LRFD Foundation Design
LRFD Foundation Design
(OP)
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?






RE: LRFD Foundation Design
RE: LRFD Foundation Design
RE: LRFD Foundation Design
BA
RE: LRFD Foundation Design
For LRFD design, you design the structure for strength using factored loads against your nominal strength φRn. You design your structure for servicability using non-factored loads (i.e. deflection control, vibrations, etc.).
When you get to your footings, you use LRFD for the strength of the footing. Then use service loads for checking soil strength capacity for sizing the footing.
RE: LRFD Foundation Design
Thanks for the response. As I said in my post, I'm an old ASD guy. I was hoping to use LRFD only for this design, but it seems that's not possible. It seems to me that, if I did this design using ASD, I would only have to do one set of calcs.
RE: LRFD Foundation Design
RE: LRFD Foundation Design
RE: LRFD Foundation Design
RE: LRFD Foundation Design
RE: LRFD Foundation Design
LOL, why not make your own deflection and drift limits that can be compared to factored loads? For wind, you could convert H/400 for unfactored, 10 year wind to H/whatever for factored, 50 year wind. It's a simple ratio. Same for 1.2D+1.6L, assuming some ratio of L/D. Knock yourself out!
The same could be done for bearing pressure also, obviously.
Honestly, I'll never understand this whole issue. At the last two firms I worked for, they adopted AISC's first LRFD Manual as soon as it came out and used LRFD ever since. I'm sure they go just as fast and make just as much money as anybody whose green book has fingernail marks on it from the desperate attempts to hang onto it as others pull it away. People just get used to carrying D, L, etc. separately and combine as needed. It's thoughtless after a while.
RE: LRFD Foundation Design
In this corner....
RE: LRFD Foundation Design
Mike McCann
MMC Engineering
RE: LRFD Foundation Design
RE: LRFD Foundation Design
For example, bearing elements have been designed for decades using a factor of safety (FOS) of 3. This is accepted as good engineering principles. Now with the introduction of LRFD, the loads are factor 1.2*D+1.6*L, lets say this averages to give us design actions 1.4 times greater than ASD. But instead of applying a FOS of 3 on the bearing capacity, we apply a reduction factor of 0.5 to the ultimate capacity. When these numbers crunch out, our total factor of safety when designing to LRFD is 2.8, making the element less safe than designing to ASD.
To me, the design should be the same if designing to LRFD as ASD. I know there is the argument that live loads are more variable then dead loads and so should receive a higher load factor, I just don't like hearing "Design to LRFD and you may get more capacity", I don't think the design philosophy of these elements should change.
RE: LRFD Foundation Design
LOL!!! No, I'm in too mellow of a mood for that today. Besides, vmirat already said in another thread a couple of months ago that he didn't have any respect for my opinion anyway.
I will say that I'm starting to wonder about folks who get so worked up over this issue. Is there nothing else to think about?! I mean, come on, there are real issues in the world to think about: taxes, religion, Middle East peace, end of the world in 2012, what happened on a reality show last night, etc. Whether I have to do two calculations, one of them D+L and the other 1.2D+1.6L doesn't show up as a blip on my radar screen. I have to submit something today, though.
RE: LRFD Foundation Design
RE: LRFD Foundation Design
In any event, even if it's just a movie, that would seem to more rightfully command attention than D+L vs having to deal with both D+L and 1.2D+1.6L, LOL. After all, I might have to get money out of an ATM and THEN drive to the movie and pay for tickets. What a pain.
RE: LRFD Foundation Design
You know...I think you're onto something.
RE: LRFD Foundation Design
Both the 8th edition (Red book printed on "Bible Paper" to make it thinner and more briefcase freindly) and the 13th edition are on my desk - but the 13th only gets used to look up sections that were not published in 1980.
