A2 - WETZEL

The goal of my design was to have a low cost and high strength. Never having designed or tested model bridges before beginning this class, my best idea was to use shorter chords as members of the bridge. Although joints each cost at least $1,000, I decided that their use in between short chords would help make the bridge stronger and tighter once weight was added. By tighter, I mean that I am imagining as weight is added to the bridge's load, the chords will push into the joints and the displacement of the joints will be towards the center of the bridge. As the members move towards the center like so, the stronger, shorter chords will lean into the joints, tightening the joint sections.

Elevation and Plan for the Knex bridge design.

I made sure that the web of the truss was composed entirely of triangles--most of which, in my case, happen to be right triangles. I am hoping that these hold up as well as equilateral triangles may have. My prediction is that if the right angle of one triangle does not have support backing it, it will give out before other parts of the bridge. This is something that I will pay close attention to so I know what the best angles are to use as the groups approach the drafting of their final designs. Something I purposely did not do was make the plan of the bridge out of triangles. I figured that the weight-bearing members all ran vertically, so I did not want to add dead weight to the bridge or raise its cost by webbing its ceiling.

The Truss Bill of Materials

I did my best to keep my design simple so that there would be no useless chords or joints in the price, but I am not yet sure that each chord that I've left will make the bridge strong enough. I was both surprised and glad to see that the total cost of my first Knex bridge design was below $200,000--a price I'd been trying to get my bridge down to in West Point Bridge Design.

A2-O'CALLAGHAN

When designing this bridge, I knew I wanted to have the truss structure on top, as opposed to underneath the bridge.  I also knew that I wanted it to be somewhat of an arc shape.  Through research, I found that this was most effective and could hold the most weight. 

 

Bill of Materials - Knex Pieces
Part Number	Description	Unit Cost ($)	Nr. of Parts	Cost ($)
1	1.25" long chord	$500	52	$26,000
2	1.125" long chord	$1,000	52	$52,000
3	3.375" long chord	$1,500	32	$48,000
4	5" long chord	$2,000	0	$0
5	7.5" long chord	$3,000	0	$0
6	chord splice	$1,000	0	$0
7	45 degree gusset plate	$1,000	0	$0
8	90 degree gusset plate	$1,000	0	$0
9	135 degree gusset plate	$1,000	12	$12,000
10	180 degree gusset plate	$1,000	28	$28,000
11	180 degree grooved gusset plate	$2,000	0	$0
12	360 degree gusset plate	$1,000	18	$18,000
13	360 degree grooved gusset plate	$2,000	0	$0
		Total Cost		$184,000

Upon creating this bridge on a scrap piece of paper, I changed a few things about it.  At first, I was going to make the arc come to a taller position, but to cut down on cost I didn’t want to have to use any long K’nex chords.  I also knew that it had to be at least eight inches flat on top and this was interfering with my design a little bit.  However, I opted to have a ‘flat’ arc instead of one that came up to two points at the top like my original plan.  Otherwise, my design was pretty much how I envisioned it.  

This bridge gave me an idea of what it is going to be like working with K’nex kits.  I had grown used to using West Point Bridge Design and it’s just not the same when you have to translate it to real pieces.  When using K’nex I had to think about what types of gusset plates were going to work best and the angles I was able to use.  I also had to think about the length of each chord piece and if I wanted long ones or many short ones.  I hope that when we get to start building our bridges in class my design works well and everything fits together.  It is quite hard to imagine a bridge that will work when you can’t play around with the pieces since they are not in front of you.