Significant Figures & and code interpretation
Significant Figures & and code interpretation
(OP)
I am a stickler to sound logic, e.g. significant figures, etc. However, My question arises from maybe a more philosophical standpoint (or poorly assumed to be). I see structural engineering as having 4 main components where variability may or will arise. 1.Loading 2.Engineering Analysis 3. Construction 4. Material Strength/general behavior.
All else equal, as engineers we have most control over the 1st and 2nd part of those 4 items as loadings may vary greatly with application (requiring our judgement) and analysis is literally performed by us. Therefore, as a young engineer, I prefer to use the code verbatim (i.e., if f'c=3000 I take it to mean that f'c has been measured to be precise to (1) significant figure(ignoring scatter of points). Then, for example, if I calculate the rupture strength (7.5f'c^.5) one calculates something on the order of 410.7919181288745850927273371006. I would say that the rupture strength is most nearly 400 psi, and I shall carry as many digits as I can from this intermediate calculation through my remaining calculations. I think my physics/chemistry professors would agree based on the standard mathematical laws or error propagation for arithmetic and single variable functions that this is valid.
However, My boss does not follow this logic when he performs calculations. He crudely & significantly rounds down whenever numbers dont seem to 'feel' right. Then in the end he selects sections with capacities as much as 20% greater than demanded by his rounded calculations. He justifies his methods of computing broadly based upon personal liability, loadings, and construction & material variability.
I find his methods to be unacceptable, as he applies this conservatism very broadly to everything he does. I understand our field is mostly experimental, but I feel that many of us (structural engineers) are so afraid of variability, we fly through analysis with grossly over estimated values and thus structural members.
In, short, how does everybody frame their logic in performing calculations as it relates to precision?
All else equal, as engineers we have most control over the 1st and 2nd part of those 4 items as loadings may vary greatly with application (requiring our judgement) and analysis is literally performed by us. Therefore, as a young engineer, I prefer to use the code verbatim (i.e., if f'c=3000 I take it to mean that f'c has been measured to be precise to (1) significant figure(ignoring scatter of points). Then, for example, if I calculate the rupture strength (7.5f'c^.5) one calculates something on the order of 410.7919181288745850927273371006. I would say that the rupture strength is most nearly 400 psi, and I shall carry as many digits as I can from this intermediate calculation through my remaining calculations. I think my physics/chemistry professors would agree based on the standard mathematical laws or error propagation for arithmetic and single variable functions that this is valid.
However, My boss does not follow this logic when he performs calculations. He crudely & significantly rounds down whenever numbers dont seem to 'feel' right. Then in the end he selects sections with capacities as much as 20% greater than demanded by his rounded calculations. He justifies his methods of computing broadly based upon personal liability, loadings, and construction & material variability.
I find his methods to be unacceptable, as he applies this conservatism very broadly to everything he does. I understand our field is mostly experimental, but I feel that many of us (structural engineers) are so afraid of variability, we fly through analysis with grossly over estimated values and thus structural members.
In, short, how does everybody frame their logic in performing calculations as it relates to precision?
-Robert Miller, E.I.T.
KPA Structural Engineers






RE: Significant Figures & and code interpretation
The philosophical 20% is an admission that you don't fully understand the situation and you are attempting to quantify that fear.
Best response to your boss would be to ask why he doubts the numbers being calc'd.
As for precision, I don't reenter the numbers in my calculator to match precision and I rarely write down more than 3 significant digits.
RE: Significant Figures & and code interpretation
Once you start leading projects of your own (and stamping your own drawings), you'll have a much greater respect for making conservative changes based on what "feels right".
It also depends on your client / project. If your project is a "fast track" job that is likely to experience lots of changes when it gets time to build the thing, then thowing in some extra conservatism is a wonderful (and potentially inexpensive) way to save the project some major headaches down the line.
I remember working with one client who wanted every footing to be optimized to the most efficient size so that they could save money on concrete. Though it didn't help them any in the long run. In the end, the contractor basically threw away our calc packets and asked if they could just standardize on one or two common footing sizes. The client may have saved some money (because the extra concrete cost was absorbed by the contractor), but they partially compromised their schedule to do it.
