GLULAM – PRIMARY CHOICE STRUCTURAL MATERIAL
Designers are influenced by a number of factors when choosing building materials, including cost, suitability and the design knowledge of those materials. Issues such as renewability of resources, energy efficiency, and environmental friendliness are also becoming increasingly important and for environmentally conscious designers, glulam is a primary choice structural material.
Furthermore, there are other advantages in using timber and glulam as a primary material.
Timber is non-corrosive material and therefore should be considered a primary choice structural material for buildings either located in a corrosive environment such as along a coastline, or enclosing a corrosive environment, such as fertiliser and chemical stores, swimming pool enclosures and reservoirs.
Timber does not require corrosive treatment in these environments, however, in environments in which there is high humidity and/or condensation, such as swimming pool enclosures and reservoirs, moisture exclusion surface coatings are needed.
In a fire, glulam has an inherent fire rating. As timber burns, a layer of charcoal forms enclosing a core of timber, which is yet unaffected. The timber core maintains its structural capacity. Hence, dependent upon the loss of material to the charcoal layer, glulam can remain structurally adequate to carry the dead load of the structure for a period of time. The thickness of the charcoal layer is proportional to the length of time of the fire. At some point, the remaining structurally sound timber core will
be unable to restrain the applied loads and the glulam member will collapse. The time required for the fire to burn enough timber thickness to reach the collapse point is deemed the fire rating of the glulam member.
In recognition of this property of timber, design standards have been produced for designers to assess member sizes required for the desired fire rating.
In general, it is significantly cheaper to increase the dimensions of glulam slightly in order to provide the required fire rating, than to enclose the member in fire resistant material. This property should make glulam the primary choice material in buildings requiring structural members to have a fire rating.
Glulam is easy to work with and this makes it the preferred material in many of the construction trades. The workability of glulam means that builders can erect structural framing without a specialist rigging trade and small builders are advantaged by this.
Roof fixers are able to nail the insulation mesh and screw the roofing more easily to glulam purlins. Furthermore, the glulam purlins are not slippery. Carpenters can easily nail wall framing to glulam columns and heads, and nail ceiling framing to glulam rafters. Plasterers can easily screw battens to glulam purlins and girts. Electricians can easily nail conduits, and fix lighting and power outlets to glulam. Plumbers can easily drill access holes and screw pipework and fixtures to glulam, particularly fire hose reels and extinguishers, to glulam columns.
As a finished, in-place structural material, glulam is cost competitive with structural steel. There are numerous examples of buildings constructed in Australia over the past decade that have been constructed in glulam rather than steel because it was the cheaper option.
A wide range of factors influence the final cost of the structural material portion of a project, including size and complexity of the building, building environment, economy, location, client contacts, supplier margins, etc. All these factors can influence the cost comparison between steel and glulam by more than 10%. Considering that the structural content of a building is approximately 15% of the total building cost, the cost comparison between steel and glulam represents only 1% to 2% of the total building cost.
It is universally accepted that an exposed glulam structure adds warmth and beauty to a building. Glulam is commonly used as the finished product, whereas other structural building materials are generally enclosed or hidden in an aesthetic application.
In structures where aesthetics are important, glulam should be the primary choice material. Community buildings, churches, sporting complexes, tourist resorts and retail outlets are all examples of buildings which house people. In terms of aesthetic appeal, glulam has a distinct advantage over other structural material choices.
In structures where aesthetics are not a material choice criterion (such as commercial warehouses), glulam can provide aesthetic appeal as an added benefit. Once they have been made aware of the advantages of glulam, many developers have converted to glulam for aesthetic reasons.
The main reason that there has been relatively little use of glulam in Australia is that designers have had very little exposure to structural timber at undergraduate level. Predominantly, they are taught about steel and concrete. Furthermore, there has been limited availability of design aids.
Recently, architectural and engineering undergraduate students have had increased exposure to timber and some universities have introduced compulsory or elective courses in timber engineering.
Design aids, in the form of load tables and computer design packages, are now readily available for the use of glulam in large span commercial structures, as well as domestic structures.