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LRFD Foundation Design
3

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

The soil capacity will be given by the geotech for either ASD loads or LRFD loads or both. Call the geotech and request your load type capacity.

RE: LRFD Foundation Design

(OP)
Yeah, I checked the soils report.  It doesn't specify which load combination system to use for the allowable soil bearing capacity given.  However, what if the soils report is based on ASD?  That's my problem.  If I have to do two separate sets of calcs to get to ASD for foundation design, then I have a real problem with LRFD as the preferred method in today's world.

RE: LRFD Foundation Design

Soil reports normally provide allowable bearing pressures, not factored.  Carry your live and dead load down separately.  For footing size, use the sum of the two.  For concrete and reinforcement design use factored loads.  It is not an onerous requirement.

BA

RE: LRFD Foundation Design

vmirat, what BAretired says is true.  Anytime you see the word "allowable" you've got an ASD situation.  Your geotech most likely is giving you an allowable bearing pressure to be used with non-factored service loads.  Usually they use grand safety factors such as 3.0 to arrive at their recommended pressures.

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

(OP)
JAE,
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

LOL, you're going to give yourself high blood pressure if you don't let it go...

RE: LRFD Foundation Design

Even if you could do the foundations in LRFD, you still need service loads for deflections and drifts.

RE: LRFD Foundation Design

Why doesn't the code specify deflection and drift limits for factored loads, thereby saving us all a lot of work?

RE: LRFD Foundation Design

Heh, heh.  Because, miecz, deflections and drift are real, and factored loads are not.

RE: LRFD Foundation Design

The code doesn't usually specify any deflection or drift limits in factored or unfactored loads.

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

ok....deep breath...here we go again....the old LRFD vs. ASD challenge/debate and general fisticuffs marquee.  

In this corner....

 

RE: LRFD Foundation Design

WSD rules...

Mike McCann
MMC Engineering

RE: LRFD Foundation Design

In 90% of the cases, the bearing capacity is meaningless - as JAE and others have pointed out it is the structure's ability to withstand settlement and distortion.  The big problem with LRFD is you spend a lot of time on the computations - to dismiss them anyway.

RE: LRFD Foundation Design

I have been raised in the LRFD era and I can't understand why there has been such a large shift in the way that foundation elements are designed.

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

"ok....deep breath...here we go again....the old LRFD vs. ASD challenge/debate and general fisticuffs marquee.  In this corner...."

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

Is the world ending in 2012?

RE: LRFD Foundation Design

Some are saying that it will.  I don't have TV, so just caught wind of this the other day.  Some kind of Mayan prediction or something.  I think there's a moving coming out about it.

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

Quote:

Because...deflections and drift are real, and factored loads are not.

You know...I think you're onto something.

RE: LRFD Foundation Design

My hat is off to you for doing a LRFD design.  No matter how many times I say it's time to make the switch, I just never have.

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

Footing sizes would be determined using "ASD" load combinations compared to the allowable bearing pressure of soil.  The concrete and reinf that comprise the footings would be designed using "LRFD" combinations (actually USD combinations because LRFD is a steel term, but they are the same).  You can also get conservative footing sizes in CRSI.

RE: LRFD Foundation Design

Am I missing something here???
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

I'm of the ASD school also but I wondered the same so today I picked up Resistance Factors for Use in Shallow Foundation LRFD by K.C. Hoye, R. Salgado and B. Scott. It says factors are needed for reliability index. e.g. CPT did better than SPT and they decrease the soil resistance factor as LL/DL increases due to uncertainty.  Resistance factor ran from .17 to .32 for sand and .6 to .75 for clay. I'm still studying it, but looks like improvement over ASD.

RE: LRFD Foundation Design

"...improvement over ASD. "
Again, I am confused here. "ASD" is a term used in steel design.

RE: LRFD Foundation Design

You're right, Stillerz.  Even though ASD and LRFD are strictly steel terms, some are using ASD to refer to unfactored load combos and LRFD to refer to factored load combos.

