What constitutes braced against sway 3

[Engrman]

As a young engineer I have a lot of questions and I'm surprised how the experienced PE's in my company sometimes dance around the issue. I understand that steel columns in a moment frame are not braced against sway and that columns in a braced frame ('X' or 'K') are. But what about columns not part of a lateral system? For example, a one story 115'-0" x 175'-0" building, steel bar joist and metal deck roof, steel beams and columns, and masonry perimeter walls. The flexible diaphragm will deflect so are the columns braced against sway? Is there PDelta effects to consider? Or are the columns designed just for axial load?

 

[JAE]

The flexible diaphragm will deflect so are the columns braced against sway?

Yes, the flexible diaphragm will deflect, especially at the "midspan" of the diaphragm. But always, the key question to ask in determining whether a steel column is "braced" is whether that particular steel frame depends upon the column's bending to remain stable. In your case the answer is NO. The interior columns do not add to or participate in the lateral stability of the structure.

Is there PDelta effects to consider?

Yes, there are PDelta effects to consider (AISC Chapter C, C1.1 and C1.2). Your interior columns will move laterally, and their vertical loads will impart a lateral force on the frame system, adding to your lateral loads.

Or are the columns designed just for axial load? The interior columns are designed just for axial load. Your remaining lateral force resisting system must be designed for the lateral loads and the added PDelta effects from the interior columns.

One way to imagine this is to visuallize your vast array of interior columns as lazy drunks, leaning against your building. Their independant stability is based only on themselves. But they sure are a load on the rest.

 

[austim]

JAE,

That description of the interior columns acting as 'lazy drunks' is about the most graphic description of structural behaviour that I have ever read (and I have read a fair amount over the years). Quite Brilliant.

 

[Engrman]

Thanks JAE. Nobody on our staff would explain it to me. Just said do it this way and don't worry about it. And they never even mentioned PDelta effects adding to the lateral load. Can I come work for you? Pretty Please?

 

[LPPE]

JAE - What a description! I needed a good laugh first thing Monday morning.

 

[adlington]

Columns within a structural frame, be they interior or perimeter columns, can provide sway resistance. For example if you were to design semi regid connections each subframe of columns and beams would effectively start to act in a 'portalised' manner and stiffen the building. It is wrong to assume that all internal columns can't resist sway forces, it depends on how you have modelled your building, i.e. simple construction with pinned beams and columns with bracing, cores or moment frames resisting horizontal forces (including sways effects from out of verticality of columns). You may find by creating a series of internal moment resisting frames stability issues can be resolved (this will depend on your geometric layout of the building)