Factor of Safety Questioned 11

[jheidt2543]

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>)

 

[Focht3]

I agree, ARH. The big disconnect seems to come when I ask for an assessment of the sustained load over time (usually 20 years.) That's when I get hammered about LRFD. (&quot;Why do you have to work in ASD? Why can't you give me soil parameters for LRFD?&quot; etc. etc.)


My head hurts! And it isn't helping the remaining hair on top...

 

[VAD]

Interesting debate. I had my initial dealings with this some topic 14 years ago and decided that to make everyone happy, I gave a slightly higher value of factored resistance for the ULS and a slightly lower one for the SLS as well as the settlement values using the unfactored structural loads. Like Focht3 I pruned or increased values, chose one method of analysis over another based on my experience, judgement and understanding based on the loading conditions, structure type, structure configuration etc,and how confident I was of the nature of the subsurface soil conditions.

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.

 

[jheidt2543]

VAD,

I can't agree with your statement in Item #1: &quot;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.&quot;

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.

 

[VAD]

Yes, understood. The statement was meant to demonstrate that vey often the Geotech Engineer is not allowed to make his judgement on the amount of test holes and testing since others dictate but those are the ones that require the best information and cry about too conservative. I can say the samething about the structural loadings as well.

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 &quot;professional judgement&quot;.

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 &quot;swiss cheese&quot; 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

 

[Focht3]

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, &quot;Just use your professional judgement&quot; is to many of us who have been practicing more than two decades. In most cases, one could substitute the phrase, &quot;I don't value your work.&quot; 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.

 

[jheidt2543]

focht3,

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 &quot;literary effect&quot;. 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 &quot;work&quot; 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 &quot;fair price&quot; 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 &quot;pressing the flesh&quot; 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 &quot;head man&quot; on his job.

That's my two cents, back to you!

 

[Focht3]

Sorry for the &quot;bandstand.&quot; Frustration with a particular little group of a contractor and his &quot;pet&quot; architects and structural engineers who do &quot;turnkey&quot; projects. They include the geotechnical fee in the overall AE services, then &quot;shop&quot; fees until they get the cheapest fee possible. How much geotechnical work can be done for a small building when &quot;the budget&quot; only allows for the geotech fee to be 0.2% of the construction cost? Ridiculous. While a minority, they are still aggravating. Sorry for the rant - the &quot;use your professional judgement&quot; comment really set me off. That's what the contractor tells me to do in lieu of a reasonable study...

I generally concur with your comments. And I've only met one marketer worth the salary. Personal contact is the best approach - by far.

 

[JAE]

Must offer my response as a Structural Engineer who knows what Pounce and Leroy are (and can use them).

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 &quot;old&quot; 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.

 

[BigH]

JAE - ask the structural types - the ones that send out bids to 4 geo firms to save $100. I've seen a struc firm put out a massive document for a $2000 investigation - to save how much they spent more than 40 hours!!

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&quot; 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

 

[Focht3]

BigH:

Welcome back from vacation! Hope you had a good time!


I noticed the &quot;star&quot; 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 &quot;dumbing down&quot; 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 &quot;weasel words&quot; 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 &quot;managing&quot; the selection of the design team. Owners do it to &quot;save&quot; 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 &quot;indispensible&quot; 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 &quot;professional&quot; services. This was, ironically, caused by the &quot;old line&quot; 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!

 

[JAE]

I really agree with the point about anyone buying a drilling rig and selling themselves out to be a geotech. I've got a guy right now calling me monthly for work who stopped by to visit one day. I asked him all kinds of questions about his geotech experience and he really didn't offer anything much. When I questioned him on his ability to perform proper evaluations of different soil types and other aspects of what a geotechnical engineer might see, he kept repeating &quot;I can do it all&quot;.

I would rather see the geotechs form in with the A/E firms and simply hire out the drilling to &quot;contractors&quot;. Separate the professional part from the &quot;labor&quot; 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.

 

[BigH]

JAE - Just a bit of a &quot;bore&quot;!!

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.

 

[JAE]

BigH - wow!

 

[Focht3]

JAE:
I don't mind your &quot;home field advantage.&quot;


About the &quot;I can do it all&quot; &quot;Klingon&quot;: 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 &quot;pie.&quot; 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 M[sup]c[/sup]Clelland 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 &quot;concrete&quot; terms.

 

[VoyageofDiscovery]

I agree with Focht3 wrt LRFD for soils. I still can't get my head around that one.

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

 

[BigH]

Yeah and the calculation of the movement depends on method of calculation and the modeling, eh? For example (from Dr. Som's new book) - using Skempton Bjerrum for 400kN coming down on a 2m x 2m footing founded at 2m depth with 4m of soft clay (Su = 50 and Cc/(1+e0)= 0.06, he calculates settlement of 43mm for consolidation after Skempton Bjerrum correction for pore pressure factor and depth correction. (plus 5mm for immediate) = total of 48mm. When he uses the stress path method, he gets 31mm of settlement. Now, if you choose to limit settlement to, say 25mm, you will get different bearing pressures permitted depending on the two methods.

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 &quot;limiting&quot; settlement, eh?