As indicated else where across my blog, I hold an idealistic view of engineering. That idealistic view states that engineering only takes place at the frontiers of science and technology. I believe that this view is held out by history, and this started to be seriously distorted by the middle of the 1900's when the focus became on engineer as a licensed profession. Still I consider the WFEO Washington Accord, defining the engineer in terms of education, to support my view that engineering is at the frontier rather than concerned with established technologies. Most people with a B.Eng don't put the intent of their education to use, and instead become guardians of safety assessing suitability of proposed adaptations and implementations of the established generic technologies: technicians with an abstract and esoteric box of tools and techniques.
I believe that people have grabbed the wrong end of the stick when they consider Telford, Navier and Stephenson to be engineer's simply because they designed and built bridges. People were designing and building bridges long before they arrived on the scene, and people without the title engineer were designing and building bridges during their time.
The important factor is that they were operating at the limits of human knowledge and past experience, they were stepping into the unknown and the potential success of their endeavours was uncertain to them: however they weren't making wild guesses, they were progressing in a disciplined and learned manner. The past experience coming from the works of Vitruvius and the means of moving forward coming from Desaguliers Experimental Philosophy.
Telford was making use of materials and structural forms not previously used, He tested the materials and otherwise built smaller prototype versions of his bridges. These prototypes provided practice in the construction process, tested the concept and provided something with which to communicate the objective to workers on the larger projects.
Navier made use of untested mathematical theory which turned out to be a big mistake. But once he had validated the theory and calibrated it against reality it became a useful and productive theory for the design of all manner of beams.
Robert Stephenson along with William Fairbairn built prototype segments of a tubular bridge in a workshop and tested, and otherwise through trial and error resolved problems concerned with buckling of plates. With the services of Eaton Hodgkinson providing a mathematical assessment of the proposed bridge based on the then developing structural theories. Still a prototype bridge was built first before tackling the main project.
Today the technical science for establishing the suitability of a proposed bridge is well established. Whilst fitness-for-function is a matter of subjective judgement, technical science is available to assess whether the desired performance can be achieved from a given proposal. The uncertainty and risk of failure are low: but only so long as persons highly conversant in the technical science and the technical characteristics of the proposed variant of a generic technology are responsible for assessing and approving the proposed design.
When at the frontier, there is no expert to turn to, no literature with the answer, the answer has to be extracted from nature itself, and that requires trial and error experimentation. But experiments themselves can be dangerous. So a disciplined, rational and controlled approach needs to be taken to the experimentation. The trials and errors are not a result of wild guesses, but thoughtful consideration.
The ingenious contriver of civilisation asks questions and goes in search of answers. They do not sit on the authority of their formal education and approved license, for such is trite and inadequate for their role as pioneer pushing forward the frontier.
Society however is not asking for any frontiers of science and technology to be pushed forward. In the main they simply seek the proper implementation of the established technologies. With respect to these established technologies people have certain expectations, some reasonable others unreasonable and impractical. In terms of the reasonable, people do not expect: the wheels to fall off cars, they don't expect to fall through the floor of their house, they don't expect bridges to collapse when they drive over them, they don't expect ships to sink, or planes to fall out off the sky. These are established technologies and we can take reasonable steps to ensure they perform as expected.
Engineering science and Engineering made these technologies feasible in the first instance, they defined the generic class of technologies from which variants can be developed. The engineering is for all intents and purposes is over. Sure there are still frontiers associated with these technologies, but its a long journey through extensive literature before bump into the current frontier. Civil, industrial, mechanical, electrical, chemical these are all technologies not disciplines of engineering. Engineering is at a frontier, it is not yet classified, and when it is, then the engineering's over: that's the point of science and engineering. Mechatronics is not a new engineering discipline, it is a new area of technology.
Industry needs people who are conversant with the established technologies and who are able to adopt, adapt and apply these technologies to achieve specific objectives. This is not engineering it is technical design. Engineers the likes of Smeaton and Coulomb operated at the frontiers where they had no access to appropriate technical science, they developed the technical science and published papers and presented lecturers to share such knowledge. The published papers could be read by others and the theories contained within put to work. But most importantly such published papers can be referenced by others, they set a benchmark. For example no one should get buried in a trench because we have the technical science to design a technical solution to avoid collapse of the trench walls. If a trench wall collapses we can reference national standards, safety manuals and industry manuals and a variety of textbooks, reference manuals and journal articles. The collapse of a trench wall is largely an avoidable event, and the literature provides the means to avoid. There may be uncertainty in the characteristics of the materials and the quality of the workmanship but such uncertainty can be kept to a minimum. If there is a trench collapse we can identify that the persons involved failed to exercise adequate duty of care.
