The main building movements factors we need to consider are deadload and live load. Simply put deadload is the weight of all the other materials added to the superstructure and façade during construction. Live load is everything else from environmental conditions like wind and seismic loadings which we calculate based on empirical data and worst-case scenarios, to the effect building occupants may have by simply using the building. It may not seem so, but the floor slabs react and deflect when people move about on them or use one particular area to keep heavy machines or stock. A typical façade needs to be able to accommodate this.
The simple answer is slots and spaces. In stick system curtain walls, we never pin both ends of a member connected to 2 floor slabs. One end is pinned and takes the dead load, the other typically has a vertical slot to allow the slab to move relative to the bar, and still resist wind load, pulling or pushing the bar in or out from the building face. Incidentally, though not insignificantly this arrangement also allows for thermal expansion in the façade members. In unitised curtain wall the slot arrangement is replaced by the stack joint which does the same thing. The phenomena generally has less effect in a unitised system, so we will concentrate on stick curtain wall.
Glass framed in a rebate needs space around it for several reasons including ventilation and drainage, but crucially to prevent glass to metal contact when the framing reacts to changes in the structure and temperature. It is also vital to maintain glass edge cover so that the glass does not become disengaged from the frame on maximum deflection and to ensure the glass can act to its full capacity as a barrier without potential disengagement from the surrounding framing.
So, to some numbers.
50mm Stick Curtain wall:
Glass edge cover 13mm (18mm on a 60mm system)
Nosing thickness 10mm
Nominal clearance 5mm
Fabrication tolerance ± 1mm
Installation tolerance ± 2mm
Allowance for thermal expansion in aluminium per linear meter 1mm
Fabrication tolerance in 8mm glass 2800×1500 + 2.2 – 2.5mm and ± 2mm respectively
Even without allowing for thermal expansion, fabrication and installation tolerances, simple geometry demonstrates that a fairly common fenestration of 1500mm mullion centres, with transoms at floor and ceiling say 2800mm on a 6000mm structural grid cannot accommodate 10mm slab edge deflection without the glass clashing with the curtain wall nosings and reducing the edge cover to 0 in some locations. See figure 1
It also follows that with only a nominal 5mm clearance between the top of the glass and the transom nosing, a 10mm drop over an expansion joint will mean the glass will hit the nosing and probably break.
This is for a standard capped 50mm stick system, but the situation is worse if a toggle fixed glazing system is selected, as the nominal clearance reduces, and the glass edge capture distance is reduced from the 13mm provided by the pressure plate, to approximately half the depth of the edge channel where the toggle retains the glass. This is all before any allowances for tolerances and expansion are added. If the glass is moved out to increase the clearance in the bottom of the channel it becomes more likely the toggle will disengage from the channel. Figure 2
While it must be said that floor slabs do not regularly deflect like this in real life we must design our facades as if they do and include the tolerances and allowances listed above. This leads us to conclude that sometimes it is simply not possible to deliver in a narrow frame curtain wall the fenestration the architect wants. Full height glazing is particularly problematic, particularly when combined with relatively wide units. Moving to a 60mm or greater framing system or mixing systems up to use perhaps 80mm transoms at spandrel zones may be necessary to provide the glass cover needed to safely retain the glass and maintain the performance required of the façade.
What is clear is that “standard, off the shelf” systems only work for “off the shelf” situations and that care should be taken when applying these solutions in situations where the wall spans several floors and in particular when the distance between the columns appears longer than normal with perhaps larger slab edge deflection than the system can handle safely.