Quote (OP)

How do you justify that its been that way for 70 years, the wood is just creeping, and that its fine, when you know the design would never check.

You could attempt to quantify what is almost certainly the reality of why the building remains sound: diaphragm action from the sheathing, particularly if there is celling framing effectively acting as a tension tie between the rafter bearing seats.

Did the roofing material material itself change from initial? People often go from cedar shake or shingle to cement tile, and that adds something like 3-5 lb/sqft and the roof surface will often show sagging. If they did, then the roofer might have pulled a permit to do so, and you can reference approval of the permit and the post-installation inspection. Is the sheathing full sheets of plywood? Cedar shake is often installed on lathing that matches the roofing course dimensions. Presumably, one can do an analysis of the roof structure to show that supports the wood sheathing and roofing material.

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I looked at a very similar setup a few weeks back. A robust, 2x3 ceiling diaphragm exists down at the batt insulation. In this instance, I didn't bother trying to quantify anything. I just said:

1) Here's what you've got and how I think it works.

2) Been there since 1910 and it isn't going any place unless something changes.

3) Revisit this with a pro if you plan to do any renos that would add load or mess with the roof or ceiling sheathing.

Just because it has stood that way for 70 years doesn't mean it will for another 70. Has it ever seen the full design load mandated by current code? Probably not. So be careful in saying that it's ok because it's been ok in the past. Say it's ok because you believe that it is, based on your engineering judgement.

Where I live, houses built after 1954 have not gone through a design event. Even most of the houses built prior to that probably didn't see the design level winds in 1954, as that was measured by a buoy on the water and not over land. And yes, that is accounting for variations in reporting based on elevation and duration of gust. So for me, a house has to be waterfront and at least 66 years old for me to consider the possibility that it was exposed to a design level event.

Thanks.
The roof covering is shingles, certainly not original. It is 4x8 sheets, with some repair areas cut in here and there.

Its south west ohio, certainly has seen it share of snow, and its not a huge pitch either, 5 or 6 on 12.

Hand-frame roof rafters? Are there collar ties? Otherwise, what Koot pointed out, the ceiling joists help keep pulling it inward. Make sure you separate the dead load on the actual roof versus on the ceiling. I think it is about 50-50 split, but it can make a difference.

Quote (phanENG)

Just because it has stood that way for 70 years doesn't mean it will for another 70.

In a probabilistic sense, a past history of successful performance absolutely implies an improved likelihood of future successful performance, even if the design loading event has not been reached. In the Canadian code, we actually have nifty provisions for this that allow us to adjust the safety factors accordingly. That, using the exact same probabilistic logic used to generate load and material safety factors in the first place.

With each passing year, a successful structure is proving its worth to a greater degree.

KootK - certainly, especially as it relates to dead and live loads and other more predictable loads. Reference design values for wood put you down in about the 5th percentile for wood strength. So once it's been built and has proven it can hold the design loads, you've proven you're at least there and likely higher, so a reduction safety factors is reasonable. I'm a little more cautious when it comes to more extreme weather/natural events, though. I'm in a hurricane prone region, and when it comes to saying an existing roof structure or uplift load path is good, I take a pretty close look and typically discount it's past performance. It didn't get ripped off during the first severe thunderstorm, and that's good, but it hasn't seen a category 2 or 3 hurricane, either.

Another thing to think about is load duration and cumulative load effects. OP is in Ohio, with pretty good snow loading. Load duration factor for snow is typically taken as 1.15, which corresponds to 2 months of cumulative time at the design snow load. I imagine a 70 year old structure has eaten into those 2 months considerably, which may undo some of the gains from the probabilistic gains based on age.

And that's neat that you have that built into the code. If anyone is aware of it in US codes, please share.

Quote (phamENG)

I'm a little more cautious when it comes to more extreme weather/natural events, though.

So is our code treatment of this. Reliability level is based on how long it's been in service and some other stuff.

Brain teaser: if a roof deigned for snow survives another year in which no snow falls, has its probabilistic capacity to resist snow increased over that year? I think so.

OP says it is "just creeping over time," but what about the possibility of creep-rupture?

I'll write the report just like your observation - bowing, sagging, no immediate danger (need some calculation here), but expecting re-roofing soon. All facts, no guess works.

Quote (KootK)

Brain teaser: if a roof deigned for snow survives another year in which no snow falls, has its probabilistic capacity to resist snow increased over that year? I think so.

It has to experience loads above that which it has previously experienced for its predicted capacity to be improved. Its capacity doesn’t improve just sitting there doing nothing.

Quote (r13)

All facts, no guess works.

