Considering the performance of renovated heavy timber structures

Nearly two centuries ago, the boom of America’s Industrial Revolution necessitated the genesis of a concurrent revolution in industrial architecture. The exceptional needs of growing American industry required building gargantuan structures capable of housing the operations which outfitted a bustling nation.

Gone were the days in which the traditions of vernacular architecture could satiate the near limitless appetite of the United States’ proliferating manufacturing base. Dawning was an era in which monolithic and function-based monuments were erected in the name of commerce. The era of “Heavy Timber” construction.

For those unfamiliar with the term “heavy timber construction”, a decidedly American designation, it’s perhaps easiest to conjure an image of the near ancient post-and-beam construction. An exterior composed of load-bearing masonry combined with internal load-bearing elements of thick solid-sawn timber. Think of a generally box-like two-story structure that is perhaps a bay or two wide, replete with an incredible amount of pronounced windows.

Now imagine that post-and-beam archetype has effectively been placed on a high dose of structural steroids, and you’ll be in possession of an accurate—yet somewhat simplistic—mental image of heavy timber construction. Its “prime” examples tipping the scales at well over a quarter-of-a-million square feet, standing perhaps in-excess of six stories, and usually possessing widths of at least three bays with lengths almost exponentially longer.

Indeed, while the concept of what heavy timber has been drawn, it’s critical to denote just what exactly the term means in today’s fire service world. That the label of “heavy timber construction” isn’t some sort of loose confederation of fortress-like industrial buildings utilizing copious amounts of brick and tree-sized timbers.

No, it’s a defined International Building Code class, a mandated classification requiring its adherents to feature exterior walls composed of noncombustible materials and its interior structural elements to be made of solid sawn (or laminated) wood devoid of concealed spaces. So let’s put the term heavy timber into better perspective.

Any building determined to carry a pure heavy timber status, or “Class IV” in the American fire service, is mandated to possess: columns not less than 8×8 inches (6×8 inches if only supporting a roof assembly), beams no smaller than 6×10 inches (8×8 inches for trusses supporting floor loads), floors measuring three inches thick with at least one inch of tongue-and-groove overlaid flooring, and partitions of solid wood formed by no less than two inches of matched boards measuring one-inch each.

Yet while the governing standards remain perfectly intact, the remaining heavy timber buildings that escaped the wrecker’s ball during the prolonged decimation of American manufacturing sit vacant and horribly neglected. No longer do they house textile mills, tobacco factories, or warehouses. Thus, the stage has been amply set for these decaying megaliths to live again.

Presenting themselves as a near perfect remedy for a younger and resourceful generation desirous of enhanced urban renewal, these bastions of a bygone era are rising from the proverbial ashes. Yet they’re not recapturing their former industrial glory, but rather, finding new purpose altogether. Today, you’re more likely to stumble upon a heavy timber building whose abundant space is occupied by residential apartments and the odd small business, rather than flour mills.

Cities—and the once endangered buildings themselves—are clearly benefiting from heavy timber’s return to an in-vogue status. However, the physical transformation responsible for the type’s resurgence is not merely rooted in a rekindled public appreciation for an icon of America’s built environment, nor is it purely fueled by nostalgia. No, heavy timber’s rebirth in industrial cities across the nation is based firmly in the type’s hallmarks, overt functionality and utility. But that repurposed utility comes at a steep structurally alterative price.

Let us keep firmly in mind the reality that many of the core structural characteristics expressed by residential occupancies are entirely anathema to the purpose-driven design of heavy timber construction. In recognizing the ever-present danger posed by fire in large buildings, the form is geared to: limit possible ignition points, contain fire spread to a lateral plane, inhibit the vertical travel of fire and its byproducts.

In accomplishing these critical objectives, heavy timber construction solidified its well-earned reputation for being, at the time, the world’s most survivable building type under fire conditions. However, and like in all buildings, outcomes capable of warding-off disaster required that any fire was contained to the floor of origin. That the unquestionable robustness of a given heavy timber building’s structural elements could withstand the damaging effects of fire long enough for successful extinguishment.

