Static beams for dummies anyone spare a moment ? 6

[Cornishengineer]

Hi all I currently have one week to prepare for a job interview as an engineering teacher . I need to prepare a 2 hour lesson for 16 year olds who know nothing about static beams ... I myself am a little out of practice on this area but obviously have some knowledge ... Anyone able to help me out with tips and what direction to take

 

[MikeHalloran]

I guess the usual way is to use a prismatical beam, say a hot rolled flat, spanning a couple of concrete blocks, measure the deflection produced by each member of the class as they stand on it, and derive an equation to describe the relation of deflection and force. Or do it for a couple of students and use the deflection to measure the weight of the remainder.

Most of the class will be bored unless, and probably even if, you frame the lesson as predicting the deformation of a homemade bridge over a swamp full of alligators or some such.



Mike Halloran
Pembroke Pines, FL, USA

 

[Cornishengineer]

Thanks mike yes it's a lesson to be based on 16 year old apprentices who are assumed to know nothing about static beams etc .... I was thinking along the same lines.

 

[rb1957]

"a 2 hour lesson for 16 year olds" ... oh, shoot me now, please !

i'd do a photoelastic problem ... plastic beam + sunglasses + load = stress

and you could show them some real pictures of Kt geometries.

possibly showing them the difference between simply supported and cantilever beams ?

Quando Omni Flunkus Moritati

 

[rb1957]

with mike's idea, you could show that an upright rectangle is more efficient than a horizontal one.

mind you the response will probably be "sooooo"

Quando Omni Flunkus Moritati

 

[msquared48]

Or you could demonstrate the principles of statics to generate new hair styles using a small VandeGraff generator with the one lead hooked to a, I beam, and the other hooked to one of the students of lesser prominence. After the brief demonstration, the remainder of the students should be well grounded on the subject.

Mike McCann, PE, SE (WA)

 

[SnTMan]

Break something

Regards,

Mike

 

[KENAT]

Have them build a bridge? You know one of those tasks where you're split into teams given a budget of 'points', certain items you can buy for those points, and have to build a bridge spanning some distance.

Then the different bridges get tested to see which takes teh most load.

2hr may be pushing it but you never know.

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[Cornishengineer]

Thanks guys im thinking of opening up with types of static beams and the types of forces applied to them ,obviously in picture forms they can relate to ... then some simple load force calculations....maybe place there phone across a gap hit it with a hammer and show them downwards force and the shear point

 

[Cornishengineer]

great idea i was thinking along the same lines using drinking straws

 

[kingnero]

spaghetti bridges (instead of straws) is a common design contest over here.
Have them span a certain distance, and whichever bridge holds the most load before failure wins.

 

[Jboggs]

Seems the thread is headed off-topic to me. The original post said the challenge is to teach kids "who know nothing about static beams". The ideas about building bridges from straw or spaghetti are great, and extremely useful projects, but they mainly focus on the geometry of trusses, not the internal mechanisms of a single beam. I would focus on some way to highlight the effects of material, shape, and orientation on load capability. For example, which is stronger: a solid round bar, a solid square bar, a round tube, a square tube, or a rectangular tube? Which one has the best strength to weight ratio? And why is a rectangular tube stronger in one orientation than another? What is the difference between a point load and a distributed load? What can we learn about moments from cantilever beams? (Example: A broomstick is not heavy. So why is it so much harder to hold it up if you hold it at the end rather than the middle? That illustrates the difference between a force and a moment.)

Just sayin...

 

[Cornishengineer]

Spot on jboggs ... I was thinking along the same lines I want to start with the most basic of principles and let them relate to how they apply in everyday life .... I like your thinking mate

 

[desertfox]

Hi

How about a 12" rule balance on two books and apply a load by hand in the middle of the rule for starters.

 

[moon161]

Assume L>10*D to simplify assumptions, stay away from shear lag.
If they will all have (or hand out) rulers and books, asking them to experiment is a neat idea.
Have they had any calculus, do they know simple integrations?
Use the Torrance incubation model to plan the lesson outline, look it up, or contact me, I can dig up a reference moon161@gmail.com

Show simple beam in various configurations, ask what things that idea might model. Pose your questions to class using their choices for what the beam is, what the loads are, etc.

