I want to build a fence 8 ft tall and I am requested by the city engineered wind load calculations for 90 miles per hour, sustained for 3 seconds.
How do I calculate this?

Thanks for any help

See if this one helps:
https://www.askmehelpdesk.com/engineering/wind-loads-27538.html#post142760

... I'd call one of the inspectors and nicely ask if he can give you more specific info on the matter too. In my area the inspectors are quite friendly about this sort of thing.