RE: Significant Figures & and code interpretation
In the calculation of a simple beam, the theory is well known, so the results should be as precise as the loading information. I would perform the calculation, then select a beam with adequate strength and deflection, but if there were fifty beams and three were a bit lighter, I would specify the same member throughout to avoid confusion.
In the calculation of the additional torsional strength provided by vertical stiffeners in a steel beam, I would neglect the added capacity because the theory is not clear to me.
BA
RE: Significant Figures & and code interpretation
I, too, in accordance with the KISS principle, add more material as warranted to help avoid the possibility of mistakes in the field. Relative to the mistakes, the cost of the additional material is minimal. Plus, it adds additional calacity to local areas of the strucgture where, if mistake is made, additional capacity is already in place so that a repair may not be required.
I never toe the line.
Mike McCann
MMC Engineering
RE: Significant Figures & and code interpretation
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Significant Figures & and code interpretation
Mike McCann
MMC Engineering
RE: Significant Figures & and code interpretation
I feel you are right. If my wife says use a W12x19 and I do not do so, the personal consequences may be greater than the professional ones. HA!
-Robert Miller, E.I.T.
KPA Structural Engineers
RE: Significant Figures & and code interpretation
Fortunately, my boss usually agrees with her.
RE: Significant Figures & and code interpretation
I do not disagree with you there. I do not think anybody would. But your decision is quantifiable and easily documented. What I was describing was haphazard calculations on projects that do not require "on the fly" decision making.
Of course, when u visit a structure with immediate bracing needs, you go conservative.
-Robert Miller, E.I.T.
KPA Structural Engineers
RE: Significant Figures & and code interpretation
Where I disagree with you is on the need to carry forward an unusually large number of significant digits during intermediate calcs. The reason being that unknown errors can "creep" in along the way. In your good example calc for concrete rupture strength here is a possibility for an unknown error:
1. Is f'c = 3000 psi accurate to one significant digit (3000 psi + or - 500 psi)?
Could be two significant digits: 3000 psi + or - 50 psi
Or three significant digits: 3000 psi + or - 5 psi
Or even four significant digits: 3000 psi + or - 0.5 psi
My point is that you cannot always make correct assumptions based only on the numbers presented, unless a tolerance is stated.
In your example equation, I would be more likely to question the precision and accuracy of the constant (7.5) and power (0.5).
For myself, I agree with woodman88 that three significant digits is all that is needed for intermediate routine calculations. In your example I would say the rupture strength is 411 psi. Of course I say THIS since three digits is the accepted limiting accuracy of a 10" slide rule.
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RE: Significant Figures & and code interpretation
I agree with you and I Like your description of the permutations with the compressive strengths. I inherently want to question those miscellaneous factors too. I guess my assumption is that the those empirical equations (when applied to the correct application) code, represent a minimum acceptable threshold deemed to be fit for design.
If you look at regression curves in the commentary and supplemental code/design books, they are never situated in the "median" of the scatter plots. They always seem to lie to the lower bound of the data. My guess is so that there is very little (-) component of (+/-) a value provided by the code and that the designer can spend more time worrying about his own assumptions and error. (Again, let me stress the proper application as so I do not sound like I ignore variances in code formulas.
From that, I frame my logic that assumes those minimums to be pseudo-exact measurements, and follow industry standard analysis methods and carry mathematical operations and their associated sig figs with them.
I must say I wouldnt argue for my point on KNOWN controversial materials or design methods, even when they are loosely specified in the code books.
-Robert Miller, E.I.T.
KPA Structural Engineers
RE: Significant Figures & and code interpretation
We tend to measure with a micrometer
Mark it with a crayon
and cut it with an axe.
That's the process from design through construction.
Significant figures mean very little in the whole scheme of things, particularly with building construction.
Why is it necessary to compute stresses to two decimal places when the wind loads you used are up to interpretation in several respects, each producing a significant difference of stress levels?
Make what you decide follow reasonable logic, common sense and a valid standard of care.
RE: Significant Figures & and code interpretation
It depends on the material. Some material parameters (including concrete tensile strength) have enormous variability, much more than 20%.
All engineers should be aware that they don't have exact values for material properties, and that code values are not always conservative, and should consider the consequences of material strength and stiffness parameters being at realistic lower or upper limits. This range will often be much more than +- 20%
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Significant Figures & and code interpretation
RE: Significant Figures & and code interpretation
1. I would remove the name of your firm and your name from your tag line. We all now know that KPA structural engineers does not provide the most effcient, cost effective designs (may be valuable information to your competetors).
