Base plate design in LRFD
Base plate design in LRFD
(OP)
To all,
I was fumbling around in the AISC 3rd Edition LRFD, Chapter F - Beams and Other Flexural Members, when I stumbled upon section F1.3 - Design by Plastic Analysis, which states:
Design by plastic analysis, as limited in Section A5.1, is permitted for a compact section member bent about the major axis when the laterally unbraced length Lb of the compression flange adjacent...
A5.1 didn't state anything relevant to base plates.
The way I understand F1.3, is that I can't assume a plastic section when designing a base plate, since the plate is bending about its minor axis. I must assume an elastic section.
That is a huge hit. I have to factor the loads up to use LRFD, and then reduce my capacity by 66% to use an elastic analysis (Z=b*d^2/6, S=b*d^2/4, Z/S = 0.667).
I am sure I am missing something. Any help would be greatly appreciated.
I was fumbling around in the AISC 3rd Edition LRFD, Chapter F - Beams and Other Flexural Members, when I stumbled upon section F1.3 - Design by Plastic Analysis, which states:
Design by plastic analysis, as limited in Section A5.1, is permitted for a compact section member bent about the major axis when the laterally unbraced length Lb of the compression flange adjacent...
A5.1 didn't state anything relevant to base plates.
The way I understand F1.3, is that I can't assume a plastic section when designing a base plate, since the plate is bending about its minor axis. I must assume an elastic section.
That is a huge hit. I have to factor the loads up to use LRFD, and then reduce my capacity by 66% to use an elastic analysis (Z=b*d^2/6, S=b*d^2/4, Z/S = 0.667).
I am sure I am missing something. Any help would be greatly appreciated.






RE: Base plate design in LRFD
Weak axis bending stength should be fine for a base plate. If it needs to be thick just make the thing thick. Note also that typically we use "S" for section modulus and "Z" for plastic modulus. Even though the numbers are right it's confusing in the post at first.
RE: Base plate design in LRFD
Thank you very much for your response. Your answer makes a lot of sense.
As a relatively new engineer, (just got out in December with my MSCE, only practicing for 3 months) I want to understand the thinking behind the code. If I understand it, it makes more sense. Your explanation makes sense.
Thanks again,
egoodwin
RE: Base plate design in LRFD
I have a follow up question.
ASD lets a designer take the allowable bending stress of a member in weak axis bending as 0.75Fy. Normal allowable bending stress is 0.6 or 0.66Fy; I can't remember.
This seems counterintuitive to not wanting plastic hinges to develop in the base plate.
ASD is an elastic type analysis. Under full service loads the base plate stress should still be 75% of the yielding stress, no where near developing a plastic hinge. But I would have thought that ASD would have wanted an allowable of 0.5Fy or at least no increase for base plates to insure no inelastic behavior.
Why does ASD allow an increase in the allowable bending stress of a base plate?
Thanks,
egoodwin
RE: Base plate design in LRFD
When you do a plastic analysis you assume that plastic hinges WILL form, i.e. when the beam experiences its design load it will form a plastic hinge or more until it forms enough to form a collapse mechanism. That's different than allowing an increase in elastic stress. Remember that a plastic hinge means the whole cross section has plasticized. The limits we have in asd, 0.6Fy for instance mean yield is when only the extreme fiber yields not the whole cross section. If the extreme fiber yields the remainder of the cross section is still elastic. The section can take more load but because the yield material has no stiffness the cross section at that yielded location has a reduced stiffness until the entire section yields. Remember yielding corresponds to the plateau on the stress strain curve (E=0). Because the section has no stiffness it has no rotational restraint and therefore behaves like a hinge. It's not a natural hinge, like a pinned support, because it has moment capacity. It simply cannot take additional moment beyond that which caused it to plasticize. All of this is incorporated into the design when you do a plastic analysis. As you can see that's very different from increasing allowable stress closer to initial section yield.
RE: Base plate design in LRFD
For some reason, I have an aversion to designing base plates. Maybe it is because I had to design hundreds of the little buggers by hand, back before the PC era.
I now use RISA's baseplate design software.
RE: Base plate design in LRFD
RE: Base plate design in LRFD
I suggest getting AISC's base plate design guide and writing a spreadsheet from that, it will make your life a lot easier. That is what I did.
RE: Base plate design in LRFD
LOL, you sound like me back in the early 80's. When you get tired of the base plate calculations, convince the boss to buy the software. If he doesn't, spend a lot of time inventing an Excell spreadsheet to do it (my life history).
RE: Base plate design in LRFD
RE: Base plate design in LRFD
I hear ya. I have a file loaded with spreadsheets for everything I do. I design my base plates with a spreadsheet that has two methods: the AISC Design guide and a method in Salmon and Johnson's "steel Structures" 2e. The results are pretty close usually. Convince the boss to buy software? Yeah right, this is a guy who can barely use ms word, can't do a search on google and can't use autocad well enough to even print a drawing and doesn't care that he doesn't know squat about a computer or the internet. If his ROUGH hand calcs are good enough for the piddly crap he does to take up his time there's no way he'll get us anything, especially while we write spreadsheets. If you can make a spreadsheet why should I buy software...
RE: Base plate design in LRFD
I would actually rather see my office spend money on books, but that doesn't happen so I buy my own.
RE: Base plate design in LRFD
UcfSE - A star for you for your in depth answers and quick response.