Information is being consolidated and organised and disseminated faster than ever before. It is important therefore that the available information is used to properly assess new implementations and adaptations of the established technologies.
Unfortunately there is also a problem of information overload which hinders getting anything done. Only the real world physical system is fully informed about itself. Anything else can only contain partial information, the importance of design is to make abstract and give consideration to the critical characteristics: not attempt to simulate a complete virtual reality due to inability to make decisions in the face of uncertainty.
Recommended Reading:
1) J.E.Gordon (1991), Structures, or why things don't fall down.Penguin
2) Jacques Heyman, (1999), The Science of Structural Engineering, Imperial College Press
3) Stephen P Timoshenko (1983), History of the Strength of Materials, Dover
4) S.C.Hollister (1966), Engineer: ingenious contriver of the instruments of civilization, Macmillan career book
A journal on everything technological and everything to do with structure: from building structures, to organisation structures, politics, education, and business. If it has structure I will essay it, if it ought to have structure I will essay it. If it don't have structure and it is chaos, I essay that too!
Showing posts with label Philosophy. Show all posts
Showing posts with label Philosophy. Show all posts
Monday, June 16, 2014
Tuesday, September 10, 2013
State of Play 2013/wk37
Been working setting up wordpress for business website. Having closed down the office and retracted the business back home, now need to make better use of the Internet. Most especially with the intended future move over to Maitland on the York Peninsula. The business exists mainly because we are local. That is people attempt DIY building development applications, council advises they need structural engineering. They ask what is that and they are given a list of consultants who submit work for the area. Only one of the consultants is actually in the area, and as both individuals and small builders, have no desire to go into the city or the other side of the city, they come to us. Moving over to Maitland therefore is potentially a major stumbling block.
On the other hand there has been a down turn in work, our regular clients have experienced a reduction in work and that has flowed onto us. As for private individuals, and small builders with one-off projects, whilst they represent some 80% of projects, they represent very little in terms of income, like around 20%, yet they consume a disproportionate amount of time. Their emergencies and urgencies tend to result in them applying pressure to jump to the front of the queue, and otherwise cause delays for our regular clients. Their projects also interfere with our capacity to pursue larger projects. They contribute to keeping our earnings low and barely scraping in incomes equal to the Federal minimum wage. That I don't so much mind, but I object to the hassle and the pressure.
I mean, there are people in the world who don't have housing and our time is wasted by people who don't follow the rules get into a mess and expect we dig them out off a hole for a low fee. Meanwhile people who try to do the right thing, have to have extreme patience, as they keep getting pushed further and further along the queue.
To me it seems there is a need for some means of providing engineering services like legal aid. Though I did take a look at legal aid services, they seem to have highly restrictive requirements as to who can get such aid.
The alternative is to design a service which properly informs people, so that they don't build stuff without approval, and so that they don't produce DIY applications. More prescriptive building solutions are required along with the necessary evidence-of-suitability.
The problem however are people who do not care, think they know what is required, and do not do any research. Such people are always going to be around, the main people to serve are those who go looking for information but cannot find anything suitable. Those who don't do the research, and think they can do what ever they want on their property no matter what hazards it poses to the rest of the community, will hit a bigger problem. That problem, will be no one to get them out off a mess. Since they don't see any reason to get building approval in the first place, they also don't see any reason to pay for the design and engineering services required to get approval. So this bunch are not really worth while clients. Can generally tell who they are from the start by their attitude, the difficulty is telling them to take a hike at the first meeting, and that not interested in their project. I believe it is preferable to refuse service to these people, Assisting them just encourages them to continue disregarding the rest of the community. Members of the Engineering team have sworn to a code of ethics, which typically places the community first, above the client and above the employer.
