OP said:
I noticed from my observation that when a problem happens (like missing details in the drawings or mistakes in the calculation), the senior engineers immediately start blaming the junior engineers for not doing it right.
You're probably just witnessing the normal phenomenon of fecal matter flowing downhill. Normally that's an internal thing and not shown to external parties. In my personal opinion, there are few jobs more difficult to do than managing the efforts of other people in the development of a quality technical product. So I kinda get this.
OP said:
I believe many senior engineers have backup plans to cover themselves in case of failure of a structural member (is this true?)
I feel that it is very rarely the case that senior engineers preemptively lay the groundwork to shift liability to their junior help. I've no doubt that it's happened on occasion, in desperate situations, but I'd not lose any sleep thinking that your leaders are out to get you in this way. In most jurisdictions, attempting to shift blame to unlicensed helpers would be courtroom suicide as it screams inadequate supervision and quality control.
OP said:
How can they sleep without checking the calculations another junior engineer did??
With great difficulty in some instances and comfortably, on a mattress filled with cash, in others. It depends very much on the constitution of the particular senior engineer in question. I tell my junior engineers exactly what I've stated here on many occasion: yeah, I'll spot check some numbers but 80% of my confidence in your work will come down to:
1) Based on our past work and conversations, do I get a sense that you "get it" and have adequate "structural vision" and a sense for load paths;
2) Do the answers that you've generated look like the answers I expected long before you got started.
3) Do I trust that you are ethical enough to seek help when you recognize that you're working outside of your comfort zone?
Junior structural engineers are often astonished at the extent to which their work is
not checked in detail numerically. I sure know that I was. Sadly, there usually is not sufficient fee or time available for detailed checking of that kind.
I used to run a little program within my own group that we jokingly called "Nerds Anonymous". Every Monday, I'd generate a new mini-design question for everybody to noodle on over the next week. Then, on the following Monday, we'd review our answers together and go over the nuances of the problem. I'd make a point of trying to generate questions that were simple enough to almost be done in one's head but, at the same time, highlighted meaningful issues in design practice that tend to trip people up. This process served me very well since, through the course of these little pow-wows, I was able to quickly gain a sense for people's strength and weaknesses. As you'd expect, it wound up burning about about [45 minutes x 4 engineers / week] worth of billable time and my own supervisor didn't love that. I didn't sweat that though since I felt the improved morale and trust more than offset the cost of the time.
OP said:
Also, I want to know how to protect myself in case a mistake happens?
1) Communicate copiously and with integrity.
2) Don't let your ego prevent you from asking for help when you need it.
3) When you ask questions, keep drilling down until the answers truly make sense to you. Demand that your mentors truly convince you of things by force of logic before giving in to their direction. Or, at the least, insist that they convince you
after the job goes out as follow up. A mentor that can't, or won't, do this isn't worthy of the role.
4) Be sure to get yourself a good mentor or six.