RC column question

[Enhineyero]

Hi all,I would like to ask if, For RC columns the category of Brace/Non-Brace and Sway/Non-Sway are two different things, because that is what my current understanding is.

for example A Non-Brace frame column can be A Non-Sway column, or A Brace frame column can be a sway column?

So a column can be (Non-Brace and Sway, Non-Brace and Non-sway, Brace and Sway, Brace and Non-sway). From my understanding the bracing depends on the lateral system of the structure (if the lateral load is resisted by a system other than the Beam and column framing, for example shear walls, then the system is braced). While Sway and Non-sway category depends on the drift of the structure (this will determine if Pdelta effects will be affecting the column design).

I just want to confirm this because I had a discussion with my colleague regarding this. He is insisting that Brace and Sway are the same..

 

[hokie66]

Braced frames and sway frames are the opposite of each other.

 

[DST148]

Reference ACI 318-02 Section 10.11.3 Unsupported length of compression members:

This section permits to assume a story within a structure as non-sway if the stability index Q is less than or equal to 0.05. Sway / non-sway term is used in the context of a story and not a column. The design parameters like k, slenderness effects and the relevant sections of the code will depend whether a column is part of a sway frame or a non-sway frame. Now a column could be a part of a non-sway frame and yet slender and vice versa.

 

[frv]

I think what your colleague may be referring to is that Braced & Non-Sway are the sane, and Unbraced & Sway are the same thing (in a sense).

I don't do a ton of RC, but I've never really known there to be a "braced" frame in concrete in the same sense that there is in steel (i.e., physical members connecting the diaphragms directly to the foundation via axial members). Rather, I think it would be more appropriate in RC to use sway vs. non-sway. Long story short Sway=moment frames, non sway=shearwalls.

 

[Enhineyero]

thanks for those who replied.

I think the brace and unbraced criteria is for the "k" factor of a column. While the sway and non-sway criteria is for the moment magnification effect or secondary moment effect on the column. Can anyone react to this?

So as i said a column can be Sway and Braced, Non-Sway and Braced, Sway and unbraced, Non-Sway and un-braced.

 

[DST148]

Enhineyero -
ACI uses nonsway (braced frame) and sway (unbraced frame) terminology. AISC uses sidesway inhibited(braced frame) and sidesway uninhibited(moment frame terminology).
As I mentioned in my previous post, braced / nonsway and unbraced / sway is used in the context of a story. There could be a scenario where one story in a structure is classified as braced based on stability index for that particular story and the story above could be unbraced. It is not necessary that the entire structure be branded either a braced or an unbraced frame.
To answer your last post - A column could be part of a nonsway / braced story and either a short (moment magnification not required) or a slender; or it could be part of a sway / unbraced story and either a short or slender for design purposes.

 

[RFreund]

DST148 Has is it right I think the confusion lies in that there are sway=non-braced and non-sway=braced systems. However columns can be short or slender in either system. Also the ACI vs AISC terminology (assuming your are using US codes)

ACI defines columns as braced=non-sway if the lateral resisting elements (i.e. shearwalls) are 12 times stiffer than the sum of the columns in that story.

EIT

 

[Enhineyero]

If the structure that you are designing does not consider lateral loads (dead and live load only) is it ok to assume that your frame is braced?

In my opinion it is braced, because there is no lateral loads that would cause it to sway to begin with. Pls comment on this

 

[DST148]

Enhineyero - A structure can undergo horizontal displacement even when subjected to gravity loading only due to unsymmetrical geometry, unsymmetrical loading etc.
Some of the codes require that structures be designed for a horizontal load equal to 0.2% to 0.5% of the factored gravity loads when designing for gravity load combinations.
To answer your last post: It is difficult to answer in an objective way, either yes or no. Most structures would qualify as braced (non-sway) under gravity load combinations but there could be exceptions.

 

[SAIL3]

It is my understanding that it has more to do with the stability of the member in question and not the loading....just because there may be no loads present(according to your calcs) to cause sidesway that does not affect the potential stability/instability of the col....

 

[Enhineyero]

DST - i think what you are saying is for notional loads on the design load combination. for a low rise structure this load combination seldom governs. I think I could run my model with notional load using an unbraced columns with rigid beam-column connection. Then run another model with just gravity loads and braced columns with pinned beam-column connection.

SAIL - i know the structure will be theoretically unstable. but it is the industry practice here. (although I also dont agree on this aproach on design)

To give you a background. I used to design on a region with Large EQ and Wind loads so all my columns are usually unbraced and all my beam-col connections are rigid (SMRF).

Now I transferred to a region without seismic load provisions and minimal loading for wind loads (only for high-rise structure). Now the industry practice here is quite different because all their perimeter beam-column connection are pinned, notional load is neglected for low-rise buildings,wind loads are only applied to high-rise structure (except for special-cases like canopy, barriers, etc). They consider columns as braced, and the design of columns is purely axial loading. I dont completely agree on the analysis approach because making a pinned beam-col connection can produce instability in the structure especially if there are only two columns.(think of a simple beam-col frame with pinned supports). another thing soil is weak is here so we usually use pinned supports at footings.

Although I don't completely agree but I have to follow their "industry standards"

 

[BAretired]

A simple beam-col frame with pinned supports top and bottom is unstable. If that is their "industry standards" they should change them.

BA

 

[hetgen]

All structures sway … how much could be safely ignored is the question. Sway, unbraced , braced… etc…etc… are terminologies used to define this.