A support is a specific building that will hold up sections of terrain to prevent cave-ins.
And, as a useful note, supports can be propping a mass of otherwise disconnected rock/regolith up, at the bottom, or holding a mass up from the top, hanging from a relevent ceiling.
By the way, bridges do not count as anything for cave-in purposes. If you end up with a platform that's held up by nothing but a bridge, the platform will collapse.
And this feature makes a convenient method to gain access to otherwise horizontally disconnected areas without adding further supports (although making it a diagonal corner-to-corner connection would also work).
Consider this (side-view) design:
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## I I I ##
## ### ### ### ##
## ### ### ### ##
_ I _ I _ I _
## ### ### ### ##
## ### ### ### ##
##~~~~~~~~~~~~~##
##~~~~~~~~~~~~~##
##~~~~~~~~~~~~~##
##~~~~~~~~~~~~~##
##~~~~~~~~~~~~~##
#################
For a start, setting up the bridges ("_") to retract, you've got yourself a standard non-expendable dumping system (provisionally with water below, but could be just a drop, or lined with spikes), but for the hardier creatures you might need something a bit more effective.
With appropriate pressure-plate and/or lever attachments to the various supports ("I") you've got options to either drop a given lower suspended blocks into the water (or magma!) below, taking any creature traversing the block with it (plus possibly some cave-in dust effects for anything it doesn't directly impact), or effectively squishing them between upper and lower blocks (which I'm betting won't help their health and well-being) by releasing the top support and letting the whole mass fall.
Having dropped one or both blocks, you could replace the remaining bridges-to-nowhere with a longer bridge, to let you still drop your enemies/awkard aquaintences into the liquid, but if you still have the upper block it can be dropped onto the bridge for further squishing-fun (aim and timing allowing). The bridges would have to be rebuilt
again after that eventuality, of course.
With enough time and effort and room to dig, a 'string' of blocks should be able to be suspended for a more protracted (though still limited) number of possible crushings, and by weaving the cross-ways path up and repeatedly through and between the strings of pearls, many more opportunities to give your enemies a bad time are to be had. Including flying creatures, who are likely to be caught in either cave-in dust or actual closing voids, given enough opportunities. If dropping native rock blocks, I think they'll stay 'intact', (whereas re-constructed blocks will disassemble, I think I found). Bear that in mind when it comes to planning the depth of the sump (and I'm unsure if water ends up being displaced by a falling block or being 'atom smashed') as you may end up with stalagmites formed of the succesively fallen stalactites.
Note, this is a combination of several different things I've done individually (hence the uncertainty at the end, but feel free to try it!), and I've not gone to such effort. (Or, at least, not so much effort in this direction.) Mostly I have tended to go for traps that snap shut
behind attacking hordes, just as their morale breaks, and forces them to take a Danger Room or Killing Zone path back out in their attempts to retreat. Or just cages, for a more controlled and personal application of Cruel And Unusual Treatment/general experimentation. But I've laid out (and not used) a basic version of the above in preparation for an ampersand-invasion that I never got (encountered Other Difficulties first) but which I was hoping to crush, literally and/or figuratively.