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

Then how do you design the concrete footing?  Do you not need 1.2D+1.6L for that?

RE: LRFD Foundation Design

For thickness and reinforcing, yes, use your factored loads. For the footing footprint, you use service loads because you are working with an "allowable" bearing pressure.

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

Right.  My point is that everybody, on practically every project has to deal with D+L and 1.2D+1.6L because every bldg has a foundation.

RE: LRFD Foundation Design

(OP)
This brings me back to my original post.  Instead of running two sets of calcs (service loads and factored loads), has anyone ever used a factor that can be applied to the factored loads (LRFD or ASD) to approximate service load conditions?

RE: LRFD Foundation Design

Yes.

RE: LRFD Foundation Design

(OP)
HA!  Message received, hokie66.  I guess I'm back to doing it the long way.

RE: LRFD Foundation Design

Not exactly.  What I have done upon occasion is the other way around.  If I know the total load (D + L) but I'm too lazy to determine the exact breakdown, I use a load factor somewhere between 1.25 and 1.5 in Canada (or between 1.2 and 1.6 in the U.S.A.).

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

I agree with BA's way as being more typical of what I have done.  My other flippant reply just means that somewhere along the way I am sure to have done that too.  I don't think it is lazy, just some cases allow for simplification.  As long as you note what you have done in  your calculations, it is all good.

RE: LRFD Foundation Design

(OP)
BAretired,
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

I consider it good engineering practice to make approximations of the sort we are talking about when more exact calculations are unlikely to affect the result in any significant way.  

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

BAretired-
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

The problem, as I see it, is that we're only going half way with LRFD.  If we're going to design with factored loads, then we should design the whole structure to factored loads, including the foundation, the drift and the deflections.  That means the geotech needs to issue bearing capacity, not allowable bearing pressure.  If we're not going to do the most important part of the design to factored loads, then forget about 'em.

RE: LRFD Foundation Design

For the bridge work that I do, our geotechnical engineers do give us factored bearing capacities.

RE: LRFD Foundation Design

LOL, I already use LRFD for drift checks, beam deflection checks, floor vibration checks, and bearing capacity checks.  The load combinations just have 1.0 factors.

RE: LRFD Foundation Design

A factor of 1.0, or 1, or 1.00000 is no factor at all.  It's a check at service loads.  

RE: LRFD Foundation Design

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.

RE: LRFD Foundation Design

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!

RE: LRFD Foundation Design

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.   

RE: LRFD Foundation Design

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.

RE: LRFD Foundation Design

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

RE: LRFD Foundation Design

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.

RE: LRFD Foundation Design

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

RE: LRFD Foundation Design

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.

 

RE: LRFD Foundation Design

(OP)
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.

RE: LRFD Foundation Design

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

RE: LRFD Foundation Design

    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.

RE: LRFD Foundation Design

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.

RE: LRFD Foundation Design

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

Quote:

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

RE: LRFD Foundation Design

Quote:

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.

RE: LRFD Foundation Design

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 soapbox

RE: LRFD Foundation Design

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.

RE: LRFD Foundation Design

(OP)
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!

RE: LRFD Foundation Design

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

RE: LRFD Foundation Design

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.

RE: LRFD Foundation Design

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.

RE: LRFD Foundation Design

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. cook on your comments (my snaps!!)

RE: LRFD Foundation Design

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.

Quote:

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.

RE: LRFD Foundation Design

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.

RE: LRFD Foundation Design

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.

RE: LRFD Foundation Design

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

RE: LRFD Foundation Design

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:

Quote:

more homogeneous and somewhat less anisotropic than sawn lumber
.  Isn't "less anisotropic" a double negative?

BA

RE: LRFD Foundation Design

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.

RE: LRFD Foundation Design

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.

RE: LRFD Foundation Design

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.

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