How to enforce wooden shelf with steel strip
This can be done with hardwood strips, but it's definitely better to go with steel. But No, the strips should not be on the top or bottom, those won't do anything...the shelf is already the universal beam's flanges, you need to add the universal beam's vertical support.

The single piece full length strips of steel should only be applied (glued &amp; liberally screwed) to the front face edge &amp; the backside face edge, use as thick a strip as you can or double &amp; triple them up. You can also use Angle or L-shaped steel. Try to keep 50kg the maximum load, but you'll have to gauge if it can take more &amp; how close to center heavy items are, lighter items in the center is the best configuration.