SUPERIOR FIRE RESISTANCE

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Buildings constructed with large structural timbers have excellent fire-resistive qualitites. U.S. model building codes recognize these qualities and provide guidelines for fire-resistive, heavy timber construction approaches. Two distinct approaches are included in the U.S. model building codes: Heavy Timber Construction and Fire-Resistive Construction.

Heavy timber construction
In Heavy Timber Construction, limitations are placed on the minimum size, including depth and thickness, of all load-carrying wood members. The design avoids concealed spaces under floors and roofs, and regulations require the use of approved fastenings and construction details. When properly sized, glulam systems meet the criteria set forth in U.S. model building codes for Heavy Timber Construction.

The performance of heavy timber structures under fire conditions is markedly superior to most unprotected “non-combustible” construction. Fire fighting is easier and safer due to the elimination of concealed spaces and the inherent structural integrity of large laminated timbers.

FIRE-RESISTIVE CONSTRUCTION
Fire resistance is the length of time a structural member can support its load before collapsing. The goal of fire-resistive construction is to provide adequate time for occupants to evacuate a building safely.

The results of ASTM fire tests for building assemblies, sponsored jointly by the American Forest & Paper Association and the American Institute of Timber Construction (AITC), enable designers to calculate specific fire ratings for glulam members. Calculations are based on a consideration of member size, degree of fire exposure, and loads on the member.

Additionally, fire-resistance ratings for glulam beams require lay-up modifications. For example, a simple-span glulam beam, having a cross section of 170mm x 340mm and exposed to fire on three sides, can be sized for a one-hour fire rating. To qualify for this rating, an inner or core lamination is removed from the beam at the time of manufacture and replaced by adding an additional tension lamination to the bottom of the beam (as shown in the figure above right).

Typical glulam beam enveloped in flames during standard ASTM E-119 fire test.

Typical glulam beam following fire testing: the outer surface of the beam has charred while the inner areas remain unburned. The charred outer material acts as an insulator during fire, reducing the rate at which the inner material will burn.

For additional information related to calculating a fire rating for heavy timber members, please refer to AITC Technical Note No. .7 Calculation of Fire Resistance of Glued Laminated Timbers (www.aitc-glulam.org) and to AF&PA Technical Report 10 Calculating the Fire Resistance of Exposed Wood Members (www.awc.org). These publications are available in English.

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