Sure much of the time, the city councils are just being picky, and its all a matter of just getting paper work in place. The city council doesn't really want to enforce a demolition order: it makes them look mean. But there are some members of the community who need to learn that their property is part of the greater environment and just about everything they do to their property has an impact on their neighbours.
Any case there has been a down turn in work load, and locality is not really a major issue any more. As most work has always flowed in from regular clients either by the telephone, fax, post, or email. In terms of post that is often in the way of clients dropping documents off in our post box after hours. We have lost our land line telephone number, but most clients already have either our email address or mobile phone number: and for a while now most work has flowed in via email. With only occasionally seeing people in person. Not getting out off the office can be some what depressing.
Still it would appear that an Internet based information resource, both with information provided gratis and other information provided for an up front fee, would resolve many issues. Admittedly there are plenty of sites already, but few providing the engineering information, and further more it is clear that people are not finding the information that is available.
Informing people has always been a problem for small business and organisations. Mail leaflet drops can be expensive, and reach few people, with the leaflets typically going straight in the bin. Yellow pages advertisements don't really attract much attention, may be good for some services but not much use for engineering: its too specialised. People may look in yellow pages when told they need an engineer, but they don't look otherwise, they don't know what they are or that they need such services. Engineers services are thus not altogether in the appropriate place in the phone directory. Being elitist not necessarily a good thing: engineers really need to be along side the drafters, fabricators and the builders.
Door knocking just presents the individual as a nuisance, as does phone calls. Advertisements in local paper can be useful, but delivery of this paper is poor, with it just being thrown into gardens, the plastic wrap is not much protection. So during winter the paper gets soaked with rain, during summer it can soaked by sprinklers watering the gardens. The result the paper often just gets picked up and thrown straight in the bin. Advertisements in other papers more useful, but can otherwise be expensive.
The fundamental problem is that engineering is relatively abstract and esoteric, and consequently of little interest and value to the community. Engineering services need to be made more accessible, more tangible and more value to the community which they serve.
Not sure how I can do that, but if do not seek, will not find.
Earlier in the year I created some additional blogs. I have now modified this blog, removing some of the pages and changing their menu tab to link to these other blogs. I have also added some extra tabs to link to other sites where I have uploaded stuff.
On the other hand there has been a down turn in work, our regular clients have experienced a reduction in work and that has flowed onto us. As for private individuals, and small builders with one-off projects, whilst they represent some 80% of projects, they represent very little in terms of income, like around 20%, yet they consume a disproportionate amount of time. Their emergencies and urgencies tend to result in them applying pressure to jump to the front of the queue, and otherwise cause delays for our regular clients. Their projects also interfere with our capacity to pursue larger projects. They contribute to keeping our earnings low and barely scraping in incomes equal to the Federal minimum wage. That I don't so much mind, but I object to the hassle and the pressure.
I mean, there are people in the world who don't have housing and our time is wasted by people who don't follow the rules get into a mess and expect we dig them out off a hole for a low fee. Meanwhile people who try to do the right thing, have to have extreme patience, as they keep getting pushed further and further along the queue.
To me it seems there is a need for some means of providing engineering services like legal aid. Though I did take a look at legal aid services, they seem to have highly restrictive requirements as to who can get such aid.
The alternative is to design a service which properly informs people, so that they don't build stuff without approval, and so that they don't produce DIY applications. More prescriptive building solutions are required along with the necessary evidence-of-suitability.
The problem however are people who do not care, think they know what is required, and do not do any research. Such people are always going to be around, the main people to serve are those who go looking for information but cannot find anything suitable. Those who don't do the research, and think they can do what ever they want on their property no matter what hazards it poses to the rest of the community, will hit a bigger problem. That problem, will be no one to get them out off a mess. Since they don't see any reason to get building approval in the first place, they also don't see any reason to pay for the design and engineering services required to get approval. So this bunch are not really worth while clients. Can generally tell who they are from the start by their attitude, the difficulty is telling them to take a hike at the first meeting, and that not interested in their project. I believe it is preferable to refuse service to these people, Assisting them just encourages them to continue disregarding the rest of the community. Members of the Engineering team have sworn to a code of ethics, which typically places the community first, above the client and above the employer.
Sure much of the time, the city councils are just being picky, and its all a matter of just getting paper work in place. The city council doesn't really want to enforce a demolition order: it makes them look mean. But there are some members of the community who need to learn that their property is part of the greater environment and just about everything they do to their property has an impact on their neighbours.
