Heavy Timber 101, Part 1: History and Design

Heavy Timber Construction (HTC), or Type IV Construction, is a term that undoubtedly every firefighter in the United States recognizes, although it might very well be largely unknown globally. Almost immediately, images of massive industrial and commercial occupancies, complete with solid-sawn timber structural frames and incredibly thick masonry walls, spring to mind. Certainly we have all learned about this historic construction type in the academy, oftentimes they are presented in the form of worst case scenarios, or veritable doomsday fire events certain to lead to line of duty deaths.

However, that is an over-simplification which highlights the varying levels of inherent familiarity with heavy timber of each individual firefighter. We often say in the fire service, that “we need to understand the enemy as well as the battlefield”. We know that fire is the enemy, and spend hours engaging in worthwhile training such as fire behavior, but comparatively little time studying the battlefield, which is building construction. While a fire in a heavy timber building certainly can be a horrific event, it doesn’t have to end in tragedy. Yet, avoiding such outcomes requires firefighters to develop a deep understanding of the iconic construction type, an understanding that spans from its history to its salient design features.

The standard industrial building design at the time of heavy timber construction’s inception made great use of small dimension lumber, a plentiful resource in New England through the late-eighteenth and early-nineteenth century. However, the exceptional fire loads present, and somewhat crude and hazardous working conditions and methodologies during the American Industrial Revolution, led to numerous large-scale fires that all but brought New England’s manufacturing centers to their knees. So commonplace and extensive were the fires during the period that insurance companies outright refused to sell insurance to factories and mills.

The story of heavy timber construction finds its official genesis in 1835 Rhode Island. It was here that a coalition of New England mill owners held a summit that sought to create a construction standard that would reduce fire-related loss. This association later became known as the Associated Factory Mutual Fire Insurance Companies, which today is better known as FM Global. This landmark convention created a distinct evolution from traditional post and beam construction. The result was the emergence of America’s first concerted effort to standardize a building system expressly designed to be fire-resistant.

Within decades of its genesis in New England, notably Massachusetts’ manufacturing epicenters of Brockton, Woonsocket, and Lawrence, and of course Providence, Rhode Island, the building type quickly sprouted up in the industrial outliers of New York City, Philadelphia, and Baltimore. Shortly thereafter, it had spread rapidly into the South and arguably its greatest bastion, the Midwest. Heavy timber finally reached the West Coast at around the turn of the twentieth century, before being firmly supplanted by modern building technologies such as reinforced concrete and steel in the first decade of the twentieth century. However, it is interesting to note that this venerable construction type persisted into the first decade of the twentieth century in most of the country, but new structures were still being erected well into the 1920s on the West Coast, where the type was heralded for its ability to withstand the stresses of earthquakes.

The original early-nineteenth century tenets of this standard were quite revolutionary. The design standards called for: large timbers of minimum nominal dimensions composing the structural frame with exceptionally heavy load-bearing walls constructed out of masonry, the elimination of sharp or otherwise protruding edges that could facilitated flame impingement, the eradication of void spaces or plenums through which fire could both hide and travel, the prohibition of unprotected penetrations of floor assemblies, and the general embrace of large open interior spans which can be in excess of 30,000 square feet. All of the aforementioned design features were instituted to not only control and contain a fire, but to ensure that fires were kept out in the open where they could be effectively combatted.

Today, the standards that regulate heavy timber construction are governed by the International Building Code (IBC), and conclusively demonstrate the utterly robust nature of heavy timber construction. For example, wood columns and trusses supporting floor loads are required to be at least 8-inches by 8-inches, beams and girders supporting floor loads are to be at least 6-inches by 10-inches, and floor decking is to be at least four total inches thick. In real world applications, particularly of traditional heavy timber construction buildings, the timber structural frame often exceeds the minimum required dimensions by a fair margin.

In terms of required performance under fire conditions, only a portion of a heavy timber building’s constituent components are specifically rated. Essentially, exterior and fire walls, along with protections for vertical openings, are required to possess a two-hour fire rating. Interior bearing walls are only required to possess a one-hour fire rating. When scrolling through the fire codes concerning heavy timber construction, one notices that the heavy timber structural members themselves do not have a specific rating. In fact, the premier fire performance testing organization in the world, the Underwriters Laboratories, ruled that such large timbers were inherently fire-resistant.

This conclusion itself highlights why heavy timber came into being in the first place, that through centuries of observation and post-fire analysis, it was noticed that timbers of large dimensions charred on the surface when exposed to flames. It was then hypothesized and later scientifically proven that the char layer insulates the interior of the timber member, thereby protecting it from the damaging effects of fire. Extensive testing in the twentieth and twenty-first centuries found that this char layer develops between 480 and 550 degrees Fahrenheit, and that such a timber loses about ¾ of an inch to charring per half-hour.

While detailed scientific experimentation and analysis can provide us with great insight into the performance of a given material, such a thought process seems to have been absent in heavy timber construction. They were designed and built in a construction era in which local resources were utilized to their fullest potential, and arguably no other building type embodied such resourcefulness. Heavy timber buildings, while constructed along common standards, made varied use of construction materials. Their masonry walls, while predominantly composed of brick, often made use of stone in areas where quarries were prevalent. In terms of the timbers themselves, it largely depended on what could be harvested locally, so timbers ranged from firs to oak. While these deviations are generally quite subtle, it is impressive that despite the particular material used, historic heavy timber buildings appear to have behaved almost identically under the stresses of fire.

What makes heavy timber construction so unusual is that it was a design conceived not by architects and structural engineers, but my business men and craftsmen. Precision engineering, fire load calculations, and ornament, simply appear to never have factored in. Heavy timber was constructed to be utilitarian, over-built to the point of structural redundancy. They were built in accordance with basic fire-resistance principles. The goal was to construct a building robust enough to withstand fire and keep it confined to a large, open, and accessible space. It was ideal if the fire could be caught in the incipient stage and quickly extinguished, if not, the stout buildings were to allow for the preservation of life through the rapid escape of occupants, with the hope that even after a large conflagration, enough of the structure would survive to allow for quick rebuilding.

These unique structures literally and figuratively carried the weight of the United States’ rise to economic might, and served as the physical manifestation of American industry. They served their purpose exceptionally well, resisting fire better than any previous American building design, successfully carving out their place in history. For firefighters, they became an integral part of American fire service culture, and a frequently trodden and well-understood battlefield. However, for the modern firefighter, they’ve become almost mythical if not entirely intimidating. They’re often viewed now as monuments to possible tragedy, as potential graveyards, rather than familiar terrain which firefighters can effectively traverse. While these buildings may have faded into obsolescence, the modern firefighter cannot allow his or her knowledge of the vaunted building type to do the same.

In the second and final part of this article we will explore how the performance history of heavy timber indicates that they are an exceptionally fire-resistant and stable construction type.