Anyone good with wood and bridges?

[Imparcticle]

Hi everyone!
I haven't been here in a while, and I see there've been a lot of changes. The place looks as groovy as ever.

I have just been assigned a bridge builiding project. We're supposed to make the bridge out of only wood that ways 25 grams or less and be able to hold 15 kilograms or more of weight. My teacher said that in the past people have built a bridge that wieghed 9 grams and held at least 15 kilograms. As a result, my group and I feel we should build one 9 grams or less...however we face but one problem. We don't know what kind of wood to use. The only one we know of is Balsawood and I've recently heard that it's not the strongest type of wood, regardless of its light weight. So if anyone here knows of a type of wood that is both light weight and strong, please suggest it. Thanks.

 

[Gokul43201]

In terms of strength to weight ratio, Sitka Spruce and Douglas Fir are among the best. Sitka spruce was the standard wood for building aircrafts until aluminum alloys came along (post WWII). Other good options are most Firs and Hemlocks, White Pine, White Cedar and Yellow Poplar.

Structurally, using members like hollow cylinders and C-channels often gives you better strength to weight performance.

Shouldn't this thread be in Civil or General Engineering ?

 

[Artman]

Balsa wood has a very high strength to weight ratio (half the weight of spruce, but greater than half of the strength in all categories. Only hickory has a greater bending strength in the list I found, but its compression strength and stiffness strength are both lower).

weigth to strength of balsa and other woods

The problem is that it is so light that it is relatively weak compared to heavier woods. To keep weight down, balsa could be used in laminated form (create your own plywood) and/or glue it in "I" beam forms.

 

[Gokul43201]

From Artman's list, black walnut and douglas fir (followed by balsa and spruce) are strong in compression, so if you're using columns, this would be a good way to go. For the span, look at the bending strength values.

Honestly, I think anyone of these woods would do it. The geometry is likely much more important.

I guess you have been given other design constraints as well : minimum span, minimum height, loading geometry, etc. ?

 

[brewnog]

We built a balsa bridge weighing around 30 grammes which spanned 200mm and supported 75 kg point load at the centre. Winner!

 

[loseyourname]

Spruce is a very good choice. I had this same assignment for a GATE summer school program a long time ago and managed to build a bridge weighing about 20 grams that was never broken. It actually held all of the weight they had available. I don't remember how much that was, but there were actually two groups that managed to do this, so my group tied for first place. Focus more on the trigonometry than the wood, though. There are ways to distribute the weight almost completely evenly through the entire structure.

 

[Imparcticle]

Thanks for the replies!

I guess you have been given other design constraints as well : minimum span, minimum height, loading geometry, etc. ?

Yes:

1.) The entire bridge must be made of wood. Particle board, laminated wood or wood products are not allowed.

2.) Any type of glue may be used to attach the sections of the bridge.

3.) The bridge may not be covered or laminated with any material.

4.) The bridge should span a distance of 30 cm.

5.) There is no maximum height for the bridge.

6.) The bridge must be constructed so that a 50mm x 50mm x 20mm high loading block, with a 1/4 inch eyebolt centered on the block, a washer and a wignut may be passed through the length of the bridge (oriented so that the 50mm dimension is horizontal and at a right angle to the bridge span).

7.) The bottom surface of the loading block must be no more than 5mm above the top surface of the test supports when placed on the load point.

8.) No part of the bridge may extend more than 20mm below the top surface of the test supports.

9.) The load point will be at the midpoint of the centerline of the bridge.

10.) The bridge must have a mass of no more than 25 grams.

Focus more on the trigonometry than the wood, though. There are ways to distribute the weight almost completely evenly through the entire structure.

How would I integrate trigonometry into the project? How should I interpret the data about a triangle's dimensions in relation to distributing the weight?

BTW, the bridge design I am interested looks like this :http://enrich.sdsc.edu/SE/images/arcbeam.jpg

The only changes I'm making is using I-beams for every thing but the arc. I think that's not such a good idea, considering the wieght it might add. I have instead considered making a simple bridge where there are two vertical beams with a horizantally extended I-beam. The vertical beams and the I-beam would all be constructed mainly with triangular shapes. Basically, I'd like to build the entire bridge out of triangular shapes, with an exeption to the basic structure of the I-beam.
good, bad idea?

 

[loseyourname]

That design might work (my geometric intuition isn't enough to tell me without looking in more detail), but the bridge I made was a cantilever bridge.

 

[Gokul43201]

I definitely wouldn't have gone with that particular design. That design is necessitated by the inability to support from below, at the midpoint, and that's where it's weakest in flexure.

Your bridge actually wants to be strong at only the midpoint.
"9.) The load point will be at the midpoint of the centerline of the bridge."

But to make it strong only there would be, in a sense, cheating the system. Nevertheless, I would design it so that the midpoint is definitely at least a local maximum for the rigidity. Uniformly distributed supports, with one below the mid-point, might be a simpler, yet, more effective approach.

 

[Gokul43201]

You could try a modified version of this :

Or this :

 

[brewnog]

From practical experience with an almost identical project, the succeeding designs used nice trusses made into rough triangles (as there was just a point load applied). I-beams were strong but lost out badly on weight, and arch designs were so flexible that they just wouldn't load up, and collapsed before any significant load had been applied. Obviously though, there's more than one way to skin a cat.

 

[Imparcticle]

How about a 3D trapezoid? I know you know what a trapezoid looks like, but I'm going to tell you just what I have in mind:

Imagine an isosceles triangle ABC, where AB and BC are the legs and B is the vertex angle. Now imagine another isosceles triangle DBF where D is between A & B. DB is 1/5 the length of AB. The relationship between F and BC is the same as that of D and AB. Now take traingle DBF out of the picture so you have isosceles trapezoid DFCA. Now imagine this trapezoid as being 3D. Segment DF would be one side of a square top of the 3D trapezoid. As explained in the specifications, I'll need to make an 50x50x20 mm enclosure. This will be basically cut out from the top part. Every lateral side of the 3D trapezoid will have Xs , so as to integrate the all mighty triangles.

This is too much isn't it?

What if the bridge was just an X with a top basically?

 

[Imparcticle]

You could try a modified version of this :

Or this :

YOU'RE A GENIUS! I'M TOTALLY GOING TO INTEGRATE THOSE DESIGNS...ESPECIALLY THE SECOND ONE!