Factor of Safety Questioned
Factor of Safety Questioned
(OP)
I found the following statement in one of the responses to a forum question. I would like to hear some opinions/discussion regarding FS relating to this following statement:
“I would calculate the bending moment based on the real load, not the safety factor. AISC has its own safety factor built in the allowable stresses. If you use a SF of 2 in your loads, and compound those SF built in AISC allowable, then you are over sizing the beam!”
For a steel beam, using A36 steel and the allowable stress in bending is 24 ksi (yes, depending on compact section provisions it could be 22 ksi), the reduction is (1-24/36) or 33%. I have always viewed this reduction as the result of some “uncertainty” in the average allowable stress of A36 steel, call it a factor of safety for material properties.
The IBC 2000 code requires that dead loads be increased by 40% and live loads by 70%, factors of safety due to the “uncertainty” of the loading conditions. Now if we combined these two (assuming for this example that DL=LL) then we have introduced a FS of 33% + [(40%+70%)/2] = 88%.
For the same beam in concrete, with a phi of .85, the material factor of safety would be 15% and the code required factored loads would still be 55% for a total of 70%. I would think that there is less variation in the material properties of steel than concrete, but this little analysis doesn’t show it.
Does this seem right??? Is the above thread response correct? It seems to me that the writer's comment leaves out the code required load factors. (Names and dates are left out to protect the innocent <g>)
“I would calculate the bending moment based on the real load, not the safety factor. AISC has its own safety factor built in the allowable stresses. If you use a SF of 2 in your loads, and compound those SF built in AISC allowable, then you are over sizing the beam!”
For a steel beam, using A36 steel and the allowable stress in bending is 24 ksi (yes, depending on compact section provisions it could be 22 ksi), the reduction is (1-24/36) or 33%. I have always viewed this reduction as the result of some “uncertainty” in the average allowable stress of A36 steel, call it a factor of safety for material properties.
The IBC 2000 code requires that dead loads be increased by 40% and live loads by 70%, factors of safety due to the “uncertainty” of the loading conditions. Now if we combined these two (assuming for this example that DL=LL) then we have introduced a FS of 33% + [(40%+70%)/2] = 88%.
For the same beam in concrete, with a phi of .85, the material factor of safety would be 15% and the code required factored loads would still be 55% for a total of 70%. I would think that there is less variation in the material properties of steel than concrete, but this little analysis doesn’t show it.
Does this seem right??? Is the above thread response correct? It seems to me that the writer's comment leaves out the code required load factors. (Names and dates are left out to protect the innocent <g>)






RE: Factor of Safety Questioned
With allowable stress design or working stress design, it was the practice to use the service loads, unfactored, and determine a required shape based on the code allowable stress, 20 ksi for grade 36 ksi steel for example.
That, of course, has changed and with load factor and LRFD brings...well, load factors and resistance factors.
RE: Factor of Safety Questioned
>
>“I would calculate the bending moment based on the real >load, not the safety factor. AISC has its own safety factor >built in the allowable stresses. If you use a SF of 2 in >your loads, and compound those SF built in AISC allowable, >then you are over sizing the beam!”
This is true assuming you are designing by ASD (allowable stress design). The factor of safety (FS) is in the allowable stresses. Do not arbitrarily increase your loads unless you are unsure of the load itself. However, the loads are usually defined by the code so why increase?
>For a steel beam, using A36 steel and the allowable stress >in bending is 24 ksi (yes, depending on compact section >provisions it could be 22 ksi), the reduction is (1-24/36) >or 33%. I have always viewed this reduction as the result >of some “uncertainty” in the average allowable stress of >A36 steel, call it a factor of safety for material >properties.
The allowable stresses are usually a fraction of the yield point of the steel. ASD is based on "elastic design". The "uncertainties" are based on several factors which are stated in just about every design text I've seen. ASD is based on service loads so you don't apply your own factors.
>The IBC 2000 code requires that dead loads be increased by >40% and live loads by 70%, factors of safety due to the >“uncertainty” of the loading conditions. Now if we >combined these two (assuming for this example that DL=LL) >then we have introduced a FS of 33% + [(40%+70%)/2] = 88%.
