Implicit Safety Factors in Wood Design 3

[Hickory]

It looks to me like the baseline design values for wood compression perpendicular to the grain are way too high. For white oak dimension lumber, for example, the design value is 800 psi (American Wood Council: Design Values for Joists and Rafters, 2005 edition, pg. 18), whereas the elastic limit found by testing small samples of clear wood is 1073 psi (Wood Handbook, pg. 4-11). This means that an implicit safety factor of SF = 1073/800 = 1.34 is hidden in the design values table. Back in the old days, however, the safety factors applied in such situations were never less than 4, and the use of 5, 6, or more was not unusual. (See

http://chestofbooks.com/architecture/Building-Trades-Pocketbook/Wooden-Columns.html.)

If we are going to take decisions about safety factors out of the hands of the engineer on the scene who is familiar with the details of the situation and place them in the hands of people in a smoke filled room thousands of miles away, shouldn't those people at least acknowledge their lack of on-the-scene information by using implicit safety factors that fall at the high end of the traditional range, rather than choosing values that fall far below the bottom of that range?

Can anybody provide a rationale for what is going on here? Is safety no longer a concern in what passes for structural engineering nowadays?

 

[csd72]

Hickory,

I read all your posts and I think you make some very interesting points, both political and technical.

In my few years I spent in the US I did find that the government seemed to be much more pro big business than most other places.

I also think the point about the effect of bearing stress failure on the overall structure is a valid one. In the eurocodes they refer to serviceability as a limit state for that reason.

 

[fel3]

WOW! I sure am glad I haven't done any wood design for about 20 years.

==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[woodman88]

"Bottom line: the only way to address this problem is the way it was addressed a hundred years ago: you need much larger safety factors for compression perpendicular to the grain...
...That's why Hurricane Andrew disproportionately destroyed the new homes and spared the very old ones."

and it could not be because there are a disproportionately more new homes existing, at the time of the hurricane, than very old homes. Also, of all the very old homes, how many were originally built and what proportion were destroyed before Hurricane Andrew because of poor design, or maintinance, or construction, or codes, etc.? Is it more or less than the percentage of new homes destroyed in Hurricane Andrew?
The Bottom Line is that only the well designed and made (and kept up) very old homes are still standing after fifty or hundred years. The same will be true of the buildings built today.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[Hickory]

Guys, I just wanted to have my say before they lower me into the ground and shovel in the dirt. Just a few more comments below, and then I leave you all to mull this over, or not, as you wish.

To those who very politely hinted that overly large design values for compression perpendicular to the grain raise mostly appearance issues, rather than safety issues, let me say that, timewise, you are correct. That is, most of the effects we see as wood structures invisibly lean this way and that under wind loads is, with the passage of sufficient time, gradual development of cracks in walls and ceilings, leaks in the roof and plumbing, annoying creaking noises in the structure when the wind is blowing, etc. These typically take years to reach the point of being regarded as a problem, and the issues raised are mostly related to money rather than safety. I don't know the actual numbers, but I would guess that maybe ten or twenty thousand homes a year nationwide reach the point where these sorts of problems are noticed and money is spent to deal with them, and, as several of you suggested, these are not really safety issues.

My point, however, is that as wind loading varies, so too does its effects. At the lower end--which is the normal state in a typical year for a randomly located house--we have only money issues. But in localities where wind loads temporarily rise above the normal range into the tropical storm region or that of one of the various categories of hurricane, these effects become safety related. And, though years can go by when that safety issue does not arise, the total number of homes for which it does arise, in my opinion, exceeds the number where only money is involved. If, for example, 35 years pass when, say, 10,000 homes a year show mere cosmetic defects due to this issue, and then, in one month, a category 5 hurricane reduces a million homes to kindling, the numbers that result over the entire 35 year period are this: 350,000 homes that show mere cosmetic effects, and 1 million homes that show safety-related effects.

Bottom line: despite the fact that most of the time these effects are not safety related, when we consider the total numbers of homes affected, we reach the opposite conclusion.

