# Ask the Structural Engineer! **URL:** \ **Category:** In My Humble Opinion\ **Created:** [May 20, 2003, 7:34pm UTC](https://boards.straightdope.com/t/ask-the-structural-engineer/176365 "2003-05-20T19:34:53Z")\ **Posts on this page:** 20\ **Page:** 5
**Author:** ![culture](https://avatars.discourse-cdn.com/v4/letter/c/958977/32.png) [@culture](https://boards.straightdope.com/u/culture)\ **Post date:** [May 22, 2003, 2:25pm UTC](https://boards.straightdope.com/t/ask-the-structural-engineer/176365/81 "2003-05-22T14:25:17Z")
> [@](#): > > \*Originally posted by ralph124c \* > \*\*I’m full of questions. My Idol (Frank llloyd Wright) once drew up plans for a 5200 foot skyscraper building (you can see the model in his 1957 book “TESTAMENT”. My questions: > -is such a building feasible? > -how much steel would be required to build it? > -and Wright designed the building like a tree-a central column with the floors being supported by the horizontal “branches”. Would such a design work? (Wright was not an engineer, but his designs worked pretty well-the tower at the Johnsons Wx Building (Racine, WI) was designed in this way. > Finally, Wright did all of this without benefit of CAD-has anybody subject his design for the "mile-high building to a modern analysis? > I’d LOVE to have an office on the top of this monster! \*\* The golden rule of engineering is that if you have to gold, we can build it. It is purely a question of economics. A mile-high building would make no sense dollar-wise but would be fun to design and construct. Keep in mind, people are talking seriously about building elevators to space. That would be a wonderful project I could sink my teeth into. No offense to FLW, he was a wonderful architect but a lousy engineer. Falling water was a pain to repair, let me tell you. ---
**Author:** ![culture](https://avatars.discourse-cdn.com/v4/letter/c/958977/32.png) [@culture](https://boards.straightdope.com/u/culture)\ **Post date:** [May 22, 2003, 2:33pm UTC](https://boards.straightdope.com/t/ask-the-structural-engineer/176365/82 "2003-05-22T14:33:03Z")
> [@](#): > > \*Originally posted by The Scrivener \* > \*\*Aside from the pun on “finite element,” is there any other meaning attached to “finite elephant”? > > * * * > > Where I live, I have a flat roof deck, approx. 17’ x 25’. How zealous do I have to be about “shovelling” (I have a roof-friendly “Sno-Brum”) snow off the roof? > > This past winter, I got rid of a large snow accumulation because we were expecting rain to follow, before the snow could melt. Authorities did encourage area residents and business owners to do this where possible, because the snow can act as a giant sponge, absorbing the rain… and sure enough, a couple of flat-roofed buildings did have collapsed roofs after the rains. Is there a structural engineering rule of thumb ratio for snow drift height:expected rainfall:size of flat roof span (assuming standard contstruction standards)? > > On behalf of my poor aching back (post-shovelling), thanks! \*\* There are standards that define the weight that a roof must be designed to carry. These standards consider expected snowfall and rainfall (based on geographic location), as well and drift developmement based on roof geometry. Once the load is known, the roof is designed to be strong enough to carry he anticipated load. ---
**Author:** ![Finite\_Elephant](https://avatars.discourse-cdn.com/v4/letter/f/a88e57/32.png) [@Finite\_Elephant](https://boards.straightdope.com/u/Finite_Elephant)\ **Post date:** [May 22, 2003, 2:36pm UTC](https://boards.straightdope.com/t/ask-the-structural-engineer/176365/83 "2003-05-22T14:36:12Z")
> [@](#): > > \*Originally posted by The Scrivener \* > **Aside from the pun on “finite element,” is there any other meaning attached to “finite elephant”?** Not really. I was on a FEA jag and someone in the office was relating their kids collection of elephant jokes when I thought it up. > [@](#): > > **Is there a structural engineering rule of thumb ratio for snow drift height:expected rainfall:size of flat roof span (assuming standard contstruction standards)?** Fairly wet snow weighs about 20 pounds a cubic foot. Water weighs 62.5 pounds a cubic foot. Your roof is probably designed for 25-30 pounds a square foot. If you track the pre-rain snow depth and the rainfall, you should be able to figure out when to break out the shovel. YMMV, of course. ---