>
You are talking LRFD or LFD which is different than ASD. Load factors are applied to the loads but ultimate values with an appropriate resistance factor are used. As far as I know the IBC still allows ASD but I may be incorrect. I've got one around somewhere but I'm not going to look it up. Again, the differences between ASD, LFD, and LRFD can be found in your steel design text, or your concrete text, or your timbers text, or your masonry text.
>
>For the same beam in concrete, with a phi of .85, the >material factor of safety would be 15% and the code >required factored loads would still be 55% for a total of >70%. I would think that there is less variation in the >material properties of steel than concrete, but this little >analysis doesn’t show it.
>
>Does this seem right??? Is the above thread response >correct? It seems to me that the writer's comment leaves >out the code required load factors. (Names and dates are >left out to protect the innocent <g>)
You need to look up the differences between ASD, LFD, and LRFD. These are very very basic design principles and it sounds like you need to figure out the difference before you do any design. Bottom line... you cannot mix and match the different methods.
MikeD
RE: Factor of Safety Questioned
You are mixing design methods as canyonrat stated. You basically have two methods to CHOOSE from - (yes - even in the IBC you can use either): ASD or LRFD
With ASD Steel, you have
Load Factors = 1.0 (or, in other words, no load factors)
Allowable stress reduction factor = 0.6 (yes it varies but let's use 0.6 for our conversation). This is applied to the yield stress - let's use 36 ksi as you used above.
This would mean that, under the actual loads (1.0 x load) you would design your member to not be stressed higher than 0.6 x 36 = 22ksi. Your safety factor then is 36/22 = 1.64.
With LRFD Steel, you have (per the IBC)
Load Factors for Dead = 1.2
Load Factors for Live = 1.6
Strength reduction factor = 0.9 for flexure
For a case where DL = LL your Load factor averages to 1.4.
So your safety factor is 1.4/0.9 = 1.56. In this case I've just used the single load combination of DL + LL (there are others of course) and looked at flexure only (shear has a different SRF).
Both are similar in their load factors but these comparisons vary if you have a structure with higher Live Load where the LRFD penalizes you (because in actuality the live load is much more uncertain; requiring a higher safety factor).
In concrete, the load factors are the same (Load factors are independant of the material) and the Strength reduction factor for flexure is a bit different too. But the overal safety factors are quite similar.
The key points to remember are:
1. Do NOT mix "allowable" stresses with load factors, or
2. Do NOT add the safety from an allowable stress with that of the safety from load factors (as you did in your fourth paragraph)
3. ASD allowable stresses include both the safety factor for the variability in loads and the variability in material properties.
4. It is incorrect to state, as you did above, that the 36 ksi yield stress is an "allowable". It is actually your limit state. The allowable is the reduced value of 22 ksi
5. Our talk here of "safety factor" is really a mis-use of these load factors and strength reduction factors. The combination of load and strength factors in LRFD simply try to develop a statistical confidence against the probability of failure. LRFD does this better than ASD due to its separate factors for dead, live, wind, etc. and the development of different strength reduction factors for different modes of failure (moment, shear, axial).
RE: Factor of Safety Questioned
I'll bet it was a one-room school too! I'll bet they had 18 year old bars with 25 cent tap beers, those were the "good old days".
Thanks for all the comments guys!
RE: Factor of Safety Questioned
Good explanation; much better than mine. One addition to number 5. I was going to mention the statistical side but was tired of typing I guess.
LRFD has been statistally 'calibrated' pretty much across the board at least for bridge design. ASD is not really as far as I know. Maybe pieces are. More of a hodge-podge.
MikeD
RE: Factor of Safety Questioned
RE: Factor of Safety Questioned
RE: Factor of Safety Questioned
RE: Factor of Safety Questioned
As you will note in reading over the entire group, there is confirmation that as the predictability of load and materials increases, the need for a "Safety Factor" decreases. This is why the FS for bearing capacity and other Geotech parameters is much higher than for steel design. Similarly, concrete (though very predictable in properties) carries a greater need for "factoring" than steel, just to compensate for the field variables and the natural variations of materials.
Controlled, manufactured products (steel, aluminum, some wood, etc.)offer greater predictability thus greater confidence in the design parameters.
