how can compression and tension make things stronger?

Answer 1

I don't really know the term but translated is pre -tensioned girder, Which happens to be stronger that the usual reinforced concrete girders which we are talking about. In reinforced concrete, the basics are these :concrete takes the compression and steel takes the tension created by the building.
Sometimes when girders are needed to cover larger spans,
they take that steel used to reinforce the concrete , now a bit thicker and with a more rugged section to be able to have a good grip to the concrete, and they grab both ends with special machines which start slowly pulling . The steel is thus a bit alongated and logically verrrrry tensioned - poor creature. At this
point they poor the concrete without releasing the steel . That is done after the concrete gets rock solid. At this point those steel beams, being released ,they kick back and try to get their previous state ,and really manifest a great compression and tension over the concrete which as a hole girder becomes increasingly stronger .

Answer 2

There are two situations you might be talking about.

First, if you know how a particular piece of a structure is going to be loaded, you can initially pre-load it with an oposite force. So if a piece of steel is going to be stretched in tension, you would start by compressing it. Then when the load starts to stretch it, you're really just returning it to it's original state. The opposing loads work to (at least partially) cancel each other out.

Or you may be referring to another use of compression/tension strength. Many materials (concrete, brick, etc) which are very strong in compression are not strong in tension. So to use these materials, you need to take a load and convert it to compression and eliminate tension - this is what happens in a dome or arch. Alternatively, you can take a material that is strong in tension (like steel cable) and load it only in tension - this is what happens in a suspension bridge.

Answer 3

By compressing molecules, you make their bond tighter thus stronger, harder..you are removing any space in between them.