Why does no one produces Aluminium façades with less the 48 mm widths for the inner profile?

why facades have minimal frame dimensions

///Why does no one produces Aluminium façades with less the 48 mm widths for the inner profile? This should be possible – and would look so beautiful!

 

F form J”

 

Dear J.,

 

This is a question we have heard earlier and so we are happy that you ask it here.

In fact, 48mm (or more common 50mm) is as small as it gets when you want to use systemised solutions. Curtain wall mullions or transoms are pretty complex constructions, serving multiple functions: Loads transfer, insulation, water and wind tightness. This means that is a minimal width of the connecter is required. The glass may not get in contact with the connector to prevent unwanted stress peaks and breakage. So there is a couple of mm space needed on each side. The glass edge itself has a minimal with (at least for insulated glass panes) which is not transparent. If you sum it up you already have about 50mm. A little sketch is attached to illustrate this.

But the most important part is that the glass in clamped in between pressure plate and mullion – with the inner and outer rebate gaskets transferring the loads. Normally, every building moves under loads. Also the primary structure can shrink or creep with time, when it is made from concrete. This leads to movement of the façade frame (secondary structure) and can result in a parallel displacement. But the glass pane will not follow any kind of movement because it is stiff. For this reason the gaskets need to form a kind of soft interface between frame and glass.

Here it get interesting: Especially with large glass panes or panels, small movements can lead to a relative large parallel displacement in the frame corners and the danger is that the glass pane slips out of the gasket. The gaskets not only need to transfer loads but also guarantee the wind and water tightness of the construction. Thus, a minimal overlap of gasket and glass must be guaranteed at all times. This simply leads to a minimal dimension of the façade frame or mullion – the 48mm you are mentioning. Obviously pressure plate and inner profile need to have the same size. The inner profile can have a T-shape though, to make it look smaller at the edge.

 

You have to consider the following when you design a façade:

Façade profiles of standard systems, as offered by different suppliers, are designed to accommodate normal movement/displacement of buildings. Always check with the structural engineer if perhaps larger movements are expected. For example: We designed a 60m steel bridge with a curtain wall façade some time ago. The glass panels were quiet large (about 2,5×3.5m). The glass overlap of 50mm profile was simply not enough for the bridge movement. Smaller glass panes and accordingly more mullions and rubber interfaces were no option. It would have resulted in less transparency at the end. We had to use 60mm profiles for larger overlap and every 7,2m an additional system joint had to be introduced. But to be honest, the difference can hardly been seen from a distance – especially with the large panes.

One other consideration: Even without frame (Just glass panes with silicone joints) constructions are not getting more transparent. Ultimately here you also have to consider movements and minimal dimensions for silicone joints. The insulated glass edges and the silicone joint will also sum up to 45 or 48mm. Of course from certain outside view angles one gets a less disturbed reflection (15mm joint vs. 50mm profile) but for the inside view it does not make a big difference.

Let´s put it this way: It is just the prize we have to pay for secure and highly insulated facades.

 

Best regards, The FacadeWorld Team

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