GJC
RE: LRFD Foundation Design
RE: LRFD Foundation Design
I woould think the ASD/LRFD design/debate would only be for the steel structure and have virtually nothing to do with designing the foundation. Allowable soil bearing pressures for sizing the footings would be based on service loads, not factored loads. From there, you essentially have to design the footings per ACI ...which is an "LRFD" approach.
RE: LRFD Foundation Design
RE: LRFD Foundation Design
Again, I am confused here. "ASD" is a term used in steel design.
RE: LRFD Foundation Design
In the ASD vs. LRFD argument, I side with ASD because there is no need to switch load combos when it comes to footing design for bearing capacity and sizes beams for serviceability conditions.
RE: LRFD Foundation Design
RE: LRFD Foundation Design
steellion: 100% totally agree with the ASD vs. LRFD thing although for steel desing I use STAAD alot so swithing load combos is not really an issue.
RE: LRFD Foundation Design
RE: LRFD Foundation Design
RE: LRFD Foundation Design
RE: LRFD Foundation Design
RE: LRFD Foundation Design
To be conservative, use 1.5 (Canada) or 1.6 (U.S.A.).
If you estimate that Dead Load = 20% of total load, then the load factor would be 0.2 * 1.25 + 0.8 * 1.5 = 1.45 in Canada or 1.52 in the U.S.A.
For concrete structures where Dead Load is much higher than 20% it is probably better to run the dead and live loads separately.
BA
RE: LRFD Foundation Design
RE: LRFD Foundation Design
Thanks for providing an answer to my question. I've actually used 1.5 on really small projects before, although I would never recommend this to anyone. This is a small (3,000 s.f.) single story addition to an existing large building. Sometimes I wonder about the amount of effort required to design things.
RE: LRFD Foundation Design
Let's face it...we don't know the magnitude of live loads precisely. The live loads listed in our code already contain a factor so they are conservative, particularly for floors. Sometimes the dead load is modified in the life of the building...more insulation or gyproc is added, more gravel is placed on the roof or some equipment is added.
Nothing about this work is precise and those who calculate with double precision on the computer are kidding themselves if they believe they are any closer to the truth than calculations produced on the good old slide rule.
End of rant.
BA
RE: LRFD Foundation Design
Spot on man!!!
I love it when engineers knit-pick the hell out of code requirements, wildly scientific factors and design procedures in order to obtain some "perfectly designed" structure all the while using educated guesses for the structures loading....especially for wind & seismic. Engineering design is an applied science, it is not a science itself. None of us here is named Castigliano or Timenshenko.
RE: LRFD Foundation Design
RE: LRFD Foundation Design
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Anyway, I was just being a smarty-pants.
RE: LRFD Foundation Design
RE: LRFD Foundation Design
The bearing capacity provided by a geotech will have a resistance factor (phi) applied to it. Which makes it a nonimal bearing capacity. Not an ultimate bearing capacity. For example, if the resistance factor phi is equal to 0.5 and the overall load factor equates to 1.5, we have an effective factor of safety equal to 3.
RE: LRFD Foundation Design
BigH's point was that regardless of the 'factor of safety' you have, if you don't design your building to accommodate the settlement which will occur, there will be problems. Dirt is not a homogeneous material, and is not subject to nitpicking load factors or pencil sharpening.
RE: LRFD Foundation Design
I agree that factored loads are not an appropriate basis for sizing footings. The use of a higher load factor for live load than dead load is inconsistent with the philosophy of soil mechanics because live loads are usually present for a short duration whereas dead loads are present continuously. From a settlement point of view, the dead load plays a more prominent role and should, if anything, be given a higher load factor than live load.
For the design of the concrete footing and its reinforcement, Limit States Design is perfectly logical.
BA
RE: LRFD Foundation Design
RE: LRFD Foundation Design
but what about the load combinations including wind or
seismic? foundation designs for loadbearing shearwalls could be controlled by any one of five different cases...
these designs really should be done based on ultimate soil
capacity and not based on settlement...the live and dead load only case would obviously be easily checked before the shear wall footing design is even begun
RE: LRFD Foundation Design
When I used the term "bearing capacity", I didn't mean to suggest that foundation design should be based on shear strength and not on movement. What I meant was, that the geotech should provide a bearing design value that can be used with factored loads. I'm not sure what to call it (the bearing design value), so I chose "bearing capacity", not knowing that the term already had a strict definition.