However, and as mentioned previously, a heavy timber building’s ability to avert catastrophe required the dutiful implementation of its core containment-oriented tenets, which in turn necessitated the faithful preservation of its fundamental physical traits. For example, the timbers themselves are to be left unaltered (much less replaced by metal members) so that if exposed to fire, a protective char layer can develop (at around 550 degrees Fahrenheit), which then insulates the interior of the timber and provides prolonged durability (capable of withstanding hours of exposure to temperatures below 212 degrees Fahrenheit). Additionally, penetrations in floor assemblies are to be entirely non-existent in order to prevent vertical fire spread.

To be sure, heavy timber always recognized that its Achilles heel rested with the durability of the metal connectors that tied its load-bearing elements together. And if fire were allowed to freely develop a purchase point, exploit the existence of plenums, or extend vertically and involve multiple floors, that the sheer size of the fire would likely require hours to completely suppress. And in that span of time, the mechanical integrity of essential structural elements would be compromised, and catastrophic failure would become a foreseeable eventuality. And with that in mind, every practical measure was taken to defend the type’s inherent vulnerabilities.

But the structural traits which made heavy timber a sentinel of stability under fire conditions aren’t necessarily repurposing-friendly, much less conducive to creating a hospitable living environment. As such, the thriving heavy timber buildings scattered across the country may well be—at least functionally—heavy timber in spirit only, veritable hybrids. And this mutated structural nature undoubtedly generates an intense need for first responders and fire prevention specialists to be cognizant of all potential deviations, as they could translate into a dangerous decrease in behavioral predictability under fire conditions.

So let’s look at a prominent example to see just what fire personnel may well encounter both during inspections, and possibly, during emergency operations. Take the Cupples Station Lofts in downtown St. Louis, Missouri. While it’s an attractively-renovated structure and the epitome of a stunning stylistic success, what ultimately makes the newly-minted Cupples Station Lofts building well-suited to house scores of residents could also drastically impact its performance, perhaps even making it a liability should a serious fire event occur.

The Cupples Station Lofts building added partitions, created plenums through the implementation of drop-ceilings, created numerous penetrations by HVAC and other utilities, installed exposed metal load-bearing structural members (which appear to be unprotected), placed heavy A/C units atop the roof, and even varnished the timbers themselves. Not only that, the decking on every floor within the building’s core was ostensibly removed, creating a centrally-located open-air atrium that rises from ground floor to roof.

Very clearly, all of the aforementioned alterations conformed to the codes governing residential occupancies. However, I’d argue that the prior laundry list of cosmetic and utilitarian changes made the building’s performance under fire conditions wholly dependent upon the successful operation of its onboard detection and suppression systems. For if those systems fail, become inoperable for any reason, are impaired due to issues like water supply faltering, or simply don’t perform adequately enough to counter a given fire, the consequences could be dire.

Take, for example, a New York City Valentine Day’s fire in 1958. FDNY personnel and New York Fire Patrol staff responded to an alarm of a fire at 137-9 Wooster Street (within a Lower Manhattan area commonly dubbed “Hell’s Hundred Acres”), with the former quickly launching its trademark aggressive attack. What was supposed to be a fairly standard response to an ordinarily-rugged heavy timber building, quickly turned from routine and toward tragedy. Firefighters quickly realized that outside of its floors being overloaded, that the fire had traveled up a shaft and involved all floors. Then, without warning, the cast iron columns added to the lower floors failed and the structure suddenly collapsed, killing six people.

Now, none of this is to say that renovated heavy timber buildings are inherently unsafe, merely that when the traits which made heavy timber a champion of durability in the face of fire are either impaired or absent, that the building may well behave differently, perhaps with greater volatility, shorter burn times, and dramatic unpredictability. And given that their immense mass and weight largely remains intact, should a heavy timber building fail, a catastrophic collapse is almost the assured result (either floor and roof assemblies cascading downward and/or masonry walls falling outward).

As such, all fire staff should carefully study any and all alterations made to these structures, and preplan accordingly. And most importantly of all, use copious amounts of caution should fire erupt within a renovated heavy timber building.

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