2. In today's world, we are all working under increasing pressure regarding price and timing. We have all experienced increases in competition from both a fee and timing aspect. If your boss feels that he can meet the budget and provide a safe structure, he make take some short cuts. Typically a 20% increase in capacity does not mean a 20% increase in cost and weight.
3. Codes are minimum standards of design. There are several areas that I routinely feel that the code is less than adequate.
4. If you feel that engineers have control over loading, then you better think again. We have control over how we model loading, but that often is not how loading is applied. I was just in a structure that by code and previous use had a floor live load of 500 psf. My obeservations were that a significant area of the floor was loaded to close to 1000 psf.
I guess in summery, engineering is not all science. There are significant buisiness and art components as well.
RE: Significant Figures & and code interpretation
I agree- it may be best to take down your name and firm from your posts. Precision, significant figures, efficient design or not- all you need is your supervisor to see that you're categorically...characterizing, his design approach on a Monday afternoon and it'll become an aforementioned personal issue for everyone.
But like all things, I think it depends. Whereas deisgning a simple structure is mainly science-- putting together a large structure is an art.
Being able to put together a large drawing set is: 3 Parts Communication, 2 Parts Drafting, and 1 Part actual analysis and design. Figuring out whether the transfer beams should be W18s or W24s is rarely as time consuming as figuring out whether either works with the architecture- and whether it can get shown on the drawings in time. Sometimes, for the project's (read: schedule's) sake-- you just need to pick the W24 and keep moving.
Further, "feels right" is important. In fact, its what our entire system depends upon. At the heart of it, the only day-to-day safeguard our structures have is the fact one person ultimately needs to stamp it. There's not always a peer review; the code department doesn't look at the drawings with a fine toothed comb. So that one person, above all, has to feel right doing it. That's it. If not, over the course of a career, he'll never be able to sleep a day in his life.
The science just helps him do that.
-MJB
RE: Significant Figures & and code interpretation
The fact that all your intermediate calculations are carried out with 17-digit precision should be irrelevant.
TTFN
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RE: Significant Figures & and code interpretation
Mark it with a crayon
and cut it with an axe.
That's the process from design through construction.
Significant figures mean very little in the whole scheme of things, particularly with building construction.
Why is it necessary to compute stresses to two decimal places when the wind loads you used are up to interpretation in several respects, each producing a significant difference of stress levels?
Make what you decide follow reasonable logic, common sense and a valid standard of care. "
Extremely well put, could not agree more
RE: Significant Figures & and code interpretation
"Structural Engineering is the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyse, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance."
The 20% being discussed may actually be 12%, or 57%. Big picture - how much do we really understand?
RE: Significant Figures & and code interpretation
Its the projects where I do add the arbitrary safety factor (there I admit it) that make me not sleep well at night. The reason, my feelings tell me I'm overlooking something. Will the 20% cover me? Not if I forgot to add the closely spaced vertical stiffeners in the torsionally sensitive beam (sorry, I had too!).
@IDS - I suppose there could be some non-conservative issues with our building codes. If there are we should discuss and have them corrected. As for materials, I'm not looking for accuracy, I'm looking for being on the safe side of failure. If I'm missing something, a 20% increase in capacity may not put me there.
RE: Significant Figures & and code interpretation
Sure, I'm not saying apply a flat 20% to everything. For instance, with the tensile strength of concrete as mentioned in the OP I'd suggest reducing by at least 50% if you want an upper bound deflection estimate, or better do a proper analysis including shrinkage, creep and differential temperature effects. The point is that just using code values won't always give reliable results, especially with non-manufactured materials.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Significant Figures & and code interpretation
Saving one's client a few bucks will probably yield more work in the future. That's a self-serving enterprise. Not wrong, just self-serving. But in that process, are we being truly efficient or is it just an exercise in risk management?
Yes, we have progressed in our understanding of materials science as applied to construction materials. We have materials that were not available a few years ago. But in our rush to "betterment", are we neglecting that sometimes performance history is our best measure of science?
Questions to ponder...individually and as a profession.
RE: Significant Figures & and code interpretation
It is often said that engineering is more of an art than a science.
When I started out in engineering I had a very narrow view of my role in design. It basically boiled down to beating the last oz. of weight out of the structure even if that meant numerous different sizes of members and footings etc.