Any case there has been a down turn in work load, and locality is not really a major issue any more. As most work has always flowed in from regular clients either by the telephone, fax, post, or email. In terms of post that is often in the way of clients dropping documents off in our post box after hours. We have lost our land line telephone number, but most clients already have either our email address or mobile phone number: and for a while now most work has flowed in via email. With only occasionally seeing people in person. Not getting out off the office can be some what depressing.
Still it would appear that an Internet based information resource, both with information provided gratis and other information provided for an up front fee, would resolve many issues. Admittedly there are plenty of sites already, but few providing the engineering information, and further more it is clear that people are not finding the information that is available.
Informing people has always been a problem for small business and organisations. Mail leaflet drops can be expensive, and reach few people, with the leaflets typically going straight in the bin. Yellow pages advertisements don't really attract much attention, may be good for some services but not much use for engineering: its too specialised. People may look in yellow pages when told they need an engineer, but they don't look otherwise, they don't know what they are or that they need such services. Engineers services are thus not altogether in the appropriate place in the phone directory. Being elitist not necessarily a good thing: engineers really need to be along side the drafters, fabricators and the builders.
Door knocking just presents the individual as a nuisance, as does phone calls. Advertisements in local paper can be useful, but delivery of this paper is poor, with it just being thrown into gardens, the plastic wrap is not much protection. So during winter the paper gets soaked with rain, during summer it can soaked by sprinklers watering the gardens. The result the paper often just gets picked up and thrown straight in the bin. Advertisements in other papers more useful, but can otherwise be expensive.
The fundamental problem is that engineering is relatively abstract and esoteric, and consequently of little interest and value to the community. Engineering services need to be made more accessible, more tangible and more value to the community which they serve.
Not sure how I can do that, but if do not seek, will not find.
Earlier in the year I created some additional blogs. I have now modified this blog, removing some of the pages and changing their menu tab to link to these other blogs. I have also added some extra tabs to link to other sites where I have uploaded stuff.
Monday, October 24, 2011
Structural Engineering
The art of moulding materials we do not really understand
into shapes we cannot really analyze,
so as to withstand forces we cannot really assess,
in such a way that the public does not really suspect.
Professor E. H Brown, (1967), Structural Analysis, Vol 1, Longmans, Green & Co.
Tuesday, September 20, 2011
Another Thought on Popper: Structural/Mechanical Design
DanQuo's applied science: Another Thought on Popper: Popper’s idea of falsifying rather than verifying a hypothesis is essentially what structural engineers do during the design process. The...
I believe the preceding article describes exactly the process which was criticised by Popper.
The whole structural/mechanical design process described concerns accumulating supporting evidence of suitability. Suitability is determined by comparison with some predefined acceptance criteria: increasingly some code of practice. The proposed structure once assessed against the acceptance criteria and found compliant is then approved for construction. Once constructed it becomes a real world experiment. Once it eventually fails, the acceptance criteria in the codes of practice are revised. {The alternate hypothesis that the beam will fail holds true.}
What I believe Popper was arguing is that need to deliberately go looking for the evidence which will falsify the proposed hypothesis, not wait for it to turn up. The evidence collected in Europe suggests that all swans are white, the evidence in Australia suggests swans are black. What ever the evidence supports it is necessary to go looking for the complement, opposite, alternative or challenging hypothesis. The evidence to date suggests that swans are either black or white: but what evidence is there to support that they cannot be blue, green, yellow or red? Other birds have these colours so what prevents swans from having these colours?
From statistics have the null hypothesis and the alternative hypothesis. The null hypothesis is that the structure is fit-for-function or suitable for purpose. The alternative hypothesis is that the structure is not fit-for-function and it will failure. Quality robust design does not seek to minimize the probability of the failure event but rather deal with the failure.
Traditional structural design was primarily concerned with gravity loads and preventing the structure from sinking into the ground or collapsing under its own self-weight. Real world failures have resulted in increasing requirements for consideration of wind loading, seismic loading and live loading.
