Mechanical Enginnering for Dummies - any suggestion on design making? Load ratings for common wood and steel materials like 2X4s etc



How do you know that whoever built the article you are copying actually spent time designing it?
When I was doing design work, the first thing I did was go to the tech library and see what others had done. But you need to run a few calculations before deciding to use some one elses design.
Dan
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I don't. Maybe he copied it, too.
But I'm talking about hobby things, not production. For example, I made a fly reel around 20 years ago (*far* more effort than it was worth), and I just copied some of it from one I had. Same for the oscillating steam engine I made. It probably was a 10th-generation copy. <g>

If I ever build a bridge, I'll keep that in mind. <g>
Actually, I agree. I considered building a spaceframe sports car five or so years ago, and I even got a simple FEA (Cadre) to analyze others' designs. (Many commercial frames suck in a big way when you analyze their torsional stiffness.) Then I got so caught up in studying and analyzing chassis design that I never built one. d8-)
-- Ed Huntress
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I also get caught up in studying and analyzing and often don't ever get around to doing any actual building. The things that I design and don't build always are perfect. The things I actually build often are not prefect.
Dan
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wrote:

Yes, we should print that and hang it on our walls.
-- Ed Huntress
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On Thu, 27 Sep 2007 22:49:17 -0400, with neither quill nor qualm, "Ed

Indeed. I came to the realization several years ago that I wasn't really a woodworker, I was merely an "enhanced tool collector". Ayup, I'll work wood and metal and plastic on occasion, but I'm more of a tool collector these days. <sigh>
'Course, that new Karcher/Honda pressure washer I bought netted me a $2,200 job which I just finished today, so I'm using a lot of the tools, too. 2k+ s/f of concrete cleaned, primed, and opaque-stained in 3 coats. She chose the first color (which turned out too light), then a second color (which I called "50s Pink"), and then the final color, one of the trim colors on their interior. It was juuust right. 300 s/f x3. Then on to the front porch/walk, then on to the lower porch and walkway, complete with 20 concrete steps & 4 landings.
-- Exercise ferments the humors, casts them into their proper channels, throws off redundancies, and helps nature in those secret distributions, without which the body cannot subsist in its vigor, nor the soul act with cheerfulness. -- Joseph Addison, The Spectator, July 12, 1711
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wrote:

That's not such a bad thing. It's better than collecting stamps.

And...you have the start of a new pressure-washer collection, doubtless the first in your neighborhood.
-- Ed Huntress
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On Thu, 27 Sep 2007 23:54:13 -0400, with neither quill nor qualm, "Ed

Ain't dat de trufe?

Nonononononono! We don't need that. Now I have to build a secure shed in which to store it (and the mowers.)
-- Exercise ferments the humors, casts them into their proper channels, throws off redundancies, and helps nature in those secret distributions, without which the body cannot subsist in its vigor, nor the soul act with cheerfulness. -- Joseph Addison, The Spectator, July 12, 1711
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Thanks guys, I'll feel better now about hacking without a definite plan, even if I have to go back and redo it a bunch of times ... will stay away from SolidWorks and AutoCad, just back of a napkin sketch :) .
Although, reading-up on basic differences in strength between building materials should help.
Ross
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wrote:

Oh, yeah, lots of interesting and often surprising stuff there. For example, plywood stacks up well against foam-core fiberglass composite, and it's stronger and lighter than uncored fiberglass-cloth/polyester composite layups. And aluminum has no strength advantage over steel on a pound-for-pound basis if the loads are strictly in tension or compression (it's better in bending). And so on.
-- Ed Huntress
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wrote:

Oh, yeah, lots of interesting and often surprising stuff there. For example, plywood stacks up well against foam-core fiberglass composite, and it's stronger and lighter than uncored fiberglass-cloth/polyester composite layups. And aluminum has no strength advantage over steel on a pound-for-pound basis if the loads are strictly in tension or compression (it's better in bending). And so on.
-- Ed Huntress
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Karl Townsend wrote:

I got an engineering degree about 20 years ago, I remember much of it and I keep studying the subject to learn more.
Yet I use pretty much the same design criteria when I can -- if I'm not doing something totally unique, I start by shamelessly copying some existing work. I'll usually do some reverse engineering and analysis on it to make sure that the original designers knew what they were up to, but that doesn't keep me from using existing knowledge.
Even if I am doing something totally unique, if there are parts of it that aren't unique -- I get all shameless, and start looking for existing designs.
--

Tim Wescott
Wescott Design Services
  Click to see the full signature.
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For your type of use I'd recommend something like Architectural and Engineering Calculations Manual (Hardcover) by Robert Brown Butler # Hardcover: 464 pages # Publisher: McGraw-Hill Companies (August 1983) # Language: English # ISBN-10: 0070093636 # ISBN-13: 978-0070093638
In spite of the broad sounding title, it has sections on sizing steel beams, wood beams, pull out strength of screws, etc etc. And the other sections talk about lighting levels for your shop (or whatever), pipe sizes, etc etc. No theory, just formulas for what you need.
djenyc wrote:

