Sadly, its not uncommon to have computer chairs snap under you. You might not even be that heavy, but chairs just arent made as well these days! Remember back when they were all metal? Those suckers withstood anything. These days, plastic is all youll find in most computer chairs. Overweight people tend to have even more problems since chairs often last only a few weeks before something breaks.
While you can keep using the chair without an arm or even if the back cracks and ends up at a weird angle (yes, this happens to many of us!), once the support column or one of the wheels or base legs goes, youre pretty much hooped. Many people cobble together Frankenchairs, made from a base that still functions with a seat that is still intact, just to save a bit of money. It might seem silly, but if you have computer chairs break on you every couple of months, this is what happens.