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RE: Wind load and pile depths for wooden fences (San Jose)
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- Subject: RE: Wind load and pile depths for wooden fences (San Jose)
- From: "Conrad Harrison" <sch.tectonic(--nospam--at)bigpond.com>
- Date: Fri, 28 Aug 2009 17:03:16 +0930
Dmitri, I don't believe there was any deliberate intent to hide so called safety factors, it is more to do with subtle use of the English language. Reliability is a statistical matter, and in statistics when carrying out an hypothesis test, an hypothesis is never fully accepted, rather it is not rejected. Thus we say: we do not reject the hypothesis that the mean yield strength of the material is 300MPa, rather than conclude that the mean yield strength is 300MPa. Also in mechanical design, I had it hammered in, never to refer to factors of safety, but to refer to design factors or factors of ignorance. A factor of safety infers safety, when no such safety is provided. Reference to factor of ignorance also not all that good, since multiplying by 100, may still only get 1% of the real load, if have high level of ignorance of the system being designed. We deal with uncertainty, and I believe those advocating the limit state approach, here, believe we should deal more directly with the risk and uncertainty, and not be misled by so called safety factors in the codes. Codes do not produce safe designs, rather they produce a design which some committee considers to have an acceptable risk of failure: the designers and end-users may have a different view. If a failure occurs the jury and coroner may also have a different view. Complying with the codes doesn't get you off the hook. The designer needs have some understanding of variability and uncertainty. My understanding of the soft conversion to limit state relative to steel structures is as follows: Z = elastic section modulus Fy = yield strength M = applied moment Permissible stress design M = 0.6 Fy*Z Or M / (0.6Fy*Z) <= 1 Rearranging the equality gives: 1.67M = Fy*Z Suggesting to some to declare that the structure is 67% stronger than it needs to be. Which totally neglects the variation in Fy and Z. It also declares that M is the actual value of the action-effect likely to be experienced by the structure, or that its maximum value is certain. Soft conversion to limit state is therefore to split the design factor, into two components: capacity reduction factor, and load factor. To then give. 1.5M <= 0.9Fy*Z Or 0.9Fy*Z / 1.5M >= 1 Where for hard conversion, (0.9Z) should be replaced by the 5th percentile estimate of the section property being considered, and 1.5M is replaced by the 95th percentile estimate of the load. Other percentiles could be used, and more rigorous consideration of reliability make more complex still. (as you say wind, snow, and seismic have different return periods, or here different annual probabilities of exceedence. Which suggests the most recent change has invalidated the 5% probability of load exceedence.) Our resistances and loads are all estimates. In some situations the standard deviations on the resistances and the loads are low, in others they are large. By improving control in manufacturing the standard deviation on the resistance can be reduced so that the 5th percentile resistance is a lot closer to the mean value. Not all structures are buildings, and for many structures it is possible to have a high level of control over the loading of the structure, so that 95th percentile load can be calculated and the standard deviation kept low so that close to the mean or nominal value of loading. The steel structures code may indicate that it primarily relates to buildings but it is used for more than just buildings. Further more increased control over the operating environment can eliminate the need to increase required design loads. Thus research into crowds and panic behaviour is leading to improved approaches for evacuating sports stadia, without need to increase design loads on associated structures. Since we largely use nominal values in design, we replace 0.9Z with phi*Z, and 1.5M with chi*M (strictly our code only uses chi for serviceability load reductions, but it is still a load factor, so for convenience I use as such). The values of phi and chi, are then supposedly determined so that nominal values are transformed into the required 5th and 95th percentile values. And a load is not taken as the 95th percentile value if it has a stabilising influence, or otherwise provides a resistance, then it needs to be a 5th percentile value. In the soft conversion phi, may provide allowance for other aspects of uncertainty other than variation in section properties. For this purpose I think there should be additional capacity reduction factors and load factors, and their purpose more explicitly identified. I don't see any problem with doing so, the timber structures code has a multitude of 'k' values to adjust resistance of a section. Also there is nothing stopping anyone writing: 1) N = phi*Fy*Z / chi*M 2) N >= 1 Or 3) n = chi*M / phi*Fy*Z 4) n <= 1 5) n = 1/N Where by the design factors (N,n) give an indication of efficiency relative to the requirements of the code, not an indication of greater or lesser safety. Equality is seldom achieved, for few sections are optimised for a specific purpose. As I indicated previously the advocates of risk based design, cite how patients are comfortable with a life or death situation and making a decision when doctor tells them operation has a 30% success rate. Engineering tends to have a far better success rate. The objective is not to obscure but to place in more familiar terms. It has also been noted, that with increased emphasis on statistical process control, and risk management requirements for occupational health and safety, that a greater proportion of the population has understanding of the problems of uncertainty, variability and risk. So explaining risk to the public is not a major issue. For the advocates of risk based design, engineers who insist on quoting safety factors of 2 and the likes, they are part of the problem, not the solution. They are the ones who distort public perceptions, and generate a public demanding safer structures. After all before building collapsed did say it had a factor of safety of 2. If it collapsed it must have been less than one. And therefore can engineers be relied on? Know the storey of the USS Pueblo? Before probability of US Ship being boarded zero. After increased to 1. If there is a 5% probability that the design load will be exceeded and a 5% probability the load will not be up to strength, then there is a risk that the structure will fail. Can we prevent failure? Generally No. But we can reduce the probability of over load and the probability of lack of resistance, and exponentially increase the cost of supply. The requirements in the code suddenly become that more preferable rather than excessive and unnecessary cost. If it fails. We didn't say it wouldn't fail, nor at what time it would fail. The question is should it have failed under the conditions experienced? Sure may get that with a safety factor, but focus then is on: the engineer failed not the structure failed: because engineer implied it wouldn't. So I don't believe risk based design makes anything more complicated to the public, nor does it hide anything. The numbers can be discarded and a purely qualitative explanation of risks can be given to the public. So I guess I am more advocate than opponent. Also from my perspective it fits in with philosophical basis of Taguchi's methods, design of experiments and principles of quality robust design. At its simplest: need to recognise variation and allow for in design. Quality robust design also tends to place a focus on the possibility of failure, and the mode of failure, the behaviour of the system at failure. So a single numerical factor of safety doesn't really inform about anything. As you indicated there is buckling, yield, fracture, progressive collapse, formation of plastic hinges, and more. These may lead to early failure, or provide reserve and early warning of impending failure. I don't believe a factor of safety would generate the better understanding that you were wishing to promote. That understanding is not about the numbers but qualitative awareness of the behaviour of the whole structure, not just its component parts. Regards Conrad Harrison B.Tech (mfg & mech), MIIE, gradTIEAust mailto:sch.tectonic(--nospam--at)bigpond.com Adelaide South Australia ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp * * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: * * http://www.seaint.org/sealist1.asp * * Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********
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