They're already built for it. The '73 Pontiac Bonneville I rode around in as a kid weighed 4300 lbs. And as sibling comment mentions, anything not a car or light truck will outweigh anything with batteries. Our Sprinter-based RV weighs 10K lbs. empty, and it is a small RV.

They're already "designed" for 18-wheel trucks which weigh 15-20 times that. Even heavy EVs are nothing compared to heavy transport.

To use your own example, they're designed to handle a certain quantity of 18 wheelers but they're not designed to handle traffic that is entirely composed of 18 wheelers.

Raising the average vehicle weight from 1.5 to 3 tons will definitely have an effect.

But not as much as it would seem, since road damage is a quadratic function of weight and one truck does as much damage as 9600 1 ton cars. Doubling the weight of the car would maybe narrow that by a few percentage points, but vehicle miles for any personal vehicle at essentially any weight is a rounding error compared to one truck.

Bridges at least need to be designed for just 18 wheelers because over 50 years that’s going to happen at least once.

Roads are designed for a certain amount of "axle weight" [1] which basically means to carry a heavier load you need to add more axles to distribute the load.

I would guess electric cars are still negligible in that metric, though.

[1] https://en.wikipedia.org/wiki/Gross_axle_weight_rating