Large timbers, because of their greater capacity to absorb heat, are  much slower to catch Þ re and burn  than smaller pieces of wood. When  exposed to fire, a heavy timber beam,  though deeply charred by gradual  burning, will continue to support its load long after an unprotected steel  beam exposed to the same conditions has collapsed. If the fire is not  prolonged, a fire-damaged  heavy  timber beam or column can often be  sandblasted afterward to remove the  surface char and continue in service.

For these reasons, building codes  recognize heavy timber  framing that meets certain specific  requirements as having fire-resistive properties.

In the International Building  Code (IBC), for a building to be classified as Type IV Heavy Timber (HT)  construction, its wooden structural  members must meet certain mini- mum size requirements and its exterior walls must be constructed of  noncombustible materials. Minimum  permitted sizes for solid wood timbers  are summarized in Figure 4.7. Glue- laminated members used in this type of construction must meet similar  requirements. Exterior walls may be  constructed of concrete, masonry, or  metal cladding. Historically, this combination of fire-resistive wood framing  and noncombustible exterior was referred to as Mill construction, reflecting  its origins in 19th-century brick ma- sonry mill structures, or as Slow-burning construction (Figures 4.8-4.11).

Figure 4.7 Minimum sizes for solid wood members used in Type IV Heavy Timber construction,
as specifi ed in the IBC.

Traditionally, the edges of the timbers  were also chamfered (beveled at 45 degrees) to eliminate the thin edges of  wood that catch fire most easily, but  this is no longer a code requirement.

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