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Why Farm Buildings are Susceptible to Wind Damage

November 1, 2021 | By: Amir Jamshidi and Mohtady Sherif

Compared to all other types of structures, our years of experience provide ample evidence that farm buildings sustain damage from severe weather and wind events far more frequently. Farm buildings are often constructed with lightweight materials and large open floor plans making adequate resistance against wind critical for safety and stability. Robust resistance to horizontal loads is needed to safely transfer wind loads to the foundation, limit damage during moderate wind events, and prevent collapse in extreme wind events. This article discusses why farm buildings seem to be more susceptible to wind damage than other structures using two cases of farm buildings that sustained damage under wind pressures within the design range.

Farm Buildings and Building Codes

Farm buildings are a specific category of buildings that are usually used for storage or to house animals on agricultural properties and are only infrequently occupied by people. In other words, farm buildings have low human occupancy load, defined as no more than one person per 40 m2. Farm Building Codes were developed to set relaxed design requirements compared to the National Building Code of Canada (NBCC), on the basis that there is a lower risk to human life for buildings that are usually unoccupied. Low human occupancy is the main justification for having special requirements for farm buildings, as distinct from other types of buildings. Farm buildings that do not qualify as having low human occupancy, and dwelling units located on a farm, are required to conform to all the requirements of the NBCC.

The first edition of the Canadian Farm Building Code was published in 1977 and contained a considerable amount of information that was outside the traditional scope of building code requirements such as structural adequacy and fire safety.  The Canadian Farm Building Code published in 1983 was a complete revamp of the code and was developed in a format that permitted its adoption by an authority having jurisdiction. The edition published in 1995 is the most updated version that remains in effect to date and was renamed as the National Farm Building Code of Canada (NFBC).  Most provincial building codes allow the construction of farm buildings to be regulated by the 1995 NFBC. However, there are still some jurisdictions that have not adopted the NFBC, which means that there are no building code requirements applicable to farm buildings in these jurisdictions.

The Effect of Wind on Farm Buildings

The magnitude of wind pressure on buildings depends on factors such as air density, the shape of the building, and the wind velocity at the elevation of the building. For design purposes, wind pressure is estimated based on a reference velocity pressure and is then modified to account for site-specific factors such as topography, surrounding terrain, the height of the building, and whether there are large openings in the walls. Farm buildings often have large openings, so the internal pressure effect of wind tends to be stronger in farm buildings than other buildings. Farm buildings are also frequently located in areas with open terrain, so they do not benefit from the shielding effect that comes from being surrounded by trees or other buildings. The NFBC requires farm buildings to be designed for the estimated 1-in-10 year maximum wind gust[1] expected at the building’s location.  The 1-in-10 year wind pressure for many Canadian cities and towns is given in the Climatic data tables of the national and provincial building codes.

For buildings to sustain wind, the walls and roof of a building must be properly fastened so they do not get blown out during windstorms. The self-weight of a roof partially counteracts the wind uplift/suction effect. However, farm buildings are often constructed with lightweight materials, so this countering effect is minimal, making proper fastening of the roof framing that much more important to prevent the roof from being blown off during a windstorm.  The walls must also resist the lateral force of the wind, or the building could be pushed over by wind and collapse. Systems such as shear walls, moment frames, and braced frames are commonly used to provide the required lateral resistance.

Case Study 1: A Large Enclosed Farm Building Keeps Lifting off the Ground

A 50-year old and 96′ long by 48′ wide farm building located in Saskatchewan sustained damage three times in a five-year period due to wind uplift (Photograph 1). The damage included uplift of some of the supporting wood posts on one side and bowing of the cladding on the opposite side of the building.

Photograph 1: General view of the enclosed farm building

The building had a gable roof, with the ridge oriented along the north-south direction, that was framed with metal plate connected wood trusses spanning in the east-west direction (Photograph 2). The trusses bore on beams at each end that were in turn supported by 6×6 wood posts spaced 8′ apart embedded into the ground with no footings.  The farm building was equipped with a 24′ wide sliding double door on the north wall (Photograph 3). Two diagonal braces on the north wall on the sides of the sliding door and one brace on the west end of the south wall were observed. No diagonal braces were observed in either the east or west walls.

Photograph 2: East wall from inside the farm building

Photograph 3: 24’ wide sliding door on the north wall

Uplift and bowing of the roof and walls were observed at the northeast corner of the farm building. Relative to the surrounding ground, the wood posts along the north end of the east wall experienced an estimated 4″ to 5″ of uplift. The uplift of the north wall of the building had been a recurring issue for five 5 years. As a measure intended to rectify the uplift problem, the soil around the timber posts had been recently replaced with 3/4″ to 1″ gravel.

Data from a weather station 30 km away from the farm building revealed that the gust wind speed on the date of loss was 89 km/hr, marginally exceeding the 1-in-10 years design wind speed based on the 1965 Farm Building Standards, Supplement No.6 to the National Building Code of Canada, the standards in effect at the time. This meant that the cause of the uplift and damage was related to the construction of the building, and not to an exceedingly strong wind. It is of note that the Province of Saskatchewan exempts farm buildings from the requirements of the National Farm Buildings Code.

Our analysis of the farm building showed that the sheet metal cladding on the walls did not provide adequate resistance to wind loads. The north and south end walls were the only members providing resistance to wind blowing in the east-west direction. The 24′ wide sliding door opening reduced the stiffness of the north wall by approximately 50% compared to the south wall that had no openings. This imbalance between the stiffness of the end walls resulted in the twisting of the building under wind pressures acting on the longitudinal east and west walls. The twisting of the building lifted the posts of the north wall east of the sliding door.

