steel in vinyl windows

what is purpose of adding steel to a vinyl window ?

It's to prevent sagging and provide reinforcement in the vinyl frame, especially in the sash frames at the meeting rail on double-hung windows.

It's used in a lot of vinyl railing, like for decks and porches, too.

It's to prevent deflection of the center rail of the window in the wind.

Both guys are right. Design Pressure increases and sagging correction if you have a really weak extrusion.

it is an easy way to increase strength in a cheap vinyl extrusion. it is also a way to increase DP ratings for a high quality window. Schuco comes to mind.

while it is used quite a bit for added strength to vinyl bar. its also used to secure hardware to the vinyl. a lock under pressure would eventually pull loose if it was only screwed into the vinyl...