Flat bar mirror frame: why flat bar fools you on a standing mirror

For the Arena do Vidro final, I gave both competitors square tube, flat bar and a welder, with just one rule: the mirror had to have a metal frame and stand up on its own. Around the 9-minute mark of the episode, the commentary table said the line that became the topic of this post: flat bar fools you, because it is easy to bend, but it is flimsy.

If you build metal frames for mirrors, you have fallen for this or you will. A flat bar mirror frame looks clean, it is thin, it bends by hand and it follows any curve. But looking good and being strong are two different things. Here I explain why flat bar fools you, what happened in the challenge and how I would combine flat bar and tube on a piece that has to stand up.

What happened in the final

Rômulo built his frame starting with the thin bar, the flat bar. The person commenting at the table said he would have followed the other competitor's approach and just added a flat bar on the back for support. Rômulo did the opposite, flipped the order, and ended up spending more energy, because the bar is very flimsy: it even got dented right there in front of everyone.

The advice was simple: use the heavy structure first and the flat bar after. When you stand flat bar up, it gives. In the last five minutes, Rômulo was still fighting his frame while Janael was already finishing. He had another problem along the way, cutting the mirror with no clearance, but that is a topic for another post. What I want to cover here is the choice of profile.

And notice this: at the start of the challenge, one of the metalworkers even suggested starting with the thin flat bar to move faster. Flat bar really is fast. That is exactly the danger: it speeds up the job and hides its weakness until you lift the piece.

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Flat bar mirror frame vs square tube: the difference

Why flat bar fools you

The reason is easy to see when you look at the profile. How well a piece of metal resists bending depends on how much material it has in the direction you push it. Flat bar is wide on one side and thin on the other. If the mirror pushes it across the thickness, which is the normal case in a standing frame, it works on its thin side and gives. It is the same bar you bent so easily on the bench: what makes it easy to bend is what makes it weak at holding weight.

Tube is different because it has a wall on all four sides. It resists in every direction and handles twisting much better, and twisting is what warps a frame when the mirror sits on a single stand. That is why tube is the skeleton and flat bar is the skin.

And do not forget the weight. Glass weighs about 2.5 kg per square meter for every millimeter of thickness, roughly half a pound per square foot per millimeter. A 4 mm mirror of one square meter already weighs about 10 kg, roughly 22 pounds. Run that math with your mirror size before you choose the profile: that is the weight hanging on the bar.

How I would combine both in a standing frame

• Heavy structure first. Close the frame and the stand with tube. That is what holds the mirror up.
• Flat bar after, on top or around it. Use it for the finish, for curves and to hide the structure.
• Flat bar on the back for support. That was the table's suggestion: a bar on the back helps hold the mirror without making the front bulky.
• Always use a stiff profile for the stand. The point that touches the floor takes all the weight. Flat bar alone there will twist.
• Stand it up before finishing. Set the bare structure upright and push it by hand. If it wobbles without the mirror, it will be worse with it.

When flat bar alone works

I am not telling you to throw flat bar away. It is great for wall mirror frames, where the weight sits on the mounting and not on the frame, and for curved shapes that would be a huge job in tube. The problem is asking it to be the structure on a piece that stands on its own. The question I ask before cutting any metal is: what is carrying the weight? If the answer is the flat bar, change the design.

That is exactly the question the planning stage of the final was testing. They had two minutes to sketch and make the metal cut plan. Deciding right there which profile is structure and which is trim saves a lot of time later, with the clock running.

Can you use flat bar for a mirror frame?

Yes, mainly for trim, curves and wall mirrors. On a mirror that stands on its own, flat bar alone gives and dents. The better approach is tube for the structure and flat bar on top or on the back for support.

Why does flat bar bend so easily?

Because it is thin in one direction. Across its thickness there is very little material to resist, so it bends by hand. That same trait makes it give when it carries the mirror's weight.

Which Arena do Vidro episode shows this?

Episode 4, the season one final. The table talks about the flat bar around the 7 and 9-minute marks of the episode, which is at the top of this post.

Watch the final and pay attention to each competitor's frame, from the planning stage to the last five minutes. If you think you would build that piece better at the bench, sign-ups for the next season are open here on the site.

Full episode 4: the final · Arena do Vidro final: a metal-framed mirror in 60 minutes · How to cut a mirror into shapes

Episode 4