Because the posts were simply embedded in the ground without any footing anchorage or uplift blocks, the only resistance to the wind-induced uplift forces was the friction between the posts and surrounding soil.  While well-intended, replacing the soil around the posts of the farm building with gravel had most likely weakened the resistance of the embedded posts to uplift as gravel has a lower friction resistance compared to native soil. These pre-existing construction flaws rendered this farm building particularly vulnerable to repeat twisting and uplift during strong winds.

Case Study 2: A Dancing with the Wind Livestock Shelter

A newly constructed (four-year-old) livestock shelter located in Nova Scotia was displaced during a windstorm in the Fall of 2019 and the cross bracing on the west end of the building was damaged (Photograph 4).  The livestock shelter was an open gabled-roofed structure that measured 180′ long in the east-west direction and 30′ wide in the north-south direction.  The roof of the was framed with wood trusses spanning in the north-south direction sheathed with sheet metal roofing panels. The trusses were supported at their north and south ends by two 2×10 wood beams supported in turn by 6×6 posts spaced 12′ apart over concrete piers. Along the length of the shelter, there were three sets of cross cable braces on the south and north sides of the building (Photograph 5).  Every post was braced to the beam above it by a pair of knee braces.  Along the width of the shelter, there were only two cross bracings, one at each end of the shelter.  At the east and west ends of the shelter, the north and south posts were braced to the end truss with a knee brace.

Photograph 4: General view of the swayed livestock shelter

Photograph 5: View of a cable cross bracing on the end bay

Based on the 1995 edition of the National Building Code of Canada for Farm Buildings, the code in effect when the shelter was built, the factored design wind load was equivalent to a 101 km/hr wind speed. According to the historical weather data recorded by a weather station 24 km away from the shelter and reported by Environment and Climate Change Canada, the maximum wind gust on the date of loss was approximately 98 km/hr blowing from south to north, which was very close to the design maximum wind speed.

Wind loads on buildings depend on the building shape, dimensions, and the direction the wind blows. The livestock shelter was 180′ in the east-west direction and only 30′ in the north-south direction.  On the day of the windstorm, the south wind had a large area to blow against, resulting in a large horizontal force in the north direction. The building was only braced in the north-south direction at the ends, which was inadequate to safely resist the wind loads. One of the cross braces broke and the building swayed significantly northward.

The existing structural drawings of the building showed that every wood post was designed to have a pair of knee braces to the beam above in the east-west direction and one knee brace to the truss across in the north-south direction.  However, the drawings were not clear on the location of the knee braces, and the contractor did not install most of the north-south knee braces between the posts and the trusses (Photograph 6). It may appear that wind caused the structural damage to the shelter, but the true cause was the pre-existing construction deficiencies.

Photograph 6: View from inside the shelter facing west

The Wind Whispers

The two case studies shared in this article show that often the true cause of the wind-induced damage to farm buildings is not inordinate wind exceeding typical design limits but rather pre-existing construction deficiencies that weakened the building’s ability to withstand the horizontal forces generated by anticipated wind pressures. Therefore, it is important not to be deceived by the apparent cause of damage, and instead engage an experienced professional engineer to properly investigate the true cause of the damage.  Although smaller and typical farm buildings that follow best construction practices may not need to be designed by an engineer, it is highly recommended that a competent professional engineer be engaged on larger and atypical farm buildings.

Farm buildings are typically lightweight structures with relatively open floor plans, subject to high wind pressure because they are usually located in open terrain and have large wall openings. A properly designed and constructed wind load resisting system is crucial. In some jurisdictions, such as Alberta and Saskatchewan, there are no design requirements for farm buildings. Without design requirements spelled out in a governing code, ensuring adequate lateral resistance for farm buildings in these jurisdictions is essentially left up to the builders. The risk of ending up with framing that is missing vital elements or simply does not have the load-carrying capacity to safely withstand the 1-in-10 year windstorm is great.  As a result, farm buildings are especially susceptible to wind damage particularly in jurisdictions that have not adopted the National Farm Building Code.

Regulation is important to ensure that safety issues are addressed. The building permit process ensures that plans are reviewed for code compliance, and the building inspection process ensures that an inspector reviews what is actually constructed in the field. Proper consideration of these processes results in safer and more robust farm buildings in most circumstances.

 

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[1] The 1-in-10 year wind gust is expected to only have a 10% chance of being exceeded in any given year. It does not imply a recurring schedule for the wind.

References

National Research Council of Canada (1965). Farm Building Standards: Supplement No.6 to the National Building Code of Canada, Ottawa, Ontario.

National Research Council of Canada (1995). National Farm Building Code of Canada, Ottawa, Ontario.

 

Amir Jamshidi, Ph.D., P.E., P.Eng.
Western Canada Forensic Structural Engineer

Amir is a Professional Engineer with over 15 years of experience specializing in structural forensic investigations. He is a licensed professional engineer in Alberta, British Colombia, Saskatchewan, Manitoba, Ontario, the Yukon and the states of Montana, Alaska, New York, Washington, Texas, and Arizona. In addition to his structural forensics expertise in investigating farm, industrial, commercial, and residential losses, he has provided engineering and technical services to the steel fabrication and oil and gas industries, including performing fatigue and fracture analyses.

 

Mohtady Sherif, Ph.D., P.Eng.
Southwest Ontario Forensic Structural Engineer

Mohtady earned both his Ph.D. and Master’s degrees in structural Engineering from Ryerson University and is a licensed Professional Engineer in Ontario. He specializes in forensic structural investigations and the rehabilitation and restoration of existing structures. Mohtady is practiced in the analysis and design of all major structural materials: wood, steel, concrete, and masonry, and possesses sound knowledge of the Canadian building codes, bridge design code, and the blast-resistant design code. His experience includes teaching structural design and conducting academic experimental research at Ryerson University.