Question:

I wonder how much the Spanclamps holding force is?

Answer

The holding force, you mean probably with this the clamping force.

It depends on the model you utilize.

In the meantime high clamping force models have been designed.

See here some possibilities for "holding force":

https://www.spanclamps.com/quarter-turn                                                                                     up to 90N

https://www.spanclamps.com/heavy-duty-quarter-turn                                                                 up to 250N

https://www.spanclamps.com/quick-accurate-positioner-knob                                                    up to 600N

https://www.spanclamps.com/quick-accurate-positioner-stud-tapered-hexagonal-head          up to 4500N

if you design with Turnlocks, depends on the series and thickness of your plates:

https://www.turnlock.eu/whats-clamping-force-turnlock

_{Holding force}