LRFD Foundation Design 3

[vmirat]

I'm attempting to design a building using only LRFD (OK, old ASD guy here). I'm down to the foundation design at this point. The problem is that the 2006 IBC allows either LRFD or ASD load combinations (1805.4.1.1). It seems unfair to use the LRFD column loads since the ASD loads would be much less. Is there a compensating factor for LRFD that equals things out or do I have to go back and recalculate everything using ASD load combinations?

 

[BigH]

dhengr - as hokie66 put forth, welcome and I found your comments most welcome. I still refer to Tschebotarioff's 1951 book on soils. Its amazing how some are just discovering what he put forth so many years ago. on your comments (my snaps!!)

 

[miecz]

BAretired-

On second thought, it seems as though the load duration factors for wood (and, dare I say, soil) belong on the resistance side of the equation, as they are related to the material.

Some of you guys don't know it, but you are making very eloquent arguments...for LRFD.

271828-

I'm still on the fence with limit states design. My argument is, if we're going to use it, we should use it for the whole structure. If we can't use it for the whole structure, then let's ditch it. It's costly having to simultaneously design for both factored loads and service loads, and, as others have pointed out, the benefits are questionable.

 

[271828]

miecz,

I think my main reason to like LRFD is that I find it easier to know what's going on with the design inequality. The items that affect resistance go on the resistance side and the items that affect demand on the demand side. There's no jumbled-up factor that uncleanly affects both.

Here's a great example of lack of clarity caused by a factor on the wrong side, going back to floor vibrations which is one of my favorite topics. The newest British method has an equation to predict the response of the floor which is compared to the tolerance to see if the floor is OK. The *response* side has a factor W that accounts for how the *tolerance* varies depending on the frequency of the *response*. (People put up with less vibe between 4 and 8 Hz than above 8 Hz, for example.) If one computes a predicted acceleration of 0.7%g, one's not predicting that it'll actually accelerate 0.7%g although it looks that way, but instead 0.7%g / W. I'd guess that 90% of people who use that inequality don't get that. W really needs to go on the tolerance side to make the equation crystal clear.

As for allowable bearing, I don't know enough about how allowables are set to have any idea how one would go about extacting the "load factor" part and setting the resistance factor to be statistically meaningful. I have no doubt that it could be done, though.

 

[dhengr]

hokie66:
ACI318-63 it was, the local ACI Rep. gave it to me as a student. And, a few years later I was using it, as a T.A., to teach Allowable Strength & Ultimate Strength Reinf. Conc. courses, including Prestressed and Shells courses. I have a number of bldgs. in the 18-20 story range which don’t even know they shouldn’t be working so well because I didn’t factor their loads. If hokie66 means what I think it means we are about the same age, and they have some really knowledgeable wood guys out at Virginia Tech, at least one.
BigH:
My ed. of Tschebotarioff’s book is newer than yours and undoubtedly not as well thumbed. Some years ago one of my mentors took back his copy of Karl Terzaghi’s book and didn’t think to will it back to me. Of course, neither of those ‘ground breakers’ needed load factors memorialized in a code for their work, and we can actually understand their concepts. My experience has been that the guy with the small bldg. didn’t want to or couldn’t afford to have a soils report done if I had the confidence to do the design without. Of course, he might have bought a couple extra yards of conc. for his footings. When I did work for found. contractors on 4-5 story deep excavations, river front structures or other large structures, I insisted on having a soils guy involved.
Thanks to both of you for the warm welcome.
BAretired:
There are some very eloquent arguments... for LRFD, but you can finesse anything to death too, and not gain much for the added effort. I have a number of friends in both the stl. & conc. fields who are making their living’s teaching, doing this research and this code writing. We are the smarter for the research and new knowledge on the subjects, but when they make my life tougher as a civil, structural and mechanical designer and don’t significantly improve the end product, I don’t particularly like it.
Sawn lumber, timbers and soils are alike in their vast variability; the one thing in soil’s favor is that there hasn’t been a decrease in its quality over time, which adds a 14th new dimension to the variability problem in wood. The duration of loading problem is not a knew one, it is a material characteristic, so it is on the resistance side of the equation, but we have been handling it just fine for a long time with some judgement and without any more new factors. Just imagine all the nice new factors we’ll have when you consider mechanical properties in at least two directions and maybe three in some cases, then let’s apply the Hankinson equation, which should certainly have its own factor, to that. The greatest attribute that some of the newer manuf’rd. wood products have is that they are more homogeneous and somewhat less anisotropic than sawn lumber. And, in thirty years we’ll discuss the longevity of the current glues being used.
Conc. is fairly homogeneous and isotropic until you start nit-picking about the difference between the cement paste and the many different aggregates. Steel is pretty homogeneous, but contrary to what most of us were taught it is not always isotropic, we just do a good job of hiding the problems from the designer in most cases. The way we use rolled shapes is generally not compromised by the dirty little secrets. Through thickness properties are slightly different than the properties parallel to the direction of rolling. Thus, some of the problems we hear about on heavy plate structures, and highly restrained welded joints, lamellar tearing and cracking comes to mind. We should be aware of some of these things to stay out of trouble, but let’s not put more factors on them which I have to carry around.
I agree with you that we should make up our minds one way or another. But, I am not totally convinced that LRFD (in any of the constr. matl’s.) is a real improvement, there is not doubt that it adds to the complexity of design. You guys chase these load factored numbers and service loads around long enough and you’ll lose a bunch of them. Then we can make the footings small without BigH factoring his bearing pressures. Just don’t forget the building, footing and soil will know the difference, even if we don’t.
miecz:
You’re right, there are important distinctions btwn. the resistance and demand side of the equation and we darn well better understand that. Part of my argument is that this might better be handled with education, a mind-set or thought process, experience and judgement, etc., just as you suggest with the floor vib. formula; instead of some cook book, recipe -like, approach which gives an inexperienced chief the idea he is doing fine cuisine. I do not mean offense by this, but I doubt that a more complicated code will make us better or more competent designers and engineers.

