Again,
ALL gliders are certified to a standard to withstand specified loads.
The standard cirrus is made before JAR 22 standards were published but for most gliders in the last twenty years, they are certified in the JAR 22 Utility standard.
The load limits at Va are +5.3G and -2.65G
JAR 22.3 states that gliders in the Utility category are cleared for spins, lazy eights, chandelles, stall turns, and positive loops.
A properly flown positive loop uses only +3.5Gs.
The plane is not the problem.
The problem is the pilot who does not know how to fly a loop and does not know how to gain control of the aircraft if a manuver fails so that the aircraft is not subjected to loads beyond +5.3G and -2.65G. Also, these are loads that are without twisting loads. A snap roll is out of the question.