Would exceeding the snow load duration factor practically guarantee a collapse? 5

[DurableEfficientGood]

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

2. 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)?

 

[dik]

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)

I suspect the 15% is to increase the load from a two-month time period to a longer time period. The material may have other requirements for duration of loading (like wood for example) which reflect the material properties.

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

If overloaded in winter, it may not wait until spring to collapse. I don't understand your question.

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

Again, I don't understand the question. Your discussion appears to be a bit convoluted to answer. Can you simplify the questions a tad... In my case, it might be old age creeping up, but I had difficulty understanding your comments.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[driftLimiter]

This is actually a really good question. ASCE 7 committees have done a bunch of statistical research and historical research to determine the loadings.

For each different loading case they basically decided on a few things....
-How confident are we that the magnitude of the loading we use is correct?
-How much variability in the magnitude of the loading do we expect? How confident are we about this?
-What recurrence interval for each load case at these magnitudes do we expect? (how often do we reach that level?)
-What is the probabilities for each of these loads to act simultaneously at their peak?

The code attempts to answer these questions by Load Combination Factors. The commentary C2.1 of ASCE716 details this out pretty well.

But that is all on the loading side, on the capacity side we employ another similar set of statistical questions to quantify the strength of elements including variability in material and workmanship.
Factors for both the loading and the strength are calibrated (for LRFD design as an example) to meet a specific probability of failure in the lifetime of the structure.

All of this background is necessary to understand the answer to your question in my view.

Now on to Cd factor for ASD only. NDS Commentary section C2.3.2 says gives plenty of information on each value of Cd presented.

TLR - The Cd factor is a scalar to be used with ASCE 7 loading. It is not a factor that accounts for the duration that a structure can safely hold the load, instead it accounts for the duration and recurrence of the loading. It is a statistical adjustment that NDS uses to make their published vales compatible with ASCE 7.

 

[JLNJ]

Structure design and response to load is not as simple as "will it collapse or will it not".

Snow load and the probability of exceedance is a whole other discussion. It's not an exact science, let's not pretend it is.

In the States, there are guidelines for using reduced loads for short-duration installations. The amount of reduction depends on the duration. If the actual built structure is in place for longer than the duration of the initial design, there is increasing risk the longer the structure is in place. Regardless, nothing properly designed is guaranteed to collapse. Most collapses involve significant design and/or construction errors, not errors in duration factors.

I have no idea if that answered your question.

 

[dik]

Thanks, drift...

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[BridgeSmith]

I understood the duration factors to account for the resilience of the wood to short-term vs. sustained loads; that creep or softening of the fibers in wood will decrease its ultimate strength when the load is sustained for a long period of time.

Even in a long snow season, the maximum snow load is unlikely to be sustained more than 2 months.

As JLNJ stated, none of this is exact, or even close to being exact. The actual tensile strength of a clear piece of Douglas Fir is around 10,000 psi, but the design stress is around 2000 psi, to account for irregularities in the structure.

I wouldn't get too hung up on whether the snow load lasts 3 or 4 months, instead of 2, but if the engineer decides that the snow load may regularly last more than 2 months, it's within their discretion to use a smaller duration factor.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[DurableEfficientGood]

I never knew and was SUPER surprised that structural engineering was not an exact science (unlike soft science, which aren't even sciences in my view and are just studies, because exactness in the crucial parameters is required for falsifiability, and falsifiability is required in order to be a real science), as otherwise given by all the math involved and especially probability distribution curves used. Thanks for letting me know. So are you implying that structural engineering is not a hard (exact) science, which means that it isn't even truly a science at all, which goes against all my prior knowledge?

 

[DurableEfficientGood]

BridgeSmith, I'm mainly wondering whether the snow load lasting 4 months (or even for a whole 12 months and then no snow for several years afterwards) will cause a roof with a standard factor of safety designed for the standard maximum snow load duration of 2 months to collapse. This is just a hypothetical scenario to test the logical consequences of the standard designs (but with perfect workmanship, the collection of parts overall not even having a single defect (not exceeding the manufacturer's stated defect rate limit even in the slightest for any set of parts), and perfect adherence to the codes) and may possibly have not even occured once in the world.

 

[Ron]

@Kwan.....structural engineering can be exact in its implementation, but inexact in its reality. The reason for this is the number of variables involved and the assumptions that must be made in the name of engineering judgment. A example: You design a beam based on its standard minimum yield strength. You calculate its capacity EXACTLY. Well, that's not actually its capacity. Chances are it has quite a bit more capacity than your "exact" calculations show because of unaccounted variability in the actual strength of the material. The same applies to steel, concrete, aluminum, wood or any other material.