Look up khan academy topics, for both content and style. If possible assign 1 or 2 khan academy lessons as homework before the lesson, but don't count on it being done.
Plan appropriate participation activities. I would say put the questions out there at the beginning, 'does it bend, how do you know, is it linear, quadratic, etc. Don't shoot down the answers, just use them as a starting point for discussion.

Get more questions out there, what to the forces look like in beam, reactions equilibrium, introduce shear and moment forces, touch on moment of inertia.

Develop equilibrium, shear, moment diagrams, check in frequently, every 5 minutes or so. Consider developing 1 point cases and using superposition.

Walk through a few examples, wrap up with them solving the same simple case you presented at the beginning of class, say last 15-20 minutes. Plan on checking in, coaching and troubleshooting method with various students.

 

[Nereth1]

Relate it to something they are interested in - unlikely that they are interested in alligator filled swamps . Some kids are interested in planes ("this lets you design the strength of a wing") some are interested in cars ("this lets you design a leaf spring") and some may be interested in sports ("you can see if the crank on your bike is well designed or not"). Anyone that you can make want to apply it now, or in the future career they have in their heads, is going to listen.

That's my opinion anyway.

Regardless, I think there are two critical things you need to cover. First, the math, for which you will need to be interesting and they will need to be interested (hence relating it to hobbies and interests of theirs) and second, the concept, the chief of which being that the second moment of area scales with cube of distance from centerline. If they already have learned about torque then that concept will be easier, but I think it's a good place for a demonstration.

To show them it happening, obviously you can just get a popstick and have them try to bend it in different ways, to show them WHY it happens is a bit harder - perhaps a cantilever beam 50-60mm tall made of rubber with some markings every 10mm up the side of it. They can bend it and literally see the strains - then it's up to you to relate how the ones on the top and bottom deforming so much more than the ones in the middle means that they are contributing more to the stiffness, and relate it to torques if they understand them, and that those ones on top and bottom also have more mechanical advantage.

And you have to do all that all without them getting bored.

Good luck!

 

[racookpe1978]

Demonstrate it! Visually attract their eyes.

C-clamp a small square beam (1/2 x 1/2 bar0 to the science desk, load it up until it bends.
Change to a 1/2 x 2 beam, same length. bend it.
Change to a angle iron or small channel, so how they twist under load.
Show how the channel resists one way, but fail in the other.
Change to a small WF shape, show how it is lighter, cheaper, but resists both ways.

Clamp the beam vertical, wiggle it to show "same problem", and then introduce compression forces. Don't go into a lot of discussion - you're working beams.

 

[KENAT]

The reason I suggested something that gets the students involved is so as to grab their attention.

Alternatively, and especially in the available time frame, demonstrations may be better.

I'd say you need to grab their attention first, maybe with some initial demo, then maybe explain some of the principles behind it, and then maybe do more demo's to illustrate what you've just taught them.

Is this just a general class, or is it kids that have already volunteered for this because they have a specific interest in Engineering? If the latter you'll have more scope to get into math and explanations without needing to worry as much about keeping their attention.

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[KENAT]

Sorry, meant to say I've seen various TV shows that had demonstration etc. like 'what the Romans did for us' etc. Maybe you can find some clips on you tube to give you ideas.

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[racookpe1978]

DO NOT "develop" the math first! most especially, do NOT begin from first principles and "derive" the math that gets to the beam equation.

Show it (beam deflection under loads, beam failure under loads, beam reaction (the beam bends but is safe one direction, fails the other direction). Beam fails if wood, beam bends if steel, beam cracks if brittle (plastic). Be exciting! Break a few wooden 12 inch rulers! Bend a few thin strips of cheap steel. Buy a 3 foot channel from Home Depot and a couple of angle iron from Lowes to let them "feel" a real steel shape. Crack a plastic "beam" to show that deflection is NOT always constant.

Only then, at the end, simply write the final equations for allowable stress, load amount, load position, beam shape values, beam thickness, beam depth as your conclusion. Not as the start of the example in oyour class.

Show which is "on top" of the formula, which is "on the bottom" of the fraction, which factors are going to make stress rise if they increase, which will make stress fall.

You are NOT going to "teach" a course here in beam theory, you are trying to excite them visually so they might (much later) remain excited about theoretical courses about beams.