Perhaps my opinion is biased because I do have a masters degree in structures. However, my BS is in a different branch of engineering and to be up to speed on even the basic civil coursework, a masters degree was purely essential if I wanted to grasp the tasks my work required.
In my experience, the best way to go through grad school is to have an employer pay as you go. This way, your main investment is time, not money, and you can structure your coursework to focus on topics that best suit your day job and make it that much more fulfilling. I think this is more beneficial than completing a masters directly after a BS with little to no work experience.
Be aware, as most academia, post-graduate studies often get a bum wrap for being too theorhetical or too focused on a niche topic. My former grad school advisor would be proud if he could read this, because I fought him tooth and nail about coursework being too theory based. I wanted to learn practical applications. Yet, he repetitively reminded me that you don't learn theory on the job, so it's best to learn it in school. In hindsight, I see the beauty in learning higher engineering theory, because it helps make everything click so much faster.
So, if you already have a BS and have a little experience and your employer is willing to help you out, I'd say you have nothing to lose by going for it.