7 Heavy

Timber and Mill Construction

Objectives (1 of 2)

• Identify and describe the similarities and differences between mill and heavy timber (Type IV construction)

• Identify the dimensions of heavy timber members

• Describe why the misnomer “slow burning” is inappropriate for heavy timber buildings

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Objectives (2 of 2)

• Identify the hazards of conflagration breeders

• Describe the hazards of vacant heavy timber buildings and structures under demolition

• Identify new structures that incorporate heavy timber construction

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A History of Mill and Heavy Timber Construction

• Heavy timber construction descendent of true mill construction

• Developed in New England

• Building design that eliminated serious faults of mill construction

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Mill Features (1 of 3)

• Exterior bearing and nonbearing walls are masonry

• Columns and beams are of heavy timber with cast iron connectors

• Floors are of thick grooved, splined, or laminated planks

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Mill Features (2 of 3)

• Roofs supported by beams or timber arches and trusses

• Adequate fire barriers

• Ends of girders are fire cut

• Cast iron box

• Corbelled beams

• Scuppers

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Mill Features (3 of 3)

• Concealed spaces are eliminated

• Vertical openings are protected

• Automatic sprinkler system

• Special hazards put in detached buildings

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Conversion, Modification, and Preservation (1 of 2)

• Remodeling

• Often detracts from the original fire characteristics

• Columns may have been removed

• New trusses do not have the inherent resistance of the old columns and beams

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Conversion, Modification, and Preservation (2 of 2)

• Changes in Use

• Conversions to tenant factories, storage, discount stores, and apartments

• Fire loads are often beyond capacity of the installed sprinklers

• Unsprinklered areas are created

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Heavy Timber Building

• Requires careful maintenance

• Deficiencies should be clearly noted

• Realistic prefire scenarios are required

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Code-Classified Heavy Timber Construction

• Type IV construction

• Often lacks at least one feature vital for fire safety

– Unprotected steel columns, part-steel trusses, unsprinklered void spaces, or no sprinkler protection

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Heavy Timber Construction Contractors

• Be wary of those who claim to be erecting a heavy timber structure

• Many do not know the actual dimensions required

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Heavy Timber Theory

• Heavy timbers are difficult to ignite

• Will char on the surface only

• Over the years the floors may have been soaked with flammable and combustible fluids

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Slow Burning?

• Proponents of heavy timber construction

• Believe “slow burning” means “collapse resistance”

• Statement true only if fire department can maintain interior offensive operations

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Definition of Conflagration Breeder

• A structure that presents severe exposure problems that are capable of initiating a large, multiple building fire which is not easily contained

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Characteristics of a Conflagration Breeder 

• Radiant heat and fire brands

• Large collapse zones

• Fire spread

• Shut-off sprinkler systems, open equipment shafts, and uncompartmented areas

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Fighting a Conflagration Breeder

• Expect the fight of your life

• You need:

• Large water supply

• Numerous heavy caliber streams

• Large collapse zone

• Extensive brand patrol downwind

• Luck

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Use of Unprotected Steel

• Unprotected steel can fail early and trap fire fighters

• Alteration permits give clues to real composition

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Beams

• Penetrated for conduits or pipes

• Long bolts and nuts are used

• Metal can provide a path for heat to reach the interior

• Watch for spliced timbers with overlapping joints and metal connectors

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Full Sprinkler Protection

• Needs to be adequately maintained

• Only fire protection measure to prevent a disaster in a heavy timber building

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Rehabilitation and Demolition Hazard

• Fire sprinkler systems

– Must be adequately maintained

– Expense may be seen as road block to construction project

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Problems with Rehabilitation

• Abandoned buildings

• Conversions to multiple occupancy

• Rehabilitations into apartment or office occupancies

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Vacant Buildings• Cities with recent massive downtown fires

– Baltimore

– Philadelphia

– Lynn (Massachusetts)

– Minneapolis

– Indianapolis

– Montreal

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Characteristics of Massive Downtown Fires

• Old combustible interior masonry buildings

• Fire barriers removed or inoperable

• Sprinkler systems removed or disabled

• Only fuel was the interior structure of the building

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Vacant Structures

• Sprinklers are often turned off

• Open invitation to arson

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Modern Heavy Timber Buildings

• Not being built in record numbers

• A niche for certain types of occupancies

• Most common use is the modern heavy timber church

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Summary• Heavy timber construction is a descendent

of true mill construction

• Remodeling has detracted from original fire characteristics

• Proponents often advance the term slow burning

• Full sprinkler protection is the only adequate fire protection measure

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