In June 2021ASME released the newest version of the Steel Stack Design Standard, ASME STS-1-2021. In this release there were some fundamental changes made. This article will discuss the major changes made.
ASME STS-1 in 2016 and prior editions, were based upon wind loads per ASCE 7-05. This version of ASCE 7 utilized Service Level Loads, meaning that the wind speed was based upon Allowable Stress Design (ASD). In ASCE 7-10 and later, those wind standards switched to using wind design speeds that were Ultimate wind speeds. In order to handle this new basis for wind speed, ASME STS-1-2016 introduced a simple conversion that would convert from Ultimate Wind speeds to Service Level Wind speeds.
Equation 4-5 converted an ASCE 7-10 wind speed (Ultimate) to an equivalent ASCE 7-05 wind speed (Service Level).
In ASME STS-1-2021 recognized that ASCE 7 was using Ultimate Wind speeds, and so it no longer made sense to convert to an ASD type wind speed. In ASME STS-1-2021 the wind basis was changed to use Ultimate Wind speeds.
The problem with this switch, is that for the exact same job site will use a higher velocity, which in turn will result in higher pressures (Ultimate wind speeds). For example, a site that would use a design wind speed of 100 mph per ASCE 7-05, might result in using an ultimate wind speed of 126 mph for the exact same job site when using ASCE 7-10 or later. Obviously the 126 mph would yield much higher wind pressures, but since these are Ultimate wind pressures to adjust to Allowable Stress Design (ASD) basis, then we will use a load factor that reduces the overall force on the structure when performing an ASD design. ASME STS-1-2021 is still an ASD code, and so we must adjust the Ultimate Wind pressures to be consistent with that ASD basis. This is done by using the following load combination factors:
In addition to the change in load combination factors, there are some other areas that utilize wind velocity and so we must adjust to a Service Level type wind velocity in order to stay consistent with the original ASD methodology.
It easiest to illustrate this change in philosophy with an example. For simplicity, we will use the example E4 provided in ASME STS-1-2021 and compare the same example worked using ASME STS-1-2016 versus ASME STS-1-2021. This is a side by side comparison of these two worked examples, using MecaStack. Looking at this comparison, there are some very different results internally. The wind pressures are significantly higher in ASME STS-1-2021, but when compared to ASME STS-1-2016 we see that the pressures are 1/0.6 = 1.66667 higher in ASME STS-1-2021; however, since the load factor is 0.6 (ASME STS-1-2021) compared to 1 (ASME STS-1-2016) and so when this is taken into consideration the wind loads are effectively the same.
There are many internal changes to he standard, but in this example we can see that the final results are comparable when we taken everything into consideration.
There are several other changes to ASME STS-1-2021, and we recommend you review the "ASME STS-1-2021 Summary of Changes" for the specific changes. You may purchase a copy of ASME STS-1-2021 at the website.