I agree with JAE that many of the accepted parameters are based on experience and observation (not a bad "proof test" as long as you have lots of data!), not statistical proof. The steel industry had an early sales motivation for consistency and, in my opinion, have turned that sales motivation into a viable, predictable engineering parameter that we all use, respect, and rely on.
RE: Factor of Safety Questioned
Thanks to all for all the good discussion in the above posts.
RE: Factor of Safety Questioned
The one point I was making - most steel and concrete design work (I presume) is based on strengths of materials - not on deflections, movements, etc. So it makes sense to use factored loads in determining sizes. For soils, as we have discussed in the geotechnical forums, bearing capacity of spread footings seldom governs the design of the footing - it is usually the permitted settlements, differential movements, rotations that govern. These, many times are dependend on sustained loading (like consolidation settlements). Such loading is not practically covered by the LRFD, in my opinion - based on my understanding of it. Geotechs need real loads for real situations to determine the movements associated with them. Tomlinson has revised his great foundation book to meet with LRFD of the Euro-Code - much to my consternation. When he dealt with the real world, it was much better book. I am afraid that geotechs are being "forced" into the LRFD situation by "designers" who probably are the dominant force in code revisions, etc. since they are the ones mostly using them.
I'll look forward to the paper - thanks.
RE: Factor of Safety Questioned
The term "allowable bearing pressure" would more properly be "the pressure at which an 'acceptable' amount of settlement will occur." That's a real mouthful; so the term "allowable bearing pressure" persists. It's an unfortunate historical term which does not have a suitable substitute.
The structure of LRFD does not seem - to me - to be structured in a way that effectively deals with geotechnical parameters. How do you use LRFD to deal with design parameters that are essentially deflection limits?
RE: Factor of Safety Questioned
I haven't read any of the LRFD Geotechnical literature lately, but the basic format of LRFD can certainly apply to geotechnical, even with the use of maximum settlements as the criteria for the design.
ASD - and traditional geotechnical procedures, as is mentioned above, use a maximum acceptable settlement as the basis for the so called "allowable bearing stress".
In this case, the maximum acceptable settlement is, essentially, a limit state. In ASD, the geotechnical engineer will apply some safety factor to that settlement to get the "allowable" value:
pressure at max. acceptable settlement/SF = allowable pressure
...and the equation for loads is:
allow. pressure < pressure from actual loads
This is no different than in steel where:
max. yield stress/SF = allowable stress
...and
allowable stress < stress from actual loads
In LRFD, you take a more statistical approach and split the safety factor into two factors - one to provide a degree of safety based on the resistance and its level of variation. The other based on the various loads and their individual levels of variation. So in steel it is:
phi x resistance > (Load 1 x SF1)+(Load 2 x SF2)+...
For soils, I would bet that it is really the same:
phi x (press. from max. acceptable settlement) >
(Load 1 x SF1) + (Load 2 x SF2) + ... etc.
The use of LRFD here with soils would do exactly what it does for steel - provide different levels of safety factor for different loads based on their variability.
To some extent, I would think that you geotechs do this already in the ASD format by using your intuition when applying a safety factor....."should I use 2, 2.5, or 3?"
In ASD, you are always faced with the problem that you NEVER provide a uniform degree of safety against failure. This is because ASD does not directly take into account the type of loads applied to the foundation. A structure with lots of dead load and little live load will have a different probability of failure than a similar structure with small dead load and lots of live load.
RE: Factor of Safety Questioned
Many thanks - I look forward to continued dialogue on both sides of the coin.
RE: Factor of Safety Questioned
Good comments. In fact, I was surprised that right after I posted the above, I came across an article in Structural Engineer magazine titled, "Foundation Engineering - Judicious Use of LRFD and ASD in foundation design", by Walter E. Hanson, PE, SE, and Donald D. Oglesby, PE, SE.
It covers the use of both systems for geotechnical design. Their website is www.gostructural.com
Don't know if the article is on the site or not -
I don't disagree with you on your points. Just want to clarify that the term "safety factor" and "probability of failure" are not the same thing. You mentioned that you might use a higher safety factor for a case with less assurance. This is exactly what LRFD does - only it does it by varying the safety factors to make uniform the statistical probability of failure of ANYTHING.
