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ASD Steel Construction Manual... 16
- Thread starter Jambruins
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vandede427
Structural
I was taught LRFD in school, then got in the real world and chose ASD.
Having learned in the 9th edition (still using) and reviewed the 13th, I realize that my issue is not with ASD per se. It is the fact that LRFD introduced too much new stuff for the sake of saving a pound of steel, and I lost many of the formulas that I was so comfortable with, not even LRFD versions. 13th ed ASD is not the same as 9th ed.
For crying out loud, we all factor our loads for concrete design. We are all intelligent enough to do that. LRFD in the 80's was merely the introduction of too much complexity, IMHO. AISC is not listening to those of us who need speed over steel savings.
For crying out loud, we all factor our loads for concrete design. We are all intelligent enough to do that. LRFD in the 80's was merely the introduction of too much complexity, IMHO. AISC is not listening to those of us who need speed over steel savings.
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I started out as an ME so the idea behind ASD is much more logical in terms of "stress analysis"; however, I did take several steel design classes in my undergraduate and was taught LRFD. I do respect the point that dead, live, and wind are treated the same under ASD. From a mechanical engineering perspective I have gained a comfort (perhaps false) with knowing the stress levels with respect to the given code allowables, so for me I love ASD. In practice, I have mainly worked on the construction side with falsework and rigging so ASD has been the weapon of choice.
Perhaps my biggest issue is that LRFD is setup so every equation produces some load or moment capacity based on lets say a 0.9 resistance factor and some combination of 1.2 and 1.6 so it is not immediately apparent what stress levels you may be dealing with. Like I said, maybe I have developed a false comfort with the idea of some allowable stress, however, I feel the methodology to determine a "capacity value" goes too far and hides the true behavior. Tell me the LRFD moment capacity of W12 X 50 and I have many additionaly questions to ask you to determine what the load situation looks like. Tell me you have a W12 X 50 with 16,000 psi bending stress and I can tell you what else we can put on it to load it up!
I also agree with the above post that we have come to a point of diminishing returns. Is it worth saving the steel if you cannot look at a set of equations and "see through" the terms to fully understand the theory used to derive the equation. I fall back on Blodgetts "Design of Welded Structures" as a clear example of presenting a clear and simple methodology to solve everyday problems. It obviously is not based on the cutting edge theory , however, it is clear, concise, and provides basic methods using ASD to solve real word design problems.
Look at Appendix D in the Concrete Code for design of embedded anchors, is that any better than the basic equations given in the PCI 1979 design manual with the basic 45 DEG breakout cone? (Discussed numerous times on ENG-TIPS)
I understand that Universities need to complete research and people want progress, however, the fundamentals have not changed and in practice I feel we are sorely lacking the fundamentals to make good solid, well-rounded structural engineers rather than techincal "experts" who can save 5-lbs of steel on a simple span beam.
Perhaps my biggest issue is that LRFD is setup so every equation produces some load or moment capacity based on lets say a 0.9 resistance factor and some combination of 1.2 and 1.6 so it is not immediately apparent what stress levels you may be dealing with. Like I said, maybe I have developed a false comfort with the idea of some allowable stress, however, I feel the methodology to determine a "capacity value" goes too far and hides the true behavior. Tell me the LRFD moment capacity of W12 X 50 and I have many additionaly questions to ask you to determine what the load situation looks like. Tell me you have a W12 X 50 with 16,000 psi bending stress and I can tell you what else we can put on it to load it up!
I also agree with the above post that we have come to a point of diminishing returns. Is it worth saving the steel if you cannot look at a set of equations and "see through" the terms to fully understand the theory used to derive the equation. I fall back on Blodgetts "Design of Welded Structures" as a clear example of presenting a clear and simple methodology to solve everyday problems. It obviously is not based on the cutting edge theory , however, it is clear, concise, and provides basic methods using ASD to solve real word design problems.
