Unfortunately, the bottom had begun to collapse, such that you could feel the sag when you sat down and also see the bottom basically scraping the floor below. Sadly I don't have any great before pictures because I always tried to avoid having the issue be visible in any apartment pictures I took, and this was an unanticipated DIY without much advance planning. Fred and my mom were visiting for breakfast the day after we hosted our families for Thanksgiving, and noticed the sag in the sofa as they were sitting on it. Fred, always eager to fix a problem at hand, asked if he could flip the sofa to examine the situation. This is the sad state of affairs we discovered:
The strapping that holds up the couch springs had come undone. We can all make some jokes about how the sofa survived for decades and then suddenly collapsed when it came to live with us and I lounged around on it basically every day, but honestly I'm surprised that the tacks that were holding the straps on stayed secured as long as they did. Fred offered to help me fix it before hitting the road, and I accepted -- "figure out what's wrong with the couch" has been on my to-do list for awhile now, but I was not at all confident in figuring it out myself and I pounced on the opportunity for his expert advice.
We picked up some screws and washers at the hardware store, and then got to work reattaching the straps to the frame of the sofa. We used two screws and washers per strap, and needed to fix all the straps along the back edge of the sofa as well as along both sides (ie both the vertical and the horizontal straps had come loose and/or sagged in a number of places). It took us about 20 minutes to get all the screws in.
Since we were replacing all the loose tacks with screws, we ended up with a collection of leftover tacks that we then used to reattach the large piece of fabric that covered the entire bottom of the couch. This fabric is just to conceal the straps and doesn't hold much of the pressure on the springs (since if the straps are tight enough, they'll do that), so the tacks should be plenty strong enough to hold it in place. Here it is as I nailed the fabric back over the strapping:
And here's the bottom of the finished sofa, nice and flat the way it's meant to be!
The sofa noticeably firmer now, and it looks so much better not sagging in the middle.
I wish I had more before photos to show how awful it was, but I'm so pleased with the finished result. Fred was a great teacher, explaining to me why we'd want to pick certain kinds of washers and screws, and showing me how to get the strapping as tight and strong as possible, all while making sure I was getting to try the techniques myself. I have to say, my curiosity in the art of upholstery is now piqued!