Whatever your material, design, etc., there are statistical data that can be measured in terms of variability. You said above - "what if we NEVER provide a uniform factor of safety against failure "....that's exactly the point. The safety factors are not (and shouldn't be) uniform. They vary with your ability to predict. For soils - I believe you are exactly right...the variability is very high...so using a more statistically accurate approach to setting your safety factors would give you a better handle on the true degree of safety that you are getting.
I know that even in Structural Engineering there are many who simply despise the LRFD approach. I don't disagree....you must use more effort to get to a higher precision answer without necessarily getting paid for it. A real bummer. So ASD can work and LRFD can work....
I would say that LRFD is "better" in that it is more accurate in getting a proper degree of safety, but ASD is "better" in that it has been successful for many years and takes less time.
RE: Factor of Safety Questioned
Thanks for the link; unfortunately the article doesn't appear to be available online. But I did find en editorial on ASD vs LRFD, which I will read this weekend. It's about 858k and in PDF format.
A major problem with LRFD lies with the selection of the risk of failure due to a particular kind of load. Unlike concrete or steel, we cannot really use lab or field tests to provide us with the kinds of information that is known about the failure modes for concrete or steel. As a result, our "tests" involving rates of failure are typically full scale structures - the completed projects. Many of the failure modes are not amenable to model tests, or result from minor variations in soil properties that the meager geotechnical studies we are forced to perform do not allow us the ability to rationally evaluate statistically. And even when we have enough tests, it's damn near impossible to know the implications of minor variations in soil properties on the design methods.
We have theories to account for all of the major ones, but there are often three or more methods that could be used to evaluate each failure mode. And statistical data on failures is often nonexistent, so we have no way to rationally evaluate most of them. Axial capacity of piles is a notable exception, thanks in large part to the offshore oil industry.
Can we do a better job of providing some statistical evaluation of the soils at a given site - which would permit us to provide a rational basis for providing the structural engineer with geotechnical parameters for LRFD design? We can. But it will take significantly larger budgets for geotechnical investigations, and inclusion of the geotechnical engineer as an active and valued member of the design team. And I mean much larger budgets - by a factor of 3 or more. Remember that there is a pretty steep learning curve involved, and you simply cannot expect geotechnical engineers to take a great deal of perceived risk - real or not - without sound engineering to back it up. And that sound engineering will require significant investments in field exploration and lab testing - as well as engineering effort to get a grip on the site and project requirements.
I am highly critical of most of the efforts by academicians to inject probability theory in geotechnical engineering design. Most involve a dizzying array of probability parameters that are completely foreign to the practicing engineer, with no way to leverage existing knowledge into the proposed design methods. Predictably, these methods have been largely ignored by all but a few geotechnical engineers. However, J. Michael (Mike) Duncan authored a paper titled, Factors of Safety and Reliability in Geotechnical Engineering that was published in the April 2000 issue of the Journal of Geotechnical and Geoenvironmental Engineering Vol. 126, No. 4. Discussions of the paper appear in JGGE Vol. 127 No. 8. This paper deals with the variability in soil properties; it does not deal with failure modes or mechanisms. But it is an excellent start. You need to read both the paper and the discussions to get a full appreciation of the issues involved - from a geotechnical perspective.
If you treat us like mushrooms (by keeping us in the dark and feeding us bovine excrement), all you'll get are toadstools...
RE: Factor of Safety Questioned
But whether you are doing ASD or LRFD, you are still dealing with that same variability. So the selection of the which method to use should be independent of how scattered your minimal data is.
With ASD, you are still assigning a safety factor to get to some perceived level of safety (probability of failure)...with ASD you are just using a "shoot from the hip" and "its worked well in the past" method to get to that level of limited risk.
With LRFD, you are doing the same thing, just separating the variability of loads (which we have a better handle on than soils) from the variability of the soils.
Both systems deal with the variability, both deal with limiting the probablility of failure....they just assign safety factors differently.
Setting a resistance factor (phi) is difficult with soils ...that's not an argument. Just how to get a good phi in LRFD and how to get a good safety factor in ASD are the difficult questions....
