Is there any way to to give a character the ability to do double damage to structures, like a seige weapon?

Adamantine weapons causes all hits against objects to be critical hits and if you can turn yourself into an earth elemental they do double damage to objects and structures.

Something like a portable ram?

Something like that. I was build a character for fun. He is an orc Fighter that is a rune knight and a Berserker Barbarian. So orc count as a size larger for their carring capacity as well as lifting, dragging and pushing. Using rune knight, they go to large size mean they should carry, lift, drag and push as much as a Huge creature would. He is also suppose to be the brawns of an evil group I'm working on and his made goal is to take out fortress walls.

A Sword of Sharpness maximizes all damage dice against objects and if you somehow can coat it with adamantine, you'll also auto crit. The ultimate would be the Axe of the Dwarvish Lords, of course, but good luck getting your hands on one of those. Coat that with adamantine and you'll be doingaranteed 32+Str damage every hit against an object at 20ft.

Oh I found something else. If you happen to get your hands one the Teeth of Dahlver-Nar and roll an 18, you can do double damage against objects. You'll have to implant a shard of a tarrasque tooth into your jaw, but you might like that.

In an unofficial game with paper books you can use the juggernaut barbarian from the Tal'Dorei Campaign Guide.

Proficiency with mason's tools. In Xanathar's guide in the section on tools it says "Demolition. Your knowledge of masonry allows you to spot weak points in brick walls. You deal double damage to such structures with your weapon attacks."

Quote from HailRobonia >>

Proficiency with mason's tools. In Xanathar's guide in the section on tools it says "Demolition. Your knowledge of masonry allows you to spot weak points in brick walls. You deal double damage to such structures with your weapon attacks."

Nice catch! I love increased utility for tools.