Do new Tzizis require Shehecheyanu? If I put on a new pair of Tzizit during the day does it require another Bracha?

Yes, on a new Tzitzit one should say Shehecheyanu. If when you put on your Tzitzit in the morning you had Kavana that the Bracha on it would not include the new one you wear later, you should make Al Mitzvat Tzitzit again. Rabbi Yosef Weitzen