In a building that's 70 years old, it's unlikely that you'll be able to take the allowable material stresses as any kind of "fact". At least not unless you bring out a lumber grader or send samples off to the lab which is rare for residential work. The line between reasonable estimation and educated guesswork can start to get a bit blurred...

Quote (tomfh)

It has to experience loads above that which it has previously experienced for its predicted capacity to be improved. Its capacity doesn’t improve just sitting there doing nothing.

I see now that I should have phrased that more precisely.

Quote (KootK)

Brain teaser: if a roof deigned for snow survives another year in which no snow falls, has its probabilistic capacity to resist specified snow loads increased over that year? I think so.

When a additional year passes with no snow, it helps to demonstrate that the original factor of safety used for snow load evaluation was unduly high. In accordance with the relation shown below, that effectively increases the specified load that can be resisted. It's just working the other side of the Demand/Capacity equation.

Maximum Specified Snow Load = Factored Specified Snow Load / Appropriate Safety Factor.

Quote (StrucDesignEIT)

OP says it is "just creeping over time," but what about the possibility of creep-rupture?

I believe that the load duration factor takes account of just that phenomenon. phamENG raises an interesting point about the using up of the assumed load duration though. When we say that the appropriate load duration factor for snow is two months, what lifespan are we assuming for the building in that statement? Fifty years? Eventually, most buildings outlive their design lifespan.

My own house is 42 years old. Does that mean that, in eight years, I'll need to reinforce my roof against an eminent creep rupture failure because I've statistically used up more than my allotted two months and the originally assumed load duration factor is no longer valid?

I'd have to do some research in order to even figure how to reinforce for an eminent creep rupture failure. Jack stuff up and reinforce it such that the original members will be exposed to a narrower range of stress in the future??

Quote:

In a building that's 70 years old...

Too many possibilities beyond a simple observation can tell. It stand for 70 years is a fact, the bow and sagging are facts, will it be standing on tomorrow after a historical event, through some calculation, it can be a fact by judgement. A warning for corrective action in the future is sufficient, unless the client asks "how long".

Quote (Kootk)

When a additional year passes with no snow, it helps to demonstrate that the original factor of safety used for snow load evaluation was unduly high

Do you mean an additional year of no snow reduces the theoretical design snow load, by reducing statistical likelihood of snow events?

Quote (Tomfh)

Do you mean an additional year of no snow reduces the theoretical design snow load, by reducing statistical likelihood of snow events?

I think so but, again, in the interest of precision I would rephrase that as:

1) The likelihood of the originally assumed, design sow event (specified load) occurring in any one year remains unchanged over time and is purely a function of the climactic record and probabilistic assessment.

2) With each additional year that passes where the actual snow load experienced does not exceed the specified snow load from #1, we have more confidence that the ultimate limit state snow load will exceed the specified snow load by a factor less than was originally assumed when the structure had no available history of performance.

3) #2 Implies that a lower factor of safety on the loads is appropriate.

Statistics isn't really my strong suit but my fundamental understanding of this is essentially this:

4) A successful structural performance history adds information that wasn't known when the structure was brand new.

5) All new information reduces uncertianty.

6) Reduced uncertainty should rationally lower appropriate safety factors since uncertainty is precisely what safety factors are meant to account for.

It's a bit like the classic Monty Hall problem. When a year passes without snow, that's a bit like opening one of the doors without a car behind it. You know more and can therefore guess better.

Quote (Kootk)

1) The likelihood of the originally assumed, design sow event (specified load) occurring in any one year remains unchanged over time and is purely a function of the climactic record and probabilistic assessment.

Our additional year of no snow increases our climate record, which could be used to recalibrate the snow probability model (however slightly), and this in my view is the only conceivable way the probabilities are affected.

It's not the Monty Hall problem. Snow events are not fixed goats and prizes behind doors that you progressively open.

Quote (r13)

..it can be a fact by judgement.

In my book, a fact by judgement is somewhere between:

1) An oxymoron and;

2) A professional opinion.

And that's really what I've been driving at. I think it overly simplistic to be giving the impression that historic evaluation work is binary stuff where an ethical engineer need merely "say what they can prove to be fact". Were that the case, we wouldn't be saying much of value.

I would temper such statements to something more along the lines of "say what you believe to be accurate and can provide plausible justification for". Structural evaluation of existing building involves a very real dimension of "guesswork". I feel that it is misleading to suggest otherwise.

Quote (Tomfh)

It's not the Monty Hall problem.

I didn't say that it was the Monty hall problem. I said that it shares one important feature of the Monty Hall problem: new information is added along the way. That information being:

1) A successful performance history in the case of an existing structure and;

2) Where the car is not in the case of the Monty Hall problem.