Over time one learns that there are many other factors that come into play...as mentioned in the above posts ie.
economies of scale, repeatition, simplicity, errors, future changes etc.
Schedules are becoming more compressed leaving less time for engineering analysis and and the vetting process required for this and thus increasing the possibilty of errors.
What concerns me is the increasing complexity of the codes and the time involved chasing down each nit-picking detail. To me this is a dangerous distraction from what is the core importance of my role and that is engineering analysis.
The 10, 15 or 20% overdesign(on paper) takes the wind out of the sails of the idea of beavering the last oz. of weight out of a design performed in splendid isolation.
What is lost in all of this is the crude approximation of the input loads and the load factors ie, 1.4 why not 1.35?, 1.6 why not 1.55? etc. Taking these input loads as gospel and designing a strutcure to evermore refinement is to me misguided.
Progression thru experience:
4 significant figures
3 significant figures
2 significant figures
1 significant figure
5% overdesign
10% overdesign
15% overdesign.....
replaced by computer!!
RE: Significant Figures & and code interpretation
A young foolish engineer would criticize his boss publicly and sign his name to it.
A devious young engineer would criticize his boss and sign someone he doesn't like's name to it.
An older devious engineer would criticize the short comings of his office and sign someone else's company's name to it.
A wise engineer, having tried the usual methods, would post anonymously (with Nom de Plume), the shortcomings of his boss on a blog he knows his boss uses in a way that the boss will recognize his shortcomings and correct the supposed error but not put his defenses up.
I think we are all wondering which one of these (or other) are you Rob?
RE: Significant Figures & and code interpretation
I try to keep intermediate calculated numbers intact as best as I can. It's easy to maintain 17 decimal places when performing calcs on a calculator or Excel, but I would never write down that many sig figs on the calculation pad. If you keep the intermediate numbers intact, then when you get to the final line of Load vs Capacity, you know how close you are to the line. As mentioned in posts above, there are lots of good reasons to be conservative, but I don't know how anyone can determine how conservative they are being if they round every intermediate number along the way.
Rob, I will spare you a lecture but it seems you could use a little more maturity. Instead of being critical of your boss, ask why he is making those decisions and you'll learn something. Least material is not always least cost.
RE: Significant Figures & and code interpretation
I think that's a serious issue in itself. One ought not be using a tool that requires you to "write down" anything. There are potentially more errors from transcribing numbers or reversing digits than anything else I've read in this thread.
I almost never have a calculation or result that's hand written. The only time that happens is if I'm without my laptop, or I'm caught in the hallway, on the way to the bathroom. Almost everything I do is in Mathcad, which allows you to set the displayable precision to a reasonable amount, AND it requires NO unit conversions ever, for any unit in its database; adding 20 inches plus 5 meters plus 2 furlongs is trivial.
TTFN
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RE: Significant Figures & and code interpretation
RE: Significant Figures & and code interpretation
I've found that it ultimately boils down to two things we must consider as engineers; our responsibilities and our practicalities.
We are responsible for public safety and the economics of the project (both our own economics and the economics of the client). What I mean is that we must "ensure" (statistically) that the structures/devices we design must not "fail" so as to cause excessive economic harm (expensive repairs) or detrimental public safety (loss of life and so on). Practicality comes into play in all the things mentioned above (assumed loads, variables, errors, constructability, time constraints, etc.). As engineers we must balance these two.
So, when I design something I evaluate these two considerations. For a "simple" project with well defined loads, limited time constraints, limited harm if it fails, I would be precise in my calculations and select the smallest size member(s) that meet the minimum design requirement. This satisfies both requirements as it meets my requirement of responsibility as an engineer while making sure that it results in the most money saved for both my own company and the client.
However, in a complex project the lines become fuzzier and I will likely choose to "round up" a lot of things. The loads might be not clearly defined; round them up. The harm if it fails might be high; round up the safety factors. The client and my firm might save more money if I can get him a slightly more expensive design sooner, round things up. The cost of going back and fixing a small error that I made might be more than the cost of the extra material and/or labor, round things up. And so on.
