My grandson and I started his homework assignment last night. He has to build a model bridge that will support 10 pounds.
No other instructions. No material, no sizes, nothing.
We decided to keep it simple and do a basic arch. We cut ot 1/2″ plywood for the arches and will glue up two for each side, with 3/4″ brace joist between them. The deck will be pieces of oak parque flooring left from a repair job I did last week.
I clamped up one set of the arches and laid 20 lbs of bean bag lead shot weight on it. I got a little deflection with the single pair, so i clamped the other pair to them and repeated the test. No deflection that time, but that was all 4 layers clamped up as one unit.
When I spread them out with the joist braces between them which results will I get? Slight deflection or zero?
Will adding a 1/4″ plywood diaphram to the bottom of the arch help stiffen it, or is the deck boards enough?
I know I could increase my arched beam depth to get more loading, but I want to keep it from looking klunky. Right now our “beams” are 1 1/8″ in height by roughly 37″ long and 6″ to the top of the arch.
What do ya think?
Replies
Do some searching here. Somebody (I forget who) posted some pics of a bridge his son made last fall, maybe? Pretty neat.
Mike Hennessy
Pittsburgh, PA
Tried, but couldn't find it. Maybe someone else will remember who posted it & when. It was a very light truss design, IIRC, and held a LOT of weight.
Edited 4/1/2008 7:08 am ET by MikeHennessy
Hubcap's son did the bridge you're thinking of.A Great Place for Information, Comraderie, and a Sucker Punch.
Remodeling Contractor just outside the Glass City.
http://www.quittintime.com/
How about this? It's the site I used for the Rainbow bridge I built. He says his models are hell for strong.
http://pacifi.ca/models.html
Forrest
Thanks Forrest.
I started with you thread and researched from there.
I wish we had the time to do a really neat model, based on what you did, but alas, time has caught us with our bridges down <g>
Calvin to the rescue -- he remembered the poster of the bridge thread. Check this out:
http://www.acmsite.com/students/loganh
Mike Hennessy
Pittsburgh, PA
My Son (Chris) made one for a class last year. Carl is making one now.
This is the one Chris made last year:
View Image
Politicians are the same the world over: they promise to build a bridge even when there is no river [Nikita Khrushchev]
When I spread them out with the joist braces between them which results will I get? Slight deflection or zero?
I'll play, just for fun: I'd say you'll still get zero deflection unless the arches move sideways.
Will adding a 1/4" plywood diaphram to the bottom of the arch help stiffen it, or is the deck boards enough?
Plywood seems like it would be better because it acts as one unit, whereas deck boards won't, unless they are T & G.
I know I could increase my arched beam depth to get more loading, but I want to keep it from looking klunky. Right now our "beams" are 1 1/8" in height by roughly 37" long and 6" to the top of the arch.
What do ya think?
Everything sounds good--I'm more for elegance and doing the most with the least material.
"... I'm more for elegance and doing the most with the least material."
Hey, that was my thought, exactly!
Allen
Just curious ... is this supposed to be a father-son (or grandfather-grandson) project? Or a student-only project? There are benefits to either approach, of course.
I recall as a kid, working solo with just a few hand tools on similar assignments, solving the problem as best I could. Then going to school to find my work "competing" against constructions obviously produced by skilled adults with well-appointed workshops!
Years later, with sons of my own, I, too, found myself wanting to be perhaps "too helpful" with some of their construction projects. I remember fighting the urge to take over completely when my son had to build a model of an English castle.
Even so, I probably did more than was probably appropriate. But when I helped carry the finished castle into his classroom, we were confronted with many unbelievably stunning, finely-crafted models that seemed to reflect more a competition between dads than a hands-on learning experience for the children.
Allen
I'm not sure what the parameters are for this assignment. Absolutely no instructions, other than build bridge that will hold up ten pounds, was given.
Since this assignment was given last Friday and due this coming Friday, there is not a lot of wiggle room for getting it done. I looked at a lot of sites for research, but given the time, just decided we needed to fly by the seat of our pants on this one. Thus the selection of the simple arch with plywood.
We made our template on a large sheet of craft paper, cut it out and laid out the cuts on a scrap sheet of 1/2" a/c plywood. Caley cut the four pieces with a jig saw, but I clamped and belt sanded everything. My belt sander is an ungainly beast from 30 years ago. Sanding an inside curve with it is challenging for me, much less a 10 year old.
Tonight we need to finish the assembly. Other than helping with the clamps and keeping things safe, he will do 99% of it.
I'll be out of commission for several days after hernia surgery tomorrow. I guess he and I both got a reprieve because the surgery was originally schedule for yesterday.
