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on a glass floored skybridge. ok, glass breaks, you fall, twenty eight storeys to the concrete below amidst a cloud of glass razors. your body strikes a plastic table, cartwheels and lands fatally hard, people eating lunch have scattered, screaming and the glass rains down in the plaza slashing and lacerating panicked civillians. but you dont care, you are face down in a pool of your own blood, your exposed skin gaping and oozing blood from the glass you shattered above. your skull is open and brain exposed, arms twisted at horrible angles. one of your eyes has ruptured and you are dead.
"Fuck!" exclaims one of your thuggish companions as he hauls a badly gashed companion up on one of the network cables that ran the underside of the skyway.
"Well that was a fucking stupid move. we better get going before the cops get here."
As a side note, im an army Engineer. we do demolitions, in particular, building dems. Grenades like your F1 and M82 frag grenades (regular old pull pin, hoik at enemy, go boom, make mess grenades) have very little impact on steel and structure. 40mm airburst and contact grenades, like those launched from an M203 or GLA, or even as you say, dedicated rotory grenade launcher likewise have very little effect as their primary use is frag. They do make a tidy mess of a wooden or aluminium door if you hit it dead on but for concrete, brick or steel walls you may as well throw stones.
Structural damage requires cutting charges, or properly placed block charges at right angles to the support, as demonstrated.
by all means let grenades blow the fuck out of I-beams in your game if its cinematic and fun, shit it works in movies and can be hilarious. but IRL, grenades dont work that way, maths or no maths.