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)?

 

[DurableEfficientGood]

The point of this discussion is to see how much over rated limit and how much over the design limit a system or an individual structural part typically fails.

 

[DurableEfficientGood]

The main thing I want to know is if heavy-duty tri-level bunk beds will overload the floor. There is currently a severe housing shortage in a not-insignificant part of the country and worldwide.

Let's say in a hypothetical (and slightly somewhat realistic) scenario that the housing shortage is so severe that people have to sleep in any indoor dry room (including living rooms, dining rooms that are not part of the kitchen, and foyers) they can find. Since all the dry rooms have already been occupied by multiple other sleepers each, they now have to resort to having only bunk beds in all bedrooms. Of course, having that many of people in any property unit is currently illegal due to timely evacuation under current fire laws, but let's say that it's been waived for this scenario. Let's also say that none of the buildings have been reinforced for this, but meet all other codes and are maintained in great condition.

The minimum load (unknown if that means rated or design limit) for "sleeping rooms" (according to the IBC) is 30 psf. Let's say that the weight of the heavy-duty steel king-size triple bunk bed is 400 lbs (very high estimate), all 3 king-size mattresses are each 180 lbs (highest amount for commercially produced ones), and that all 3 mattress topping sets are comprehensive and are each 30 pounds (extremely high estimate). This means that the bed setup has an overall empty operating weight of 1030 pounds, which is the same as the curb weight of a 4-seater golf cart. A king size bed has dimensions of 76 inches by 80 inches.

Let's also give an aisle on one side of the bed so that people can get on without passing through other beds. Let's make the aisle have the width as that in a school bus, which is 12 inches. However, the aisle is shared between facing beds, so that the aisle width per unit cell is only half that. Assume that each bed corner support post is 2 inches by 2 inches (fairly small estimate). That gives a bedset unit cell of 86 inches by 84 inches, which is an area of 7,224 in^2 = 50.16667 sqft.

Let's say that everyone who sleeps in the bed is the heaviest person who is healthy. The tallest height for a healthy person is 6'6", the heaviest sex for any given height is male, and the heaviest weight for 6'6" is 230 pounds. That means everyone sleeping in the bed is an extremely muscular (but not insanely, because being too muscular is unhealthy) 6'6" tall male. 2 of these men are able to fit with plenty of space remaining in a queen size bed, while 3 of them cannot fit in it without one turning his body or sleeping on top. So, 3 of these men sleeping on every king-size mattress was chosen because they give the greatest density within a standard size while being able to totally fit inside. The weight of clothing and shoes are insignificant, so they will be ignored here. The total weight of all men on the bed is 2070 lbs. This gives a total system bed weight of 3100 pounds, which is the heavy-duty bed can definitely support. This means each unit cell has a night average load concentration of 61.794 psf. Let's also say that they do not have to work and genuinely enjoy sleeping with each other during free time (and they all quickly become best friends forever within a week from that), so they spend an average of 20 hours laying in bed, 12 cumulative of them being sleep, for every calendar day including weekends and holidays.

If all dry rooms (including bedrooms) were loaded to a 20-hour average of 61.794 psf and all wet rooms loaded to a 24-hour average of 40 psf, both in every calendar day, in a well-maintained, light wooden-framed multi-storey building that was built to the minimum standard (30 psf for bedrooms and 40 psf for other dry rooms) in a location that is seismically stable, has little wind, and has little snow, will it have a not-insignificant chance (e.g., greater than one in a million chance of collapsing within any given year) by itself of causing the bedroom floors to pancake on top of each other? Will it even have a not-insignificant chance of causing the entire building to collapse? Given that it's well maintained and located in a geographically easy place, external factors such as natural disasters, water damage, erosion, and termite damage will not be a thing here. The only factor remaining will be structural. Also, this is way different from college students crowding together and dancing in a room because there are severe impact loads and significant resonance there but no impact loads here.

 

[BridgeSmith]

The point of this discussion is to see how much over rated limit and how much over the design limit a system or an individual structural part typically fails.

It varies so widely, there is no typical limit where that can be predicted.