I'd especially like to point out the last thing. Personal errors are my biggest pet peeve. I will never hesitate to point out my own errors and accept the consequences. However, it just looks bad to myself if I was at fault. Even if the design was ultimately still "safe" (well within a safety factor), I feel it reflects poorly on myself if I keep having to revise myself. How can anyone trust me as an engineer if I keep coming back to them saying "I'm sorry, that design wasn't safe, but this one is. Trust me, I got it right this time." That just doesn't sound good in my head.
So, for that reason I will always round numbers in a conservative fashion when it seems appropriate. As long as I "feel" that I've satisfied my responsibilities as an engineer while making sure to keep costs down (remember, we're here to make money too) then I'm satisfied. Researchers are the only people who need to truly care about significant figures in my opinion.
P.S. Rob, I'm hoping that you're showing that you have a lot of spunk signing your post while you criticize your boss. If you really do have the confidence to accept responsibility for your statements in a public forum then I applaud that, too many people hide behind the anonymity of the internet these days. However, if that was unintentional then I really hope you consider the repercussions of this and think more carefully before you speak in the future.
EIT with BS in Civil/Structural engineering.
RE: Significant Figures & and code interpretation
Would that include transposing the "e" and "h" in "The" in your handle, or does "Teh" actually mean something?....or have I made an assumption that is not based on authoritative evidence?
Just pullin' your chain a little to point out the human factor in all of us.
RE: Significant Figures & and code interpretation
I was wondering how long it would take before my usual internet alias drew a comment. You would laugh at the back story behind my "mighty pirate". ;)
I suppose that's actually a reasonable point which is semi-relevant to the topic. A third requirement to engineering design would be to enjoy it. Sweating over the difference between 4.92185 and 5.0 just takes all the fun out of a nice design, just call it 5.0 and enjoy it. Leave the 4.92185 to the mathematicians and statisticians out there.
EIT with BS in Civil/Structural engineering.
RE: Significant Figures & and code interpretation
(Back on topic now, forgive me).
EIT with BS in Civil/Structural engineering.
RE: Significant Figures & and code interpretation
RE: Significant Figures & and code interpretation
I imagine that kips came into favor about the same time the metric starting showing up but perhaps it was a slide-rule thing. A brief internet search on kips yields nothing of real note.
EIT with BS in Civil/Structural engineering.
RE: Significant Figures & and code interpretation
I admit it's odd. I think it stems from my difficulties in college with figuring out significant figures and getting in trouble using kips in one place and whole numbers in another place, resulting in the wrong answer. I found it easier to just use whole numbers for everything versus keeping track of where I used kips and where I didn't.
RE: Significant Figures & and code interpretation
I have little confidence in significant places.
Dik
RE: Significant Figures & and code interpretation
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RE: Significant Figures & and code interpretation
The hidden meaning behind my post was that most engineers are taught to do calcs in kips, so there is already a built-in sigfig factor.
It's interesting how we continue certain practices as a carry-over from antiquated design, such as the spacing of railroad rails (Snopes.com has an interesting article about that story).
RE: Significant Figures & and code interpretation
Structural engineering involves understanding some of the basic assumptions behind the statics, strength of materials, statistics, etc. Based on these assumptions it is apparent that our calculations are very approximate in nature. Increased number of significant figures will actually give a false sense of accuracy and may be misleading.
Now, to be a bit philosophical - if you already use 17-digit precision - why stop there, why don't you use 20, 30, or 10000 digits?
RE: Significant Figures & and code interpretation
Because that would actually be more, or quite, difficult. If you are using Excel or Mathcad or TKSolver, or even a programmable calculator, why would you waste your time truncating intermediate calculation results to 3 sigfigs? For that matter, why even use a calculator when you can quite easily restrict yourself to 3 sigfigs by using a sliderule?
"Everything more than that is practically waste of time and money. "
The cost in time and money is exactly zero, unless you're using pencil and paper and using a sliderule. Even the lowliest calculator can do it, and if you have your calculation routines programmed in, there is no need to transcribe anything, and there is no cost in maintaining the precision. And, no one is suggesting that one litters one's worksheets with 17 digit numbers, if for no other reason than it being visually cluttered. All my worksheets are set for 3 decimal place displayed precision. The inner workings of the math package are completely hidden and transparent to the user.
There are plenty of structual engineers using Mathcad, which does all the unit conversions on the fly, kips or otherwise, documents the calculations, and is fully repeatable and reusable, time and again. The worksheets can be neatly arranged and are suitable for customer documentation, without any transcription or typing, or whatever. And, because you can do calculations directly with the units, unit and dimensionality checking is automatically done.
