Need a book? Engineering books recommendations...
Re: asd vs lrfd (not lfrd)[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaoc(--nospam--at)seaoc.org>
- Subject: Re: asd vs lrfd (not lfrd)
- From: "Bill Allen" <BAllenSE(--nospam--at)mail-gw3.pacbell.net>
- Date: Sun, 12 Oct 1997 17:37:06 -0700
-----Original Message----- From: Lynn Howard <lhoward(--nospam--at)silcom.com> To: seaoc(--nospam--at)seaoc.org <seaoc(--nospam--at)seaoc.org> Date: Sunday, October 12, 1997 4:39 PM Subject: Re: asd vs lfrd >Jon turner wrote: > >> I was wondering why allowable stress design is still being used in >> the United States. I am a practising structural engineer in Canada and have >> been using limit states design, which i believe is called lfrd in the US, >> since I graduated from university. I do not have much knowledge about asd, >> so can somebody clue me in on the advantages and disadvantages between the >> two systems? Also I am wondering why you have so many exams to become >> registered as a professional engineer. Here in Canada, after you graduate, >> you are registered as an engineer in training. After a period of two to >> four years, depending on your province of residence, as an engineer in >> training you become a professional engineer after passing a civil law exam >> related to engineering. Many people have told me the reason for the >> differences in our systems is that your engineering programs are not >> regulated from university to university, so the quality of students is >> judged more from whether they can pass the exams after they graduate. Our >> engineering programs are regulated by a central body insuring that all >> students are taught the same courses and that all of our engineers are on >> equal footing. >> > >Jon- >It has nothing to do with the ability of the graduating engineering students. >All recent engineers can do lfrd design. >The problem is with the Boss. In our case, I am the boss, and I don't want to >learn lfrd. It is just a waste of my time. I have been using working stress >for almost 25 years now, and it works just fine thank you. If I am going to >supervise engineers doing structural calculations for me and put my stamp and >signature on their calc's, I need them using a design method that I am familiar >with. > >I know that it seems like we may never switch over to lfrd, but with time it >will happen as it did with concrete. When I went to school we were taught both >working and ultimate methods for concrete. My first job at Bechtel we used >ultimate strength. At my second job, the boss had no idea now to do ultimate >strength method, and would not allow me to use it, so I used working stress. At >the present time in our office, either method is acceptable. I tend to use >working stress the most because I know it the best. However, I am equally >familiar with both methods. > >I suspect that as time goes on, lfrd will become more common, as the "boss" >becomes more familiar with it. > >Lynn I too learned in school to design steel in allowable stress design (asd) and concrete in working stress design (wsd). I soon learned (in practice) to design concrete in ultimate strength design (usd). However, my mentors cautioned me early on not to forget design for service loads and the relationship between the applied loads and states of strain. For example, footings that have to be certain sizes for soil bearing pressure and other practical considerations would still be in an elastic state of strain even when the design loads are factored. I must admit that I do not do very many (if any) structures where the economy of usd would be warranted. Younger graduates who only learn usd do not have a feel for crack control and other serviceability requirements that wsd addresses. With regards to lrfd (it is lrfd, not lfrd - load and resistance factor design), I never learned this method in school and I have never used it in practice. Outside of the "old dog" syndrome, I always found it odd to apply "ultimate" forces to a frame in a linear elastic distribution rather than using plastic hinging, etc. BTW, many years ago when I served on the seismology subcommittee of SEAOSC, a senior member of the committee reported that, with high rise steel framed structures, only the bottom six stories ever go inelastic. I don't remember his name, so don't ask me for documentation on this conversation. The point is, one should correlate the design loads with the state of strain of the design member before deciding which analysis method is appropriate. On a concrete section, this concept is fairly easy to visualize. Upon first loading, the section is uncracked and a transformed section is the most appropriate method of analysis. Once the section cracks, a cracked section elastic analysis is appropriate. When the strain hits approximately 0.003, then plastic deformation begins and usd is appropriate. Regardless of the structure, I caution all to use the method of analysis appropriate for the state of strain in the structural section. Regards, Bill Allen
- Prev by Subject: Re: asd vs lrfd
- Next by Subject: Asphalt Overlay
- Previous by thread: Re: asd vs lrfd
- Next by thread: Fwd: Roark and Young ...