 

[SlideRuleEra]

I'm mainly wondering whether the snow load lasting 4 months (or even for a whole 12 months and then no snow for several years afterwards) will cause a roof with a standard factor of safety designed for the standard maximum snow load duration of 2 months to collapse.

There is such a thing as unintended consequences. The building could collapse, but not for the reason you are expecting (wood member "breaks").

When wood exceeds its' elastic limit (2000 psi in BridgeSmith's example) but is less than its' failure point (modulus of rupture, 10,000 psi in the example), over time, the wood deflects, distorts, bends, etc... a lot.
This unanticipated movement could overload these connections (nails, bolts, screws, split-rings, etc), causing the connections to fail... bringing down the structure.

 

[rb1957]

"surprised that structural engineering was not an exact science" ... and therein lies the difference between engineering and science. Science wants to deal with absolute statements; "material science" is thus correct for defining the strength of materials. Engineering on the other hand has to deal with ill-defined issues ...
1) I know how strong steel is, but how strong is this piece of steel ? what is the manufacturing process like ? Sure I could test each piece (like matches) but it's easier to apply a factor to the strength of steel to be (somewhat) confident that no (well, very, very few) pieces will be under strength.
2) how much snow (and at what density) will fall on this building ? After a bunch of research a number is agreed, actual snow load can possibly (but not often) exceed this.
3) similarly with other loads ... approximations that have stood the test of time. It may be interesting to see if climate change brings changes to weather related loads.

I keep telling the young guys at work "there is no truth", but sometimes we should keep looking for a closer approximation to it !? In your case, I think if the building fell down then the investigation will show that either ...
1) the code had been correctly applied, and reality exceeded the code (so the building was doomed), or
2) the code had not been correctly applied, but even still reality exceeded the code, or
3) the code had not been correctly applied, and the building should've withstood the loads applied.

Be happy we don't live in Hammurabi's time !

another day in paradise, or is paradise one day closer ?

 

[BridgeSmith]

So, would the roof certainly collapse in the snow stays on it for 4 months? No.

Is there a possibility of it collapsing under that scenario? A microscopically thin possibility, but yes.

Is there a possibility of collapse with the the snow load on it for one day, or no snow load at all? Again, a microscopically thin possibility, but yes.

As rb1957, stated, nearly all structural members are stronger than we assume them to be for design, and nearly always, the loads will never exceed what is anticipated in the design. In order for there to be a collapse of a properly engineered structure, would require a combination of several unlikely events. Could several wooden roof trusses all have a bottom chord with an undetected structural defect that renders their tension capacity smaller than the capacity used for design? Of course it's possible. Is it possible that all of them are put into a roof system side by side? Sure. It would be like the odds of throwing snake eyes on a pair of dice 20 times in a row, but it's possible. Then, if you had large and usually dense snowfall on that very rare roof system, it could collapse.

The collapse of a structure can be the result of unnoticed or ignored deterioration of some critical part of the structural system. Sometimes collapses are due to unexpected external loading (look up the Sunshine Skyway bridge collapse), and occasionally by a failure to anticipate how the loading would affect the structure.

Most often, when a collapse happens, it's the result of a combination of factors. Again, I'll use bridge example: The I35W bridge in Minneapolis. When it was designed back in the 1960's there was an error made in the design, which resulted in some of the gusset plates being undersized. The error went undetected until it collapsed into the river 40 years later. At teh time of its collapse, the thickness of the concrete deck weighed 33% more than it was designed for, and the barrier rails weighed twice as much as the original barriers. There were vehicles bumper to bumper in 4 of the 8 lanes all the way across the bridge, and a pile of gravel weighing 120,000 lbs and 2 full 11 CY concrete trucks were all right together on the bridge. By the numbers, there was no factor of safety, the bridge should have collapsed the moment the loading exceeded the original design load by even a few pounds. The estimates I saw indicated at the time of the collapse, the loading exceeded the original design loading by almost 30%.

If you want to really understand the mechanisms and causes of structural failures, dig into the Engineering Failures & Disasters forum.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[IRstuff]

From a purely statistical perspective, consider the BridgeSmith's 1st three comments. There is some probability distribution that describes the collapse probability vs. load. Ostensibly, you do not want to design such that the rated loads are infinitely safe, because every structure would be outrageously expensive and grossly over designed; nor do you want the rated loads to produce anything other that "microscopically thin possibility" because too many such failures would cause a lot of lawsuits for "poor" design. And given you don't know what each individual structural element is capable of, and nor do you have assurance that every connection meets its torque, or whatever spec, the design "rules" are constructed such that any structure, correctly designed, will likely survive its rated loads and possibly exceed them sufficiently often that you won't lose a lawsuit if such a structure does fail.