**Author:** ![ralph124c](https://avatars.discourse-cdn.com/v4/letter/r/8797f3/32.png) [@ralph124c](https://boards.straightdope.com/u/ralph124c)\ **Post date:** [May 22, 2003, 2:39pm UTC](https://boards.straightdope.com/t/ask-the-structural-engineer/176365/84 "2003-05-22T14:39:28Z")
Regarding Falling Water, you are correct-the cantilevered flat roofs are collapsing, due to inadequate rebars in the slabs. I think that FLW had a tendency to ignore costs-untill a project was seriously over budget (then he had to scrimp). But, his deign for the roof columns at the Johnsons Wax Hdqrs in Racine was brilliant-all of the engineers were amazed by how strong thje “musroom” columns were. Unfortunately, the pyrex-tube windows were a disaster (they still leak). ---
**Author:** ![culture](https://avatars.discourse-cdn.com/v4/letter/c/958977/32.png) [@culture](https://boards.straightdope.com/u/culture)\ **Post date:** [May 22, 2003, 8:37pm UTC](https://boards.straightdope.com/t/ask-the-structural-engineer/176365/85 "2003-05-22T20:37:27Z")
> [@](#): > > \*Originally posted by Tuckerfan \* > \*\*Is it just me, or is there a serious lack of _talented_ engineers out there? More and more, I keep running across things which are capable of doing the task that they’re designed for, but could be _vastly_ improved in their design (and produced at a lower cost than what they currently are). In some cases, it’s simply a matter of moving a part a few 1/16ths of an inch over to one side. > > Admittedly, I’m not an engineer, but I am studying to be a machinist and have read engineering texts for fun, so I do have _some_ understanding of engineering principles. \*\* Like any other field, there are talented engineers and not-talented engineers. The reality is that most engineers are average. We do not seem to have trouble hiring what we need, which is about one talented engineer for every five average engineers. This is good, as someone has to do the production work. ---
**Author:** ![Tuckerfan](https://avatars.discourse-cdn.com/v4/letter/t/eada6e/32.png) [@Tuckerfan](https://boards.straightdope.com/u/Tuckerfan)\ **Post date:** [May 30, 2003, 7:22am UTC](https://boards.straightdope.com/t/ask-the-structural-engineer/176365/86 "2003-05-30T07:22:25Z")
Another question, if you folks don’t mind the bump. I was watching a program on building rollercoasters and the one company that they profiled was boasting about having tolerances of 1/8th an inch. Now, in machining, one generally has to be within a few thousandths of an inch or millionths of an inch, and I realize that such close tolerances are probably not necessary in a building, but considering that the “hi-tech” machine they were using to cut the sections of the rollercoaster probably dated from the 1970s (I’ve not seen one of the machines IRL, but my instructor mentioned one. You stick a blueprint on a light table and a photoeye follows the lines on the print), and they showed workmen beating the hell out of a part in order to get it to line up with another one, I’ve got to ask what kind of tolerances are standard in your line of work. I’d think that an 1/8th is a bit much, but maybe not. ---