Look at Appendix D in the Concrete Code for design of embedded anchors, is that any better than the basic equations given in the PCI 1979 design manual with the basic 45 DEG breakout cone? (Discussed numerous times on ENG-TIPS)
I understand that Universities need to complete research and people want progress, however, the fundamentals have not changed and in practice I feel we are sorely lacking the fundamentals to make good solid, well-rounded structural engineers rather than techincal "experts" who can save 5-lbs of steel on a simple span beam.
connectegr
Structural
I little of the AISC history on ASD v LRFD. Research in the late 80's had shown that LRFD provided a more economical design. However the saving where seen primarily in commercial design with relatively high dead loads. Little savings are obtained in industrial design with high live loads. With the publication of the 1st Ed LRFD, AISC intended to phase out ASD. Schools began teaching LRFD exclusively and new research data was established using ultimate forces and LRFD. The AISC approach was that no new publications of ASD would created unless errata was necessary for unsafe applications of the ASD Manual. All new research was applied to future publications of the LRFD Manuals. Even with this ASD persisted as the primary manual for industrial design. Due to the confusion of fabricators over service v ultimate forces, commercial firms often designed using LRFD and provided service loads on the drawings to avoid unnecessary increases in fabrication estimates.
Engineering firms also have a large investment in software and training of their engineering staff. Without significant savings industrial firms were not motivated to covert th LRFD.
After twenty years AISC began a change in philosophy. The primary goal should be the promotion of steel design over alternative materials. A survey of the industries leading designers found that the primary concern with ASD was that the manual was antiquated with respect to the previous 20 years of research. Thus the 13th Ed combined ASD/LRFD manual. Note that the manual lends no preference to either code and provides parallel design examples.
The 14th Ed Manual has been completed in a similar format and should be available later this year.
Engineering firms also have a large investment in software and training of their engineering staff. Without significant savings industrial firms were not motivated to covert th LRFD.
After twenty years AISC began a change in philosophy. The primary goal should be the promotion of steel design over alternative materials. A survey of the industries leading designers found that the primary concern with ASD was that the manual was antiquated with respect to the previous 20 years of research. Thus the 13th Ed combined ASD/LRFD manual. Note that the manual lends no preference to either code and provides parallel design examples.
The 14th Ed Manual has been completed in a similar format and should be available later this year.
connectegr
Structural
Also note that the current Standard (Specification for Steel...) refers to ASCE for load combinations.
Lets be clear - the new "ASD" - Allowable Strength Design - is not what was set forth in the 1989 Green Book "ASD" - Allowable Stress Design.
There is a substantial difference in the methodology and AISC has made this distinction. We now have three different animals.
Fundamentally I do not like the idea of an "allowable strength".
I am not questioning that significant research has been incorporated and that it may provide a more efficient design in commercial construction. I understand the merits, however, I am not convinced on the benefits.
There is a substantial difference in the methodology and AISC has made this distinction. We now have three different animals.
Fundamentally I do not like the idea of an "allowable strength".
I am not questioning that significant research has been incorporated and that it may provide a more efficient design in commercial construction. I understand the merits, however, I am not convinced on the benefits.
The Aluminum Association has had parallel ASD/LRFD design procedures for over 10 years. I've done a few projects where the EOR dictated LRFD in the subsystem design.
One of my associates, graduated in 1997, treats his LRFD manual like we diehards treat our ASD manuals. When I'm reviewing his work, I have to check things a bit closer than if he had used ASD, but overall it's a good exercise for me.
One of my associates, graduated in 1997, treats his LRFD manual like we diehards treat our ASD manuals. When I'm reviewing his work, I have to check things a bit closer than if he had used ASD, but overall it's a good exercise for me.
LSPSCAT nailed it on the head. There are currently 3 different methodologies.
Unfortunately, I understand connectegr to say that the next edition doesn't address industry which has largely decided that steel has won the material battle. ASD still is not taking off in the 13th edition because it doesn't address the REAL issue. Therefore, 14th ed will be more of the same.
I certainly can't recall a discussion or an article in Modern Steel Construction (by AISC) which has addressed the real heart of the argument of 9th ed vs. 10-13th ed, ease and speed vs. unnecessarily tedious calcs. AISC ignores like minded engineers and presumes that we just want to stay behind the times. Insulting!
OK, time to get off the soap box. Jambruins - I'm sorry if some of us hijacked your thread.
Unfortunately, I understand connectegr to say that the next edition doesn't address industry which has largely decided that steel has won the material battle. ASD still is not taking off in the 13th edition because it doesn't address the REAL issue. Therefore, 14th ed will be more of the same.
I certainly can't recall a discussion or an article in Modern Steel Construction (by AISC) which has addressed the real heart of the argument of 9th ed vs. 10-13th ed, ease and speed vs. unnecessarily tedious calcs. AISC ignores like minded engineers and presumes that we just want to stay behind the times. Insulting!
