Engineers follow a code like those of doctors. They must, to the best of their ability ensure that human safety is a priority. Structural Engineers must keep this in mind more than most. News of building failures is common regarding earthquakes and flooding. However, when a building fails then there is an underlying issue of checks and balances of which it falls on the structural engineer to maintain.
Familiarity with supports, braces, and building materials is of the highest importance; as well as a team of experienced contractors. Just weeks ago, a floor collapsed on a University campus during a fraternity party. I even recollect a party at my own University years ago where we were asked to stop dancing because the floor was caving in. While historical buildings tend have such issues with infrastructure, it is essential when doing any Structural Engineering Building Design to consider future use on and plan accordingly in order to upkeep buildings in the future to avoid collapses that could injure or even kill. Many floor collapsing issues occur due not following building codes, for in the example of my university is was fair for them to expect a gym to hold up to DJ competition. Aside: It was a University that condemned dancing but what could they have possibly expected?
While the engineer is not strictly responsible for unreasonable stress on something he has designed, there are certain things called factor of safety that should be built in. Each member should, in a building, have a factor of safety of 2.0, meaning that each member should be able to hold twice as much as it is ever expected to. With the right choice of materials, expert assembly, and frequent inspection collapses should not happen without a catalyst. Fatigue failure is common in older buildings, often caused by temperature changes. What structural engineers must now look at towards the future is expecting these temperature changes as weather patterns become increasingly more radical and prepare for them.
One may propose members that have backup permanent members with a lower factor of safety, so the standard supports could be removed frequently and inspected for fissures, which are what cause fatigue failure. Once there is a fissure (crack) the stress increases at the point as the member becomes no longer uniform. This problem can be shown with the example of bending a wooden popsicle stick that is solid vs a one with a crack in it. The solid one will bend, and the cracked one may bend as well, but the crack will expand. The next time it will extend further. Until the cracked wood simply breaks along the crack, folding and thus, collapse.
The responsibility of the Engineer is to do their best. But with proper study, knowledge, and the innovative spirit that inspires the career: many failures that we see in older buildings can be prevented when we build new structures that will last longer by having more accessible preventative maintenance.