BAretired
You make a good point. I think that you're argument applies to wood structures as well as to soil. I wonder how the wood designers are coping with LRFD. I think the elegant answer is to add (still) another limit state. Or, put another way, substituting a limit state for the service load limit state that's geared toward soils. Except, if we added limit states based on material response, then we could end up with a lot of limit states.
RE: LRFD Foundation Design
The current code for wood design has an option for designing with LRFD, similar to what AISC did for the steel manual long ago.
I think all the material codes will eventually go to LRFD as the primary design methodology. It's frustrating why they don't just make the break now and be done with it. But there are probably some things, such as geotechnical, that need to be addressed a little more along with everything going LRFD.
RE: LRFD Foundation Design
BA
RE: LRFD Foundation Design
Interestingly in geotechnical, almost everyone acknowledges that the factors are calibrated to the traditional way . . . but some soils are highly different in behaviour - do you handle highly sensitive clays the same way you do insensitve clays? Is such behaviour included? Most structural engineers believe that soils guys are "way too safe" - too conservative; there is one regular contributor that has said this numerous times. I look at the foundation aspect as insurance - if you screw it up - the rest is down the tubes.
RE: LRFD Foundation Design
OK. So for wood, a load duration factor is applied to adjust for short/long term effects. It seems that this could be done with soil to make limit states design more consistent with the philosophy of soil mechanics.
BigH
Seems to me that, to be consistent with the limit states approach, there would need to be different resistant factors for the different materials.
RE: LRFD Foundation Design
I think the concept you are suggesting has merit, but it would need careful study by both geotechnical and structural engineers.
BigH
You said
I don't know what most structural engineers believe, but I believe that the study of soil mechanics deals with materials of such variable properties that the use of a high factor of safety is just common sense. Perhaps a Limit States Design approach could give the geotechnical engineers more scope to vary the load factors according to their judgment than Working Strength Design. It could be worth looking at.
BA
RE: LRFD Foundation Design
Some of you guys don't know it, but you are making very eloquent arguments...for LRFD. The idea is to take the statistical variations into account in a formalized manner. Quantities like allowable bearing pressure BEG for a LRFD approach. Quantities that have tight variations are well suited for simple factors of safety.
LOL, this reminds me of a conversation I had with a friend the other day. He pointed out that SEs nowadays are required to sharpen their pencils far too much considering the wide variation in numerous quantities. What he didn't realize is that he was making the argument for full blown probability-based design methods. I spend a lot of time reading technical journals and it's clear that such methods are coming. It's just a matter of time before these work their way into the codes.
RE: LRFD Foundation Design
RE: LRFD Foundation Design
All I'm typing is that problems that are dependent on highly variable parameters are really probabilistic problems, not determinstic ones. We fake them into deterministic problems because that's currently easier for us. I don't read geotech jnls, but I'd bet good money that someone's already working on methods to handle the kinds of issues mentioned above in a probabilistic format.
This might end up being better in the long run. I wish there was a good probabilistic method for one of my specialities, floor vibrations evaluation. I've had several projects in which I end up telling the client that there's a "low chance of complaints" and the guy almost always shoots back "How low?" It would be very nice to say that with the type of occupancy, it would be 2% in 10 years or whatever. Floor vibe has many highly variable parameters also: damping ratio, weight of the walkers, frequency of the walkers, tolerance of the occupants, material properties, etc. I'd guess that floor vibe analysis has about the same level of uncertainty that you deal with. There are papers out there indicating that people are already looking at probabilistic approaches.