OK, time to get off the soap box. Jambruins - I'm sorry if some of us hijacked your thread.
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"...Tell me the LRFD moment capacity of W12 X 50 and I have many additionaly questions to ask you to determine what the load situation looks like. Tell me you have a W12 X 50 with 16,000 psi bending stress and I can tell you what else we can put on it to load it up! ..."
False comfort for sure. With old timey ASD, you could have fb=16 ksi (from all DL) and compare that to Fb=24 ksi and get the idea that the situation is equivalent to fb=16 ksi (from all LL) vs Fb=24 ksi.
"I also agree with the above post that we have come to a point of diminishing returns. Is it worth saving the steel if you cannot look at a set of equations and "see through" the terms to fully understand the theory used to derive the equation."
I don't follow this logic. If I look at 2005 Chapter F equations, I get a much better idea of physical behavior than what's apparent from the 1989 counterparts. Someone in here going to claim to see warping and pure torsion in the Fb=12000Cb/[whatever] equation? I think this is the single biggest positive leap that's been made from 89 to today.
False comfort for sure. With old timey ASD, you could have fb=16 ksi (from all DL) and compare that to Fb=24 ksi and get the idea that the situation is equivalent to fb=16 ksi (from all LL) vs Fb=24 ksi.
"I also agree with the above post that we have come to a point of diminishing returns. Is it worth saving the steel if you cannot look at a set of equations and "see through" the terms to fully understand the theory used to derive the equation."
I don't follow this logic. If I look at 2005 Chapter F equations, I get a much better idea of physical behavior than what's apparent from the 1989 counterparts. Someone in here going to claim to see warping and pure torsion in the Fb=12000Cb/[whatever] equation? I think this is the single biggest positive leap that's been made from 89 to today.
I disagree.jsdpe25684 said:
Sure, but only because one document came out in 1989, and the other 16 years later. If AISC hadn't abandoned ASD back in the '80s, we'd all be used to the "current" ASD as the natural progression of the green book. How many threads are on this forum asking how to calculate something that is not addressed in the green book, that is addressed in later volumes? The specification has developed to include more scenarios. The fact that it has grown more complex is analagous to the observation that ASCE 7 has doubled in size the last five years. It is not indicative of a third design philosophy.LSPSCAT said:
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paddingtongreen
Structural
Letting my mind wander.
When I started working in the drawing office in 1951, there was only allowable stress. Analysis of statically indeterminate structures was generally by Slope Deflection or Moment Distribution methods and took only primary load paths into account. We didn't try to pretend that we could calculate with any great accuracy using a slide rule so we didn't skimp on material. Secondary and tertiary load paths provided redundancy.
Then "Plastic Design" came along, invented and pushed by some university types. A fixed ended beam could be allowed to go fully plastic at the ends and the middle before it would fail. Marvelous, but columns had to have greater moment capacity than the beams or the column would develop the plastic hinge first and fail as a column. It disappeared.
Along came computers that calculate stresses to thirteen decimal places from guesswork loads and use all of the available load paths. With them, came Ultimate Design so that resulting structures had been skinnied down to the borderline of failure. Now we have to deliberately add redundant girders to bridges and other structures.
I keep hearing that we save much steel over the old designs but I wonder how much money we actually save. If you reduce the weight a beam by 10%, you save only the 10% of mill cost and freight, the labor cost remains the same, and that is the largest part of the cost, by far.
To my way of thinking, structures live in a service load world and should be designed as such and tweaked to provide the additional load capacity for the possibility of extraordinary conditions. My body is designed for everyday conditions and I have some inoculations to help me fight extraordinary conditions. I do not want my body to be rebuilt to best fight the extraordinary instead of the everyday, I like it when it is equipped for the everyday.
Rant over, highjack done,carry on chaps.
Michael.
Timing has a lot to do with the outcome of a rain dance.
When I started working in the drawing office in 1951, there was only allowable stress. Analysis of statically indeterminate structures was generally by Slope Deflection or Moment Distribution methods and took only primary load paths into account. We didn't try to pretend that we could calculate with any great accuracy using a slide rule so we didn't skimp on material. Secondary and tertiary load paths provided redundancy.
