Lost in Load Factors? Navigating Wind Design with Mean Recurrence Interval

To view the figures and tables associated with this article, please refer to the flipbook above.

Wind design provisions across codes and standards, including ASCE 7 Minimum Design Loads and Associated Criteria for Buildings and Other Structures (2005 through 2022), ASCE 37 Design Loads on Structures During Construction, and Factory Mutual (FM), often appear inconsistent due to differing wind load factors used in load combinations.

Focusing solely on these factors can obscure an unchanging design principle: the target wind design is governed by the mean recurrence interval (MRI), which remains consistent even as code formats and load factors evolve.

Rather than getting “lost in load factors,” engineers can use MRI as a compass to interpret wind design provisions and communicate design intent more clearly with architects, contractors, and design-assist partners.

Design wind pressure begins with the basic wind speed, obtained from the wind maps in ASCE 7 or the online ASCE Hazard Tool based on project location. The basic wind speed is a three-second gust speed at 33 feet above ground in Exposure C terrain, such as airports and similar open country exposures.

These wind speeds are derived from statistical analysis of historical meteorological data from more than 500 stations, each with at least 15 years of record, and correspond to specific MRI depending on the building risk category.

The design wind speed for different risk categories of buildings corresponds to the MRI of the wind hazard. For example, the ultimate design wind speed corresponding to risk category II buildings represents a 700-year MRI wind event.

A common misconception is that a 700-year event occurs once every 700 years. In reality, the term corresponds to an annual probability of exceedance, P_a, of about 0.14% (P_a ≈ 1/700).
The probability of experiencing at least one such event during the lifespan of the building varies with the targeted risk category. For example, over a 50-year building lifespan, a 700-year wind event has roughly a 7% chance of being exceeded (Fig. 1).

P=1-(1-P_a)⁵⁰=6.90%

This probabilistic framework reflects the reliability objectives embedded in ASCE 7. Buildings with greater consequences of failure (Risk Categories III and IV) are assigned higher MRIs (1,700 and 3,000 years, respectively), resulting in lower probabilities of exceedance.

In general, design wind pressure, W_T, in codes can be expressed in a simplified form as:

W_T=C_F(V)²W_LF

Where, C_F is a generic coefficient that accounts for factors such as building geometry, height, air density and terrain as defined in ASCE 7 and WLF is the wind load factor. Because wind pressure is proportional to the square of the basic wind speed, V, velocity is the primary driver for calculating wind pressure.

In ASCE 7-05 and prior, the basic wind speed was provided in a single wind speed map based on a service level, 50-year MRI wind speed, V50. Wind pressure calculations included an importance factor, I, and a load combination factor of 1.6 to amplify this wind speed to appropriate ultimate design wind loads based on risk category:

W_T=CF(V50)21.6I

For example, I for a risk category II and III building was 1.0 and 1.15, respectively.
To understand how these factors connect to MRI, Peterka and Shahid (1998) conducted research to establish a relationship between MRI, T representing time in years, and the ratio of wind speed for any return period, VT, relative to V₅₀. They developed the following equation after analyzing long-term wind speed data for non-hurricane zones:

Solving for T, this relationship can be expressed as: T=0.00228e¹⁰ $√$W_LF

As an example, ASCE 7’s target MRI for a risk category III building is 1,700 years. Solving this equation with T as 1,700 results in a W_LF of 1.83, which is equivalent to the 1.6 load factor multiplied by the 1.15 importance factor from ASCE 7-05.

One disadvantage with ASCE 7-05 was that amplifying service level wind loads to reach ultimate level MRI targets was not consistent with the procedure for other hazards, particularly seismic loads.

To address this, ASCE 7-10 transitioned away from the single wind speed map and introduced separate maps based on each risk category’s target MRI. Since the updated maps provided the ultimate level basic wind speed, the importance factor was removed and WLF reduced to 1.0. However, the target MRI for each risk category remained the same. The wind pressure calculation was simplified as:

W_T=C_F(V_T)²1.0

For a risk category III building in ASCE 7-10, V_T=V₁₇₀₀, which is read directly from the 1,700-year MRI wind map. This applies regardless of whether a building is in a hurricane or non-hurricane prone region as the maps incorporate hurricane wind speed data. ASCE 7 versions since 7-10 have maintained this framework, with slight revisions to the basic wind speed values as new wind data analysis became available. ASCE 7-22 also introduced wind speed maps for tornado zones which follow a similar logic to the previous chapters.

If building owners choose to pursue FM insurance, certain building assemblies must be designed to meet FM’s load criteria. FM believes that property losses can be mitigated by identifying hazards using an engineered approach like ASCE 7.

