Elevated Bridge B/C

[13bridges]

The pictures are uploaded under the bridge building section...I didn't see the "Elevated Bridge" category until I was done uploading them...sorry.
The two horizontal members at the top were put in to hold the legs from expanding outward....I think that means tension.
The load block was placed in the center.

[Dark Sabre]

I moved them to Image Gallery

1975|10/1.1.jpg
1976|10/1.2.jpg
1977|10/1.3.jpg
1978|10/1.4.jpg
1979|10/1.5.jpg

[andrewwski]

The top member is going to be in compression. The outside members of the legs are also going to be in compression, probably a good amount too.

You want lateral bracing throughout the entire distance of your compression members. Straight, horizontal pieces are not effective lateral bracing. X's are. From the picture, it doesn't look like you have lateral bracing down the entire sides.

I'm still not sure what you mean about it breaking though. Did the top piece bow? Where did the failure initially occur?

[nejanimb]

Okay, so, that top piece is definitely in compression. You will need to provide extra bracing for it - I think making it an I-beam would work better.

I do think your compression pieces on the side however, supporting that truss along the box, might need some beefing up too.

[SLM]

I need a few suggestions on how to improve my design....anyone?

So basically, my bridge has a A frame and there are two parallel beams on the top, designed to hold the legs from expanding outward. (They undergo tension....I think)
I have been using 1/16x1/8 in pieces to build these parallel bars. It looks a bit like this (this is just showing the top; the problem members are red):

However, when I tested, my bridge has a problem in which the top began to distort. The thin members I used were able to hold the weight, but they deformed, breaking the top of the bridge. I'm not sure whether to make this an I beam or add specific pieces to reinforce the member....and suggestions??

Clearly, the top members of the bridge are under compression. One reason for the distortion of your bridge at the top is probably due to lack of adequate lateral bracings along the length of the compression members at the top. Solution: connect the two members along the top of the bridge laterally using either a zig-zag pattern or an X pattern.

Also, you may want to calculate the magnitude of the compression force in the top member. My guess is that the force in that member is higher than the compression force in the outer member of the leg. Since it appears that the bridge failed along the top, you may have to increase the size of the member or modify the geometry of the bridge in order to reduce the force in the member. There are a number of software that you can use to find member forces in your bridge. Example:

http://www.nexote.net/nexote/Structural ... 0Notebook/

[13bridges]

The top member is going to be in compression. The outside members of the legs are also going to be in compression, probably a good amount too.

You want lateral bracing throughout the entire distance of your compression members. Straight, horizontal pieces are not effective lateral bracing. X's are. From the picture, it doesn't look like you have lateral bracing down the entire sides.

I'm still not sure what you mean about it breaking though. Did the top piece bow? Where did the failure initially occur?

I believe the failure occurred at the top of the bridge. Basically, if you were to look from an aerial view of the bridge, the two horizontal members seemed to bow and twist. Instead of being I I they were ( (....this is a slight exaggeration but I hope you get my point.
Also, my bridge seemed to be weak as soon as i built it. When I pressed down on the model, the legs of the bridge expanded outward. How can I prevent that?

[AlphaTauri]

Thicker/wider/denser wood usually deforms less, but at the price of greater bridge mass. It may be worth it or it may not. Your choice. But be careful- not all wood is the same. Some pieces are stronger than others, like denser balsa is usually stronger than less dense wood.

Also, try using something like JHU Bridge Design program (here: http://www.jhu.edu/~virtlab/bridge/bridge.htm) to calculate forces, then adjust what size wood you're using accordingly. It even tells you what members are under compression or tension.

[andrewwski]

The top member is going to be in compression. The outside members of the legs are also going to be in compression, probably a good amount too.

You want lateral bracing throughout the entire distance of your compression members. Straight, horizontal pieces are not effective lateral bracing. X's are. From the picture, it doesn't look like you have lateral bracing down the entire sides.

I'm still not sure what you mean about it breaking though. Did the top piece bow? Where did the failure initially occur?

I believe the failure occurred at the top of the bridge. Basically, if you were to look from an aerial view of the bridge, the two horizontal members seemed to bow and twist. Instead of being I I they were ( (....this is a slight exaggeration but I hope you get my point.
Also, my bridge seemed to be weak as soon as i built it. When I pressed down on the model, the legs of the bridge expanded outward. How can I prevent that?

They're going to bow downward, somewhat, as that's the direction of the load.

If they're bowing outward (horizontally), you need to increase the strength of your lateral bracing.

The legs are going to go outward somewhat as well, as there's no horizontal supports for the bridge. Using thicker wood and/or more trusses may cause the effect to be lessened, but chances are it's going to want to go outward.

Sounds like your bridge is bowing quite a bit - not a bad thing but you may want more strength from thicker wood. But the important question would be your efficiency, as that is what matters, not load.

Have you tried to draw out your truss in a truss analysis program? It's incredibly useful.

[SLM]

Thicker/wider/denser wood usually deforms less, but at the price of greater bridge mass. It may be worth it or it may not. Your choice. But be careful- not all wood is the same. Some pieces are stronger than others, like denser balsa is usually stronger than less dense wood.

Also, try using something like JHU Bridge Design program (here: http://www.jhu.edu/~virtlab/bridge/bridge.htm) to calculate forces, then adjust what size wood you're using accordingly. It even tells you what members are under compression or tension.

The bridge designer software at jhu.edu is very good but is a bit limited in the type of bridges you can analyze. It can be used only to analyze the so called "statically determinate trusses." For example, it can analyze this truss:


But, it cannot analyze this bridge:


So if your bridge is not designed as a statically determinate truss, you cannot use the JHU's bridge designer. Also, the software does not give you any indication as to how much the bridge is going to move when it is loaded. The online software I mentioned in my previous message does not have these shortcomings. It can analyze any type of bridge, it calculates displacements at the bridge joints, and it draws the displaced shape of the structure.

[dragonfly]

When I pressed down on the model, the legs of the bridge expanded outward. How can I prevent that?

I had a few scary experiences with some of my bridges because of the legs doing something that looks more or less like 'skipping' as more weight was loaded. If it's only a minor bit of movement like mine, a way I found effective was to make sure when placing your loading block on the bridge make sure to slightly pull outwards on the legs while putting the bridge on the table. It doesn't seem like much, but it works.