Statement on required load combinations for wind uplift.
Late last year, the Structural Engineers Association of California (SEAOC) learned that some engineers designing ballasted rooftop solar arrays using Allowable Stress Design (ASD) have not been applying the appropriate load combination for wind uplift (i.e., Load Combination No. 7a from ASCE 7 Section 2.4.1), either ignoring it or modifying it based on an incorrect understanding of its basis. In response, the organization issued the statement shown in the inset on the next page.
The statement was authored by the SEAOC Wind Committee and approved by the SEAOC Board of Directors in November 2022. (The statement is also posted at https://tinyurl.com/SEAOC-Statement)
In the cases that prompted SEAOC’s statement, the engineers ignored ASD Load Combination No. 7a (i.e., 0.6D + 0.6W) from Section 2.4.1 of ASCE 7 and instead considered only ASD Load Combination No. 5a (i.e., 1.0D + 0.6W). For Risk Category II, ignoring the load combination intended to govern uplift (i.e., Load Combination No. 7a) essentially means that the ballast is designed for wind speeds with a return period of approximately 40 years instead of the intended and required 700-year return period. Thus, a ballasted solar array designed this way would have an annual probability of uplifting from the roof approximately 17 times higher than a code-compliant design. (Over a 30-year service life, the probability of uplift of a solar array would be about 50% rather than the intended 4%.) This is a substantial reduction in the stability of the solar array, and, as emphasized in the SEAOC statement, it is not appropriate to use the full dead load (even if it is known with complete certainty) to counteract the ASD-level wind load.
Basis of ASD Load Combinations
ASD is intended to achieve reliability by providing a factor of safety between ASD-level load effects (informally called “service level”) and nominal resistance. In Load Combination 7a, 0.6W is the ASD-level load effect, and the dead load D provides the nominal resistance. The 0.6 factor applied to D represents the factor of safety against uplift, equal to 1/0.6=1.67. Using 1.0D instead of 0.6D would eliminate the factor of safety, which is an integral part of ASD. Just because the dead load is well known does not mean that a coefficient higher than 0.6 can be used for dead load.
Although the basis for the 0.6 factor on dead load may be unclear to some, the requirement to use it in Load Combination 7a is unambiguous. Section 2.4.1 clearly states that whichever load combination “produces the most unfavorable effect … shall be considered.” Thus, all potentially governing load combinations must be considered; none can be ignored.
Getting the Word Out
The authors have recently found cases in which this erroneous design practice is still taking place; thus, we are working to get the word out to engineers and agencies that work with solar energy structures and building officials. Please help by forwarding this article or the link to the SEAOC statement to anyone you know who might be interested.
Because of the high probability of uplift for such designs, any solar arrays that may have been designed with incorrect load combinations should be reviewed and, if necessary to meet code requirements, be redesigned and retrofitted. Retrofitting would typically require additional ballast weight to hold down the array, with further modification or measures needed if the array configuration, the racking system, or the capacity of the roof structure cannot accommodate the additional ballast.■
Statement by the SEAOC Wind Committee on Wind Uplift and Allowable-Strength Design Load Combinations for Ballasted Solar Array Designs, November 11, 2022
(Authored by the SEAOC Wind Committee, and reproduced here with permission from SEAOC)
The Structural Engineers Association of California (SEAOC) Wind Committee has recently learned that some engineers designing ballasted rooftop solar arrays for wind uplift have been using the Allowable Stress Design (ASD) load combinations of the ASCE 7 standard, the International Building Code (IBC), and the California Building Code (CBC) incorrectly.
These engineers have proposed ignoring the required Load Combination No. 7 in Section 2.4.1 of ASCE 7-16, (0.6D + 0.6W), which leaves Load Combination 5, (D + 0.6W), governing the design for uplift1.
In the above equations, D is the dead load of the ballasted panel system, and 0.6W is the allowable-stress design-level design wind uplift force.
The justification that SEAOC has seen for this practice assumes that the 0.6 factor on dead load is intended only to represent uncertainty in the dead load. This assumption is incorrect. Rather, the factor was derived so that ASD load combinations would give results similar to designs that use Load and Resistance Factor Design (LRFD). The degree of certainty that one may have in the dead load is not a justification to change this factor.
It is SEAOC’s opinion that a design that ignores or modifies the 0.6D + 0.6W allowable stress load combination does not comply with ASCE 7, the IBC, or the CBC, and is in direct conflict with code provisions. Further, such an approach cannot be justified by any code provisions that allow alternate design approaches. Failing to use the required 0.6D + 0.6W load combination in the allowable stress design procedure is unconservative and will lead to designs with a substantially higher probability of wind uplift failure than a code-compliant design.
The 2009 paper “Counteracting Structural Loads: Treatment in ASCE Standard 7-05” by Ellingwood and Li2 directly addressed the issue of ASD Load Combination No. 7. The paper states, “the factor applied to dead load in situations where its effect is counteracted by the effects of other lateral or uplift forces should not be increased above 0.6.”
In summary, ignoring or modifying the 0.6D + 0.6W allowable stress load combination violates the building code, and use of 100 percent of the dead load to resist allowable-stress design-level wind uplift will result in markedly unconservative designs and, consequently, a higher probability of solar array failures.
1 ASCE 7-16, the 2018 IBC, and the 2019 CBC all have identical ASD load combinations.
2 Ellingwood, Bruce R and Li, Yue, “Counteracting Structural Loads: Treatment in ASCE Standard 7-05”, Journal of Structural Engineering, January 2009, pp. 94-97.