I don't think that truck could have caused so much damage. So I am going with charges placed on the supports of the bridge.
But this brings another question with it. If Ukraine special forces were able to place charges then why only on one side of the bridge? Why not take down both sides and also the train side?
That leads me to believe the attack was symbolic and to cause panic in Russians. There are genuine reasons for keeping the bridge up, including allowing Russia to retreat without fighting every man to the death, ease of monitoring supplies entering Crimea and usefulness after the war. But we already see that there has been panic with Russian civilians wanting to leave Crimea. It would have sent a shock wave through the Russian leadership and the news will spread back in Russia among civilians that the bridge which was supposedly so well protected, could be hit.