What's The Difference Between ASD & LRFD Design?
Put in layman’s terms, think of a wood plank designed to stretch between two supports.
You’re going to walk across the piece of wood.
The person that designed that plank needed to figure out if it will hold you.
They had two options when designing:
1) Reduce the calculated capacity of the plank to provide the necessary safety factor (so say a plank that can hold 300lb is ‘rated’ for only 200lb) – This is the ASD Method.
2) Increase your weight and design for that higher load (hence the elephant in the image), so a 200lb person becomes a 300lb person. This is the LRFD (Ultimate Design) method.
It’s important to understand which method is being used in a chart, table, calculator, or design, and to utilize that method across the design.
ASD History
Many years ago, the ASCE wind speed map was replaced from ASCE to LRFD.
The new wind maps are now specified at the strength design level (Load Resistance Factored Design, or ultimate load) rather than the service design level (Allowable Stress Design).
A load factor increase was integrated into the design wind speeds specified in the ASCE maps.
…You may recall one used to hear ‘140mph winds’ used to describe category 5 hurricanes in Florida which is now ‘170mph winds’. It’s the same force, only ultimate instead of allowable.
Use In Product Approved Building Component Design
Most product evaluation approvals are tested in service design conditions (ASD), but should be confirmed within each product approval’s limitations of use.
Generally speaking, test protocols provide a test pressure and/or failure load which already include a safety factor for the design pressure. Thus, by the very nature of having built-in safety factors during testing, most of product approvals are inherently forced into ASD.
It’s recommended to consult a design professional before using any result from a table in your project.
This table is an example of one of the minority of uses of ultimate load values in product approvals.
The user is instructed to multiply the table loads by 0.6 before using them with ASD wind pressures. This format for designing is difficult to understand, out of the ordinary, and will lead to safety installation issues during design events.
It is NOT recommended that tables be displayed in this manner without significant instruction and warning.
Most construction building products sent to product approval agencies use the ASD method for design.
A designer may ask, “Aren’t the design pressures for these components reduced by the 0.6 factor?” The answer is very simple: the pressure is not reduced, it is a completely separate type of analysis.
If a building component was analyzed with a demand such that the LRFD factor was used, the component itself would have an LRFD capacity that would be much higher than if the capacity was analyzed using the ASD method. As an example, a component could have a demand of 100 kip-in using LRFD, or a demand of 60 kip-in using ASD.
The component can ALSO have a capacity of 1000 kip-in using LRFD, OR a capacity of 600 kip-in using ASD, as each method uses separate design factors. The component still has the same strength and is approved in both scenarios. They are apples to apples and oranges to oranges.
The Allowable Stress Design approach seeks to maintain serviceability by using safety factors to ensure that applied loads do not exceed the elastic limit. This is why ASTM E330 and others require a safety factor of 1.5 up to 2.0.
IMPORTANT: With Load Resistance Factored Design (strength design) the approach is to avoid failure by increasing required loads through the application of load factors to obtain the ultimate required strength load and compare this to the ultimate factored strength capacity, whereas with Allowable Stress Design (service design) the approach is to avoid failure by decreasing allowable loads through the application of safety factors to obtain the allowable service capacity and compare this to the required service demand.
Applied To Building Codes
2023 FBC Section 2404.1: (other code versions are similar to date) The design of vertical glazing shall be based on the following equation:
0.6Fgw ≤ Fga (Equation 24-1)
where:
Fgw is the wind load on the glass computed in accordance with Section 1609
Fga is the short duration load resistance of the glass as determined in accordance with ASTM E 1300.
From 2023 FBC (other codes similar to date)
1609.1.2.4 Impact resistant coverings.
1609.1.2.4.1 Impact resistant coverings shall be tested at 1.5 times the design pressure (positive or negative) expressed in pounds per square feet as determined by the Florida Building Code, Building Section 1609 or ASCE 7, for which the specimen is to be tested. The design pressures, as determined from ASCE 7, are permitted to be multiplied by 0.6.
See more on IBC 2021 Building Code in section 1609.1.2.
Last Update: June 9, 2025