To woodman88:

I would suggest that if the more recently built homes were of comparable quality to the very old ones, then since there were vastly more of them when Hurricane Andrew rolled through, vastly more of the homes that remained standing should have been of relatively recent vintage. However, that was not the case. Virtually no wood homes built after 1950 survived in the footage I saw, despite the fact that they comprised almost the entirety of what was down there at the time. Nor could this have been some sort of sampling error. As I said, it was live footage that went on for approximately half an hour. And the helicopter was just following the track of the storm. There was no discussion of the age of the structures that survived or their quality. One of the things that amazed me the most, in fact, was the utter lack of awareness, on the part of the newscasters, of the fact that only the very oldest of the wood homes had survived the catastrophe. To me, that fact was so glaringly obvious that it seemed to jump off the screen in my face, yet the reporters did not notice. Frankly, I was shocked by the massive extent of the difference between the old structures and the new, where survivability was concerned.

 

[Lion06]

I just looked at this thread for the 10th time............................. and lauged for the 20th.

 

[MiketheEngineer]

War and Peace was smaller than this forum!!

 

[woodman88]

Money vs safety?
Shall we (engineers) force people to live in holes in the ground because they can not afford to live in a home or apartment? Cheap light-frame construction is the only home many people will ever be able to afford to live in. If we decide to force all homes and buildings to survive an hurricane like Andrew or last more than hundred years than where will most people live and work?
I certainly could not afford to live in my house if it were built at the level of safety Hickory seems to want it to be.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[Hickory]

To Woodman88:

You said:

"Cheap light-frame construction is the only home many people will ever be able to afford to live in. If we decide to force all homes and buildings to survive an hurricane like Andrew or last more than hundred years than where will most people live and work?"

My response:

If a customer who has been living in "cheap light-frame construction" in Arizona moves to Florida and wants to be equally safe, he is going to have to live in a smaller house with a heavier frame. The reason: hurricanes are routine in Florida, but unheard of in Arizona. Unfortunately, we have "one size fits all" building codes and the prevalent mindset is to "build it up to code" rather than "build it safe." Result: we have "cheap light-frame construction" in Arizona, where it is appropriate; and we have "cheap light-frame construction in Florida, where it is wildly inappropriate.

I believe the building trades worked far better a hundred years ago under unregulated laissez faire than they work today, under the regime of "building codes." As I said the other day:

"In a free market economy, standard conditions are assumed. If you build a house for a customer, there is an implied warranty of serviceability unless you expressly indicate otherwise. It is expected that you have produced a product which will perform its intended function under the conditions in which it is intended to be used, provided only that it is properly maintained."

That means if a customer in Florida insists on "cheap light-frame construction," you can build it for him, but there will no building code for you to hide behind. To protect yourself against future lawsuits you will have to inform the customer, in a very explicit, "on-the-record" way, that the house he wants probably will be destroyed by a hurricane, and that you will not be liable for damages if that happens.

In an unregulated free-market economy, in short, builders would be free to throw up the same sort of nice-looking crap all over Florida that Hurricane Andrew blew away in 1992, BUT ONLY IF THEY INFORMED THEIR POTENTIAL CUSTOMERS, IN ADVANCE, THAT THIS WAS WHAT THEY WERE DOING.

But, of course, if the customers had been informed in advance that their houses would be nice-looking crap, few would have opted to purchase them. Having that information, in most cases, would have influenced their decision making. They would have either opted in favor of smaller, more heavily built structures, or else they would have decided to move to some state other than Florida.

THAT'S WHY I HAVE SAID, REPEATEDLY, THAT THE EFFECT OF BUILDING CODES IS TO CLEAR A PATH THROUGH THE ECONOMY FOR "BUSINESSMEN" WHO INTEND TO ENGAGE IN FRAUD.

 

[woodman88]

“...we have "one size fits all" building codes ...”

Hickory,

You need to read a building code. All the codes for the last twenty or more years would require major changes to be done to a house designed for Arizona to be built in Florida.

