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?

 

[271828]

1.0 is not a factor?! I'll have to go check the dictionary to be sure of that.

Anyway, I was just being a smarty-pants.

 

[BigH]

So . . . you want bearing capacity and NOT the allowable bearing pressure. Okay. But when your footing settles 75 mm and the distortion with the adjacent footing is such that the windows don't close and the doors can't open and cracks are all over the brick work, fine - you didn't exceed the bearing capacity but you certainly have a problem on your hands!! You exceeded the allowable bearing pressure which is that pressure given for to meet the structure's service constraints. Good luck living with that!

 

[DWHA]

BigH,
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.

 

[hokie66]

DWHA,

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.

 

[BAretired]

hokie,

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

 

[BigH]

DWHA - I want to be clear - at least in my point of view - on the meaning of bearing capacity. Bearing capacity is a term associated with shear strength. You might very well have an adequate factor of safety (and I don't care if it is ASD or LRFD) against a shear failure. Bearing pressure is not related to shear but is related to movement/distortion - and safety factor on shear, while typically may cover and ensure that the structure is not "overloaded" - is no guarantee - especially as the size of footings are increased and bring into play significantly deeper strata.

 

[ucat]

designing for live and deads loads is easy....
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

 

[miecz]

BigH

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.

 

[vmirat]

miecz,

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.

 

[BAretired]

The National Building Code of Canada has been using LRFD (we call it Limit States Design or LSD) for wood design for a good many years now. There are many modifying factors to be applied but the Load Duration Factor, Kd is 1.15, 1.00 and 0.65 for short term, standard term and permanent loads respectively.

BA

 

[BigH]

My question is, with LRFD: You have all these neat modifying factors - as BAretired says 1.15, 1.00 and 0.65. You design using these factors - then the design runs into problems in practice because of some nuance that the factors are not really correct for a particular situation with a particular soil type. What's the solution? Who do you now blame? I've said it before and will again, soils have coefficient of variations of 30 to 50% compared to 10% for steel and slightly more for concrete.
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.

 

[miecz]

BAretired

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.

 

[BAretired]

miecz

I think the concept you are suggesting has merit, but it would need careful study by both geotechnical and structural engineers.

BigH

You said

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 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

 

[271828]

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.

I was going to type this, but you beat me to it.

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.

 

[BigH]

Yes, I am afraid that probability will be coming and mathmeticians will rule over those who know the soil. I have no doubt as I am of some political going-ons. How will LRFD take care of varved clays? slickensided planes of old slides within a soil mass? jointing patterns within moderately to strongly weathered rock? The fact that I "know" from the geological history that I have some buried stream channels or infilled depressions at a site but the borings - even though I made minced meat of the site - didn't turn them up? Yes, as some suggested, I can modify the load factors - maybe at this site, use 0.88 rather than 0.85 - but how do I know that? The codes do not make those distinctions. I'll take my mentor's experience and his peers any day over some hot shot mathematicians played applied math for a geotechnical problem. But I am old school - not real old - but old enough (I know and used a slide rule (sorry SRE - I know that is your moniker). Thanks for the

 

[271828]

BigH, not being a geotechnical engineer, I have no idea how those variables should be taken into account.

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.

 

[vmirat]

I would like to offer something that I found a long time ago, written by Doug Loos, PE:

"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!

 

[BigH]

There is a great quote from Karl Terzaghi - I'll try to find and post.

 

[dhengr]

vmirat, for this discussion, your quote by Doug Loos should be extended thus: “....have no reason to suspect the extent of our ignorance, so then we apply load and resistance factors to prove that we are.” Anyone here ever heard of the KISS principal (keep it simple stupid)? I still have a couple slide rules which, if still used, and combined with some good experience and judgement could produce very good structures. The first conc. design code I used measured 6"x9"x3/8" and my buildings are still standing and functioning just fine. I like my hand held calculator and run my calcs. to enough precision so that my geometry or forces do self check at a joint. I do not think that this precision amounts to a more accurate or better design, I’m just assuring myself that I haven’t lost 10° or 5k at a connection. I even use a computer, but try not to let it do my thinking for me or let it cloud my judgement when my experience suggests there may be a problem. Let’s not complicate BigH’s life by making him factor things up and/or down; I go to him for his knowledge and experience with soils and local geology when my foot print in the footing excavation or my thumb print in the clay isn’t enough for me to be comfortable designing around them.

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.

 

[hokie66]

dhengr,

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.