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QUESTION: Hello Sir I want to know that what is major and minor axis concept in frames.. And in 2 or 3 storey frames what value of Lb must be used for columns? (whole height or storey to storey height)

ANSWER: Dear Nouman

There is a direction in each geometrical shape in which the moment of inertia of X-X and Y-Y are minimum and maximum. Then this a properties of the sections not the frame. Usually the minimum axis is governing the stability of the column.

Lb must be defined story per story, normally or calculated by software (etabs or sap) for more accuracy.

Regards
BEH

---------- FOLLOW-UP ----------

QUESTION: Hello Sir. I want to ask that as we know the limiting values like bf/tf and h/tw so I want to ask that are there any limiting values of hw/bf (height of wen to thickness of flange)? If yes, then kindly tell me the values. Thanx.

Answer
Dear nouman

To control the twisting mode of instability of sections there is a limitation for ratio of flange width to web height. I will let you know a range of it on Saturday because I have no access to the codes right know.

Regards
BEH

according to aisc as to consider the twisting buckling of beams we have:

Mb=840000Cb/(L.d/ Af) <0.6Fy

Cb= 1.75 + 1.05 M1/M2 + 0.3 (M1/M2)^2

L= Unbraced length of beam
d= height of the beam ( appr.)
Af= flange area

M1 , M2 bending moments of 2 ends of the beam located between 2 braced point. M1 and M2 are opposite sign to each other if the make same curvature to the beam.

units in kg, cm

regards
beh

please dont hesitate to ask further in the case of any doubt.

Civil Engineering

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