The Ronan point disaster, was the consequence of failing to consider a potential failure event. The gas explosion lifted the floor up, and blew the wall out, and the floor then had no wall to sit on. Having a mechanical and manufacturing engineering background, I have always found the structural codes to be some what deficient when it comes to loading considered. I believe there are typically considered to be 6 degrees of freedom, though I have a tool design handbook which identifies 12. There are three axes of translation, and 2 directions along these axes, and three axes of rotation and 2 directions of rotation about.
Structural codes have a tendency to only consider a given axis and only consider movement in one direction relative to. So tradition check gravity loads and prevent movement towards the ground, but otherwise ignore upward wind loading, or ignore upward loading from an explosion. A failure event occurs and codes revised to consider the other direction: thus now consider wind uplift. As I understand recent discussions on SEAint listserver, the international building code (IBC) ignores vertical seismic actions and only considers horizontal, whilst the nuclear industry apparently considers the vertical. However requirements for robustness in structural are starting to introduce more consideration of qualitative acceptance criteria into structural design, if not explicitly identified in the codes. As I understand it good seismic design is more to do with detailing than altogether resisting the magnitude of the forces involved.
Resisting forces is a problem. The alternative hypothesis that the structure is not fit-for-function and will fail holds true: the structure will fail. It is not possible to design earthquake resistant buildings, or hurricane resistant or flood proof buildings. The so called earthquake resistant buildings will be destroyed by earthquake, the hurricane resistant buildings will be destroyed by hurricane, and the flood proof buildings will be damaged by flood. Because structural design is based on supporting evidence rather than falsification these failure events are not considered.
The BP oil rig was always going to fail, and it was always going to leak oil into the Gulf of Mexico. Good design is not to make its failure a low probability event, but to design for the failure event. If failure events are made low probability then people become complacent about the hazards, and emergency services are otherwise cut. What could just be an inconvenience escalates into a disaster. It has been suggested that earthquakes and floods in undeveloped countries are less of a problem than in developed countries. The argument being that the people in the undeveloped countries have less to loose and also consequently less to recover. Whilst the developed countries have highly integrated and interdependent systems of supply: life style and life support is entirely dependent on the infrastructure of the city: loose the infrastructure and life is severely affected. If struggling to survive in the first place, then life after the earthquake, is still the same struggle for survival.
Engineers are sued and/or prosecuted because they put forward false propositions, such as the building is safe or that the building is earthquake resistant, when the building is no such thing, and cannot be made so. The point of moving towards limit state probabilistic design is so as to avoid such false propositions, to be more explicit that the design is based on low probability of failure, not zero probability of failure. What is the probability that a US ship will be boarded by the enemy: until the USS Pueblo was boarded it was zero, afterwards increased to one. The failure event needs to be dealt with, not ignored because of low probability. Good design does not involve increasing the design loads every time a structural failure occurs: it requires consideration of the mode of failure and the response to failure. {NB: not suggesting buildings be designed to resist meteor impacts or aircraft flying into them: the cause of the failure is a different issue. The issue here is that failure has occurred by what ever cause.}
When the multi story buildings in a crowded city collapse, there is no clear space to escape to, there is little clear space where a person can stand and a collapsing building will not fall on them. Increasing the resistance of the structure doesn't provide protection from the failure level event. There is little merit in strengthening existing hospitals to new higher magnitude loads, if the day after strengthening is complete, an earthquake of greater magnitude occurs and destroys the strengthened buildings. Far better to consider the response for the failure event in the first place. Cities have more buildings not compliant with current codes than are compliant, buildings are typically compliant with older now obsolete codes. Strengthening existing buildings may satisfy insurance companies who do not wish to pay out for replacement and therefore want to minimize such pay out, but it does little to really address safety.
Consider the operation of a submarine without instrumentation, all that the captain knows is that there is a 5% probability that he will operate the submarine at a depth which will collapse the submarine: the magnitude of the collapse depth is unknown, and the operating depth is unknown. The captain is unlikely to operate the submarine. When it comes to buildings people either believe their buildings are resistant to extreme events, or aware that there is potential for failure but not under what circumstances. People are poorly informed relative to the decisions they have to make. It would be preferable that people know that a structure is going to collapse before they start hearing the creaking and feeling the movement. When to shelter in the building and when to evacuate the building or when to evacuate the area altogether? If know when the building or other structure is at risk it is not necessary to strengthen to comply with current codes: simply respond accordingly to the current state of the dynamic environment.
