The team set the explosives at the bottom of the supports but when the detonators are triggered the explosions occur along the bridge deck.
As the two choppers take off for the beginning of the main mission of the episode (to blow up the bridge), there's a reused sequence from a previous episode where a door gunner can be seen laying down some fire next to Sgt. Anderson on the chopper - only there's no sound of machine gun fire heard.