Vertical Brise Soleil Fixed to Mullions – Is it even more complicated than you think?
As the fight between fully glazed elevations and the issue of trying to minimise solar gain intensifies, we get a lot of projects and enquiries incorporating all types of vertical brise soleil blades.
As a rule, we really don’t like them, and advise only to consider this form of solar shading as a last resort. That’s because structurally, they are far more complex than they look.
All engineers look to see how they can model the effects of “sideways” wind loading on these. There is limited design guidance for assessing the magnitude of this loading, so it is often open to some level of interpretation. Lots of engineers seek clarification in the all-powerful book that is Eurocode 1, with all sorts of approximations ranging from assuming they are free sign boards, to parapets “on their sides, to lattices and structural elements with sharp edges. Literal interpretation then follows that unless there if full shielding, all fins can be loaded at the same time – in can lead to a difficult discussion with the project engineer when you advise them that they may need to revisit the loads in their diagonal bracing..
The CWCT gives more specific, measured advice taking into consideration that these are not stand-alone structures, but are dictated more by the building to which they are attached, but still suggests that all fins should be assessed for a quite high-pressure differential, with a particular uplift for fins on corners of buildings. In the absence of hard testing, we find our CFD models do align with the CWCT in that corner fins do get a tough life, but others much less so, which is probably why so may fins that look like they shouldn’t work, actually do.
At this stage there is appears to be a distinct divergence in how engineers deal with structural validation of the system.
Let’s consider the simplest of these forms, which would be a deep cap extrusion fixed along its length to a pressure cap. Lots of manufacturers produce them.
Engineers often take a view of how this net sideways load is transferred back to the structure. Again, there are several trains of though depending on the specific design. The most conservative method is to neglect the stiffness of any infill and to check the mullion for “sideways bending”, taking into consideration the contribution of the cap. So far, so good.
However, thereafter it gets a bit messy. There seems to be a consensus that when designing the mullion section, it is conservative to neglect the contributing stiffness of the cap. We often find that the cap is significantly stiffer than the mullion to which it is attached, so on first glance, neglecting its stiffness would indeed be conservative. But it isn’t as simple as that.
If we consider wind suction, the load from the glass is transferred into the stiff cap. The mullion just goes along for the ride because it’s not very stiff. The result is that most of the screws in the pressure cap don’t do much. However, at the ends the load and stress in the cap then needs to find their way back into the mullion (and back to the bracket) so we find that the loads on the end screws and pressure cap are very high.
If we then look at the pressure case, we find that the load from the infill does go directly into the mullion. However, the mullion immediately deflects and that results in the screws at its most flexible part (near mid span) working much harder – causing high stress in some screws and not others. Again, this load gets transferred back to the top and bottom so stress again at these locations is higher.
That means that the generalised analogy of tension loads on the screws being uniformly distributed may not be suitable for this sort of situation.
If we then consider the implications of “stand off fins” with multiple brackets back to the mullion, there is again likely to be unforeseen implications. If the external fin is stiffer than the mullion, then there will be a tendency for higher loads to be transferred through the brackets, and it may not be a simple “one way thing” thing. That is because unless specifically detailed, it doesn’t know that it isn’t a Vierendeel type structure– so that’s what it is.
Here at Structic we are currently working with our partners to undertake some further research and testing of brise soleil fins of this type, with a view to supporting the theoretical values with practical demonstration. In the meantime, we continue to advise our clients of the complexities associated with this type of solar shade design.
Please consider the brise soleil before you specify the system. Its never as straight forward as it looks!
By Rob Whalley & Chris Lee
