The argument has been made that a truck bomb of a few hundred kilograms could not have caused the damage, due to the heavy mass of steel in the bridge. This is incorrect.
Truck bombs are usually employed to destroy buildings. For this purpose, the blast effect is intended to be isotropic, without preferred direction. To damage a bridge, a directional, downwards explosion is desired. This can be achieved with a tamper, a mass of dense material placed over and/or around the charge to multiply the effect in a specified direction. Some materials used in construction fit the bill. The explosive multiplication is limited only by the degree of concentration.
If you google “tamper”, you’ll find references to nuclear weapons technology. Tamper predates this; it is the basis of all modern directional munitions and shaped charges. See Gurney Equations if you’re a techie.