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wrote:

I was going to suggest an architectural reference or an architectural structures course as well. I helped my daughter a bit with her course and it integrated statics and structural design, as opposed to my engineering courses where statics, strength of materials and structures were spread across four or five courses.
Another good practical book is Blodgett's "Design of Weldments," published by Lincoln. And a bargain at $15. https://ssl.lincolnelectric.com/foundation/item.asp?prodnum=DW&PID
--
Ned Simmons

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Roy, Ned - will check those books. Thank you for recommendations. Ross
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I trust that you will come to appreciate that credible structural analysis consists of rather a lot more than plugging numbers into formulas and hoping for the best.
For example:
- Measuring, calculating, guesstimating the loads and the uncertainty thereof.
- Determining the interaction effect of multiple, simultaneous loads.
- Determining whether loads are constant (static) or fluctuating, and if the latter, characterizing that fluctuation numerically.
- Considering less obvious sources of loading (e.g. earthquake, snow loads, steady wind loads, wind gusts flowing fluid loads, people clambering on your baby).
- Determining whether a 2D analysis is adequate to describe a 3D situation (or when nothing short of a valid finite element computer analysis is required or when the situation is so complicated that an established empirical approach (e.g. lugs)or outright experimental load testing of a prototype is called for).
- Determining how loads distribute themselves among redundant structural elements (such as multiple fasteners).
- Determining all the possible modes of structural failure and which is the critical one (e.g. excessive deflection, non-linear deflection, permanent deformation, tension/compression/shear failures, ductile rupture, brittle fracture, fatigue cracking, stress-corrosion cracking, hydrogen embrittlement, creep, elastic buckling, elastic/plastic buckling, plastic buckling, delamination).
- Locating the appropriate properties for the material in the condition pertaining to its planned use (e.g. Tensile yield strength, in the direction of the grain, of Southern yellow pine, air dried, at a moisture content in equilibrium with its worst-case planned environment).
- Statistical variability and possible directionality of material properties.
- Environmental degradation of material properties over time (e.g. wood decay, sunlight and atmospheric effects on PVC).
- Materials that behave as composites (e.g. concrete slabs with rebar).
- Estimating the effects of material imperfections (e.g. holes, knots, notches, changes in section thickness, corrosion penetration) and whether this is relevant to the critical failure mode.
- Understanding how joints and support points affect stress distributions (and whether that's relevant to the critical failure mode).
- Understanding the assumptions and simplification inherent in the derivation of strength formulas (even the most sophisticated analyses contain simplifying assumptions, such as, for example, linear material behavior) and recognizing situations in which these assumptions are simply not tenable.
- Considering how serious are the possible consequences of a structural failure.
- Are there liability issues associated with structural failure (anbody but you going to go near it) ?
- Knowing when established codes or standards should be (or must be) applied (e.g. residential/commercial building codes, AISC structural steel design codes, timber design codes).
- Considering all the things that I forgot to mention in the absence of a specific design problem.
- Given all of the above, determining the appropriate factor of safety to apply Note: The appropriate FS arrived at logically by an engineer is often much greater than one the layman might choose and feel to be adequate.
All that said, some references are:
Roark, Formulas for Stress and Strain
Baumeister & Marks, Standard Handbook for Mechanical Engineers
AISC Manual of Steel Construction (ASD)
http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/fplgtr113.htm
http://trs.nis.nasa.gov/perl/search?abstract%2Fkeywords%2Freportno%2Ftitle=structural+manual&abstract%2Fkeywords%2Freportno%2Ftitle_srchtype=ALL&authors %2Feditors=&authors%2Feditors_srchtype=ALL&year=&_satisfyall=ALL&_order=byti tle&_action_search=Search
http://www.knovel.com/knovel2/Toc.jsp?BookIDX9
http://www.efunda.com/materials/materials_home/materials.cfm
http://www.matweb.com/index.asp?ckck=1
http://www.knovel.com/knovel2/Toc.jsp?BookIDu4
David Merrill

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David,
You raise very good points.
But, nothing would ever get built if all the i's got dotted and the t's got crossed.
It is EXPERIENCE that allows shortcuts to the desired result. Many, many designs begin with sketches of what it should look like, along with likely material sizes. A quick analysis then would verify the strength of the design and allow adjustments.
Use large factors of safety, an FOS of 5 to 10 is not too much...this is also called factor of ignorance among the initiates. :-)) Steel is cheap and you're not building an airplane, are you?
Good judgement comes from experience, and experience comes from bad judgement.
Wolfgang
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David, I see there are a lot of design factors too consider- with major ones to not get sued or get hurt :) Will try to keep that in mind when applying theory to practice :)
Those pdf links are great! Thanks a lot.
Ross

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