Then "Plastic Design" came along, invented and pushed by some university types. A fixed ended beam could be allowed to go fully plastic at the ends and the middle before it would fail. Marvelous, but columns had to have greater moment capacity than the beams or the column would develop the plastic hinge first and fail as a column. It disappeared.
Along came computers that calculate stresses to thirteen decimal places from guesswork loads and use all of the available load paths. With them, came Ultimate Design so that resulting structures had been skinnied down to the borderline of failure. Now we have to deliberately add redundant girders to bridges and other structures.
I keep hearing that we save much steel over the old designs but I wonder how much money we actually save. If you reduce the weight a beam by 10%, you save only the 10% of mill cost and freight, the labor cost remains the same, and that is the largest part of the cost, by far.
To my way of thinking, structures live in a service load world and should be designed as such and tweaked to provide the additional load capacity for the possibility of extraordinary conditions. My body is designed for everyday conditions and I have some inoculations to help me fight extraordinary conditions. I do not want my body to be rebuilt to best fight the extraordinary instead of the everyday, I like it when it is equipped for the everyday.
Rant over, highjack done,carry on chaps.
Michael.
Timing has a lot to do with the outcome of a rain dance.
Best I can tell, that is not a fact...and that's a fact LOL. New stuff has been added to fill in holes. There are dozens of design checks that are missing from the 89 ASC Manual that are in the 13th Ed. Until someone invents a way to make something bigger on the inside than it is on the outside, that's going to require a bigger and more complex book.
Here are a couple for you guys who still have your fingernails digging into your green book as AISC maliciously drags it away: How do you calc the allowable axial load for an I-shaped column with different flange thicknesses? Second, how do you check a W-shape with cap channel for lateral-torsional buckling? If I was a betting man, I'd bet good money that all of you are checking these with ASD 89 equations that don't apply to those situations. Further, nobody can tell from looking at those equations that they don't apply without digging into Salmon and Johnson or some other book.
Examples? I think there is only one obscure design check that existed in 89 that doesn't exist now--could be wrong about that, though. Of course the equations look different because different models are used. Take unbraced beams for example. Look in Salmon and Johnson, far enough back that they cover both ASD89 and AISC86 or 93. The Fb vs Lb curve for LTB is a real mess for ASD89. It's far, far cleaner and more logical using the modern formulation.
I've never understood this objection. I used ASD89 for 3 years then LRFD since 98. I've designed many projects with very tight early release schedules with LRFD and it doesn't cause problems because I very rarely crank through sizeable equations from the Spec. It's either done with a program or a table. If someone's still doing lots of manual calcs in 2010, then IMO that's an issue with how the engineer chooses to do the calcs. If the issue is that the mean ole' 13th Ed. has a design check (that applies) that wasn't in the 89 Spec., then no sympathy on needing to check it, right?
dcarr82775
Structural
I disagree with LSPSCAT. In ASD you are dealing with stresses well below yield. In LRFD you are dealing with a stresses at Yield. To get similar performance in the first case you ratio down the yield, in the 2nd you ratio up the loads. It gets you to the same place in almost the same manner. I happen to think LRFD is better because it deals with actual performance (plastic) at high loads, not elastic performance which isn't what applies when loads approach failure. Of course I also learned LRFD in school, and ASD later on.
It comes down to whatever you are more comfortable with.
It comes down to whatever you are more comfortable with.
No, I cannot, since I was taught LRFD in 95/96 at my school.
When I went to the PE review class, the teacher was appalled by the size of the LRFD manual. The next week, I brought in the connections book that he did not even realize existed. When he saw that, he vowed he would stop teaching the review course when the balance tipped to more students knowing LRFD.
When I took the PE, I took both ASD and LRFD books. That worked out well, since there was one question easier to answer for each method.
paddingtongreen
Structural
Those of us who already had work experience before LRFD arrived, had to try to learn this new method while still producing a week's worth of work each week. It was not enough for one person to switch, the checkers and reviewers also had to make the change.
It was much easier to continue with a system of design that was not broken. Making the change was similar to changing direction in an ocean liner, it takes time and distance, and it takes guts to switch from a proven system to an unproven, counter intuitive, system invented by the pointy heads at the universities.
Michael.
Timing has a lot to do with the outcome of a rain dance.
It was much easier to continue with a system of design that was not broken. Making the change was similar to changing direction in an ocean liner, it takes time and distance, and it takes guts to switch from a proven system to an unproven, counter intuitive, system invented by the pointy heads at the universities.