RE: Factor of Safety Questioned
Spread footings, mat foundations and piles/piers/caissons which rely on "significant" tip bearing/bearing pressure are a different story. When using ASD for the foundation types, I seldom calculate an ultimate bearing capacity and apply a factor of safety to arrive at an allowable bearing pressure. I usually look at the expected movements and reduce/increase the bearing pressure accordingly. Why don't I calculate the ultimate bearing capacity? Simply because my experience tells me that it doesn't control the design, and that simply relying on pultimate divided by F.S. = 3 will get me into trouble - the foundations may experience too much settlement.
I guess that one way to handle this would be to calculate pultimate and choose reduction factors that limit the allowable bearing pressure to the value of pallowable to something no greater than my other calculations tell me will result in acceptable performance. My concern is twofold: 1) that the LRFD procedure won't necessarily use the reduction factors in the manner I have envisioned, and 2) I will end up in endless arguments with structural engineers telling me I'm too damn conservative. (I'm not - but I use more sophisticated approaches than most of my competitors, and end up with more reasoned and consistent answers.)
Unfortunately, AISC has used the same brute force method to "educate" geotechnical engineers ("cram it down their throats") that they did with USD. (I wonder how many structural engineers realize that LRFD is really repackaged USD - another design method that has failed to achieve widespread acceptance?) A brute force method that, frankly, I resent. AISC still hasn't addressed the concerns of the majority of the geotechnical engineering community. We haven't been engaged at all in the LRFD development process. And then they wonder why we continue to resist USD/LRFD!
Anyway, enough ranting. None of this was directed at you, or any other structural engineer, in particular. But you've gotten a small taste of how many in the geotechnical engineering community feel about LRFD. And AISC.
RE: Factor of Safety Questioned
RE: Factor of Safety Questioned
My head hurts! And it isn't helping the remaining hair on top...
RE: Factor of Safety Questioned
It did not seem to me that anything else but the reciprocals of the traditional FOS's were being used as resistance factors with some polish here and there based on the experience of the geotechnical experts on the code committee. In fact the resistance factors have been calibrated against the traditional FOS's leaving the geotechnical Engineer to make the choice on the method to use in determining the load capacity. This choice is often based on experience. Here is where values are often judged to be very conservative etc by the Structural Engineer.
For pile foundations we are still using between 25 and 50% of the load capacity derived from calculations unless settlement is of concern. For hard unyielding ground our capacity should not exceed that of the pile material for which we can use an appropriate resistance factor as prescribed.
What I would really like to see is
1. That the structural and geotechnical codes define the number and type of tests to be done for each foundation type and make these mandatory in our practice. Hence, no longer would we be doing one or two testholes and providing values for an entire set of foundations but we would be doing as many testholes as there will be foundations, along with as many insitu tests, consolidation, shear strength, Atterberg limits,moisture contents etc etc as are required to fully define the subsurface stratigraphy. This should be applied to all jobs big or small.
2. The provision of a range of capacity values, settlement values etc by the Geotechnical Engineer rather than single values so that the Structural Engineer and Geotechnical Engineer can sit down and discuss the project and the Geotechnical Engineer can finally decide what values are applicable for the situation . As well, the Structural Engineer can discuss his choice of limit states and his/her spectrum of numbers that he/she has adopted i.e whether strength or extreme etc as the codes may stipulate and the reasons for his/her choice. This way both parties can be educated on how each other approaches the design and hopefully this collaboration can lead to understanding, practical, safe and cost effective foundation designs and judicious selection of foundation types. This collaboration should extend to the construction phase as well.
With the present concept of the LRFD design approach the preferred approach should be different than the Structural Engineer asking for a factored resistance, and settlement from the Geotech Engineer and the Geotechnical Engineer asking for the factored and unfactored loads from the Structural Engineer with both parties going along their different ways. Both parties have a responsibility to ensure that the resulting structure performs as intended.
RE: Factor of Safety Questioned
I can't agree with your statement in Item #1: "Hence, no longer would we be doing one or two test holes and providing values for an entire set of foundations but we would be doing as many testholes as there will be foundations, along with as many insitu tests, consolidation, shear strength, Atterberg limits,moisture contents etc etc as are required to fully define the subsurface stratigraphy. This should be applied to all jobs big or small."
