Canoes and other water sporting equipment in a storage area in front of the building caught fire and by 9am was sending "thick black smoke" along the road.
Among other things, the students claimed the union was failing to represent them on issues such as government funding for higher education, and rises to tuition fees.
This includes the Welcome Desk, Academic Advice Service, elected SU Officers and other staff right to the heart of campus.
The union is now a multi-site organisation, with spaces in Senate House and the Richmond Building, Queens Road, Clifton.
[10] Bristol SU is run by a team of seven officers who are elected by the student body annually in March.
Bristol SU currently offers over 400 societies, sports clubs and networks covering a wide range of activities.