Answer by Patrick Barry:
A ‘span’ in engineering parlance means ‘the gap between two supports’
This bridge is single span:
A single span slab is a slab that is supported at either end. And that’s it.
This bridge is (very) multi-span.
A multi-span slab has a support at either end and one or more additional supports in between.
BUT if the slabs on the above bridge were cut (see below) each becomes a single span slab. The bridge is then made of many many single spans that share supports.
^^ This is NOT continuous multi-span ^^
Why do multispan slabs achieve greater spans?
Because they’re more efficient.
Let’s look at the Bending Moment Diagram (BMD) for a single span (simply supported) beam or slab. We generally design the slab to be able to resist the maximum Bending Moment, so it’s a very important value.
Looking at the diagram below, it’s zero at either end, and largest (most negative) in the center. Basically, it’s trying to bend downwards (sagging – something we’ve all seen with a fat man on a plank). If it breaks, it will break in the center.
Now let’s look at the bending moment diagram for continuous multi-span (simply supported) beam.
At the very ends, it looks a bit like a single span beam, but at all of the supports in the ‘middle’ section the bending moment is positive! This means in this area the beam is trying to bend the opposite way to the single span beam. (Imagine a chair with a plank balanced across it, and a fat man on either end).
If you look at the shape of the BMD curve between two supports, you can see it’s the same shape as for the single span, just ‘slid’ up the diagram vertically so some of it’s positive and the rest stays negative. This means the largest (most positive or most negative) value is actually smaller for the continuous multi-span section then the single span.
So for the same size span, the continuous span is stronger. Or, as you’ve found, for the same slab thickness, the continuous slab can span further safely.
Incidentally, if you’re doing this as a DIY project, please consult a civil/structural engineer. It’s not really something you can learn to do safely the first time from Quora, and if it goes wrong there won’t be a 2nd time.