Most all products are evaluated using the ASD method (unless noted otherwise). ASD method in, then ASD method out.

Among the notable changes in ASCE 7-10, the wind speed map and the importance factor for wind loads have been replaced with three wind load maps based on difference and newly defined return periods and for structures with different risk categories, which replace the former occupancy categories. The new wind maps are now specified at the strength design level (Load Resistance Factored Design) rather than the service design level (Allowable Stress Design). A load factor of 1.6 for strength design has already been integrated into the design wind speeds specified in the ASCE 7-10 maps, so the wind load factor for the strength design load combinations has actually gone from 1.6 in ASCE 7-05 to 1.0 in ASCE 7-10, and the wind load factor for the allowable stress design load combinations has gone from 1.0 to 0.6 in correspondence. It is important to note that the previous wind pressure requirements for product approvals was rarely, if ever, multiplied up by the 1.6 factor, even though that is shown in ASCE 7-05 for the LRFD method. The reason is that the 1.0 factor for 7-10, just like the 1.6 factor for 7-05, are for LRFD strength design applications only (typically used in concrete design and certain types of anchor testing).

Most product approvals are tested in service design conditions, and should therefore be in ASD, however that will need to be confirmed with each particular product approval. Since previous editions of the ASCE 7 code returned wind pressures in ASD, those pressures could be directly compared to the product approval’s allowable pressures. Now that ASCE 7-10 returns LRFD factored (increased) wind pressures, a design professional will need to convert the wind pressures to ASD before comparing them to the product approvals. An exception will be if the product approval has pressures listed in LRFD. The bottom line is that if the product approval shows service level allowable pressures (allowable stress design), then the applied wind pressures will have to use the ASD load combination in order to be comparable. In the future, engineers should specify whether given design pressures are in ASD or LRFD, and product approvals should denote if their allowable pressures were found using strength (LRFD) or service (ASD) conditions. Generally speaking however, existing test protocols provide a test pressure and/or failure load which already include a safety factor for the design pressure (typically either 1.5 for wall systems or 2.0 for roof systems). Thus, by the very nature of having built-in safety factors during testing, 95% of product approvals are inherently forced into ASD. The use of LRFD combinations may result in overly-conservative designs, applying a load factor to a product that already has safety factors multiplied to its failure load.

One 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 theultimate 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.

The 2010 Florida Building Code, as well as the 2009 and 2012 International Building Code and International Residential Code, reference either the 2011 or the 2008 edition of AAMA/WDMA/CSA 101/I.S.2/A440 – NAFS North American Fenestration Standard/Specification for Windows, Doors, and Skylights for Fenestration Testing, depending upon which edition of the codes are adopted in a particular jurisdiction. Both editions of NAFS evaluate a product’s ability to resist uniform loading based mainly on compliance testing, and assign Performance Grades based on allowable stress design.

2010 FBC Section 2404.1: The design of vertical glazing shall be based on the following equation:

Fgw ≤ Fga (Equation 24-1)

Fgw is the wind load on the glass computed in accordance with Section 1609multiplied by 0.60
Fga is the short duration load resistance of the glass as determined in accordance with ASTM E 1300.

2010 FBC Section

1609.1.2.3 Impact resistant coverings.
1609. 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.


Mar 7, 2016   6628    Codes & Standards    
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