**Author:** ![Finite\_Elephant](https://avatars.discourse-cdn.com/v4/letter/f/a88e57/32.png) [@Finite\_Elephant](https://boards.straightdope.com/u/Finite_Elephant)\ **Post date:** [May 30, 2003, 12:29pm UTC](https://boards.straightdope.com/t/ask-the-structural-engineer/176365/87 "2003-05-30T12:29:47Z")
That’s the difference between machining and construction. 1/8" is a pretty loose tolerance for something that’s 6" long, but if you talking about a part of a building that’s 40 feet long, 1/8" is pretty tight. You’re doing well to get the material from the mill that straight. As far as the automated machining you were describing, there are a lot of shops that use CAD/CAM driven machines for fabricating steel, but bear in mind that the machining involved in most building structures is of the cut-to-length, coping, and drilling holes variety, so there isn’t a great need for really sophisticated 3D tooling. ---
**Author:** ![Aro](https://avatars.discourse-cdn.com/v4/letter/a/b3f665/32.png) [@Aro](https://boards.straightdope.com/u/Aro)\ **Post date:** [May 30, 2003, 2:24pm UTC](https://boards.straightdope.com/t/ask-the-structural-engineer/176365/88 "2003-05-30T14:24:05Z")
> [@](#): > > \*Originally posted by culture \* > \*\*Definately designed by an architect! \*\* Hey! I resent this remark! Even if it is no doubt true. 😉 On the Mile High tower idea, i think the most difficult part in making them economical is actually getting people to the upper floors. Due the restrictions on lifts (elevators) travel distances and the amount of floor space eaten up by their inclusion the top floors would be astronomical cost to construct compared to the Sq footage of usable space actually obtained. So simply, as **Finite Elephant** said, its a matter of economics. FYO, here’s an example of another proposal for a [Mile High Tower](http://www.retrofuture.com/skyscraper.html), this time by Lord Foster (& partners). I have a queston: have you worked on a specific project where you have designed to the minimum limit for the materials you have been working with, solely to achieve a desired aesthetic effect? (Most St. Engs I work with build-in a factor of x20 to their designs (rather than, say x2), just to be completely sure of no future issues). What was that project, and was it completely successful? Oh, and how do you rate the work of [Santiago Calatrava?](http://www.calatrava.com/indexflash.html) He blends the boundary between architect and structural engineer successfully, IMHO. ---
**Author:** ![tramp](https://avatars.discourse-cdn.com/v4/letter/t/4491bb/32.png) [@tramp](https://boards.straightdope.com/u/tramp)\ **Post date:** [May 30, 2003, 3:04pm UTC](https://boards.straightdope.com/t/ask-the-structural-engineer/176365/89 "2003-05-30T15:04:42Z")
Hey now, all you engineer people… Are you ASCE members? If not, you should be! And you should join their Key Contact Program so I can send you lovely eamils about legislations concerning engineers!!! ---
**Author:** ![culture](https://avatars.discourse-cdn.com/v4/letter/c/958977/32.png) [@culture](https://boards.straightdope.com/u/culture)\ **Post date:** [May 30, 2003, 3:21pm UTC](https://boards.straightdope.com/t/ask-the-structural-engineer/176365/90 "2003-05-30T15:21:28Z")
> [@](#): > > \*Originally posted by Aro \* > **I have a queston: have you worked on a specific project where you have designed to the minimum limit for the materials you have been working with, solely to achieve a desired aesthetic effect? (Most St. Engs I work with build-in a factor of x20 to their designs (rather than, say x2), just to be completely sure of no future issues). What was that project, and was it completely successful?** If I told you the project, I would have to kill you. However, it is true that there is a relationship between engineering cost and FOS. For example, take a look at the long-span roofs on stadiums. They are carefully engineered and the FOS is a reasonable 1.6-2.0. However, when the client does not want to pay for engineering, the FOS starts to go up as CYA fopr the engineer, and can easily reach 5 (I have never personally seen 20). Therefore, the client pays on the backend. It drives engineers crazy that owners will pay a contractor an extra 10% to avoid paying engineer 0.25% To provide an example, typical fees for common commerical structures are about 0.75% for the structural engineer. If paying an extra 0.25% could cut construction cost 1% (via more careful engineering), you get a 400% return. However, owners bid the work to the lowest cost engineer. You can imagine the results. ---
