Questions & Answers

This month Tim Burton, General Manager of K2 Architectural Systems, addresses concerns around the use of bi-folding doors.

My customer wants Bi-folding doors in a conservatory. Can I just fit them instead of windows?

The simple answer is probably not!

Although the majority of PVC-u window system companies insist that their windows are not load bearing, the PVC-u conservatory industry has always relied on reinforced frames to support the roof.

There are systems on the market, like the K2 Konnect system, that utilise structural mullions to take the lateral load of the roof away from the frames However UK standard conservatory systems generally have eaves beams that are designed to be supported at regular intervals, typically between every two and three metres.

With Bi-folding doors we also need to look at the deflection of the roof eaves beam. The conservatory industry generally uses deflection of L/125 for polycarbonate and L/175 for glass. This means it is acceptable that under full load a 3 metre beam can deflect 24mm with polycarbonate and 17mm with glass. This sort of deflection would have serious implications on the operation of the doors... they just wouldn’t open!.

Typically, K2 uses a maximum, fully loaded deflection of 6mm for any “clear span” opening, to ensure that the operation of the doors is not hindered.

Ok, so we need to beef up the eaves beam. How do I do that?

There are different ways to increase the strength of an eaves beam. For smaller loadings we could insert a reinforcing bar. For larger loads we could position a structural beam under the eaves, on the internal face of the eaves or we could replace the eaves completely.

So that’s it. I just put an RSJ or something under the eaves beam?

Well yes... well sort of... Well actually NO!

In certain circumstances this will suffice. For example, if you were building a lean-to with brick walls down either side then the simplest solution would be to build a beam into the two walls, as you would when knocking two rooms together. You would need to think about the extra height required, and you would need to get an Engineer to specify the beam size. However, if you don’t have walls to attach to, or wanted to create a Georgian or hip back style roof you would need to look at the support in more detail.

Where there aren’t any walls to take the load of the roof we need to use posts. These should be structural and be bolted down to the foundations. As they have to take the whole load of the roof from over the doors they definitely should not be sitting onto of a PVC-u cill. However taking the weight is not their only purpose.

When we design a conservatory the window frames themselves assist in preventing racking. If you built a roof supported on bay poles it would be incredibly unstable and would push over. The window frames prevent this happening by using the strength of the glass in shear. This is known as the diaphragm principal. If we remove the frames we remove the strength.

So what we must do is to create a connection between a post and the eaves which stops the roof from falling over?

In traditional construction this would be created using a diagonal brace or flange plate. However we cannot have this on the conservatory as again it would infringe on the door opening. So we need to create a connection between a structural post and the beam.

We do this by increasing the depth of the eaves beam, continuing the post up to the top of the beam and creating a two bolt moment connection.

This all seems to be getting a bit confusing and complicated. Should I just tell my customer that they can’t have them?

Definitely not. Creating this extra support is not complicated and is not expensive.

We at K2 have dedicated systems and software to work out the best way to allow you to fit any door system. We have a full range of purpose designed aluminium beams for various spans and loading, with our largest capable of a 5m square Georgian style glass roof with all three sides completely open. All our Bi-fold supports are manufactured in-house and on each job we check that not only is our product structurally suitable, but also that the doors will not be obstructed by beams or posts.