The cost of construction would skyrocket! IMHO, the geotect's job is to use his professional judgement, including project cost as a factor, and decide on reasonable testing program, without making swiss cheese out of the site.
RE: Factor of Safety Questioned
I have been to battle with this topic for many years and there is a lot more to this subject than what we can publish professionally in this forum. For example, I have noted that the client rarely ever gets to understand what the purpose of the geotech work is all about since either an architect , developer, or structural engineer is awarded the project and the geotech is often held as a sub who because of a job has to use his "professional judgement".
Well often the less you pay the geotech the more is in the pocket of the other. Yes it does happen despite what we say. On the other hand there are some situations where both sides behave professionally but this is often rare.
Well it is better to make "swiss cheese" rather than spending lots on iron and concrete. At least it is far cheaper and makes economic sense. I have heard that term too many times before and generally am amused.
Regards
RE: Factor of Safety Questioned
VAD made a lot of very valid points. The issue of the cost of the geotechnical study was raised by jheidt2543 in response to VAD's point #1. While I won't go so far as to suggest that we put a boring under each footing, it is quite clear that the current state of affairs is not acceptable for many (most?) projects. Even by tripling the geotechnical engineering fees on most projects, our fee will still be a lot less than the structural engineering fee. I don't expect to hear too many SE's complain that their fees will break the piggy bank! And while this will seem heretical to many of you, most of a structural engineer's work is heavily automated - and it seems that the SE fees could be trimmed. Our work will never see the level of automation that structural engineering offices have achieved these last 20 years. How many of the SE's that have graduated since 1988 know what Pounce is, or how to Leroy a title block? Damn few. Yet structural engineering fees, as a percentage of the cost of construction, have not eroded that much. The geotechnical fees have.
And most of you have no idea how offensive the phrase, "Just use your professional judgement" is to many of us who have been practicing more than two decades. In most cases, one could substitute the phrase, "I don't value your work." And the attitude doesn't come from the owner - it's from the other professionals! I cherish the projects where the owner really understands the various professions involved in building a project, and hires the geotechnical engineer first - sometimes before the land is purchased. Those owners really get their money's worth, and have fewer construction and post-constructions problems.
RE: Factor of Safety Questioned
That's not a soapbox, it's a whole bandstand!
First of all, my comments regarding VAD's suggestion of a boring for each and every footing stands, even though he said it for "literary effect". Somewhere there is a balance that has to be struck between prudent design and cost, just as you noted. A good example of not doing so is that $155 million dollar high school in L.A. They skimmed on the geotechnical work and had one soil related problem after another and they are still not out of it!
Secondly, I surely won't defend a good number of the structural guys either. Many of them produce "work" that doesn't deserve the name.
I have yet to find an engineer (in any field), an architect or a contractor who has said he earns enough or that some other SOB cut his fee and got the job he had work so hard to get. And when you talk to an owner, well whatever the first price is it's too high. That fella's is the industry we CHOOSE to work in, mainly because even on a bad day it's a great way to earn a living (it still doesn't beat fishing though!). I don't believe the construction industry owes us engineers and contractors a living, we have to go out and earn it every day.
When I hire a soils engineer, I generally hire the same one, out of the 4-5 firms available in the area. The reason is I trust their professional judgement regardless of price (honest). Price is a factor, but not the only one.
The plain fact is, the guys that do good work, at a fair price get additional work. The definition of a "fair price" may be questioned, but that's the way it is. We have to sell ourselves first, then our services.
Most of us engineers don't like going out and "pressing the flesh" so we hire a Marketing Manager. Personally, I think that is money wasted. Even in these cynical times, personal relationships can make a difference and an owner likes nothing better than to see the "head man" on his job.
That's my two cents, back to you!
RE: Factor of Safety Questioned
I generally concur with your comments. And I've only met one marketer worth the salary. Personal contact is the best approach - by far.
RE: Factor of Safety Questioned
I would have to take issue with you a bit, Focht3, on the part about automation in structural design. Yes, things are automated. But here's what has occurred in the last 15 or so years:
1. Design of members such as steel beams, columns, footings (the concrete limit state, not the soil) are all acccomplished much quicker thanks to the PC which has taken away time spent on tedious hand calcs and allowed more time to be spent on details, concepts, and true design.