Michael.
Timing has a lot to do with the outcome of a rain dance.
Paddington's rant above makes a lot of sense to me.
I never could understand how engineers or university types could honestly believe that they have refined something down to the knats ass.
Mike is right, the loads are mostly guess work. I have worked on Super heavy industrial projects where, as the structural engineers, we had little or no loading information from the mechanical vendors yet we were constantly pressed to issue foundation loads to the foundation engineers. We literally had to come up with equipment loads on our own. Here is what happens:
Structural guy is SUPER conservative on estimating weights in his model or calc's.
The guy responsible for issuing "not-to-exceed" loads for the foundation guys factors the loads up to add some cushion in case things change (like they always do) and to be safe.
Foundation engineers receive the loads and again factor for both design and to be sure they are safe.
In the end the loads are almost entirely "guesstimated" and we actually believe we saved steel or concrete? No, we worked hard to save our ass and the clients power plant from collapsing for the sake off getting the plant online quick.
I understand that since almost all of my work has been in industrial settings, it can be a lot different in commercial construction.
In industrial, usually the structurals are the last guys to get info and the first that have to be done.
I never could understand how engineers or university types could honestly believe that they have refined something down to the knats ass.
Mike is right, the loads are mostly guess work. I have worked on Super heavy industrial projects where, as the structural engineers, we had little or no loading information from the mechanical vendors yet we were constantly pressed to issue foundation loads to the foundation engineers. We literally had to come up with equipment loads on our own. Here is what happens:
Structural guy is SUPER conservative on estimating weights in his model or calc's.
The guy responsible for issuing "not-to-exceed" loads for the foundation guys factors the loads up to add some cushion in case things change (like they always do) and to be safe.
Foundation engineers receive the loads and again factor for both design and to be sure they are safe.
In the end the loads are almost entirely "guesstimated" and we actually believe we saved steel or concrete? No, we worked hard to save our ass and the clients power plant from collapsing for the sake off getting the plant online quick.
I understand that since almost all of my work has been in industrial settings, it can be a lot different in commercial construction.
In industrial, usually the structurals are the last guys to get info and the first that have to be done.
Several years ago I attended an AISC seminar to introduce the AISC 13th edition. During this seminar they drew a business card and offered the winner their selection of an AISC manual. I requested the 9th edition ASD. I could tell he wasn't very happy, but I did receive my 9th edition shortly thereafter.
ASD vs. LRFD is so tiring don't you think?
I was educated with ASD - used it for many years in practice. Then LRFD came out. I managed to learn it very easily (not an ocean liner changing direction at all).
It didn't take me but a few weeks to get into it and understand how to use it. And I'm no super-exceptional engineer. I just don't buy all this talk about how difficult it is to learn or use LRFD.
I'm amazed at the reluctance of engineers to learn new things once they get into practice. I once worked with an engineer who used 1950 era ACI codes (this in the 1980's). We used to chuckle about how he was an old stick-in-the-mud.
I can understand that you might just "like" ASD more and use it. I have no problem with that. But the 9th edition of ASD has changed to the 13th edition. At some point you have to use the current version under legally adopted codes.
I was educated with ASD - used it for many years in practice. Then LRFD came out. I managed to learn it very easily (not an ocean liner changing direction at all).
It didn't take me but a few weeks to get into it and understand how to use it. And I'm no super-exceptional engineer. I just don't buy all this talk about how difficult it is to learn or use LRFD.
I'm amazed at the reluctance of engineers to learn new things once they get into practice. I once worked with an engineer who used 1950 era ACI codes (this in the 1980's). We used to chuckle about how he was an old stick-in-the-mud.
I can understand that you might just "like" ASD more and use it. I have no problem with that. But the 9th edition of ASD has changed to the 13th edition. At some point you have to use the current version under legally adopted codes.
paddingtongreen
Structural
JAE, I was referring to a large company, one large enough to have a couple of nukes and a couple of large fossil power plants in design and construction at the same time, when I spoke of turning an ocean liner. When the liner turns, everybody on it turns with it. With the company, the company isn't turned until almost everyone in the company is turned.
Michael.
Timing has a lot to do with the outcome of a rain dance.
Michael.
Timing has a lot to do with the outcome of a rain dance.
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