**Author:** ![culture](https://avatars.discourse-cdn.com/v4/letter/c/958977/32.png) [@culture](https://boards.straightdope.com/u/culture)\ **Post date:** [May 30, 2003, 3:25pm UTC](https://boards.straightdope.com/t/ask-the-structural-engineer/176365/91 "2003-05-30T15:25:44Z")
> [@](#): > > \*Originally posted by tramp \* > \*\*Hey now, all you engineer people… Are you ASCE members? If not, you should be! And you should join their Key Contact Program so I can send you lovely eamils about legislations concerning engineers!!! \*\* Yes, I am a member of the secret cabal of engineers bent on world domination known as the ASCE (also a key contact). For those of you who do not know, it is kind of like the Trilateral Commission, but for engineers. ---
**Author:** ![Emilio\_Lizardo](https://avatars.discourse-cdn.com/v4/letter/e/898d66/32.png) [@Emilio\_Lizardo](https://boards.straightdope.com/u/Emilio_Lizardo)\ **Post date:** [May 30, 2003, 4:05pm UTC](https://boards.straightdope.com/t/ask-the-structural-engineer/176365/92 "2003-05-30T16:05:03Z")
[HIJACK] Hey **Tramp** do you work for ASCE? If so, are you in DC or Reston? [/HIJACK] ---
**Author:** ![MSU\_1978](https://avatars.discourse-cdn.com/v4/letter/m/ac8455/32.png) [@MSU\_1978](https://boards.straightdope.com/u/MSU_1978)\ **Post date:** [May 30, 2003, 4:52pm UTC](https://boards.straightdope.com/t/ask-the-structural-engineer/176365/93 "2003-05-30T16:52:08Z")
I’m a bridge engineer and I’m just curious what software you’ve used for any bridge design/analysis. My personal feeling about Virtis/Opis is that it’s a piece of (_%(_&%\*&(. ---
**Author:** ![Finite\_Elephant](https://avatars.discourse-cdn.com/v4/letter/f/a88e57/32.png) [@Finite\_Elephant](https://boards.straightdope.com/u/Finite_Elephant)\ **Post date:** [May 30, 2003, 4:53pm UTC](https://boards.straightdope.com/t/ask-the-structural-engineer/176365/94 "2003-05-30T16:53:44Z")
> [@](#): > > \*Originally posted by tramp \* > \*\*Hey now, all you engineer people… Are you ASCE members? If not, you should be! And you should join their Key Contact Program so I can send you lovely eamils about legislations concerning engineers!!! \*\* I’m not a member of ASCE. Nothing against that organization, but since I don’t to transportation or “heavy” civil projects, I feel that my dues budget is better spent at SEAOI, and through it, NCSEA and CASE. I’ve always wondered about the national organizations for design professionals, like AIA, NSPE, etc., and how they justify having extensive lobbying efforts in D.C. when their respective professions are regulated entirely at the State level. It must just be to ensure that a large number of tax dollars are spent on construction and research. ---
**Author:** ![tramp](https://avatars.discourse-cdn.com/v4/letter/t/4491bb/32.png) [@tramp](https://boards.straightdope.com/u/tramp)\ **Post date:** [May 30, 2003, 5:28pm UTC](https://boards.straightdope.com/t/ask-the-structural-engineer/176365/95 "2003-05-30T17:28:33Z")
Kudos to you culture! Now lets get you set up for some back home visits… Emilio… I’m in lovely and scenic DC. Finite, to answer your question, ASCE is a 501 c 3 org. and can’t have a PAC like NSPE. Our lobbying efforts are different because we just can’t hand over a bunch of cash to an elected official like other orgs. that have PACs. We lobby to ensure that all you engineers get money for big projects financed by some federal dollars like bridges, highways, dams and whatnot and the cash for research. ---
