Strengthened Frame is a technology that can be researched at the Hvy Weapons Factory in Tech Level 3. It costs 100 Food and 75 Carbon, and gives Hvy Weapons Factory units +10% Hit Points. It must be researched before Reinforced Frame becomes available.
The Hit Points increase is useful, and since the technology is so cheap, there is little reason not to research it as soon as it's available.
"This Tech Level 3 upgrade grants Heavy Weapons a bonus to hit points, rendering them more resistant to enemy attack."
—Manual description