Similarly design structures themselves to respond accordingly to loading events. For example a Mongolian Yurt is unlikely to crush its occuppants if collapsed by an earthquake, whilst a concrete apartment block will. Whilst a yurt unlikely to resist a hurricane, it can be packed up and occupants and dwelling evacuated together.
Additionally to be quality robust the structures need to be fabricated and constructed using processes which have low variability, maintained by processes that have low variability, and otherwise designed to have acceptable performance no matter what the variability of the operating environment. Acceptable performance does not mean equal performance no matter what the operating conditions, but some type and level of performance which is considered acceptable for the operating conditions.
Designing and constructing reinforced concrete apartment block in a region where steel is in short supply is not quality robust. No matter how much inspection is provided the required steel is not going to get into the structure if it is not available. A welded steel structure is not going to be fabricated by certified welders if none are available. The null hypothesis is that the structure can and will be constructed to the specification, the alternate hypothesis is that it will not. If not built to the specification how defective and hazardous will the structure be when placed into use?
Normal design process is largely built around supporting evidence that the proposal is fit-for-function and can be made to specification. Design is a justification process, not a process of falsifiability or refutability. Interpreting Popper as business as usual for the design process, misses the point, we have to deliberately search for that evidence which refutes the hypothesis that our final design is fit-for-function, and further more design for the failure event. Failure of the production process, failure of the product in service and failure of the maintenance process. Whilst the failure event is not possible to avoid, an appropriate response can be determined: and we return to square one: the null hypothesis that the structure is fit-for-function backed up by our supporting evidence of suitability for our predefined acceptance criteria. At some point have to decide that have done all that is practical with respect to supporting and refuting evidence.
When a decision is made it shouldn't just be on the basis of the benefits obtained but also the detriments incurred. Politicians put forward supporting evidence for the benefits of a proposal but leave out the detriments, the opposing politicians present only the evidence supporting the presence of the detriments. In a court of law the coroner and prosecution will accumulate the supporting evidence that a design was defective, the defence will have to provide evidence supporting fitness-for-function in the face of evidence to the contrary.
As far as I am aware thus far no designer has been held responsible for a design which could not be made fit-for-function using the resources available. Thus far manufacturers and builders are held responsible for not making and supplying to specification: for thus far few have put forth the alternate hypothesis that the specification was not fit-for-manufacture, not fit-for-fabrication and not fit-for-construction.
So to recap: null hypothesis structure or design is fit-for-function. The alternate hypothesis the design is not fit-for-function and will fail. How will it fail, what is an appropriate response? When consider resolved this issue, then have returned back to the null hypothesis so reconsider the alternate hypothesis once again. Repeat whilst practical and refuting ideas available.
{NB:
The difference between DanQuo's article and what I describe, is subtle. I took DanQuo's description to be the stock standard structural design process, which reaches a predetermined conclusion of fitness-for-function. That is the process is iterated until a design-solution which meets the existing acceptance criteria is met. I am suggesting this process does not involve the falsification process, it is all justification. I am saying that when this conclusion from the routine process has been reached, that is the point at which the alternate hypothesis should be addressed, and should attempt to prove the conclusion is false and the design is not fit-for-function. Further more it can always be proven not fit-for-function, and therefore helpful to be aware of those situations. Failures are important.
TEDxYYC - David Damberger - Learning from Failure - YouTube
}
I believe the preceding article describes exactly the process which was criticised by Popper.
The whole structural/mechanical design process described concerns accumulating supporting evidence of suitability. Suitability is determined by comparison with some predefined acceptance criteria: increasingly some code of practice. The proposed structure once assessed against the acceptance criteria and found compliant is then approved for construction. Once constructed it becomes a real world experiment. Once it eventually fails, the acceptance criteria in the codes of practice are revised. {The alternate hypothesis that the beam will fail holds true.}
What I believe Popper was arguing is that need to deliberately go looking for the evidence which will falsify the proposed hypothesis, not wait for it to turn up. The evidence collected in Europe suggests that all swans are white, the evidence in Australia suggests swans are black. What ever the evidence supports it is necessary to go looking for the complement, opposite, alternative or challenging hypothesis. The evidence to date suggests that swans are either black or white: but what evidence is there to support that they cannot be blue, green, yellow or red? Other birds have these colours so what prevents swans from having these colours?
