Facade Fixings And Substrate Capacity

Facade panels get specified carefully and fixed into whatever happens to be behind them.

Which is the wrong way round, because the panel almost never fails. The fixing does, or the substrate the fixing went into does.

What a facade fixing resists

Wind suction, which is the governing case. Wind pulling a panel off the building is a larger force than wind pushing it on, and it peaks at corners, edges, parapets and setbacks where local pressures are far higher than the general face value.

Those peak zones are why [wind loading](/blog/wind-loading-on-buildings-explained) is resolved by zone rather than as one number, and why a fixing pattern adequate in the middle of a facade is inadequate two metres from the corner.

Self weight, which for heavy cladding is substantial and acts permanently.

Movement. Thermal expansion of the panel, movement of the structure behind it, and differential movement between the two. A fixing that restrains movement it was not designed to restrain fails in fatigue or tears the panel.

Impact, at ground level and in trafficked areas.

The substrate is the variable

The same fixing has completely different capacity depending on what it goes into.

Concrete is the most reliable and it depends on the concrete strength and on edge distance. A fixing close to a slab edge or an arris has a fraction of its rated capacity, and reinforcement position matters because drilling into a bar or through a post-tensioning tendon is a serious matter. On a post-tensioned building, nothing gets drilled without locating the tendons first, which is the substance of [core drilling and penetrations](/blog/core-drilling-and-penetrations-in-existing-slabs).

Solid masonry is reasonable and variable, and capacity depends on whether the fixing lands in a brick or in a mortar joint.

Hollow block and hollow terracotta need specific hollow-wall anchors and offer much less capacity than they appear to.

Lightweight steel framing carries load only where the fixing hits a stud, and the stud gauge governs.

Autoclaved aerated concrete needs its own anchors and is frequently fixed with the wrong ones.

Render over anything carries nothing. A fixing into render is not a fixing.

Existing buildings are the hard case

On a new building the substrate is documented and specified.

On an existing building nobody knows what is behind the wall until it is opened, and the drawings frequently disagree with reality. Which is why a recladding project starts with investigation: opening up at sample locations, establishing the actual construction, checking the concrete strength and the reinforcement position, and then designing the fixing.

And then testing. Pull-out testing of sample fixings in the actual substrate is the only way to convert a calculated capacity into a demonstrated one, and on a recladding job it is routine rather than exceptional. The broader approach is the one described in [assessing an existing building with no drawings](/blog/assessing-an-existing-building-with-no-drawings).

Corrosion is the long-term failure

A fixing that is adequate on day one and corroding is a fixing with a service life.

Which means stainless steel of the appropriate grade in coastal and marine environments, and it means no mixed metals in contact, because dissimilar metals in the presence of moisture corrode galvanically and the weaker one goes first.

Sydney's coastal suburbs are an aggressive environment for facade fixings, and the consequence of a corroded fixing at height is severe. The same exposure logic that governs [concrete durability](/blog/coastal-exposure-and-concrete-durability) governs the fixings.

The detailing that prevents problems

Allow for movement. Slotted holes, sliding connections, and a considered decision about which fixing point is fixed and which ones float. Restraining everything is the most common detailing error.

Keep water out and get it out. Every fixing penetrates the weather line. Flashing, sealing and drainage at fixing points is part of the design.

Support at the right points. A panel supported at the wrong locations carries bending it was not designed for.

Provide access for inspection, because a facade fixing that cannot be inspected will not be inspected.

Who is responsible

Facade design sits between the architect, the facade contractor, the structural engineer and sometimes a specialist consultant, and the fixings are where the responsibility most often falls in the gap.

Setting that out clearly, including who designs the fixing, who verifies the substrate and who certifies the installation, is worth doing before the panels are ordered. It is a version of the coordination question in [how engineers and architects work together](/blog/how-engineers-architects-work-together).

ACSES provides structural engineering and facade fixing assessment across Sydney. Talk to us about a building.