RE: LRFD Foundation Design
"Structural engineering is the art of molding material we do not wholly understand into shapes we cannot precisely analyze to withstand forces we cannot totally assess; in such a way that others have no reason to suspect the extent of our ignorance."
In other words, "Measure with a micrometer, mark it with a crayon, and cut it with a chainsaw."
I can see the handwriting on the wall. LRFD is what we will eventually wind up using. However, I still wonder if we engineers are measuring with the micrometer when the construction contractor is building with a chainsaw.
As far as soil mechanics goes, I'm with the geotech guys. It's such a variable material to deal with, I appreciate a healthy factor of safety. Allow me to offer yet another saying, this one from Rudyard Kipling:
The careful text-books measure (Let all who build beware!).
The load, the shock, the pressure
Material can bear.
So when the buckled girder
Lets down the grinding span,
The blame of loss, or murder,
Is laid upon the man.
Not the stuff - the Man!
RE: LRFD Foundation Design
RE: LRFD Foundation Design
We certainly should understand the probabilistic nature of our work; but let's not let code writing and re-writing and the science and math behind that, turn into the be-all and end-all that they are becoming. All the research and testing is wonderful because we end up smarter about what we are doing for having followed this research. But, introducing much of this into new versions of the codes every few years, when they all refer to each other, but none of them are in sync. is just making our life tougher, not really producing better structures. I can hardly afford to keep up with the latest codes the way they are churning them out, and I suspect that many of us don't really learn their new intricacies before they are replaced by a new ed. Then we need the 2009 version of one code, the 2006 version of a code it refers to, etc. etc., and finally a ASTM standard that we haven't purchased yet, just to get a partial picture of what we should do and think to comply with the latest code. I always thought the commentaries were as important as the exact code wording or numbers, because I might come away with a better understanding of what the numbers meant and why I was doing what I was doing. For all the complexity we have added to the design process, we are not producing a sufficiently superior end result or saving a heck of a lot of material. Again, our improved knowledge of design, materials, etc. are important for our judgement; and our better understanding of seismic and wind loading, etc. are important enough to be included in the codes once they are well enough understood; but inclusion of some of the other miscellanea just keeps the code publishers in business and taking our money. A good code would incorporate most of this new knowledge into a few 'thought adjustment factors' which, when applied, meant that I could design a little closed to ultimate strength here, or I better leave a little more margin of safety there, judgement and experience again.
I dare-say that most of the structure problems that I investigate have little to do with not following the latest code to a tee. But, they do show plenty of lack of understanding of good details, how structures or soils really work, or just plain lack of good engineering common sense. The code doesn't cover that; did your education, did your mentor? The guy who can run the computer fastest should be relegated to that function alone, until he gains enough experience to know when his modeling is wrong and the guy who is very good with CAD shouldn't be left to check the details. I submit that we might save more by paying attention to, how a member is most easily fabricated and erected if using steel or how easily the member is formed and the reinf'g. stl. assembled and installed if using conc., than most of the new code machinations ever will.
We might be smart to go back to the days when the codes didn't cover every possibility, every new hypothesis or idea, and all the very latest misc. new knowledge. Then critically important findings can be covered by an addendum until the next edition ten years hence. $50k of experience will trump $100k of new codes and software almost every time, and I don't care whether you use a factor of 1.2 or 1.6 on that fact. BigH and I could probably work well together. That's my introductory rant.
RE: LRFD Foundation Design
Welcome to Eng-Tips and thanks for your eloquent first contribution. There are many of us old timers here who agree with you. Your first concrete code was probably ACI318-63. I still refer to it.
RE: LRFD Foundation Design
RE: LRFD Foundation Design
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.
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.
RE: LRFD Foundation Design
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.
RE: LRFD Foundation Design
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.
RE: LRFD Foundation Design
RE: LRFD Foundation Design
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: . Isn't "less anisotropic" a double negative?
BA
RE: LRFD Foundation Design
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.
RE: LRFD Foundation Design
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.
RE: LRFD Foundation Design
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.