Quote (Tomfh)

Snow events are not fixed goats and prizes behind doors that you progressively open.

I disagree. The goats and the prizes are not in fact fixed until after they are revealed. Rather, they exist as outcomes with associated probabilities just like Schrodinger's cat and next year's snowfall.

Quote:

2) With each additional year that passes where the actual snow load experienced does not exceed the specified snow load from #1, we have more confidence that the ultimate limit state snow load will exceed the specified snow load by a factor less than was originally assumed when the structure had no available history of performance.

Not sure I buy this line of reasoning; there have been lots of instances where nothing significant happens for many years, and then, we get hit with multiple "100 yr" events. Moreover, given climate change, it's arguable that the previous weather events were less challenging than weather events going forward.

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Quote (IRstuff)

Not sure I buy this line of reasoning; there have been lots of instances where nothing significant happens for many years, and then, we get hit with multiple "100 yr" events. Moreover, given climate change, it's arguable that the previous weather events were less challenging than weather events going forward.

Such is the nature of probabilistic events that pretty much anything can happen at any roll of the metaphorical dice. At worst, this just means that recent and future extreme events will need to be incorporated into our statistical models going forward. And such updating was always the plan.

Quote (Kootk)

I disagree. The goats and the prizes are not in fact fixed until after they are revealed. Rather, they exist as outcomes with associated probabilities just like Schrodinger's cat and next year's snowfall.

The goats and prizes are chosen in advance, are fixed, and are known to the presenter, and are progressively used up like a deck of cards - which is the essence of the Monty Hall problem. It is probability without replacement. The future probabilities are strengthened as a door is opened.

Random quantum mechanics outcomes and future snow events are random events. Probability with replacement. You don't change the future odds by knowing the result of this year. You do potentially improve your statistical model by adding to your data set, but that's all.

Due to my own, limited knowledge of statistics, I'm likely to run out of ammunition with which to support my positions here. That said, the National Building Code of Canada contains a procedure that does just what I've described: it reduces load factors based on a demonstrated history of non-overload. I encourage anyone who questions the approach to check out both the procedure and the references that accompany it for more information. It's neat stuff.

Quote:

given climate change, it's arguable that the previous weather events were less challenging than weather events going forward.

It is true regionally only. Climate change causes extreme weather pattern changes around the globe. Where I live has just experienced the snowy winter and coldest spring in decades.

Quote (Kootk)

That said, the National Building Code of Canada contains a procedure that does just what I've described: it reduces load factors based on a demonstrated history of non-overload.

Yes, the structure has already been loaded to working loads and thus a proportion of the original safety factor is now redundant. We know it's good for at least working loads. So we needn't add the same amount of fat as a completely untested structure.

KootK - maybe I'm just tired, but I'm a little confused by your wording in number 2. I'm going to take my own shot at rephrasing it. Instead of:

Quote (KootK)

2) With each additional year that passes where the actual snow load experienced does not exceed the specified snow load from #1, we have more confidence that the ultimate limit state snow load will exceed the specified snow load by a factor less than was originally assumed when the structure had no available history of performance.

Should we say:

2) With each additional year that passes where the actual snow load experienced does not exceed the specified snow load from #1, the probability of a design snow even randomly occurring within the remaining service life of the structure is reduced.

This approach makes sense to me. As Tomfh mentioned, we're dealing with probability with replacement. It can occur an unknown number of times, at unknown intervals, and with an unknown upper limit. However, we can use the probabilistic forecasting to determine approximately what interval and intensity we think will occur. The probability of an event occurring within a specified period of time is proportional to the interval considered. For instance, what's the probability of a 100 year snow occurring in the next 5 minutes? Extremely low. This winter? Higher, but still pretty low. Then it'll shift until you get to considering 100 years - we'd be looking at something closer to 99% that it would occur in our considered time period. So going back to our building - as we progress through the service life the remaining service life is getting shorter. So we can say that the probability of one of these random events occurring within the remaining service life is being reduced. The chances of it happening in a given year hasn't changed, but the probability of that year being in our subject time frame does change. Statistics isn't my strongest subject either, so I don't remember the academic term for this - compounding probabilities? Something like that.

As for using up the load duration, it does add an interesting dimension to the evaluation of existing structures, doesn't it? Might even warrant its own discussion. Load duration factors do, indeed, exist to prevent creep rupture, but what happens when a building outlasts it's "design life" and what is the standard design life? In most cases, it seems like it won't matter. See page 29 and 30 of this PDF. Way up north in the ice and snow, however, you may have a problem if you regularly see design snow events. If you're regularly below the full design snow load, then I wouldn't worry about it. If you are, and there's a legitimate chance you're approaching your "limit" then you can analyze the existing for a Cd=0.9 and reinforce as required.

