Wind loads can affect skylighting in a number of ways,some of which should always be considered: Wind can distort the overall shape of a skylight and can cause he rafters and other parts of the roof to bend excessively.Wind can also cause uplift,possibly resulting in the skylight being ripped off the building. The direction in which the wind is blowing has a distinct effect on whether the wind will impose an inward (pressure)or outward (suction)force on the system.Both inward and outward forces can cause excessive bending of the skylight.Outward forces can also cause the structure to be torn completely off the building.
If the pitch of the skylight is steep enough,the wind will cause an inward force on the windward side of a skylight,and an outward force on the leeward side.
Consideration at design stage should also be given to load distribution other than wind.These loads can be divided into two groups,namely:snow or hail loads;and imposed loads caused by people carrying out skylight maintenance.
The design of glazing bars on extrusions should make allowance for water penetration and effective drainage to the outside of the structure (condensation outlets).In essence,one of the most important functions of a skylight is to resist the penetration of rain. The most common complaint surrounding skylights is leakage -a problem that can be easily avoided with correct design and installation methods.On roofs with a pitch more than 15 degrees, one has the option of pre-made flashing with a lead apron.On slate roofs,a soaker system is used.
Condensation is also an important consideration in the design of skylight systems.These systems should be mechanically designed through the use of a guttered weep system to control condensation.
Excessive air leakage will cause drafts,wind noise and discomfort to the occupants of the building and can also be a major source of energy loss in a building.
Glazing Material Properties
Careful consideration of glazing materials to be used is of the utmost importance -some factors to consider are: Thickness of glazing material in relation to wind loading;weight of glazing material;impact strength;fire risk;bending properties;ultra- violet properties;security risk.
Durability & Maintenance
To be assured of satisfactory performance,materials used in the manufacture of a skylight should have good structural properties, excellent resistance to corrosion and the ability to withstand deterioration under climatic and environmental conditions to which it will normally be exposed. Material finishes should be attractive in appearance and durable. Properly selected and specified skylights can be expected to perform throughout their life at a relatively uniform level when given reasonable care and maintenance. Maintenance that may be required includes washing,repairs to and replacement of operating parts (if any)and glazing material replacement.The homeowner ’s decision on how this is to be done may affect the choice of skylight to be used,which in turn affects the maintenance cost.
Proper selection of glazing materials – the attention of specifiers is drawn to the proper selection of glazing materials for use in skylight applications above human traffic in occupied areas. Definitions – inclined glazing includes the fenestration of skylights and space enclosures,which are tilted more than 15 degrees from the vertical.Sloped glazing systems should be inclined a minimum of 15 degrees from the horizontal to ensure proper condensation and water infiltration control,and to minimise accumulation of dirt above horizontal or purlin framing supports.Systems inclined less than 15 degrees from the horizontal may require special consideration.
As the impact strength varies considerably from product to product, this should be considered and checked prior to selection.Inclined glazing installations may be situated above areas where people pass or work.This raises safety and liability considerations for the owner,designer,glazing and skylight manufacturer.Breakage can result from any of the following causes: Excessive loading:wind,snow or concentrated Impact loads from falling (hailstones)or wind borne (roof gravel) objects.
Thermal effects generated within the glazing material itself (i.e. heat absorbing tinted,reflective)due to inclined position Inadequately designed glazing system,which does not provide proper support,clearance or drainage Edge or surface damage to glazing material during manufacturing, handling,installation or maintenance.(In glass products,edges should be polished). Vandalism or destructive accidents Effects of long-term weathering
The choice of an appropriate material for a given application with regard to uniform load strength is dependent upon the design load in the building to which the glazing material will be subjected.The selection of the required design load is the responsibility of the specifier in conjunction with the skylight designer. Once the design load has been determined,the appropriate glazing thickness can be chosen by using published strength information. Glazing materials are normally designed for uniform loads.It is important to note that most published strength charts are based on a uniform one minute wind loading and may not address the additional factors that affect performance in a sloped installation.
The design factor is a minimum statistical factor that is often used by the designer in selecting glass for buildings.If the situation warrants,the designer may choose to use a design factor greater than 2.5,such as 5.0.The probability of glass breakage failure at a design factor of 2.5 is 0.8%– or 8 panes per 1000 tested when subjected to an initial,uniformly applied wind load.At a design factor of 5.0,the probability is 0.07%or 1 pane per 1000 tested.
For most orientations,incident solar radiation or inclined glazing applications will be subsequently higher than on vertical glazing due to the angle of inclination.Also,thermal stresses may be higher as they are dependent on glass centre to edge temperature difference and not solely on the level of solar radiation.The increased level of solar radiation can affect the performance of the glazing material. As the thermal performance between thermoplastics differs considerably,from between less than 80 to 120 degrees centigrade, and as heat build up in the sheet can reach very high temperatures, the designer should consult with various thermoplastic manufacturers for additional technical information.Generally,a glazing material manufacturer ’s brochure will include performance criteria (shading coefficients,U-values,etc)for the various types and combinations of glazing materials.All glazing materials expand and contract to a greater or lesser degree
The primary function of a skylight framing system is to provide a structurally sound support and water control system,which will accommodate the glazing infill under load.The allowable deflection of a glass support-framing member must not exceed the length of the span divided by 175 of “L/175 ”.Deflections exceeding this limit on two edges can effectively reduce glass support from four edge to two edge.For plastics glazing materials,the allowable deflection must not exceed the span divided by 100 of “L/110 ”. The engineering staff of skylight or metal framing system manufacturers should handle deflections and stresses associated with design of the support framing.The structural aspects of the inclined glazing system must be carefully integrated with the glazing rebate and drainage details to ensure proper performance.
