Column Bracing 1

[slickdeals]

Folks,
I have a very ridiculous question. Will a column bracing ever lead to additional moment at that location?

The column buckling is prevented at the bracing location, but does this bracing lead to a moment at that location?

 

[slickdeals]

Oh, I must mention that this is a initially curved column and not a straight column.

 

[civilperson]

No additional moment.

 

[slickdeals]

@CivilPerson,
I tried to model a initially curved column in ETABS and specified a beam framing into the curved column and applied an axial load to it.

There was moment being developed at the bracing node. I would appreciate it if you could provide me with more insight as to why a moment won't be developed?

 

[civilperson]

No ADDITIONAL moment, did not say no moment at all.

 

[slickdeals]

I apologize, my OP should have read "will it induce any moment". Thanks.

 

[Mrgum42]

@slickdeals:
- Pls think logical way: if is there any restrain of rotation, it will induce the moment.

 

[abusementpark]

I guess it depends if the column is theoretically a pure axial column. If the column has transverse loading, then bracing will change the moment diagram, but shouldn't ultimately increase the moment in the section.

 

[Lion06]

Let's think about this. If the column is curved there will be moment if there is no brace because of the initial out-of-straightness effects. That's a no brainer. I would argue that the brace REDUCES the moment in the curved column because it prevents/reduces (depending on the location of the brace) the horizontal displacement of the curved column. This horizontal displacement would happen because of the moment induced by the out-of-straightness. That additional moment would not be present if the column is braced.

 

[Lion06]

I will go a step further to say that if the curved column is braced at its mid-height (assuming that is the location of maximum out-of-straightness), that it will significantly reduce any first-order moments in that column compared to the unbraced condition. This is assuming a lateral brace only with no moment restraint or restraint in the direction of loading.

 

[msquared48]

I would have to diaagree here about the statement of no additional moment.

In the case of a knee-braced column, the column, which is braced in the strong axis at the knee-brace, definitely sees more moment at that location with the presence of the knee brace. However, with the lower unbraced length, it can take more vertical load as well as moment. This arrangement is commonly used in pole type structures to control the size of the columns - the primary lateral resisting members.

Mike McCann
MMC Engineering

 

[asixth]

Slickdeals,

Can you please expand further on your situation. I will assume that you are referring to steel construction because I don't think any contractor will form a curved r.c. column.

Also, why is the column curved? Is it an architectural camber or is it a diagrid structure. A curved braced column will induce P-small delta effects but not P-large delta.

My thought is that bracing will reduce the moment thrown into the column because there is less reliance on the structure to go into frame action.

The brace itself will not induce additional moment into the column if there is no rotational fixity and the column is concentrically braced.

Finally, what computer package are you using to analyse the structure? Is it FE? Does the FE package have the capabilities to model a curved 3D beam elements and if so do you have any meaningful technical info on curved 3D beam elements?

 

[Lion06]

Mike-
I was getting at a gravity only column with a discrete brace at mid-height, not a knee brace frame. I also assumed the only moment to be from the axial load and the curvature of the member. Under those conditions, the brace will significantly reduce the moment in the column, right?

 

[msquared48]

Right.

Mike McCann
MMC Engineering

 

[ChipB]

This isn't really a straight forward question. I think StructuralEIT's initial post is on the right path.

If the column is bent initially, and you install the brace while the column is in an unloaded condition, when you load the column, it is going to try to deflect further. The brace is going to resist this deflection by going into either tension or compression. It will help the column by all means, but, the column now has a horizontal force being applied at this brace location, which is either taken out through the lateral and vertical bracing system, or as shear in the column, shared with the other column the brace is attached to. So, if there is no vertical bracing present, while it is not inducing moment at that location, it is inducing shear which will result in moment. Either way, this horizontal force has to get down to the foundation, whether through vertical bracing or shear.

The point is, you have to look at your entire load path. As such, the way I read msquared48's post, it could be misinterpretted. In a pole type structure, adding a knee-brace UP (like a T-pole supporting a utility bridge), will not reduce your kl/r. Adding one down to the foundation, does reduce your kl/r.

 

[slickdeals]

Ok, What I have is a curved truss whose bottom chord is in compression.

Hence, it is an initially curved column which is deflected at unloaded condition and braced laterally by beams. In essence, a curved column braced by beams.

 

[asixth]

Architects like curved trusses.

I think with any truss the primary resistance will be by axial stiffness and not much bending. So I would design the bottom chord for compression including the second order effects the StructuralEIT has indicated. Ensure that there is additional capacity for combined actions.

AS4100 which is the Australian steel code requires the designer to design for bending, compression and combined actions and does not require the engineer to determine whether the element should be designed as a beam or column as required by the concrete code.

 

[msquared48]

ChipB:

Adding a brace up does decrease the kl/r as an inflecton point in the column is created, also helping to decrfease the lateral deflection.

Mike McCann
MMC Engineering

 

[Ahmadradzi]

there is minimal moment at this bracing areas and can be consider as non moment area.