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When selecting the length of beams for a bridge, the better approach is to choose multiple full-length beams.

bridge spans

Unless there is a compelling reason, one would not choose multiple shorter spans.

shorter bridge spans

An elementary exercise by sketching the bending moment diagrams would verify why the first is superior to the second.

Now suppose that said spans are for a backyard patio. (So far I'm only considering 16-ft long spans of natural wood, but composite/synthetic materials are an option.)

If the span is slightly longer than 16 ft, then one option is to use full-length spans, and splice the remaining part, while of course staggering the cuts. (Here the joists—our beams—are not the subject. We look instead at the structural flooring material.)

staggered full-length spans

Another option is to use 8 ft-long cuts. That would simplify transportation, but let's ignore this convenience factor.

using shorter spans

Are there structural (or code) reasons for choosing longer or shorter beam spans (besides the obvious bending moments, and ignoring cosmetics)?

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  • $\begingroup$ you should go look at a bridge that's more than a couple dozen feet long, exactly none of them has a single span member. $\endgroup$
    – Tiger Guy
    Commented Jan 15 at 7:20
  • $\begingroup$ @TigerGuy Was what I wrote somehow ambiguous regarding this issue? $\endgroup$
    – Sam7919
    Commented Jan 19 at 2:28

2 Answers 2

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Actually the shorter the span the stronger the structure. As a rule of thumb, a beam doubled in length supporting the same load requires four times more material and will deflect more proportionally to the power of four of length. 16/2=8 times more. eg, The deflection of a supported UDL, simply supported, beam is:

$$\delta=\frac{5\omega L^4}{384EI}$$ Also, A longer beam is heavier and in an earthquake will vibrate with greater amplitude and will likely be cut off by the shear at supports. same as has happened in Los Angeles 1994 Northridge quake.

For small projects such as a deck or a roof other parameters such as spacing of the columns or bearing walls, availability of rafters or joists or cost of the material dictate the span.

'

nrthridge q 2

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northridge q 1

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As a rule of thumb, a longer span is usually preferred, the primary reason is "costs" - shorter spans will require more support/abutment, thus foundations. The construction of those structures is costly and time-consuming.

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