving them as templates suddenly makes a lot of sense. Not because the geometry will be identical, but because the logic of the detail stays the same. The way forces flow, the chosen connection philosophy — those are things worth repeating.

And when two nodes finally are similar enough, applying a ready‑made template feels almost suspiciously easy.

New release of IDEA StatiCa 26.0 - a game changer?

Well, not really. But!

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Final thoughts

Designing steel connections for membrane structures is demanding. It forces you to be precise, to respect real behaviour, and to stop hiding behind simplified assumptions. But it’s also deeply satisfying, especially when you have tools that let you model what you’re actually thinking.

If my previous blog was about speed, repetition, and efficiency, this one is about control.

Control over geometry.
Control over boundary conditions.
Control over how forces really move through steel.

And once you have that, even the weird side of steel starts to feel manageable — and maybe even fun.