Some launchers, like Falcon 9 recently as in a video clip linked to below, and I think some other launchers since long ago in the space race history, test their engines before launch by firing them while remaining strapped on to the launch pad. I mean firing the actual vehicle days or hours before a real launch, not any horizontal engine test under development. F9 weighs about 500 tons on the launch pad and the engines firing are powerful enough to go to orbit.
How is it possible to hold something like that anchored to the ground without breaking it? Are such launchers especially sturdy and heavy, or is it maybe not as hard as it looks because the forces are similar to those a launcher encounters during a real launch anyway?
What are the main trade offs when deciding if a launcher should be able to do such static engine firing before each launch? How large is the penalty to the payload it can deliver to orbit?