TTFN (ta ta for now)
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[DurableEfficientGood]

For the I-35W Mississippi River Bridge, did it exceed the rated load capacity (after the factor of safety is applied) by 30% or the ultimate load capacity (before the factor of safety is applied) by 30% (meaning that it exceeded the rated amount by well over 50%)? Also, what was the overall system factor of safety applied?

 

[IRstuff]

Bridges require constant maintenance because of their environment; it's not a fair comparison. And we know most bridges and overpasses are undermaintained, so it's likely that most bridges are not operating to their original design strengths.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[BridgeSmith]

For the I-35W Mississippi River Bridge, did it exceed the rated load capacity (after the factor of safety is applied) by 30% or the ultimate load capacity (before the factor of safety is applied) by 30% (meaning that it exceeded the rated amount by well over 50%)? Also, what was the overall system factor of safety applied?

The bridge was intended to have a factor of safety of approximately 2, but due to a change in the design, the implications of which were not fully examined, the load carrying capacity of some of the gusset plates were half what they should have been, therefore the factor of safety was negated.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[DurableEfficientGood]

Was the factor of safety just mostly negated, totally negated, or did or did it over-negate and become net negative instead of just neutral right after the final change in design before initial completion? If so, how many percent of the factor of safety was remaining (it will be negative if the actual load is greater than the limit at the factor of safety), or what was the ultimate net factor of safety (will be less than one if actual load greater than load at limit of factor of safety)?

 

[BridgeSmith]

I don't know how I can be any clearer, but I'll try once more. The original intent was that the capacity would be twice the expected loading at the time it was designed. The design error made it so that, by the numbers, the capacity was approximately equal to the original design loading (half of 2 equals 1). There was apparently, in reality, more capacity (about 30% more) than was expected, since it did not actually fail until the load was about 30% more than the original design loading.

Of course, no one knew this, since only the truss members, and not the connections, were checked when the various rehabilitations were done that added to the load on the trusses.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[DurableEfficientGood]

I see. So,
1. the actual load capacity (actual load during catastrophic failure),
was 30% greater than
2. the original plan's rated (also known as "expected" in this case) load capacity
2.1. which was roughly equal to the changed plan's overlooked (actual rather than stated) design ultimate load capacity
2.1.1. which was twice the changed plan's overlooked (actual) rated load capacity (engineers forgot to recalculate or miscalculated the rated load after the design change, so they didn't spot that error), assuming that the same factor of safety is used.

So, if I use the original rated load as the index, then the following will have these factors:
1. original rated load == 1 (defining point)
2. original design ultimate load == (1)*2 = 1*2 = 2
3. original design breaking point == (2) = 2
4. changed stated (also known as "intended" in this case) design ultimate load == (2) = 2
5. changed stated design breaking point == (4) = 2
6. changed actual design ultimate load == ~(1) = ~1
7. changed actual design breaking point == (6) = ~1
8. changed stated rated load == (4)/2 = 2/2 = 1
9. changed actual rated load == (6)/2 = ~1/2 = ~0.5
10. changed actual breaking point == (6)* ~1.3 = ~1* ~1.3 = ~1.3
11. changed actual ultimate load == (10) = ~1.3

So, if the changed plan's actual design ultimate load is precisely 97% of the original plan's rated load and the bridge actually failed at precisely 31% over the changed plan's actual design ultimate load for example, then the following factors will be:
1. original rated load == 1
2. original design ultimate load == (1)*2 = 1*2 = 2
3. original design breaking point == (2) = 2
4. changed stated (also known as "intended" in this case) design ultimate load == (2) = 2
5. changed stated design breaking point == (4) = 2
6. changed actual design ultimate load == (1)*0.97 = 1*0.97 = 0.97
7. changed actual design breaking point == (6) = 0.97
8. changed stated rated load == (4)/2 = 2/2 = 1
9. changed actual rated load == (6)/2 = 0.97/2 = 0.485 = 0.48 (after rounding to 2 sig. figs.)
10. changed actual breaking point == (6)*(1+0.31) = 0.97*1.31 = 1.2707 = 27% over the original rated load (after rounding to 2 sig. figs.)
11. changed actual ultimate load == (10) = 27% over the original rated load (after rounding)

So, am I right on everything? If not, then what did I get wrong?

 

[SWComposites]

Kwan - what is the point of this discussion? Structural engineering relies on all sorts of assumptions and factors, some of which are arbitrary. Trying to make it a precise science is ridiculous.