 

[BigH]

I also have the 73 edition of Tschebotaroff's book!

 

[BAretired]

dhengr,

I too welcome you to Eng-Tips. In many respects, I agree with your stated position. I do not seriously suggest that we should move toward using LRFD in soil mechanics. I am not sure it would be helpful, but I think that free discussion of the topic is always a good thing as one or more of us may have missed something along the way.

You said:

more homogeneous and somewhat less anisotropic than sawn lumber

. Isn't "less anisotropic" a double negative?

BA

 

[dhengr]

BA:
My original comment was; “The greatest attribute that some of the newer manuf’rd. wood products have is that they are more homogeneous and somewhat less anisotropic than sawn lumber. And, in thirty years we’ll discuss the longevity of the current glues being used.”

Sawn lumber is anisotropic, as we use it as a building mat’l.; it is certainly not an isotropic mat’l., which has identical mechanical properties in all directions. However some of the manuf’rd. wood products, given the methods of manuf’r. and the use and orientation of the constituent mat’ls., are being made more homogeneous, and in this process they are somewhat less anisotropic, maybe somewhat more isotropic, but still a long way from isotropic.

Now, having said the same thing three times and having added “somewhat more isotropic”, do we have a hexa or an octa negative? Maybe ‘it’s gooder’ would have been a better phrasing than ‘it’s less anisotropic’. I’m pulling your leg, of course. Thanks for the welcome.

 

[271828]

dhengr typed "You're right, there are important distinctions btwn. the resistance and demand side of the equation and we darn well better understand that. Part of my argument is that this might better be handled with education, a mind-set or thought process, experience and judgement, etc., just as you suggest with the floor vib. formula; ..."

I agree with the bold part, in theory, but I don't see how new knowledge works its way into widespread practice without being put into a code which makes people use it. Most folks I know would stick with the ways they learned first for their entire career, regardless of what new info was learned. They're human and have the desire to stay within their comfort zones, get their work done, and still have a life outside of work.

It's a philosophical point. Maybe that's not a bad thing if someone never changes practices because their first designs were fine at the time and are still safe. On the other hand, do we want new knowledge to get into widespread practice or not?

...or perhaps you have an idea for getting new knowledge into widespread practice without code changes. Just because I don't know of a way doesn't mean that there's not one.

 

[paddingtongreen]

I'm still finding my way around here. I'm retired and I've done the working strength/ultimate strength, ASD/LRFD, battles.

LRFD was introduced because there was a large disparity between the actual safety factors over member failure for different members designed in ASD, notably, simple beams versus fixed end beams.

What caught my eye was the discussion of foundations. If you have a large moment and small vertical load at the base of a column and try to apply the factored loads you may find that it cannot be done; the resultant may land outside the foundation. The only reasonable way that I know is to use the service loads, separately to get the separate soil pressures and then multiply the effects on the concrete, by the load factors.