Wind Load for Tightly Spaced 3 Storey Houses 1

In jurisdictions that use ASCE 7, there is no additional reduction. You can't guarantee the houses next door will always be there or that they won't get knocked down in a storm. So you get reduced wind loads in urban/suburban areas, but no direct considerations for shielding effects. Our code explicitly prohibits it.

Yeah shielding is explicitly forbidden. I think in reality if you didn't have a lateral system along the lots, you'd have accidental torsion when the wind load comes at the face and the twisting would create issues or at least cracking, so it has some justifiable benefit particularly since the lateral analysis of residential structures isn't all that rigorous.

I agree that we never design for shielding. It's not allowed and also it's not a permanent thing in case a building gets taken down. But if your code allows it, I don't see why you can't use it. Hurricane events tend to damage houses, but if they're properly designed for wind at all, they'll survive. It's not as much of a life safety issue as one would think. Most of the time, it's just roofing getting pulled out. In the case of racking and uplift, that's when you don't have any properly designed shear walls or uplift connectors at all. A reduced wind load isn't going to make that much difference, especially when you have uncalculated redundancy. For example, we assume only the shear walls work for lateral, but non-structural partitions have a bit of stiffness. We also assume that uplift is taken solely by uplift connectors, but you also have nailing between members and plywood sheathing.

I'd just use what the code allows and not go beyond that, because there is a very good argument against not using any reduction at all.

I don't know anything about Canadian codes, but is the reduction you're talking about in Urban areas similar to what we have in the US as exposure categories?