In Boxed Column, there is hollow portion inside timber planks but in built-up column the hollow portion is either entirely empty or filled. All boxed column are built-up columns but all built-up columns are not boxed columns.
IS 800:2007 is silent about the design of angle section purlin.Note: In angle section purlin, both wind load and gravity load (dead + live) are assumed to be normal to the roof truss. So, calculated load is taken as factored load. (not multiplied by Gamma_f)
Peak gust velocity averaged over a short time interval of about 3 seconds and corresponds to mean heights above ground level in an open terrain (Category 2). Basic wind speeds have been worked out for a 50 year return period.