Concrete: Column Slenderness Ratio

[student71]

slenderness ratio

Slenderness ratio of concrete columns.
Author: student71, member of CivilEA

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[concreteok]

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@aslam, since we are at this, do u thk there should be an effective length factor maximum limit for unbraced frame?

sinha and kim elliot do have them in their book but those figures (limit) are not in the codes (at least bs8110:2 and aci 318).

wats your opinion on this?

[aslam]

@ concreteok,

First of all, we shall not mix braced/unbraced & sway/no sway frames.

May people considers column as braced if framing beam exists and unbraced if framing beam does not exists in particular direction. But this is very approximate assumption and most of codes do not use this terms(i.e. braced/unbraced).

Most codes uses sway/no sway terms for consideration of effective length factors (slenderness). There are also some approximate formulas to estimate moments due to slenderness.

Regarding your question on limit on max. effective length factor for unbraced (may be sway) columns. Code has limit upto infinity. But you can not go upto that, because it will result in very large cross section of member with very high amount of reinforcement. Thus, member size and reinforcement will indirectly control max. effective length factor.

[student71]

I will say only that when i get slenderness ratio more then 4, i make changes to structure, larger cross section of column or make shear wall that will make my structure no sway. For minimum slenderness ratio i use 0,84*l, if anyone have DIN regulations they don't recommend using less then 0,84*l.
And aslam is wright people should not mix braced and sway. Braced is every structure that have system for lateral forces (shear walls, concrete core, etc), and every frame is sway frame, but in some cases there is no need to calculate like sway, because movements of joint are small, and results won't difference much from no sway.

Regards!