There are two situations in which a talit is held aloft in a possibly tent-like manner that make me wonder whether they present any danger of transgressing the prohibition of "Building" on Shabbat and Yom Tov:
During Birkat Kohanim, many men cover their faces with their talit, and some gather their children under the same talit. When there are multiple people under the talit, it can begin to resemble a tent over them, supported by their heads at the vertices.
For the special aliyot at the end of the Torah reading service on Simchat Torah, people tend to hold a talit up in the air, parallel to the floor like a chuppa, over the head of the honoree.
Do either of these situations have the potential to create a transgression of "Building"? If so, are there steps that should be taken to avoid transgression?