|California Header. Held in place by nails.|
Walls can be classified as either partitions, which means they only serve to separate rooms and therefor only need to support themselves, and supporting, which means they are holding up the stuff above them, like the upper stories, or the roof. Look at a simple square room on the ground floor of a two story house. The floor above will be supported by joists that will run clear across the room. The walls that the ends of the joists sit on are going to be supporting walls. The other two walls are basically partitions. They may have other supporting duties like handling sideways forces, but they are not supporting the floor above.
In a proper wall, nails are just there to hold things in place. They should never be saddled with support duties. Here's a properly framed stud wall.
|A typical wall section in platform framing 1. Cripple 2. Window header 3. Top plate / upper wall plate 4. Window sill 5. Stud 6. Sill plate / sole plate / bottom plate - Wikipedia|
If this was a supporting wall, then the header would need to substantially thicker. Depending on the span (the width of the door or window) you can sometimes get away with a 2 x 8, but we usually used 2 x 12's. Two by twelves can be good for several feet, but it depends on how much stuff is above them. Walls that are supporting three more floors, like some apartment buildings, might not be allowed to have any openings at all.
A friend of mine was building condos in Houston about 40 years ago. They had framed a three story building that was going to contain half a dozen townhouses all packed together, so there were big solid walls separating the individual condos, but the cross walls had lots of opening for doors and windows. And somebody wasn't paying attention. They got the building up and it started to lean sideways. Either the engineers had failed to specify enough diagonal support, or the carpenters had left it out, figuring they could go back in later and put it in. I suspect it was the later. I worked with a crew of yahoos for a while. They would just throw shit together willy-nilly and then spend days afterward going back and fixing the things they had skipped in their hurry to get the roof on.
The tilted condos? Pickup trucks, winches and come-a-longs weren't enough to straighten it up. They might have had to get professional help.