Quote (Phameng)

So going back to our building - as we progress through the service life the remaining service life is getting shorter.

That’s valid too. You generally design for about 10x your expected service life. 50 year life you design for about 1 in 500 year event. Less remaining life means you can design for less extreme event for the remaining period.

It’s no way near linear though. It’s not until you’re almost thru the lifespan that it makes much difference.

jstruct.....you mentioned the house is 70 years old. If it has plywood or OSB sheathing, then those were added at a much later date. Houses built 70 years ago had individual dimensional lumber as the roof substrate.....similar to the photos KootK showed.

[ ]

Firstly, I feel that a public service announcement is in order here regarding the NBCC Chapter L provisions. Based on the turn that this conversation has taken, one might get the impression that it's principally about justifying lower specified loads. It's not. Rather, it's a wholistic approach that integrates many aspects of structural reliability including:

1) load variation
2) excess structural capacity
3) performance history
4) field / plan verification that details aren't load path abominations
....
....
75) in a very real way, society effectively deciding "meh, maybe we just don't care that much after all for this particular situation".

Quote (phamENG)

I'm going to take my own shot at rephrasing it.

Your point is interesting and valid in my opinion. It is not, however, an accurate restatement of my position. I'll try again, by way of example.

1) Assume a structure located where the code flat roof design snow load is 30 PSF.

2) Assume a structure where analysis has indicated that the roof is only capable of supporting a 20 PSF snow load.

3) With each passing year of successful performance, does it not become increasingly probable that some combination of the following is true:

a) The reserve capacity of the roof has been underestimated and/or;

b) The flat roof snow load specific to this site and this building should be lower than the 30 PSF value determined elsewhere in the state?

That's it. Similar thinking is apparent in the NBCC Chapter L provision shown below.

In my opinion, it is not correct to be asking:

"What will be the peak snow load next year at the meteoritical station where we collect our data?"

But, rather.

"What site specific, peak snow load should be considered given that this structure has 20 years of service lift left and we know that it hasn't collapsed in the last 30 yrs even though analysis would indicate that it is 10 PSF under capacity?"

Those are very different questions, the latter incorporating new information only made available with the introduction of a successful building performance history.

If the house you are describing is in an area controlled by Roof Live rather than Snow, that is the primary reason it has stood for 70 years. Where I live, I have seen at least a 100 roofs that would have failed in a 20 psf snow load area but have stood here for many years. Some of these roofs are less than 10 years old. See the photo below of a newer house. Note the ridge board running front to rear only has rafters on one side. Has not failed and shows no obvious signs of problems at this time. However, it has never had more than the weight of shingles, decking and probably one person on it since the temporary bracing was removed. While it has some vertical braces (not many) it has nothing to inhibit sideways movement.

I am fairly sure 2x4 rafters are maxed out at about 7'. In the past I have had to pass judgement on roofs such as you depict.

"It has functioned for years with the loads that have been imposed on it but that is no guarantee larger loads will not be imposed on it. It currently only provides x% of what a modern code requires."

Depends who you're writing it for to a certain extent as to how you frame the recomendation.

The seller wants a simple clean bill of health, nothing wrong, no problems, you don't need to do anything type of report.

The buyer / mortgage company wants something similar, but with an obvious sag, you may want to add something along the lines of

"Doesn't / wouldn't comply with current code design, no current signs of failure / distress, but may benefit in in future from some strengthening to arrest any further sagging which may occur over time or after a particularly high snow load". This likely to cost in the region of $xxx.

Then leave the two to haggle a bit over the price.

I bought a house with a visibly sagging roof / valley beam and cracked ceilings due to replacement of slate tiles with concrete ones ( 4 x weight) and minimal reinforcement of the roof structure. Got a report which said something like that and we negotiated a reduction in price to accommodate the works which we got done a few months later.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Quote:

Structurally Its in no worse shape than the 100 other houses built in the neighborhood. Just need to find a way to say in a report.

Lack of care and maintenance.

KootK - I see where you're going with it now, and I don't disagree - but I feel like there's a bit of a gap that has to be bridged before I completely agree.

I certainly agree that site specific micro-climate effects can reduce (or increase) probabilistic snow load, and if possible to incorporate that information it should be done. But it seems like a leap to say that, because a design snow event hasn't taken place in the first 30 years of the buildings service life, it must be estimated as too high. I think there needs to be more information. Did the measuring station for which this region is based experience the design snow, or did other surrounding areas experience it? If so, then I'm with you 100%. This site should likely be reduced. ASCE 7 uses a 50 year MRI (2% annual probability of being exceeded), so it seems there would be no reason to reduce the loading at 30 years unless there's supporting evidence that it has seen it's local peak.

