How to strengthen Ikea Billy bookshelves?

[JellyWednesday]

I have two 80cm wide Billy bookshelves & the shelves sag under the weight of my books. I only found out today, that the shelves are designed for a maximum weight of 30kg each. My books weigh 40kg per shelf. I suspect that the shelves would still bow with 30kgs on them as they're only made of fibreboard. I've experimented with adding a batten along the back just under the shelf for extra support, but I now suspect the shelf will still sag because of the material it's made of. Has anyone had any success truly beefing these up?

There are lots of ideas on the web; like this. I hope you can find something that works for you.

Unless you are deeply in love with those bookshelves I’d search on eBay or fb marketplace for solid wood ones-if you’re happy to paint unfashionable dark wood so much the better!
Otherwise you need a supporting piece in the middle of each shelf-ideally sane depth as shelf and matching thickness to the rest of the wood. Mdf would probably do but I’m not a builder!

I saw that metal bracket but I'm not convinced that it will stop a fibreboard shelf from sagging when it's totally full of large hard-back books. It doesn't address the intrinsic weakness of the shelf material. The open front edge could still sag potentially,

Chipboard basically creeps a lot under load, you would need some normal wood as a shelf as it's more springy and stronger. For remedial assuming the shelves haven't permanently deformed, depending on aesthetics you could glue a metal strip underneath or even use a router to make a groove and glue some threaded rod into the groove.

I don't buy Billy bookcases for this reason. I buy kallax as the cubes are much stronger. Books at the back of the cubes and photos and other stuff at the front or two rows of books and they don't sag at all.

Jali.co.uk do bookshelves made to order. If you speak with them on the phone, they will tell you the upright spacing needed to support all of your books .