(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

The problem statement:

Bridge of length: 200m

Construction can expand horizontally to allow thermic expansion

Masses of beams: small in comparison to the car

Mass of the car: 1360 kg.

The answers: Na= 11,0 kN, Ne= 3,67 kN, Fab = Fde = 7,35 kN, Fac = Fce = 6,37 kN, Fbc=Fcd= 4,24 kN, Fbd = 8,49 kN

Question: Calculate the forces in points A,B,C,D,E

2. Relevant equations

We've already solved a similar bridge, but there was not the possibility of horizontal expansion.

In the similar bridge we stated that, as the beams were in static equilibrium, the forces should be parallel to the beams, either pointing outwards or inwards:

3. The attempt at a solution

My problem is I haven't got a clue how to incorporate the ability to move horizontally. Do I have to allow the forces to be non parallel to the beams?

What I find hard to understand is the fact that the normal force in point A (Na) is different form the normal force in point E (Ne)? I thought that, since the massa is equally distributed along the bridge, the normal forces would be the weight of the car divided by 2.

I have made a drawing with the forces, knowing that when I found a negative force, I assumed the wrong direction.

I thought to calculate Fab by saying that there are only forces having a vertical component, so those two should be zero when added.

I stated Fn is = 6670,80. , so Fab . sin(40°) = - 6670,80, Fab = -10377 N, which apparently is wrong ?

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# Calculating forces in bridge

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