I can buy the reserve capacity argument. It doesn't "feel right" but focusing on the logic - it makes sense.

Not to hijack my own post, here is a picture too of a 'bracing scheme' or attempt to truss the roof. the bracing shown and the metal strap are mirrored about the center line. The come down to meet on a metal clip. I see these a somewhat original to the house, as the hardware is older. (perhaps someone had old hardware, square nuts, etc laying around)The lumber does not seem to match the rafters exactly like it was from the same lot.

The buyer is asking for money to add these braces at each rafter. In my mind that does not fix the bowing, as they are all bowed, even those with bracing.

The only thing that will fix the bowing is to replace the roof. To prevent future, additional bowing, trussing out all of the rafters may be beneficial. Right now, the trusses are stiffer than the rafters. So as the rafters deflect, the load is transferred to the trusses through the sheathing (there is a bit of load sharing through the deck). So the truss sees a disproportionate amount of the roof load. So then it deflects. If you stiffen all of the rafters by turning them into trusses, then you'll require less load sharing to get the load to a stiff element and the aggregate load path will be a bit more spread out.

Quote (phamENG)

I can buy the reserve capacity argument. It doesn't "feel right" but focusing on the logic - it makes sense.

Quote (phamENG)

But it seems like a leap to say that, because a design snow event hasn't taken place in the first 30 years of the buildings service life, it must be estimated as too high.

The reality of what the NBCC procedure is doing is likely to feel even "less right" than what you're thinking:

1) Firstly, the method does get into load testing and, as one would think, treats that as the gold standard.

2) When load testing isn't done, they still give you a way to use the lowered load factors, albeit to lesser effect.

3) Although the relaxations are expressed as reductions to load factors, my understanding is that those reduction are meant to include the effects of a number things in addition to a possible reduction of applicable loads. The load factor was just a convenient bucket into which to slop everything into. Maybe a better expression would have been:

phi_resistance x nominal resistance >= alpha_load x specified load / kappa_voodoo. Truth in advertising.

4) The NBCC procedure, as far as I can tell, actually makes no attempt to separate or quantify the relative impacts of reserve capacity and excessive snow loads in the absence of load testing. It just says "hey, the numbers predict a failure that isn't occurring so something must be up".

As you know, statisticians are able to do something resembling magic in that they can often extract meaningful results from data without actually knowing all of the the details of what's going on. Like the difference between Newton and Schroedinger. Rest assured that I find it as intellectually unsatisfying as you do. I've tried to read some primers on structural reliability but I'm afraid the math bucks me before I get to the reveal and I lose my grasp of the real world meaning long before that.

For all its dark magic character, I do respect the NBCC method and find it to be uncommonly pragmatic for code stuff. I did my first ten in Wisconsin with lots of historic Milwaukee renovations and all the usual struggles with quantifying capacity and being the bearer of bad/expensive news. When I returned to Canada, I discovered chapter L and was like "Where has this been all my life??". I go into most renovations with a gut feel for whether or not I really think that invasive work is necessary. With chapter L, I find that the calculated answer jives with my gut feel far more often. And I like that.

Load testing! Excellent. Good to know that's in there and actually makes it feel a little better. The link I posted early to the Timber Frame Engineering Council gets into some load testing, though it cautions against some of the accepted standards as being overly conservative and potentially harmful to the structure (going back to the cumulative damage/load duration/creep rupture discussion).

In the absence of load testing, you're right - probabilistic voodoo. Statistics is intriguing, infuriating, fascinating, and frustrating.

I keep waffling on my master's research - maybe I'll play with something in this vein as I do a good bit of historic work here in Virginia and this is a constant issue.

Quote (phamENG)

The link I posted early to the Timber Frame Engineering Council...

And thanks for that. Killer info to have on hand for timber renovation and the consideration of the load duration factor.

Quote (phamENG)

...maybe I'll play with something in this vein as I do a good bit of historic work here in Virginia and this is a constant issue.

I feel that structural reliability in general would be a great choice. I'm in oil country and I know of a couple of guys that make a killing doing reliability work for industrial applications. They'll get paid more to figure out if a pipe will last 50 yr or 30 yr than I'll get for an entire building. It appears to be kind neat "sciency" work too for still being nominally within the realm of civil engineering. They don't have the supply and demand problem that we do (yet). I kinda feel like AI might be able to make hay in that space when the time comes.