TTFN
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RE: Significant Figures & and code interpretation
Just because Mathcad maintains 17-digit precision that doesn't mean your calculation will be more accurate than hand calculations using 3 significant digits.
The point is that by increasing the numerical precision to more than 3 significant digits you are not increasing the accuracy of your calculations.
RE: Significant Figures & and code interpretation
"1.Loading 2.Engineering Analysis 3. Construction 4. Material Strength/general behavior."
I might rate them in the order of 1, 2 & 4 together, and then 3 (which I have very little control over).
I was always conservative with my load assumptions; which then meant that I did not have to get hung up on something being at 103% (or 105%) of allowable. Too many supervisors get so hung up on being less than 100%, or now days, never seeing anything turn red on the computer output.
I know which loads I started with and so trying to maximize design results always seemed ridiculous.
Picking steel sizes that are economic and/or more readily available; coordinating sizes so that connection detailing can be done more efficiently; and incorporating uniformity throughout the structure can all add material cost to a project, while reducing overall construction costs.
CIP concrete design is as much art as science. I love the flexibility it allows in solving problems.
As noted by others above, I always treated the clients money as if I was spending my own. Way too many engineers get hung up on feeling self-important and have no problem spending the Owner's money for "cheap concrete or steel" to pad the Safety Factors, etc.
gjc
RE: Significant Figures & and code interpretation
No one said there was an increase in accuracy. What's being discussed is "increasing numerical precision" as if there something that had to be done differently. There isn't; the precision comes for free and costs nothing.
Since one calculation flows directly into the next, you would have go out of your way to decrease the precision at that point, so why would you?
TTFN
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RE: Significant Figures & and code interpretation
One of my mentors was a little, bald headed, bearded, English professor (actually an electrical engineer), from the U of Southhampton. He used to do a skit, pretending to use a sliderule, and with a wonderful German accent, he used to mimic using a sliderule. He would say (in a German accent)... for example we will multiply 2 x 2... and he would proceed with two, und ve vill multiply it by 2, und the answer is 3.996... ve'll say 4... <G>
RE: Significant Figures & and code interpretation
There seems to be the slide-rule learned types vs the computer types (and those who think codes are absolute). In the end, one must think of what makes sense. As for footing design, I certainly would add a few extra square feet on the footing size (assuming a reasonable number of footings) rather than trying to cut it "close". A few hundred or even thousand of dollars on the foundation concrete is small insurance compared to what would happen if there was a sizing problem due to exactitude and the structure was almost completed.
When I see 3000 psi, I pretty well assume that they are talking about plus or minus a 100 psi. Only if someone wrote 3000. would I take it at four significant figures. I've seen this on my current job where the client's safety protocols required a "rest" platform if the ladder was more than 6 m high. So we end up with 6.2 m. Do we provide the platform or say, 6 m - could be up to 6.5 before we would "round" to 7.
I've been having a fit with our accountants who don't use the actual exchange rate on their calcuations (Indonesian rupiah - as stated on the credit card statement divided by the Canadian Dollars quoted as the expense - because the exchange rate quoted with the rupiah has truncated figures - and I always get the "bohica" over this!
RE: Significant Figures & and code interpretation
Dik
RE: Significant Figures & and code interpretation
Lets be clear, if designing a beam, I first find the size the numbers require, then I consider it's fitness for purpose, is it wide enough for what sits on it etc,, it's fitness for decent connections (indecent connections cost money). If I'm at the top weight of a series of steel beams, I try to step up to the bottom of the next series, if it passes my other tests, above. Why do I do that? because a change in load can be covered by a weight change only, details still hold, if I have to change to another series later it may affect the detailer, the architect and/or other disciplines.
Most of the cost of structural members is in the fabrication and/or field labor, the raw materials are are a small part of the cost.
I advise all young structural engineers to look at the acceptable tolerances for structural members, thickness, out of true, sweep etc. Understand that when they reach the field they may be worse, but they are accepted. Take a look at the blemishes accepted under ASTM A6 and it's cousins in other jurisdictions.
This business is as much an art as it is a science. we do not build fine clockwork mechanisms.
Michael.
Timing has a lot to do with the outcome of a rain dance.