The reason I'm asking is because my daughter gets frustrated occasionally and slams her hands down on the keys, you know, "kraaannng!!" and I was always told, when I was a kid, "Don't do that, because you might break the strings." So I find myself dutifully repeating it to her, but I got to wondering if it was true. Or is the danger more to the hammers and other widgets under the hood?[/QB][/QUOTE]
It is possible to break strings while playing but it usually only happens with pianos which are played long and hard every day and which develop deep grooves in the hammers. It is really just metal fatigue.
Once this occurs, the strings may literally start breaking right and left. The pianist may claim that he/she wasn't playing loudly at all at the time and it may be true. It may just be a case of the "straw that broke the camel's back".
It is often associated with Jazz or Rock and Roll playing because those forms of music can be loud, hard and repetitious but it is also found among classical pianists and church pianists who perform shall we say, the more "spirit filled" kinds of music.
One of the most chronic string breakers I know of is the shyest, inarticulate, most weak looking, slight of build teenager you've ever seen. Yet when he plays his classical repetoire, he turns into an absolute monster. I guess it's all in the speed and the leverage. He won a full scholarship to Indiana University not as a performer but a composer.
This kid and his family and a young lady at the "United Community Bethesda Christian Evangelical Fellowship" were the people in my clientel who got the most upset about the chronic problem. They just could not buy my explanation of what was happening.
In the case of the church, it was the pianist before the gentle young lady who did the damage. They fired him when the strings started breaking and hired the young lady because her music was so gentle. Everyone thought it would be the end of the "Ka-SPROING" sound and the wound strings literally shooting out of the piano during church service. Metal fatigue cannot reverse itself. It made matters all the worse when this happened during her soft and gentle playing.
The young classical pianist's parents finally came to understand and accept that their son was rather, "special". After all, Dad didn't have to pay for college even though he could afford to easily. Now that he is away most of the time, it is giving the piano a well deserved rest. Dad is planning to buy a brand new Shigeru Kawai as a graduation present and will sell the Yamaha C6. (Let the buyer beware). Meanwhile, he'll just pay to have the broken strings replaced when they need to be.
Having said all of that, let me state that this is a very rare problem. Most people never come close to the point where they would cause the strings to break during playing. What your daughter does in a moment of frustration might sound terrible but it is probably not forceful enough and repeated enough to be of any undue wear and tear at all.
One suggestion might be to find a really terrible sounding chord that can be played with all 10 fingers. When feeling frustrated, play that chord instead of just banging. It won't be of any more stress to the piano than keys which are just struck at random. It may well vent the frustration a little better since the pianist will know that it has structure that comes right from herself.
It is, of course, possible to hit piano keys so hard that something might break, a string, the key itself, the hammer shank, a jack tender but it is usually only a piano technician who ever hits a key quite that hard.