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I'm quite new to Mechanical Engineering and I'm currently writing a report for a project that is to make a pedestrian truss bridge. I decided on the materials and dimensions of the deck, truss, and safety rails then I proceeded to calculate the dead load of the structure.

However, I'm stuck in calculating the dead weight of the Truss because I'm not sure about the dimensions of a chord in order to calculate its volume. Is there a way to determine cost-effective truss chord dimensions? Or else is there a way to assume the Truss's weight regardless of its volume?

I would really appreciate the help! Thanks.

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  • $\begingroup$ You should assume the sizes and then refine member size through iterations. $\endgroup$
    – r13
    Sep 12, 2022 at 20:40
  • $\begingroup$ don't design truss components (no one does this), get values for structural members online $\endgroup$
    – Tiger Guy
    Sep 12, 2022 at 21:29

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This is a standard issue in any civil engineering project. The structural dimensions are a function of the structure's dead weight... which is a function of its dimensions.

It is simply impossible to "solve" this problem. The solution is to use one's experience (or your boss') to come up with "reasonable" dimensions. Use those to calculate the structure's deadweight and therefore whether it is stable. If some elements are over- or underutilized (strength below or far above requirements), then you can modify those elements and recalculate everything. And again, and again, until all elements are properly designed with reasonable utilization ratios.

Of course, it's entirely possible (or probable) that you end up at a sub-optimal stable point. That is, that changing the dimensions of a beam here and there you could come up with a more efficient structure.

But this is simply one of the many ways in which engineering isn't a science, but an art. You have to use your intuition to come up with something that'll get you... close enough to the "perfect" structure.

Of course, as @Tiger Guy mentioned in his comment to your question, for some specific and simple use-cases you'll frequently find that someone else has already done the work of trying to reach perfection.

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This is a civil engineering problem early solved using simple analytics. A bridge is designed according to to its loading design. You need to take account of materials dead weight, live loadings and variable factors. These are set out in Eurocodes. Depending on structural material chosen, it will either be steel, reinforced concrete, or wood. Wood is seldom used as it is subject to highly variable continuum. Steel is the most often used material for pedestrian bridges and rc the most common.

To get an idea of what is involved, I highly recommend getting your hands on a book that deals with this.

BRIDGE DESIGN IS NOT STRAIGHT FORWARD.

Design of Structural Elements (3RD ED) by Chanakya Arya. ISBN 9780415467209

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