Under the Load and Resistance Factor Design (LRFD) model, materials are assigned resistance factors based on the variability in the material strengths, and the loads are assigned load factors based on the variability in the magnitudes expected. In the case of bridge design, the combination of those are calibrated with the aim of reaching a consistent reliability index value, currently 2.5. That is not to say every new bridge would fail at a load 2.5 times it's design load, or any value particularly close to that. It's really all a best guess, because despite our efforts at quality control of the materials, we don't know what the actual strengths of the components are. Also, despite improvements in structural modeling We don't know with any great degree of accuracy how the applied loads will affect the stresses in the structural members.

The main thing I want to know is if heavy-duty tri-level bunk beds will overload the floor.

Then why did you begin by asking about snow loads, and then broaden the scope to include all structural engineering?

I'm done. I've wasted enough time on this.

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

 

[RontheRedneck]

Regarding the comment by Kwan Kok Ko that a 30 PSF live load is allowed for 2nd floor sleeping rooms -
I have never worked for a truss company that was willing to do that. I'm not saying that it's absolutely never done. But I don't think it's common.

 

[rb1957]

1st problem ... 9 230lb persons should not (in any reasonable world I know of) be sleeping in a triple bunk bed.

You want to put 1 ton on four point forces ... there's your second problem !

yeah, I'm with Ron ... I'm out, see you in the pub.



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

 

[DurableEfficientGood]

I never knew the difference is strengths between true and nominal varied so widely. On another topic, that's because the extended snow load is a much more realistic scenario than 9 large men in a triple bunk bed.

 

[DurableEfficientGood]

As for a 1 ton force on 4 point sorces, I forgot to say that the bed posts have a significantly wider base, meaning that the load concentration directly under there would be only a small fraction of what it would have been otherwise.

 

[DurableEfficientGood]

In other words, I'm trying to know whether or not a light wooden-framed building built to the minimum safety factor allowed by the laxest manual from the International Code Council (probably the International Residential Code), in good condition, without structural defects, and having dynamic loads below 20% of that of static loads, being sustained statically overloaded to 2.06 times the rated load for all bedrooms and being sustained statically overloaded to around 1.86 times the rated load for the average among all floor space will have a greater than negligible chance of collapsing, either over the the short run of a week or over the building's design life of 30+ years.

 

[DurableEfficientGood]

RontheRedneck,

Then how about 6 230-lb 6ft-6in tall highly fit men in a full size (54" * 75" = 4050 sqin = 28.125 sqft) triple bunk bed, where it is actually a decently comfortable size for those highly fine men to sleep in? Add 2 inches on all sides for the support posts and 6 inches on one side for half of a standard school-bus aisle (there will be a full-width aisle because of another unit cell with a reflected orientation on the other side), and you've got a unit cell dimension of 64" * 79", which is 5056 sqin, or 35.111 sqft. Also, the support posts are highly flared into the space under the bottom bunk, so that it ensures the load will be spread to a drastically larger area to ensure that it won't punch through the floor board while not increasing the footprint of the bed in the slightest.

Since the bed's structure will be super heavy duty, it will be very heavy for its size, weighing in at 300 pounds, in order to ensure that it can support all loads. All mattresses will use the heaviest full size one commercially available on the market, which is 60 pounds. Each bed-topping set is the heaviest commercially available full size full-featured set, clocking in at 30 pounds. That gives a total operating empty weight of 570 pounds and a total weight of 1950 pounds when including the handsome men. The weight of their clothing can be ignored because they will only be lightly dressed at most from being super warm under all the blankets. This means the concentrated load within the footprint of the bed is 69.333 psf, with the average load for the unit cell (which is the room average floor load) being 55.5 psf. The last figure is 85% over the bedroom floor's rated load of 30 psf, with the floor being built to the standard factor of safety. There is an inconsequential amount of dynamic load since there is not enough room for the humans to jump.

1. Will that cause the floor to fail if it is built to the IRC's minimum factor of safety?
2. How about if it is built to the IBC's minimum factor of safety?
3. How about if it is built to the minimum industry standard (industry standard's minimum rated load and industry's minimum factor of safety) in the USA and Canada?

Some people would actually want (given a hypothetical super severe housing shortage) to sleep in this bed with all the people in the other beds sharing the same room if they're all very close friends (or gay partners) and get to stay there long-term for almost free (because they own the condo unit located on the second story or higher and initial purchase price and recurring taxes divided evenly among all 36+ fully grown men), so this case is orders of magnitude more realistic than the last one.