2. With the advent of PC's, code writers across the country have no qualms about increasing the building code complexities (primarily in wind and seismic design) to the point where even understanding what calculations are necessary is extremely time consuming. The code from even the 1980's is extremely simple compared to the IBC of today.
3. General Contractors lower capacity to understand and build structures has diminished overall (not everyone - there are great contractors out there) such that there is increased pressure on the structural to detail every little bolt. We see "old" plans of say a 15 story building taken up on 5 sheets where today we would require 40.
4. With lower contractor understanding and a larger population of lawyers comes increased risk. Thus, higher efforts are required to ensure better drawings ( I know and agree with you to a point that a lot of engineer's drawings are really awful....but there are also firms that put out really detailed stuff these days).
5. Owner schedules for their buildings have reduced to a point that is quite ridiculous, requiring enormous design efforts over a short period of time. This, I believe, is the primary culprit behind the sick looking SE drawings you mentioned. With faxes, email, digital conferencing, the business community has shifted to a NOW mentality and they expect us to engineer at the same speed. Yes, I've got a computer, but I also have to THINK through all aspects of the design just like I did many years ago.
You have a point, perhaps, that SE fees as a % of construction haven't changed that much over the years...I don't know about Geotech fees - but one thing I do know: there are a LOT of Geotechs who are ready, willing and able to submit a fee, knowing full well that the owner is choosing the lowest. This vs. Quality Based Selection which is how you all should be selected in the first place.
The Geotech community has slipped into a bidding culture somehow...I believe that that is the problem....how you got there I'm not so sure.
RE: Factor of Safety Questioned
I've been away a bit - on a vacation. ... but isn't it interesting that only the SE's get the stars in this debate!!
Focht3 and I know about clients not understanding geotechs - 'ell, half the SEs don't either. (VAD, Ron, jheidt, too).
I plan to do something about this - am submitting a paper to the Indian Road Congress on the Role and Value of geotechnical engineers - for a conference later this year. This forum helps, in some ways to see the sides. I know of one case where an entire area (several sites) was forced to do ground treatment for support of retaining wall foundations because the geotechs and SEs didn't understand - or compartamentalized too much. Even after long discussions, some strong debates, SEs still want to put a 'new" 6m high reinforced concrete retaining wall on the same soil (without ground treatment to increase Su) that won't support, even, 5m of retaining wall!! This, too, in an area where the 6m of fill may cause up to 500mm of settlement!!
There is a lot falling through the cracks between geo practice and Design firm practice.
But, take care and
RE: Factor of Safety Questioned
Welcome back from vacation! Hope you had a good time!
I noticed the "star" pattern, too. But I'm not posting for accolades - only to foster understanding and communication. I have a pretty tough hide...
JAE:
Good post. Since I agree with most of your comments and observations, I don't see a point in trying to address them all. Let me hit a few key points (before I'm called to supper):
The "dumbing down" of the construction business
It's real. And real frustrating. But keep in mind that geotechnical engineers often (usually?) provide CMT services also; we often get to deal with jobsite ignorance on a daily basis. And the need to CYA in our reports and designs affects the cost of our work, too. Our only advantage is that we were forced to confront litigation much earlier than any other construction professionals. ASFE was formed by ten geotechnical engineering firms in the early 1970's (if memory serves me right) because insurers walked away from our profession. We couldn't get E&O insurance - at any price. But we adapted - and formed our own insurance company! A lot of SEs used to make fun of the "weasel words" in our reports; now I get requests to peer review theirs!
Quality Based Selection
I'm a big fan of QBS. Smart owners use it. Structural engineers - and architects - should urge owners to use it for all their projects. We all have to explain the benefits to owners - you can't expect the geotechs to do it alone.
My biggest concern is the growing trend toward contractors "managing" the selection of the design team. Owners do it to "save" money on design fees, but end up shooting themselves in the foot (or other anatomic extremities...) We have to do a better job of education.