From statistics have the null hypothesis and the alternative hypothesis. The null hypothesis is that the structure is fit-for-function or suitable for purpose. The alternative hypothesis is that the structure is not fit-for-function and it will failure. Quality robust design does not seek to minimize the probability of the failure event but rather deal with the failure.
Traditional structural design was primarily concerned with gravity loads and preventing the structure from sinking into the ground or collapsing under its own self-weight. Real world failures have resulted in increasing requirements for consideration of wind loading, seismic loading and live loading.
The Ronan point disaster, was the consequence of failing to consider a potential failure event. The gas explosion lifted the floor up, and blew the wall out, and the floor then had no wall to sit on. Having a mechanical and manufacturing engineering background, I have always found the structural codes to be some what deficient when it comes to loading considered. I believe there are typically considered to be 6 degrees of freedom, though I have a tool design handbook which identifies 12. There are three axes of translation, and 2 directions along these axes, and three axes of rotation and 2 directions of rotation about.
Structural codes have a tendency to only consider a given axis and only consider movement in one direction relative to. So tradition check gravity loads and prevent movement towards the ground, but otherwise ignore upward wind loading, or ignore upward loading from an explosion. A failure event occurs and codes revised to consider the other direction: thus now consider wind uplift. As I understand recent discussions on SEAint listserver, the international building code (IBC) ignores vertical seismic actions and only considers horizontal, whilst the nuclear industry apparently considers the vertical. However requirements for robustness in structural are starting to introduce more consideration of qualitative acceptance criteria into structural design, if not explicitly identified in the codes. As I understand it good seismic design is more to do with detailing than altogether resisting the magnitude of the forces involved.
Resisting forces is a problem. The alternative hypothesis that the structure is not fit-for-function and will fail holds true: the structure will fail. It is not possible to design earthquake resistant buildings, or hurricane resistant or flood proof buildings. The so called earthquake resistant buildings will be destroyed by earthquake, the hurricane resistant buildings will be destroyed by hurricane, and the flood proof buildings will be damaged by flood. Because structural design is based on supporting evidence rather than falsification these failure events are not considered.
The BP oil rig was always going to fail, and it was always going to leak oil into the Gulf of Mexico. Good design is not to make its failure a low probability event, but to design for the failure event. If failure events are made low probability then people become complacent about the hazards, and emergency services are otherwise cut. What could just be an inconvenience escalates into a disaster. It has been suggested that earthquakes and floods in undeveloped countries are less of a problem than in developed countries. The argument being that the people in the undeveloped countries have less to loose and also consequently less to recover. Whilst the developed countries have highly integrated and interdependent systems of supply: life style and life support is entirely dependent on the infrastructure of the city: loose the infrastructure and life is severely affected. If struggling to survive in the first place, then life after the earthquake, is still the same struggle for survival.
Engineers are sued and/or prosecuted because they put forward false propositions, such as the building is safe or that the building is earthquake resistant, when the building is no such thing, and cannot be made so. The point of moving towards limit state probabilistic design is so as to avoid such false propositions, to be more explicit that the design is based on low probability of failure, not zero probability of failure. What is the probability that a US ship will be boarded by the enemy: until the USS Pueblo was boarded it was zero, afterwards increased to one. The failure event needs to be dealt with, not ignored because of low probability. Good design does not involve increasing the design loads every time a structural failure occurs: it requires consideration of the mode of failure and the response to failure. {NB: not suggesting buildings be designed to resist meteor impacts or aircraft flying into them: the cause of the failure is a different issue. The issue here is that failure has occurred by what ever cause.}
When the multi story buildings in a crowded city collapse, there is no clear space to escape to, there is little clear space where a person can stand and a collapsing building will not fall on them. Increasing the resistance of the structure doesn't provide protection from the failure level event. There is little merit in strengthening existing hospitals to new higher magnitude loads, if the day after strengthening is complete, an earthquake of greater magnitude occurs and destroys the strengthened buildings. Far better to consider the response for the failure event in the first place. Cities have more buildings not compliant with current codes than are compliant, buildings are typically compliant with older now obsolete codes. Strengthening existing buildings may satisfy insurance companies who do not wish to pay out for replacement and therefore want to minimize such pay out, but it does little to really address safety.
