Risk Versus Reward: Understanding ASCE 7 Windload and Serviceability

By Taylor Hogan and Marty Trainor

Design of curtainwall components often comes down to two primary metrics: structural capacity and serviceability. Structural capacity is the maximum amount of applied load that a given component can withstand in a given direction based on limits set by tested material properties. This analysis is objective, standardized and considered universally applicable for a given material. The serviceability of a given component is a subjective analysis with parameters that require explicit specification, without which, the result is subject to interpretation.

Understanding serviceability (or deflection) of a component is not as straightforward. It’s typically defined as a relative ratio based on the unrestrained span of the component. For example, a curtainwall specification may require the following:
• For spans less than or equal to 13 feet-6 inches, deflection must be less than L/175¹ or ¾ inch, whichever is less.
• For spans greater than 13 feet-6 inches, deflection must be less than L/240 + ¼ inch.

For many years, this was the extent of specification for curtainwall component deflections. However, a secondary consideration is noted with ASCE 7 that more specifically outlines the windloads for these ratios.

Considerations

Prior to ASCE 7-10, nominal windloads were calculated as service-level windloads, i.e. a measurement of the actual windload expected for a given region based on historical data. For strength analysis, the applicable load combination factor for wind was 1.0 for Allowable Stress Design (ASD) and 1.6 for Load Resistance and Factor Design (LRFD). Additionally, per Commentary C, the nominal service-level windloads could be further reduced by 30% (0.7 factor) for checking serviceability.²

Nominal windloads are calculated relative to a given Mean Recurrence Interval (MRI). This is a statistical value stating the probability of a given event occurring in any given year within the noted time period. In ASCE 7-05, the nominal design windload was based on a 50-year MRI, meaning that in any given year there was a 2% probability of a windload equal to or surpassing the noted design load. Utilizing the noted 0.7 reduction would bring the windload required for deflection only to that of a 10- year MRI (10% probability).

The code appeared to drastically change with the release of ASCE 7-10 in which nominal design wind speeds were increased to LRFD ultimate-strength loads and based on a nominal 700 year MRI (and increasingly further for higher risk category buildings). This also meant that the applicable load combination factors for windloads changed to 0.6 for ASD and 1.0 for LRFD.

The most current version of ASCE 7 maintains that reductions for windload in consideration of serviceability may be applied based on windloads calculated from a separate series of wind region maps presented in Appendix C. This is a change from ASCE 7-05 that allows designers to determine the MRI that they deem acceptable for deflections of cladding components. These maps are given in increments of 10, 25, 50 and 100 year MRI windloads.

Although this change seems quite different from the flat reduction of 0.7, the ultimate result is practically the same. For example, in ASCE 7-05, the base design wind speed for most of the interior of the country is based on 90 mph for a 3-second gust wind at 33 feet above ground for Exposure C. In ASCE 7-16 and versions since, this equivalent map for Risk Category II would be in the range of 105-107 mph, varying by region.

The service-level wind speeds of ASCE 7-05 are most closely similar to the 50-year MRI wind speeds for serviceability. Wind pressures used for design are calculated from wind speed maps based on the following equation:
(26.10-1) qz =0.00256×Kz×Kzt×Kd×Ke V² (lb/ft² ); V in mi/h

Assuming all factor variables to be the same for a given location, the difference between these serviceability maps may be determined as the ratio of the base wind speeds squared. For example, for a project in Chicago the ASCE 7-16 base design wind speeds would be 107 mph. The equivalent wind speed for a 50-year MRI serviceability analysis would be 90 mph. The ratio of 50-year MRI to design wind pressure would be 90^2/107^2 => 0.707. Conversely, the equivalent wind speed for a 10-year MRI serviceability analysis would be 74 mph. The ratio of 10-year MRI to design wind pressure would be 74^2/107^2 => 0.478.

Understanding and Analysis

In the example above, a 50-year MRI deflection criteria requirement would ultimately be a more stringent design requirement than the service-level windload required for Allowable Stress Design (0.6 factor). This means that the lowest reduction allowed by code (10-year MRI) has slightly increased from its ASCE 7- 05 equivalent of 0.7 * service-level windload (0.7*0.6 = 0.42 on LRFD windload from ASCE 7-16). This also means that simply analyzing components for serviceability based on ASD loads (0.6 * design wind pressures in ASCE 7-16) is still highly conservative per the allowable reductions by code.

It should be clearly noted that the example provided is but one case study for a particular set of wind speeds. As the industry’s understanding and analysis of historical windload data has developed, the information has become more refined and localized for specific regions of the country.

Because of this variability in allowable windload reductions, it has now become imperative that designers, owners, and engineers come to a consensus on the specified MRI that components must be designed to. In most curtainwall applications, deflection and serviceability will be the controlling design parameters. Requiring designs to meet a more stringent MRI windload criteria may provide peace of mind, but at the expense of greater engineering, material and construction resources to provide an appropriate system. This is antithetical to several of the major aims of the AEOC community—the use of excess material resources when they are not required structurally would be a step backwards from the primary goal of reducing embodied carbon in our buildings.

1. L/175 is a span ratio used for defining acceptable deflection of a framing component. L = span of that component (in inches).
2. ASCE 7-05 Appendix C

Taylor Hogan is a professional engineer with Façade Concepts in Memphis, Tenn., and Marty Trainor is senior vice president of Ventana based in Chicago.

To view the laid-in version of this article in our digital edition, CLICK HERE.