Choosing materials for a bridge

Ngineer

Hello everybody,

We need your advice with a project for building a simple one-beam bridge. We have figured out the dimensions, required load, max allowable stress and safety factor. We need to choose a material that would support our requirements.

Which formula should we use to compare these requirements, and which property of a material should be compared to?

Your help is highly appreciated

Thank you very much .

Enthalpy

A bridge builder seeking advice on a forum, that's a bit unexpected... Who goes to jail if the bridge fails?

Maybe you can tell more about this bridge. Half a metre high to step in your own home? Or 30 metre high to carry 1,000 drunk supporters to a soccer match? How cold and marine a climate?

In any case, if it must carry people, I'd use only perfectly known materials, which I translate into: no concrete (in accordance to my personal knowledge limits), no fibers, only traditional alloys AND traditional manufacturing processes, especially for welding.

Ngineer

This made me smile on a bad day!

I'm extremely sorry for sounding too serious, by project I meant university project, and by the bridge I meant just designing a bridge.

Its supposed to carry 1 truck of 265 kips of weight, we found that the these bridges have a factor of safety of 5-10.

Our problem is relating our results to formulas in which actual material properties are variables, so we can determine the ideal material.

I apologize again for the wording in the original message.

SteamKing

What is the max. allowable stress and safety factor from your design?
How did you settle on an allowable stress without consideration of the material of construction?

Studiot

Hello Ngineer, what university course are you studying?

You have chosen to place this question in the materials section so I assume it is about materials properties rather than structural engineeering.

Nevertheless an appreciation of the location and types of stresses involved is needed for a university level analysis of your question.

I suggest you look up some stress trajectories or contours and find out where the significant stresses are in a beam and equally importantly in what direction they act.

http://www.google.co.uk/imgres?q=str...r:18,s:0,i:143
Click on the blue x in the white box in the centre of the page to see the image of the trajectory.

This will allow you to consider what materials would be useful in what position ie what is strong in tension and what is strong in compression.
This is how designers of reinforced concrete place the reinforcement in suitable positions.

You should then consider the distribution of stresses at a vertical section and examine the effect of introducing an axial prestressing force.

Finally you can look at beams of composite construction such as flitch beams (steel and timber composite)

go well

Ngineer

Our idea was getting the stress from the force acting on the beam, and then finding a material that supports this stress at a given cross sectional area, is this incorrect?

Ngineer

Hi Studiot,

The course is mechanics of materials, I posted in this forum because our main problem was determining the materials.

Thank you for the valuable link, we're accounting only for the bending stress and we have to figure out a material given the loads.

I will have a deeper look into the topics you mentioned and see how it turns out

Thanks again

Studiot

You could say that there is no such thing as bending stress.

Bending moments (including torsion) apply tensile, compressive and shear stresses to the materials that make up a structure.
Different parts of the structure are subject to different levels of these stresses.

So finding the level and position of the maximum is important.

Ngineer

I meant the stresses applied due to bending, sorry again for the wording

Enthalpy

If it's a school exercise you can choose the materials with more freedom... No corrosion in your paperwork/computer, no cyclic stress, no wind, no cold...

Reasonable material: more recent high-manganese construction steel, like S700MC.

Half-reasonable: alloyed steel, tempered at low temperature, like Xabro and others. Already used at construction and mining vehicles.

Also: weldable but hard aluminium alloys like 7022. Thicker than steel, hence sounder against buckling. More expensive.

Not reasonable to my taste, but exists already for years, and the bridge hasn't collapsed: fiberglass. Though everyone would choose carbon-polymer presently.

Personnally, I wouldn't take concrete nor wood, but only because I know metals better.

Ngineer

Enthalpy, very useful post, thank you very much for your valuable help.