Consider the operation of a submarine without instrumentation, all that the captain knows is that there is a 5% probability that he will operate the submarine at a depth which will collapse the submarine: the magnitude of the collapse depth is unknown, and the operating depth is unknown. The captain is unlikely to operate the submarine. When it comes to buildings people either believe their buildings are resistant to extreme events, or aware that there is potential for failure but not under what circumstances. People are poorly informed relative to the decisions they have to make. It would be preferable that people know that a structure is going to collapse before they start hearing the creaking and feeling the movement. When to shelter in the building and when to evacuate the building or when to evacuate the area altogether? If know when the building or other structure is at risk it is not necessary to strengthen to comply with current codes: simply respond accordingly to the current state of the dynamic environment.
Similarly design structures themselves to respond accordingly to loading events. For example a Mongolian Yurt is unlikely to crush its occuppants if collapsed by an earthquake, whilst a concrete apartment block will. Whilst a yurt unlikely to resist a hurricane, it can be packed up and occupants and dwelling evacuated together.
Additionally to be quality robust the structures need to be fabricated and constructed using processes which have low variability, maintained by processes that have low variability, and otherwise designed to have acceptable performance no matter what the variability of the operating environment. Acceptable performance does not mean equal performance no matter what the operating conditions, but some type and level of performance which is considered acceptable for the operating conditions.
Designing and constructing reinforced concrete apartment block in a region where steel is in short supply is not quality robust. No matter how much inspection is provided the required steel is not going to get into the structure if it is not available. A welded steel structure is not going to be fabricated by certified welders if none are available. The null hypothesis is that the structure can and will be constructed to the specification, the alternate hypothesis is that it will not. If not built to the specification how defective and hazardous will the structure be when placed into use?
Normal design process is largely built around supporting evidence that the proposal is fit-for-function and can be made to specification. Design is a justification process, not a process of falsifiability or refutability. Interpreting Popper as business as usual for the design process, misses the point, we have to deliberately search for that evidence which refutes the hypothesis that our final design is fit-for-function, and further more design for the failure event. Failure of the production process, failure of the product in service and failure of the maintenance process. Whilst the failure event is not possible to avoid, an appropriate response can be determined: and we return to square one: the null hypothesis that the structure is fit-for-function backed up by our supporting evidence of suitability for our predefined acceptance criteria. At some point have to decide that have done all that is practical with respect to supporting and refuting evidence.
When a decision is made it shouldn't just be on the basis of the benefits obtained but also the detriments incurred. Politicians put forward supporting evidence for the benefits of a proposal but leave out the detriments, the opposing politicians present only the evidence supporting the presence of the detriments. In a court of law the coroner and prosecution will accumulate the supporting evidence that a design was defective, the defence will have to provide evidence supporting fitness-for-function in the face of evidence to the contrary.
As far as I am aware thus far no designer has been held responsible for a design which could not be made fit-for-function using the resources available. Thus far manufacturers and builders are held responsible for not making and supplying to specification: for thus far few have put forth the alternate hypothesis that the specification was not fit-for-manufacture, not fit-for-fabrication and not fit-for-construction.
So to recap: null hypothesis structure or design is fit-for-function. The alternate hypothesis the design is not fit-for-function and will fail. How will it fail, what is an appropriate response? When consider resolved this issue, then have returned back to the null hypothesis so reconsider the alternate hypothesis once again. Repeat whilst practical and refuting ideas available.
{NB:
The difference between DanQuo's article and what I describe, is subtle. I took DanQuo's description to be the stock standard structural design process, which reaches a predetermined conclusion of fitness-for-function. That is the process is iterated until a design-solution which meets the existing acceptance criteria is met. I am suggesting this process does not involve the falsification process, it is all justification. I am saying that when this conclusion from the routine process has been reached, that is the point at which the alternate hypothesis should be addressed, and should attempt to prove the conclusion is false and the design is not fit-for-function. Further more it can always be proven not fit-for-function, and therefore helpful to be aware of those situations. Failures are important.
TEDxYYC - David Damberger - Learning from Failure - YouTube
}
Subscribe to:
Posts (Atom)