FM’s procedure is like ASCE 7-05 in that the basic wind speeds are typically based on a 50-year MRI wind speed map. The ultimate design wind load pressure calculation includes an I of 1.15 and a safety factor of 2.0, written as:

W_T=C_F(V₅₀)²(1.15)(2.0)

Unlike ASCE 7-05, FM assigns a uniform 1.15 importance factor, making wind pressure determination independent of risk category. The 2.0 safety factor also results in higher pressures compared to ASCE-7. Using the Peterka and Shahid (1998) relationship, the equivalent return period for ultimate level FM wind pressures is about 8,800 years (Fig. 2). Recent updates to FM include separate wind speed maps for tornadoes, which follow the 10,000-year MRI maps from ASCE 7-22.

Although FM pressures are amplified, FM requirements are only intended to be used for components and cladding elements such as roof assemblies, roof decks, and the securement of roof decks to structure below. FM pressures do not apply to the design of main wind force resisting system elements such as braces and shear walls. See Table 1 for a comparison of load factors and MRI between ASCE 7-05, ASCE 7-10, and FM.

The discussion of load factors and target MRI thus far has focused on ultimate level wind loads intended for life-safety design. MRI targets such as 700 years or 1,700 years represent extreme wind events that the structure’s main lateral force-resisting system must be designed to resist.
However, these extreme events are not representative of day-to-day wind behavior. When evaluating serviceability criteria such as frame drift, ASCE 7 and AISC Design Guide 3 Serviceability Design Considerations for Steel Buildings recommend engineers to use a 10- or 25-year MRI wind speed, rather than the 0.6W used for allowable stress design, as the 0.6 factor leads to higher serviceability wind speeds than suggested.

For example, using Boston wind data from ASCE 7-22 for a risk category III building, the 1,700-year MRI wind speed is 124 mph, while the 10-year MRI wind speed is approximately 74 mph. The load factor is $(74/124)^2$=0.36 which is significantly less than 0.6 (Table 2).

Ultimately, the target MRI wind speed used for drift may require input from the building owner for the target performance goal. For the deflection of individual structural members, the International Building Code (IBC) typically recommends using a 10-year MRI wind speed with a wind load factor as calculated above, or a roughly equivalent factor of 0.42 (after ASCE 7-05). Member deflections should be limited to the IBC requirements and coordinated with the facade manufacturer based on the selected facade product.

Although it is not adopted by the IBC, ASCE 37 provides guidelines for determining wind loads on structures during construction. Per ASCE 37, buildings during construction shall be considered as risk category II unless otherwise required by the authority having jurisdiction. ASCE 37 provides reduction factors that modify the ASCE 7 basic wind speed based on the anticipated length of the construction period, reflecting the shorter duration of exposure.
These factors are calibrated so that the probability of exceedance during the construction period is approximately the same as the probability of exceeding for the permanent structure during 50-year service life.

For example, a Risk Category II building designed for a 700-year MRI has approximately a 7% probability of exceedance over a 50-year service life as discussed before. A theoretical probability match for a 2-to-5-year construction duration would correspond to a wind speed reduction of roughly 0.82: approximately a 70-year MRI wind.

ASCE 37 instead specifies a 0.9 reduction factor, which corresponds to a higher effective wind hazard level and therefore provides a conservative margin. Recent discussions for ASCE 37-25 have explored presenting construction wind speeds with equivalent MRI, in addition to the traditional reduction factors.

Design standards will continue to evolve as new data and research improves our understanding of wind hazards. Although code formats and load combinations may change over time, concepts such as MRI offer a useful framework for interpreting wind design intent. By focusing on these underlying principles, engineers can better navigate future code developments. ■

About the Authors

Pengcheng ‘Alex’ Zhou is an Associate at LeMessurier. (pzhou@lemessurier.com).

James Fong is a Designer at LeMessurier (jfong@lemessurier.com)

References

Peterka, J. A., and Shahid, S. (1998). “Design gust wind speeds in the United States.” J. Struct. Engrg., 124(2), 207–214.

American Society of Civil Engineers (ASCE). (2006). “Minimum Design Loads for Buildings and Other Structures.”, ASCE/SEI 7-05, Reston, VA

American Society of Civil Engineers (ASCE). (2010). “Minimum Design Loads for Buildings and Other Structures.” ASCE/SEI 7-10. Reston, VA 

American Society of Civil Engineers (ASCE). (2017). “Minimum Design Loads and Associated Criteria for Buildings and Other Structures”, ASCE/SEI 7-16, Reston, VA

American Society of Civil Engineers (ASCE). (2022). “Minimum Design Loads and Associated Criteria for Buildings and Other Structures”, ASCE/SEI 7-22, Reston, VA

American Society of Civil Engineers (ASCE). (2015). “Design Loads on Structures during Construction.”, ASCE Standard ASCE/SEI 37-14, Reston, VA

FM Global. (2024). “Data Sheet 1-28: Wind Design (July 2024 Interim Revision).” Johnston, RI

Duntemann, J. (2025). “ASCE 37-25 Updates [M4] 2025”, NASCC: The Steel Conference.