**Author:** ![Tuckerfan](https://avatars.discourse-cdn.com/v4/letter/t/eada6e/32.png) [@Tuckerfan](https://boards.straightdope.com/u/Tuckerfan)\ **Post date:** [May 31, 2003, 3:45am UTC](https://boards.straightdope.com/t/ask-the-structural-engineer/176365/96 "2003-05-31T03:45:38Z")
> [@](#): > > \*Originally posted by Finite Elephant \* > \*\*That’s the difference between machining and construction. 1/8" is a pretty loose tolerance for something that’s 6" long, but if you talking about a part of a building that’s 40 feet long, 1/8" is pretty tight. You’re doing well to get the material from the mill that straight. > > As far as the automated machining you were describing, there are a lot of shops that use CAD/CAM driven machines for fabricating steel, but bear in mind that the machining involved in most building structures is of the cut-to-length, coping, and drilling holes variety, so there isn’t a great need for really sophisticated 3D tooling. \*\* See, this just blows my mind. Because the machine they were using was cutting pipe in segments using a torch. I don’t know what the thing cost new, but really, today, I could go out, buy something similar for a home shop, that let me use AutoCAD drawings saved to a floppy, for a few grand. Seems to me that converting a dinosaur of a machine, as shown in the program, to being able to use AutoCAD drawings (or a similar program) should be relatively inexpensive. ---
**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\ **Post date:** [June 3, 2003, 7:41pm UTC](https://boards.straightdope.com/t/ask-the-structural-engineer/176365/97 "2003-06-03T19:41:55Z")
Does general framing lumber come in lengths greater than 10’ ? Suppose I was building as post and beam shed 14’ or 16’ in length, would I be able to run 4x4’s that are longer than 10’ across as top plates and 2x4s as perlins? Or am I going to have to run two 8’ long 4x4’s with some kind of support in the middle where they meet? Doesn’t this detract from the post and beam look? ---
**Author:** ![culture](https://avatars.discourse-cdn.com/v4/letter/c/958977/32.png) [@culture](https://boards.straightdope.com/u/culture)\ **Post date:** [June 3, 2003, 8:02pm UTC](https://boards.straightdope.com/t/ask-the-structural-engineer/176365/98 "2003-06-03T20:02:08Z")
> [@](#): > > \*Originally posted by Philster \* > \*\*Does general framing lumber come in lengths greater than 10’ ? > > Suppose I was building as post and beam shed 14’ or 16’ in length, would I be able to run 4x4’s that are longer than 10’ across as top plates and 2x4s as perlins? > > Or am I going to have to run two 8’ long 4x4’s with some kind of support in the middle where they meet? Doesn’t this detract from the post and beam look? \*\* 4x4 come in at least 14 foot lengths, I am not sure about longer lengths as I have never needed to use 4x4s any longer. 2x10 and 2x12s can be found as long as 26 feet. They may also come onger, but I have never needed one any longer. Remember, the strength of a columns and beam decrease with the square of the legnth, so long beam and tall columns are contraindicated (i.e. a beam 10 foot long can carry four times the load of a beam 20 foot long, measured in pounds per foot of length). Timbers, such as 4x6, 8x16, etc. comes in pretty much any length you want. BTW, dont use 4x4 as beams. They are ok as plates (i.e. the top sill). ---
**Author:** ![leonardengr](https://avatars.discourse-cdn.com/v4/letter/l/b77776/32.png) [@leonardengr](https://boards.straightdope.com/u/leonardengr)\ **Post date:** [February 9, 2005, 12:51am UTC](https://boards.straightdope.com/t/ask-the-structural-engineer/176365/99 "2005-02-09T00:51:40Z")
can any one please tell me how to design a compression ring or just tell me some refrence for design of a compression ring. thanks leonardeng ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [February 9, 2005, 2:51am UTC](https://boards.straightdope.com/t/ask-the-structural-engineer/176365/100 "2005-02-09T02:51:19Z")
A lot of conspiracy-minded folks believe the damage done to the Murrah Building (OKC bombing) could not have been caused by a truck bomb. Have you looked into this? If so, what is your opinion? [Previous page](https://boards.straightdope.com/t/ask-the-structural-engineer/176365.md?page=4) [Next page](https://boards.straightdope.com/t/ask-the-structural-engineer/176365.md?page=6)