Thanks for your reply. I had seen the 15kg bars described as "women's bars" -- e.g., on the Roguefitness site
Barbells - Weight Training Barbells from Rogue Fitness | Rogue Fitness -- so I thought their smaller diameter and lighter weight was to accommodate women's needs in all barbell training. From your answer I see that a 15kg bar is NOT a "women's bar" for women to use for squat/press/DL, but a bar for women who are Olympic lifting - which I understand to be the kind where you are doing "power" moves. Those are not in my plans (being almost 60 years old and having zero strength training up to now, I'm doing the squat, press, DL, and bench press, but no power clean) - so now I know that a 15kg bar is not what I need to buy. I thought smaller diameter might be easier to grip for me, since I have relatively short fingers. Also I need something lighter to start on than 20kg. Up to now I've been using the equipment at the Y - they have ~4' long bars with smaller-diameter collars, and plates that are 1kg or 2.5 kg. (I don't even count the weight of those bars in my lift totals since they are pretty light and not marked as to what they actually weigh. They're actually designed for group "fitness" classes and I have to sneak into the studio and grab a bar and plates to bring with me into the free weight room.) For DLs I use kettlebells, but I'll soon be up to 50 lbs on that lift. Might need to get a "technique bar" and then a 20kg bar later.
Thank you for all the work you put into the books, and the SS website, especially the "on the platform" training videos. Your books and Dr. Sullivan's are the most useful I've had in my hands since at least 1985 - and I can say with great confidence that nothing this useful was ever communicated to me in many years of "physical education" classes in public school, in the 60s - 70s. We used to do the President's Physical Fitness Test every year, but there was never any training to develop the abilities being tested!