# Would exceeding the snow load duration factor guarantee a collapse?

The snow load duration factor is 1.15 times the standard 10-year sustained maximum load rating as long as the accumulated time under this load is not more than 2 months. Does this mean that

1. a. the snow load being at the duration factor of 1.15 for 2 months continuously (experimental case) is at the same level of safety as

b. permanently being at the standard load rating (control case)

when assuming that the experimental case is well under the under standard load rating for the rest of the year, or

1. does the experimental case impart additional risk compared to the control case from the long-term perspective?

Basically, this is asking whether the standard practice allows the control case to use up the safety margin (factor of safety minus one) of the standard load rating only because its short term means that the probability of other loads happening simultaneously will be negligibly small, or whether the safety margin of the standard load still applies on top of the maximum load at full duration allowed by the duration factor.

For example, if the snow season is especially long one year but not especially intense at any given moment, meaning that the roof at any given time is at the forseeable maximum snow load accounted by the duration factor but that it is at that load for a whopping 4 months rather than the expected maximum of 2 months, does that mean the roof will cave in, probably causing the pancake effect to collapse the entire building too?

As for another example under main question, will having the load at any given time within the timeframe moderately exceeding the snow load duration factor, but not having the duration exceed that, make the roof or even building collapse?

Under the hypothetical case that load duration factors in standard practice already use up all of the safety margins for the standard load ratings, this means that exceeding the load duration factor also exceeds the safety margin. Since this exceeds the safety margin, does this mean that exceeding the load duration factor will guarantee a collapse?

In civil engineering in general, does being at the rated limit mean that there is a negligible chance of failure if everything is done perfectly (too small to be measured, such as 1ppb) and does being at the upper limit of the safety margin practically guarantee a failure if everything else is done perfectly (too large to measure a successful case, such as 99.9999999% failure rate)?

• Would only looking at 10 years of data be the issue or should 100 years be examined? ie has snowfall amounts been reducing over time? Jul 1, 2022 at 5:31
• Yes, 100 years and even 1000 years should be examined. Also, this is only a hypothetical question for logical purposes, so the conditions would be perfectly controlled. As a result, average annual snowfall over any 10-year interval remains constant, just that once every so many (arbritrary amount) years, the snowfall wildly exceed expectations so it overloads the roof to a twice the difference between the load duration factor (with either one at the limit and the other at twice the amount over (how much the load duration factor is over the standard load limit allowed). Jul 1, 2022 at 6:00
• Also, all other factors, such as humidity, live loads on the rest of the structure besides the roof, and foundation settlement, remain the same between the different scenarios at any given time, so they are control variables even though they will change somewhat within each scenario over time. Jul 1, 2022 at 6:14
• Snow (over time from wind and sun) will compact and wet snow (from rain or melting) is heavier and you must factor in slope, roof shape, drifting, sun exposure. So exceeding safety margin does not mean the roof will collapse, but does increase the chance of collapse. Jul 2, 2022 at 17:05
• @StainlessSteelRat Okay, I see. I always thought thought the rated amount is practically 0% chance of failure (for example, lies outside 7 sigma confidence interval), the interior of the safety margin is just a sliding scale (transition zone) between totally safe and totally dangerous, and the outside edge of the safety margin is practically 100% chance of failure (for example, lies within 7 sigma CI). Apparently, I was wrong. So what is the typical/standard percentage chance (or confidence interval number of standard deviations) of failure rate at the upper limit of the safety margin? Jul 3, 2022 at 3:30