It's not important what powers the movement of the device's piston. The piston must have some amount of mass to it, but doesn't need significant mass (ie not tons).

The device is affixed firmly to the structure. The steam power causes the piston to cycle through it's range of motion at a high frequency rate. Being affixed to the building structure, the vibration energy is transmitted throughout the structure harmlessly at first. As the cycle speed (aka hertz) increases, you will eventually come upon one or more resonant frequencies of the building. At a resonant frequency, the building will no longer dampen the vibration effects, but will instead begin amplifying the vibration effect. If the device is not shut down, this amplification of the vibration effect will eventually cause the building to shake violently and basically tear itself apart.