Time to THINK
It's a big problem. I can remember working in the days before the Internet, GPS, cell phones, pagers, total station, fax machines, laser printers, CAD, PCs, memory typewriters - all those "indispensible" tools. When you had a few days to think about a problem before you had to provide an answer. A lot has been lost as the time schedule has been compressed. I don't mourn the passing of a lot of it - but I do think we should insist on adequate time to think about a problem before we provide an answer.
The competition among geotechs
This has a lot of causes. One is the availability of just about anyone to hire a contract driller; so anyone with a P.E. can call himself a geotechnical engineer and offer "professional" services. This was, ironically, caused by the "old line" geotechnical firms in the 1970's. Many of them began selling off their old rigs and buying new ones. Drillers - the smart, hardworking ones - bought them at prices that approached their value as scrap metal.
Another is the lack of a practice act for geotechnical engineering. Frankly, the biggest impediment to a geotechnical practice act - aside from those engineers that wouldn't make the grade - are structural engineers. California has a practice act for structural engineering, but only a title act for geotechnical engineering. As I understand what happened, the California SEs gave only token support to the GE practice act, and it was gutted in the California assembly's subcommittees. Help us get a practice act - with very limited grandfathering, and you would see the quality of the work go up. Yes, the price of a geotechnical study would go up. But the geotech fees have significantly eroded over the last 20 years - now running less than 0.5% on fairly large, complex projects. They ran from 0.5% to 1% in the late 1970's and early 1980's. They need to go up, anyway - if you want to be provided with quality information.
Anyway, good discussion. Gotta go - dinner bell's a-ringing!
RE: Factor of Safety Questioned
I would rather see the geotechs form in with the A/E firms and simply hire out the drilling to "contractors". Separate the professional part from the "labor" part.
Anyway....about those stars...I don't know who's popping them down here but I don't believe my comments are any different than the others above...just must have some home field advantage as this is the STRUCTURAL ENGINEERING FORUM.
RE: Factor of Safety Questioned
One point of Focht3 I would like to address:
Case History: the erosion of price.
My company in Canada is an old one in the geotechnical business. Long ago, before Vic Milligan, John Seychuk (great engineers) joined forces with H.Q. Golder to form Golders, they worked for our company. Rumaging through old files, I saw an invoice for a one story warehouse in Brampton Ontario that Vic worked on (his charge out rate in 1967 was something like $15 Canadian/hour - sorry don't know the exchange rate at that time). The work consisted of drilling a few shallow borings to prove clayey till and bedrock. The price? About $3000 (1967 yr). I did a warehouse right next door in 1993 - my price including 4 borings, etc. - $1450 CDN; if I had bid $1750, I would have lost it, no question. Vic had something like 70 hours to view and write the report; after drilling, I had 5. Interesting, eh? This is no wonder that the pioneers had time to have visions.
RE: Factor of Safety Questioned
RE: Factor of Safety Questioned
I don't mind your "home field advantage."
About the "I can do it all" "Klingon": everyone starts somewhere. But be sure he's qualified before you hire him! (Although it sounds like he has already failed the pre-screening...)
On blending geotechnical engineering into AE firms: it has been tried before - with few successes and a lot of failures. I don't know why, although it probably has a lot to do with the lack of opportunity for advancement and raises when geotechnical engineering is a smaller part of the total fee "pie." Hard to compete for partner when there's 20 senior architects, 15 senior structural engineers, and one or two senior geotechs. I think it's a numbers game.
BigH:
I saw the same trend with McClelland Engineers in Houston - but couldn't quote numbers I no longer have access to. As I recall, fees were pretty steady with respect to construction cost from the early 1950's through the early 1970's. Thanks for the case history - it helps to explain the problem in "concrete" terms.
RE: Factor of Safety Questioned
My understanding is when certain soil parameters are determined, one inch defines a failure. From here a factor of safety is used to determine allowable limits.
Does this one inch movement define an ultimate condition?
I understand that due to the inherent variability of soil, a blanket statement of ultimate soil parameters based on limited boreholes make geotechs cringe.
VOD
RE: Factor of Safety Questioned
I also remember a paper by Brown in Ground Engineering where he compared the results of settlement computations in sand for some 15 methods - he basically said, pick three you like and take the average. Doesn't make you all that warm and fuzzy on giving a bearing pressure for a "limiting" settlement, eh?