I'm looking at attaching shelves to a brick wall using single-track metal strips with adjustable brackets. The metal strips will be 78 inches long and I'm planning on using pine shelves 47 x 12 x 3/4 inch. There will be 5 shelves in total. How much weight do you reckon I could put on each shelf? And what length screw would be the minimum I would need?
There will be no practical limit. Shop for screws first: each will have a rating for load. The more the better obviously, the thicker the better especially in softer brick. Most of your load will be shear, some will be pullout load. You'll want to figure out how deep the brick is: there is little cost to going most of the way through each brick.
If your brick is solid and does not seem to be crumbly (like in +100 year old house) then I would always suggest and even my self use and Anchor bolt - In some countries called a robolt - These are on of the best load and pull rated bolts. And I always try to use these bolts because you can never be sure about who will put what on a shelf. Using these bolts if installed correctly(not difficult) are rock solid.
All you have to do is:
And because this is for your home and not a building site you would finish it off with a bolt "nut" cover. This will give you some extra points with the wife for paying attention to the finer details. You can get various shapes, colours and finishes. Or just paint over them.
Using anchor bolts makes you only have to worry about the bearing load of the brackets and shelves you use. But it is obvious the shelf max load should not exceed the anchors load capacity to be safe.
I have seen shelves that collapsed because the bracket could not hold the weight but these bolts stayed put!
Disclaimer: Just a bit of humour but entirely factual.