Annealed,wired,heat-strengthened,tempered and laminated glass of equal size and thickness has the same deflection behaviour under short-term (one-minute)uniform load conditions.Under long-term loading and normal temperatures,laminated glass will deflect in proportion with its component make-up. Glass –-greater flexural strength or resistance to impact loading does not indicate greater stiffness.The designer should also consider the differences in stiffness between glass and plastic glazing materials. Plastics – under normal use,,the deflection does not exceed 50mm. Impact Impact Impact Impact Impact In general,skylights are not designed to resist point type loads.The designer should examine the surrounding environment for possibilities of impact loading (human,missile,etc).Precautionary measures such as handrailing or protective screens are recommended whenever the probability of any such loading exists. Point type impact load resistance varies considerably from one type of glazing material to another. Undetected surface or edge damage in glazing materials may,in some cases,be incorrectly assessed as spontaneous or mysterious breakage.The designer must also consider the hazards of human impact or falling object impact from higher roof areas.In some cases,the designer may specify that a protective white screen, shield or alternate glazing material be situated above the skylight to protect installation.In some cases,the designer may specify that a protective wire screen,shield or alternate glazing material be situated above the skylight to protect installation.In general,the designer should contact the glazing material manufacturer for recommendations regarding impact resistance.
When considering glass or plastic glazing,the specifier should ensure that whatever material is chosen complies with relevant building regulations.The specifier and owner should be aware of the potential glazing materiel replacement factors and associated costs for the various glazing material options,before the specification is agreed upon.It should be noted that irregular shaped panes or patterns do not easily lend themselves to efficient high volume production.Specifiers should discuss this matter with designers at an early stage in order ascertain any added problems and expenses which may be incurred with custom shape glazing units. The following table outlines the basic properties and breakage characteristics of commonly used glazing materials.Generally, most glazing materials are available in a wide range of transparent tints and in some instances,reflective finishes – manufacturers should be consulted in this regard.
There are currently two methods of manufacture whereby two or more sheets of ordinary annealed glass are bonded together by using one or more sheet of a plastic interlayer such as polyvinyl butyral (PVB)under heat and pressure,or alternatively,using a resin liquid interlayer.The latter is called the cast-in-place method.
Laminated glass of proper strength and interlayer thickness has excellent retention capability.When broken,the fragments of glass will tend to the interlayer,thereby affording increased protection against falling glass.If broken by impact,the interlayer will resist penetration of the impacting object.
This glass is produced by subjecting ordinary annealed glass to a process of heating and rapid cooling which produces high compression in the surface and compensating tension in the centre of the glass.Toughened glass for this reason cannot be cut and should not be drilled,surface or edge worked.The treatment endows the glass with greatly increased resistance to impact, loading and thermal shock.Toughened glass is 4 – 5 times the strength of ordinary annealed glass of the same dimension and thickness.
Laminated toughened glass
This is glass that has already been pre-stressed (by heat and controlled cooling,or chemical treatment)which is then laminated together using PVB or resin liquid
Plastic glazing materials
There are two types of plastics commonly used in skylighting, namely acrylic and polycarbonate (perspex).The support framing system must be properly designed to accommodate different thermal movement and glazing flexibility under load as opposed to that required for glass.The designer should consider that there are also differences in stiffness between glass and plastics,and should be certain the proper thickness is used.Reference should be made to appropriate sheet thickness tables and graphs published by manufacturers once the design load has been determined.
Extruded aluminium members,where used,should be of the best commercial quality and proper alloy and free from defects impairing strength and/or durability.Commercial alloy designation 6063 in extruded form is one of several alloys,which will meet the above requirements. Aluminium used for sill,flashing,or similar,shall be formed from flat sheet.Commercial alloy designation 3103 is one of several alloys, which will meet the above requirements. If anodised exposed surfaces shall comply with SABS 999 and be of 25-micron thickness for coastal areas and a minimum of 15 micros for inland areas. If powder coated,shall comply with SABS 1274 Wood members,where used,should be of a suitable strength material which has a moisture content between 6%and 12%at the time of fabrication.
All exterior wood parts shall be treated with a suitable water- repellent finish Steel members,where used,shall conform to either:SABS 727, SABS 1200II,or SABS 1200 IIA that will meet the requirements of SABS 0162 All exterior steelwork shall be coated with a suitably corrosion resistant treatment.
A suitable corrosion resistant,malleable sheet material shall be used for flashings,saddles and drainage channels at abutments, junctions and valleys. Note:the following materials are considered suitable:copper,zinc, alloy,lead,aluminium alloy,galvanised steel,fibreglass,or bitumen based material.
All fastening devices incorporated in the fabricated skylight,or used in the installation process shall be of sufficient strength and quality to perform their intended function and shall be compatible with any adjoining materials.
Hardware shall be of aluminium,stainless steel or other corrosion resistant materials,and be compatible with any adjoining materials
Weather-stripping and gaskets shall be manufactured from any suitable material compatible with adjoining materials,shall withstand atmospheric conditions and mechanically function under use.
Sealants used in the fabrication,glazing or installation of the skylight shall be compatible with other materials used in the construction of the skylight and shall have the properties necessary to perform the function for which it is intended.
Installations &Glazing Procedures
The skylight shall be securely anchored,sealed and undamaged, ensuring that it will meet with any performance test necessary. Glazing shall be carried out in accordance with SABS 0137.
Appointed Skylight Contractor
The appointed skylight contractor shall: Provide drawings showing design and installation details for approval by the specifier
Provide a test performance certificate.