structuresguy
Structural
- Apr 10, 2003
- 505
I am designing a 70 foot tall glass atrium, which is basically the shape of a football in plan view, with straight vertical walls and flat roof. The "football" passes through an office building, with 5 stories on one side, and 2 stories on the other. The framing sizes are all preliminary right now, but I am having some problems with the columns. I am modeling it in Staad in full 3D. See attached file for plan view at one end of the atrium.
My columns are HSS12x12x1/2, spaced about 10 feet apart. The columns will be rigidly connected to the foundation and to HSS roof trusses, forming a portal frame, in effect, across the width of the atrium.
I have horizontal "ribs" every 14 feet, which are HSS20x12x3/8. The ribs are fully welded to the columns, to make a continuous rib. Due to the shape, the ribs form a peaked arch in plan view, with the base of the arches tying into the floor slabs at the first two levels on both sides, and on one side only for the full height of the atrium.
So my question is, what do you think the unbraced length of the columns is for the strong axis (out of plane of the wall) direction? Full height of the atrium (70 feet)? Or do you think the rigid "ribs" will contribute to bracing the columns against strong axis buckling? Weak axis is obviously braced by the ribs every 14 feet.
I know that the ribs will contribute to some extent, and that full height is probably too conservative, but I am not sure how to justify a rational approach to reducing the unbraced length.
If you imagine the arch that the ribs form, that is a very stable structure that could withstand an out of plane buckling load from the columns. So can I figure on using 2% of axial compression to calculate a horizontal OOP point load at each column-rib intersection, and then check the model with those additional loads? If the arch can handle that additional lateral load, I think I am good to consider the ribs as bracing the columns out of plane, due to the stiffness of the arch. What do you think?
I told the architect that HSS12x12 would work (based on some prelim hand calcs), and it does in most cases using 70 foot unbraced length. But where I have columns tied to vertical x-bracing at the ends of the atrium, the axial forces due to overturning are exceeding my column capacity (based on 70ft). So if I can justify a reduced unbraced length, then I can make the 12x12 columns work.
Thanks very much.
My columns are HSS12x12x1/2, spaced about 10 feet apart. The columns will be rigidly connected to the foundation and to HSS roof trusses, forming a portal frame, in effect, across the width of the atrium.
I have horizontal "ribs" every 14 feet, which are HSS20x12x3/8. The ribs are fully welded to the columns, to make a continuous rib. Due to the shape, the ribs form a peaked arch in plan view, with the base of the arches tying into the floor slabs at the first two levels on both sides, and on one side only for the full height of the atrium.
So my question is, what do you think the unbraced length of the columns is for the strong axis (out of plane of the wall) direction? Full height of the atrium (70 feet)? Or do you think the rigid "ribs" will contribute to bracing the columns against strong axis buckling? Weak axis is obviously braced by the ribs every 14 feet.
I know that the ribs will contribute to some extent, and that full height is probably too conservative, but I am not sure how to justify a rational approach to reducing the unbraced length.
If you imagine the arch that the ribs form, that is a very stable structure that could withstand an out of plane buckling load from the columns. So can I figure on using 2% of axial compression to calculate a horizontal OOP point load at each column-rib intersection, and then check the model with those additional loads? If the arch can handle that additional lateral load, I think I am good to consider the ribs as bracing the columns out of plane, due to the stiffness of the arch. What do you think?
I told the architect that HSS12x12 would work (based on some prelim hand calcs), and it does in most cases using 70 foot unbraced length. But where I have columns tied to vertical x-bracing at the ends of the atrium, the axial forces due to overturning are exceeding my column capacity (based on 70ft). So if I can justify a reduced unbraced length, then I can make the 12x12 columns work.
Thanks very much.
