Today the word Gothic is often associated with dark arts and gloomy music. But its original use was quite the opposite. Gothic architecture was renowned for, among several features, its larger windows. Prior to its arrival, inside spaces were gloomy affairs with little natural light.
Modern buildings have taken this to a whole different level. Large glass facades often enhance or even cover the outside of skyscrapers, malls, churches and other places. Glass has become a very popular building material for architects which can be used as the building envelope. Architectural glazing includes windows, doors, shopfronts, window walls, structural curtain walls, clerestory’s, skylights and balustrades.
Glass is extremely strong under compression but is brittle in flexion. However, glass has a molecular memory and can withstand repeated flexing over many years. It is important to ensure that the physical characteristics of glass are carefully considered with regard to wind load.
What is wind load?
‘Wind Load’ describes the forces that the wind exerts on a building or structure. Importantly, wind exerts variable load and the load is measured in 3 second gusts. There are two types of wind loads:
- Uplift Load is an upwards force that could affect roof or similar horizontal structures in a building, such as canopies or awnings. Wind can push up the face of a building creating positive load or the wind flowing over the horizontal structure can cause negative load, thereby pulling the roof upwards.
- Shear Load is a wind force that exerts loads onto walls or other vertical structural elements. Again, positive and negative pressures can cause failure of the vertical elements.
Engineers can calculate wind load. These calculations are dependent on several criteria. Coastal wind load tends to be higher than inland. Exposed sites are susceptible to greater loads than buildings shielded by trees or other buildings. The aspect of the building is important relative to prevailing winds.
What to consider when using glass
Centre deflection refers to the area in the middle of the glass pane, which will distort the most. Imagine holding a towel by all four corners in the wind: it’s the centre that ends up with the most pressure. Glass can withstand years of centre deflection without failure. However, the size of the glass pane, height from the ground and site conditions will determine the glass thickness to ensure that failure does not happen.
The glass must be held in place by some mechanism. The support structures and the fixings of those structures also need to be designed to ensure that wind load cannot cause catastrophic failure.
Planning for wind load requires professional advice to ensure the right glass systems are specified for a project. Standards help guide the selection process as well. In South Africa, we rely on the SANS 10400 standard, which defines the type of glass for different situations as well as the acceptable tolerances for deflection and how the glass must be supported on its edges. Glazing of structures in excess of 10m height, overhead or sloped glazing, glass flooring, three and one edge supported glass, toughened glass assemblies and entrances, glass for balustrading, frameless shower doors, cubicles and enclosures etc. must be signed off and approved in writing by a Competent Person Glazing duly registered with the South African Glazing Institute. Aluglass Bautech is proud to have 5 Competent Person Glazing 2 (CPG2) for professional advice and technical planning.
Architectural glass can help create a stunning building full of light and vibrancy. But one big storm could shatter the windows by the force of its winds alone, leading to damage, loss and injury. Correct design will allow the results to speak for themselves…