CROSS Safety Report
Erroneous design snow loading assumption for indoor sports facility causes structural distress
Overview
An indoor sports facility in a mid-western city suffered snow-related damage one winter. The pre-engineered structure was comprised of a fabric shell stretched over a series of structural steel arches and was reportedly installed ten years prior. The shell reportedly exhibited visible deflections in several of the arches, causing the owners to close the facility.
Key Learning Outcomes
For structural engineers
- When using proprietary fabric clad buildings, ensure that snow, and other, loads are suitable for the site location in all conditions
- Check assumptions made in the design against the circumstances that are likely to be encountered
- If the full snow load, following storm conditions, cannot be accommodated by the structure withing normal limits have a fail-safe method of removing snow, e.g. heating
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An indoor sports facility in a mid-western city suffered snow-related damage one winter. Averaging 76 inches (1.9m) of snow annually, the region recorded over 132 inches (3.4m) of snow and fewer warming/thawing cycles during this season, thereby leading to more significant overall snow accumulations. The pre-engineered structure was comprised of a fabric shell stretched over a series of structural steel arches and was reportedly installed ten years prior. The shell reportedly exhibited visible deflections in several of the arches, causing the owners to close the facility and engage snow removal services. During the following spring, a structural engineering professional was engaged to document the extent of damage and provide a causation analysis. Field investigation and deflection measurements along the length of the frame segments found plastic deformation in six of the nine arches, ranging from less than 1/8 inch (0.32cm) to as large as 5 inches (12.7cm).
A review of available proprietary information, followed up with lengthy phone conversations with the pre-engineered system provider, revealed that the designer, manufacturer, original installer, and the presumptive repair contractor were all the same entity. The manufacturer’s published deflection criteria provided more stringent deflection criteria than the International Building Code (IBC) general serviceability criteria.
The provider’s technical representative explained the arches were not intended ever to hold snow, and the restrictive deflection criteria were intended to ensure snow accumulations on the fabric did not occur. That statement of assumption was both enlightening and concerning, considering the structure is near a historically documented heavy snow zone. The provider’s designer contended that the fabric surface, structure’s slope, and arch stiffness collectively prevent snow from accumulating. Actual events and structure performance suggested otherwise. Simply, snow accumulating on the roof caused expected deflection, which allowed for additional accumulation and additional deflection. Later reviews of sliding snow criteria and the roof slope indicated that indeed this structure should have expected snow accumulation.
In this case, the designer applied incorrect assumptions. The proprietary design/manufacture/install model was generically applied in a location susceptible to snow loading contradicting the baseline assumptions. Anticipating that the sloped segment of the arches would shed the snow prior to deflections led to improper application of the code-required loading and, eventually, poor material selections. Ultimately, the flawed assumptions were exposed when snow fell and accumulated, liberally selected materials plastically deformed, and design parameters were unacceptably violated.
As structural design professionals, it is imperative to revisit assumptions regularly, especially those used repetitively. Different locations and situations always require renewed attention and verification of previous expectations. In the end, this site narrowly avoided a collapse and instead only suffered the arch deformations. All arches with permanent deflections out of manufacturer’s strict tolerances were replaced, operational heating recommendations were refined, and the building management implemented removal instructions for any future snow accumulations.
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Expert Panel Comments
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This is another legacy report about a snow loading problem with the added complication that the design did not, apparently, consider the full effects of a severe fall.
There does not appear to be any indication of fabrication or erection error with the frame. Further, there appears to be no indication of design error except for significant underestimation of snow loads and the ability of those loads to accumulate over time.
Fabric structures are becoming increasingly common. The design/build method of procurement for these structures is a natural consequence of the proprietary, or claimed proprietary, nature of their design and construction. There is nothing inherently wrong with this practice. However, there have been too many failures of fabric structures. One prominent example was the 1988 collapse of the Dallas Cowboys Practice Facility (Anatomy of a Collapse, the National Institute of Standards and Technology’s Final Report on the Collapse of the Dallas Cowboys Indoor Practice Facility, May 2, 2009 (NIST IR 7661) | NIST
In this case there was an assumption that the nature of the fabric, the slope, and the inherent shape and stiffness would prevent any snow accumulation. However there is a code to follow; sections 3102 and 3102.7 of the IBC are dedicated to membrane structures and they require consideration of snow load. ASCE 7-16 has numerous instructions for snow load including cold and warm surfaces, shapes and slopes. Only in the cases where the slopes exceed 70 degrees is the designer permitted to omit snow, per ASCE 7-16 7.4.3 and the related Fig 7.4-3. It may be noted that the relevant text existed even in ASCE 7-05. Snow load based on locality and shape, material or roof are not assumptions, but data to be used in design. The behaviour of snow and its movements is a specialist subject and any deviations from code recommendations should be deferred to an expert in the subject. Any loading condition that doesn’t apply, particularly if it is typically prevalent in the area of use, should be backed by evidence.
Another example is the failure of the Metrodome fabric roof in 2010. A storm came in fast and workers trying to remove snow via firehoses had to be brought down for safety. Because the dome was flattening and increased heating couldn't keep up with the storm, the fabric failed and was captured on dramatic video. Story and multiple video at The Moment the Metrodome Roof Collapsed (vikings.com)
In the event of abnormal accumulations thought should be given to avoiding the consequences of severe overload. These might include operational heating procedures or other methods of shedding load. Furthermore, especially since this structure was apparently a repeatedly manufactured product, the assumption that snow would slide off the fabric and not accumulate could have been verified by field testing.