I think you are highly likely to be in breach of tay bridge byelaws and liable to a fine. This is a civil offence, but if the rossers attended and wanted your name etc so they know where to send the fine to and you refused that would be grounds for arrest and fine in itself.
You could potentially be arrested for creating a danger/breach of the peace for creating a danger (to yourself)- but more likely to be cautioned, as long as you dont get into being chased by the rossers if they tell you to ‘gerrof the brigde’.
I doubt it could be trespass because it is a public bridge and subject to public right of way