Once you have read the code, then I might be willing to read the rest of your last comment.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[Hickory]

To Woodman88:

If you will deign to search this thread using the word "local" you will find that I noted early on that there are local variations in building codes. Such variations exist for a number of reasons, prominent among which is the desire of local regulatory authorities to appear to be "doing something" in the aftermath of adverse events that occur periodically in their areas of responsibility. What they never do, however, is address the real issue, which is the fact that they have that area of responsibility--i.e., the fact that laws have been passed giving them the authority to violate the property rights of private individuals who have committed no crimes, by telling them in vast detail how to design and build, and threatening them with fines and jail if they do not comply.

In short, it is no surprise that the Florida building code was changed subsequent to the Hurricane Andrew disaster; and it is no surprise that, as usual, the regulatory authorities failed to point the finger of blame in the proper direction: at themselves. What they should have done was admit that Hurricane Andrew proved, beyond any hope of redemption, that regulation of the building trades was, itself, the cause of the disaster; and they should have recommended its immediate abolition.

That, of course, would have put them and all the hordes of code-writing committees, building inspectors, and sundry other busybodies and enforcers out of work, and would have removed the competitive advantage enjoyed by the big builders who use building codes as a mask to hide the reality of shoddy construction practices. Thus addressing the actual cause of the disaster was an option that was never considered or even mentioned.

Bottom line: the short-term self interests of the big builders and the regulators whom they control always trumps the public interest, where "regulation in the public interest" is concerned.

I laughed out loud when I read your comment about not reading past my "one size fits all" statement, by the way. The reason: an image immediately popped into my mind of you standing in front of me holding out a crucifix and shouting: "Stand back, creature of the night! Stand back!"

It may be hard for you to believe, but (a) it won't hurt you to read things with which you disagree, and (b) people who know me well generally consider me to be a nice guy.

 

[IRstuff]

Seriously?

No building codes == Haiti, and Port-au-Prince in particular

We already know what unregulated building looks like...

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[Hickory]

To IRstuff:

Yes, seriously.

A lack of building codes is a wonderful thing, in a country where private property rights are secure, as they were in the U.S. a hundred years ago. Such a lack, however, is of no use in a country where property rights are pretty much nonexistent, as in present-day Haiti. Below, between the lines of asterisks, you will find a short quote which addresses this state of affairs.

**********

Economist Hernando de Soto’s celebrated book, The Mystery of Capital, gives some specifics about the pathetic state of private property rights in Haiti. He estimates that 68 percent of Haitian city dwellers and 97 percent of their rural counterparts live in housing for which no one has clear legal title. No one, mind you, no one!

Tell me, if you were building a house for which you do not have legal title, how interested would you be to use your resources to build a more durable structure? Not very, I’m sure. Certainly less interested compared to if you had clear title to the structure. After all, you’re unsure about whether someone can come along and take away “your” house, and you’re unsure about your ability to sell the house in the future. The resulting shabby construction won’t cause earthquakes, but it’ll make earthquake-related damages more extensive. Even fatal.

This lack of property title in Haiti is not surprising, says de Soto. For Haitians to settle legally on government (!) land, they must first lease it from the government for 5 years. Finalizing a lease requires 65 bureaucratic steps, taking 2 years on average. Then things get worse! Subsequent purchase requires another 111 bureaucratic steps, taking 12 more years — nineteen years of red tape in a country where, to compound the problem, illiteracy is pervasive.

**********
[Source:

http://www.thefreemanonline.org/headline/sowing-reaping-haiti/.}

 

[IRstuff]

I'll repeat; no building codes equals Port-au-Prince. We know that model does not work. We know that when left to tehir own devices, people cheat, people lock firedoors, people disable explosive gas warning devices, people don't bother to maintain fire sprinklers, etc.

These are all things that have happened even in the presence of regulations AND laws. Elimination of regulations and laws is not going make these people suddenly do the right thing.

The only other way to make it happen is summary lynching, i.e., when a building falls down or otherwise fails, the people responsible are strung up on the nearest tree or telephone pole. But, that is obviously contrary to the Consitution of this country.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize