Will house insurance cover main support beam broke in your rental apartment building?

It depends on the covered "perils" listed in your policy but I definitely would not rule it out. For example, if your policy covers the peril of "collapse" which generally covers collapse of the building due to the weight of ice or snow, and your main support beam broke due to the weight of ice and snow, it would then be covered and the insurance company would pay to repair the damages, less your deductible.

Furthermore, if, by "rental apartment building" you are alluding to the fact that you own the building and rent out apartments within it, the policy will have been rated differently than a single family structure. Additionally, a commercial form of policy would have been issued. This may or may not track the same sort of perils included in a standard HO-2 or HO-3. Therefore, it is critical that you consult with a licensed commercial lines insurance agent in your state.