I'll have DW get a couple of pictures of someone called a "simple but elegant designed" bridge. Maybe get them posted this weekend.
Absolutely no instructions, other than build bridge that will hold up ten pounds
He could try and win the "simplicity of design" category and bring in a 2X4.
I was thinking a small log propped up on a couple ogf rocks.
"It's so easy a Caveman can do it."
I was thinking the same thing."Make a bridge that will hold ten pounds."Hold 10 pounds ? Absolutely no deflection, or just- hold 10 pounds ?How long ? Does it span half an inch, or half a mile ?How long ? 2 seconds before collapse ? Never collapse ?I'm thinking it's possible that someone didn't take the time to get all the instructions, or simply doesn't want to do this, or is simply having fun working with grampa, and is playing dumb...;o)I'd be calling the teacher to find out the details.If the instructions truly are that simple, then just lay a 2x4 down. Or one of those 1/2" pieces of plywood. The amount of effort the teacher put into the instructions, doesn't deserve any more effort in return, than that.
Life doesn't often leave a very easy trail to follow.
Of course you realize that my post was tongue-in-cheek. I do agree that it seems unusual that there were no constraints or design guidelines, but remember, it's a model. A 2X4 doesn't really qualify because it can't reasonably be scaled up. What's that? They're selling 200X400's at the big box? Well, never mind then.
ROFLMAOThat be true.=0)
Life doesn't often leave a very easy trail to follow.
I'm thinking it's possible that someone didn't take the time to get all the instructions, or simply doesn't want to do this, or is simply having fun working with grampa, and is playing dumb
Not the case with Caley. He knows gramps to well for that.
It is more like the teacher. I am way under impressed with her efforts this year. In fact, under impressed with her when she was a special needs teacher the last four years, before switching to a regular classroom this year. She is also the coach of the academic challenge team and assist with the technology team which DW helps with. The lack of information supplied to the kids on those teams is appalling, so the lack of information for this assignment is not surprising.
Can we say "tenure"?
Oh well, we got it finished before my surgery. He spent the last two days painting it, and it went in this morning. I was going to send in 10 lbs of bean bag weights from my dive gear, but noticed there is a lot of lead dust seeping out of them. I need to thoroughly rinse them out before I let anyone (especially kids) handle them.
I know one little girl is bringing in a Warren Truss bridge. Her Dad and older brother are model rail road buffs. They already had everything they needed to build it, so it became a family project.
I'm going to take a ride with DW when she picks up Caley today. Maybe get to look at everything that was turned in. If the teacher will allow it, I'll also take some pictures of all of them.
I think the whole assignment was pretty cool. I'm also sure there was more information that could have been supplied, but like the real world, the and their helpers had to work with what they were given. (that sounds familiar, doesn't it?)
I'm looking forward to the (hopefully) pictures !=0)
Life doesn't often leave a very easy trail to follow.
Dave,
Sounds like you're handling the project admirably. A nice chance to work with your grandchild.
I suppose some might say the "no instructions" aspect of the assignment encourages creativity, but, personally, I've always found such open-endedness annoying. Perhaps I'm just not a creative person!
Maybe Don's suggestion of a two-by-four is the most creative solution!
Kudos to you for encouraging Caley to do most of the construction work.
And best wishes regarding your upcoming surgery.
Allen
I suppose some might say the "no instructions" aspect of the assignment encourages creativity, but, personally, I've always found such open-endedness annoying. Perhaps I'm just not a creative person!
That's me. DW says I am linear. I say that is why I married her. She is the creative and artistic half.
BTW, Caley is my grandson (step daughters first child), and he is also my adopted child. We adopted him when he was two. Best thing we ever did. I can't imagine being without him now.
Just maybe, DW and I have matured enough to raise him without the pitfalls we faced when we were young.
Dave,
Regarding your design, it sounds like your bridge will easily support many times the required load. And will look great, too.
I recall a similar assignment I had as a freshman in college. The twist was that we were allowed just a minimal number of toothpicks to build our structure. The results were elegant, open, lightweight constructions. By using triangles and arches, great strength can result from very little material.
Given your assignment, I would have looked at various bridge styles (truss, suspension, etc.), and designed the simplest, lightest construction possible. The reaction I would want is "Wow, I can't believe that skimpy bridge holds that much weight!"
Allen
Edited 4/1/2008 12:05 pm ET by WNYguy
The American Society of Civil Engineers sponsors model bridge building contests, as part of their "E-week", in February, but they always have pretty stringent rules, as to materials, span lengths, support locations, etc.
Typically, an arch bridge is supported by the end supports on the banks to keep it from flattening out. You could try some string or wire across the bottom, like a strung bow. This would put the wood in the arch in compression, and the string in tension.