Again.. an asteriod on the smaller end of the scale will "burn up" in the atmosphere causing no damage. One on the larger scale would end up creating a ton of mini bombs if you will. But an asteriod of that size would be an Earth killer anyways.
Siderites don't burn up in the atmosphere unless they are tiny. Aerolites can burn up in the atmosphere whether they are broken into pieces or not, even if they are large enough to cause major damage. The Tunguska object--apparently an aerolite--burned up entirely before hitting the ground, and it still devastated a vast area.
So smashing a small object doesn't help.
One on the larger scale would end up creating a ton of mini bombs if you will. But an asteriod of that size would be an Earth killer anyways.
If it hits the Earth, yes. But if it remains intact, it can be steered away from its collision course.
So smashing a big object doesn't help.