Which construction type is most accurately described as heavy timber with high heat tolerance but heavier collapse risk?

• A. Type V Wood Frame: lightweight and prone to rapid failure under fire.
• B. Type II Noncombustible: good fire resistance but can fail in heat.
• C. Type III Ordinary: exterior masonry with interior wood.
• D. Type IV Heavy Timber: high heat tolerance but heavier collapse risk.

Answer: (D)

Heavy timber construction uses large, solid wood members whose size lets a protective char form on the surface, insulating the interior and slowing heat penetration. That char layer preserves enough strength for a longer period, so the structure can resist fire better than lightweight wood frames. At the same time, the enormous mass of these timbers means they carry a lot of weight, so when they finally fail, the collapse can be more substantial and heavier than in lighter constructions. This combination—strong fire resistance thanks to the timber’s mass and charring, paired with the potential for a more dramatic failure due to the heavy members—is what defines heavy timber construction. The other types either burn through more quickly (lightweight wood frame), rely on noncombustible materials with different failure modes, or mix exterior masonry with interior wood without the same heavy-timber characteristics.