Dynamic Facade Testing: When and Why should you do it?

 

It’s a tough question really because a façade is rarely a single entity with no variations.  Usually there are edges and interfaces with other materials or construction types.  A glazed assembly – unitised or stick system runs into a sheer wall with rainscreen cladding, or precast concrete for example.  Maybe the curtain wall has some opening vents with specific performance criterion or is penetrated by support members for a canopy or other external features on the building.  Are you then testing the façade system?  The window?  The penetration detail and the sealing – often this will come down to workmanship on site and the correct application of perimeter membranes and their fixation to the structure. Or are we testing the façade as a collection of systems and details behaving in a unique way because of their positioning within the specific collection of neighbouring components?

Logically it makes sense to test the entire façade and to build a representative sample on a rig big enough to take examples of all the interface details and system types on a project irrespective of whether there is a test certificate for the façade system itself.  This logic however would be extremely expensive adding tens of thousands of pounds to a project, probably unnecessarily.   What we would be testing would be the interfaces – edges and corners where one system meets another.  In most cases detailing these interfaces, potentially using 3D software and ensuring the drainage and ventilation layers are consistent and unbroken will remove the need for any laboratory testing.  Structural calculations and FEA can predict the behaviour (usually conservatively) of a curtain wall system under dynamic test conditions, at least in theory.  What cannot be tested on any laboratory rig is site workmanship on the project.  One would expect the level of workmanship on a test to be optimum and that the fabricators and installers would be the best available to the façade company paying for the test.

System suppliers will generally provide test data for their standard assemblies which will be accepted by façade consultants and architects as evidence that the systems are performance compliant and will often negate any specification requirement for testing.  Systems have changed very little in the past 30 years aside from the addition of more cavities and improved thermal breaks to accommodate ever tightening thermal performance requirements.  We still rely on secondary lines of defence for water ingress and pressure equalisation to allow it to freely drain back to the outside while maintaining a relatively airtight seal to the inside.  However, we use membranes at the edges which all, irrespective of the system type rely on workmanship and the correct application of appropriate sealants for both air and weathertightness, while the perimeter of the test rig will certainly be sealed using membranes with or without clamping strips and adhesives or sealants, the rig itself will not be made from the same materials or configuration, as the conditions on site and system tests usually exclude this area from any test results.

Bespoke systems designed for landmark projects are rarely different in any significant way to any other system tried and tested many times before by the façade company involved.  Changes are usually the addition of architectural features for fins or brise soliel externally or maybe blind tracks internally or non-rectangular shaped mullions and transoms.  The principles of drainage and statics remain the same as always and these principles can be tested virtually by ensuring the drainage paths, slots and holes are all sized and positioned correctly.

So; when and why?

If you are going to do this, it must be done before it is too late to change anything in the event of failure.  As a curtain wall system this would normally already have been done within most realistic parameters by the system supplier so it’s probably not necessary in the first place.  If it’s a “bespoke” system with new dies cut for aluminium and gaskets, it must be recognised in the project program that these may need to be altered if the test fails.  This would generally be because drainage lines were not aligned, or gasket junctions did not seal correctly.  Particularly vulnerable to this would be areas like corners where the lines may be out of plane as the transoms connect to the mullions with single or compound mitre cuts.  Also, potentially problematic is the aesthetic of how bespoke profiles connect.  Operating parts of a façade like windows and doors, will usually be standard with very little in the way of modification, and will therefore be covered by the system supplier’s data.  Some consultants however, will contend that a window glazed into a curtain wall frame behaves differently to an identical window fitted into a punched opening for example.  I would counter that the difference is in the interface at the edges where one is glazed into a drained and equalised zone and the other is fitted into an admittedly more rigid structure but is reliant on the perimeter membrane and the correct application of sealants to prevent “failure” of the window and that the window in the curtain wall is therefore safer than the one in the punched opening.

Type testing of façade systems by the system suppliers (or “bespoke” fabricators) is necessary for validation of the design.  Although with 3D modelling and FEA it could all be simulated there can never be any substitute for real empirical data.  Laboratory testing of interfaces although logically tempting is of limited value as it can all be reviewed in detail in VR and is entirely dependant on site workmanship and quality control.

As discussed above system test data doesn’t cover the edges.  Lab testing of interfaces doesn’t replicate real site conditions and so only validates design detailing and membrane positions, which competent designers / reviewers should be able to do on paper.  There is therefore I believe, a strong argument for system suppliers type testing at their expense (yes, they pass it on) and clients / consultants insisting that the money, which may have been allocated to dynamic lab testing etc. on interfaces be spent on more site QA/QC with site hose testing where appropriate by competent and experienced façade engineers.  These could be either independent façade consultants with the relevant experience employed by the design team / main contractor, or quality control professionals employed by the façade contractor using systems such as FieldView to record installation and sign off at hold points. This kind of real time quality control is much better value for the client I would suggest than most of the albeit entertaining aero engine tests on artificially constructed test rigs.

 

Photo-credit Wintech Engineering for the picture.