column bracing 1

[calculor1]

I have a multi storey stair tower running parallel to the exterior column line. Due to the location of the tower I can only brace the exterior column with the intermediate stair landing. Has anybody encountered this situation, btw the building is steel construction.

 

[DaveAtkins]

I wouldn't count on stair landings as bracing for a column.

I would design the column as unbraced for its full height.

DaveAtkins

 

[frv]

DaveAtkins-

Why not? Your bracing only needs to carry something like 2% of the axial load. With a properly detailed brace and proper load path to the LFRS, this would greatly reduce your column size. We have a responsibility toward our clients to provide safe AND efficient designs.

 

[DaveAtkins]

Explain how a stair landing can take the 2% force to the LFRS.

DaveAtkins

 

[JAE]

I recently did a tall stair tower outside the main building and all three exterior sides of the tower were diagonal braced frames. Those braces didn't, of course, brace the frame from laterally deforming into a parallelagram (sp?).

I depended upon moent connections at all the horizontal beam-to-column connections.

 

[calculor1]

Maybe I was a little vague in my description of what is happening, the attached sketch may clarify my question. The problem I have is the column would be laterally unsupported for 6 stories (approximately 80 feet)and it existing fabricated from A7 steel. Half of the column is embedded in concrete which may provide some lateral restraint but I don't feel comfortable not bracing at each level. Any suggestions?

Thanks

 

[JAE]

calculor1,

The exterior beam from the left would help brace the column in weak axis bending - cantilevering off the diaphragm and framing just to the left of the stair shaft.

 

[frv]

DaveAtkins-

I did state "properly detailed" in mi response. Just follow your load back to the LFRS. If your stair landings are not connected in any way to the LFRS, then obviously you cannot. But if the landing is supported, say, at a shearwall, then it's fairly simple.

 

[calculor1]

JAE
Yes I was counting the bracing about the weak axis but how do you brace about the strong axis? If the intermediate landing was directly connected to the column using a beam and hung from the floor on the inside of the landing and design the hanger as a cantilever taking the brace force?

 

[kslee1000]

Not quite sure I have enough info to think about.

Is the left end landing an intermediate platform between 2 stories, or level with the floors? If it is the latter, is there an edge beam at S end of the opening to intercept the interupted floor beams? Is this a steel stairway?

 

[JAE]

No, calculor1, what I meant was that beam from the left cantilevers out in horizontal fashion (bending about its weak axis) to brace the column in its strong axis bending direction.

If you have a landing at the main floor level, with a stair beam from the column to the main floor on the inside, that would also brace the column.

But the edge beam also participates - might have to check against strength and stiffness of it as a brace, but it might work.

 

[calculor1]

Okay just to remove some confusion, JAE are you saying the beam from EF to FG could brace the column at FG about its strong axis (column) with beanding about the beam weak axis? I didn't really consider that a possiblity.

KSLEE In my situation the intermediate landing would be located near gridline FG with the landing beam supported at column FG on one side and using hangers from the floor edge beams above to support the rest of landing. Yes this building is steel construction.


Thanks for taking time to provide feedback

 

[hokie66]

I think if you make the hanger and the landing support beam into an L shaped piece, that should brace the column. If you are not comfortable with that, instead of just hanging the landing, use a post from floor to floor and brace off that. That way you would avoid the need for the moment connection at the knee.

 

[kslee1000]

From the drawing, it looks like the stair well is not fully occupied by the stairs, there appears a space along the north edge. If this is the case, would you consider to add a column inside and tied together with the main column to for a vertical truss. Then, at each level, add horizontal bracket on eack side of the additional column to stabilize it.

 

[calculor1]

Hokie thanks for the great ideas, I think I'm going to use a post from floor to floor and use that as my brace. When you look at these things too much you tend to miss the obvios solution. Once again thanks.

 

[hokie66]

I know what you mean. Forest for the trees. Same problem here. Anyway, you are welcome for the small contribution.