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# Civil Engineering/Biaxial moment coulmn design per BS8110

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Question
Dear Sir,

When I design a column subjected to biaxial moments, Bs8110 proposed a simplified methods as below:

"When it is necessary to consider biaxial bending and in the absence of more rigorous calculations in
accordance with 3.4.4.1, symmetrically-reinforced rectangular sections may be designed to withstand an
increased moment about one axis given by the following equations:

if Mz/h' > My/b' then M'z = Mz + b(h'/b')My
if Mz/h' < My/b' then M'y = My + b(b'/h')Mz

....."

The area of rebar As then is calculated as single axis bending with an increased moment M'z or M'y.

Could you please explain my question?

Rebar shall be arranged as single bending which concentrate on top (As/2) and bottom (As/2) of axis bending or it will be arranged regularly around 4 sides (As) with equal spacing or we can accept every arrangement

Thank you very much

Answer
Dear Alex

I suggest you to calculate the Rebar area for both sides and then arrange the bars so that you have the needed area in each direction without adding them up. Each direction must have its own area arranged as you said : As/2 on the top and As/2 on the bottom.

Regards
BEH
Questioner's Rating
 Rating(1-10) Knowledgeability = 10 Clarity of Response = 10 Politeness = 10 Comment Thank you sir, The design may be conservative. Anyway, I would like to understand the philosophy design of BS 8110 because it is not very clear about the arrangement after increasing the moment in the critical direction. I found that if we arrange in the manner symmetrically around the perimeter of the column, the capacity shall be stronger than if the rebar are disposed in two layer top and bottom of critical direction. Thank you very much for you answer

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