Ample evidence shows farm buildings sustain wind damage more often than every other type of structure.
So, as weather events become more frequent and intense, it’s truly surprising that a substantial portion of farm buildings are still regulated — if at all — by relaxed building code requirements. That makes it truly important for insurers to recognize and manage farm buildings’ inherent vulnerability to wind damage.
Building codes define farm buildings as structures located on land devoted to farming that do not contain a residential occupancy. If a farm building is occupied by no more than one person per 40 square metres of floor area, it is classed as a low human occupancy (LHO) farm building.
Building codes consider LHO buildings to be of low importance, allowing these structures to be designed for 80% of the wind loads for which other buildings must be designed. The rationale for this classification is that lower human occupancy reduces risk.
Due to LHO buildings’ distinct use and occupancy, a national code was developed for farm buildings. The most recent update, the National Farm Building Code of Canada (NFBC), was published in 1995.
Most provincial building codes allow construction of farm buildings to be regulated by the 1995 NFBC, although it was not adopted by all jurisdictions, most notably the prairie provinces. In places where the 1995 NFBC is not adopted, no specific structural design requirements apply to LHO farm buildings. That essentially leaves ensuring adequate resistance to wind, and other loads, up to the builders.
Until 2005, buildings were designed for the maximum hourly wind speed expected at the building’s location within a 30-year period (known as a one-in-30-year wind speed). Since 2005, the design wind speed has been increased to the maximum hourly wind speed expected at the building’s location within a 50-year period (known as a one-in-50-year wind speed). But the 1995 NFBC permits farm buildings to be designed for the maximum wind speed expected in a 10-year period (known as a one-in-10-year wind speed), which is approximately 80% of the one-in-50-year wind speed.
The compounded effect of the low human occupancy factor, which allows farm structures to be designed for 80% of other buildings’ wind loads, plus the one-in-10-year wind design, means that when LHO farm buildings are designed to code, they can only resist two-thirds of the wind load used to design any other building.
Before the 2020 National Building Code of Canada (NBC), the size of LHO farm buildings did not factor into structural wind load requirements. The 2020 NBC introduced requirements for large farm buildings, defined as buildings more than 600 square metres in area, or more than three storeys in height.
As a result of the 2020 NBC changes, large LHO farm buildings are now designed to resist the one-in-50-year wind like any other building. But LHO farm buildings at or below 600 square metres, and three or fewer storeys high, are still covered by the 1995 NFBC, which permits design for a one-in-10-year wind.
Farm buildings are more susceptible to wind because of their long, narrow layouts that result in extreme length-to-width ratios. They’re further plagued by their open floor plans, large door openings and lightweight materials.
These construction characteristics are dictated by functionality and affordability. Compared to any other type of building, farm buildings are much weaker at resisting wind that pressures their long sides. What’s more, most farm buildings don’t include a robust wind resisting system, which leaves them highly vulnerable to moderate wind and at risk of collapse under strong wind.
In recent years, our firm has investigated multiple cases of repeat wind damage to LHO farm buildings. These cases clearly demonstrate that restoring damaged farm buildings to their original conditions will only result in a recurrence of damage. Considering the devastating impacts of climate change and the growing need to build more resilient communities, it won’t be surprising if the 2030 edition of the NBC adopts the design for a one-in-100-year wind speed.
Insurers should be wary of damage claims from LHO farm buildings reported to have been caused by strong wind. Inadequate wind-resistant framing, non-engineered field modifications, harsh service environments, and lack of maintenance can all result in building failure. In turn, that could lead to catastrophic losses for LHO farm buildings under moderate wind that properly constructed structures would not suffer.
Pre-coverage structural assessments of LHO farm buildings and inspections of other farm buildings on sites where LHO structures have sustained damage can help underwriters better assess the elevated risk of these farm buildings and save insurers considerable sums of money.
Dr. Yasser Korany is a consulting forensic engineer and managing principal of KSI Engineering. This story is excerpted from the December 2024-January 2025 print edition of Canadian Underwriter. Feature image by iStock/ftwitty
