Expert: Arshad Khan - 1/12/2010

Question
QUESTION: Dear Arshad,
While designing a building, we generally analyze a frame consisting of footing, columns and beams.Say we analyze frame along "x-Z" Plane. My question is, we don't anylize frame in orthogonal direction i.e, "y-z" plane. How does the structure guarantees the strength in Y-z plane?

ANSWER: Hi Ananta,

Normally if you design a frame as 2-D, then you have to assume that stability in the orthogonal direction (for lateral loading)is provided by either shear walls or X-bracing, etc. As far as gravity loading is concerned, 2-D analysis is fine as the loading acts vertically down. However, even with 2-D analysis, you should design the columns and bases for minimum code specified moments due to eccentricity in the orthogonal direction.

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

QUESTION: First of all, thank you for the answer.I have some more doubts.The nature of stress in orthogonal direction must have different value, as such, as you say, we have to follow code and consider eccentricity in that direction. In that case, is it true that the Reinforcement will asymmetrical?While designing shear wall or X bracing coming in orthogonal direction do we take into account the stress developed by columns in that direction? My other question is doesn't it give good design if we analyze frames in both direction?And my last question is how do we know which (out of two) frame analysis give better design solution?

Answer
Hi Ananta,

1. Yes, column rebar can be different in different planes/axes- it is normally equal in opposite faces, but even that can vary, e.g. if you have a cantilever column with uni-directional high moment.

2. If the columns are braced, then there is nominal moments to account for only, and the forces required for the bracing should be resisted by the walls or X-bracing.

3. 3-D design will probably result in most economy, but in simple low-rise buildings the savings may not be substancial.

4. This comes from experience and engineering judgement, as each building configuration is unique. Remember that most of the tall buildings in the world that were designed/built before 1980 were analysed using 2-D models, so this method is still good enough!