So there you are, designing away, when you come across a strange bird: a very long column. Or is it a very short wall? Where exactly is the line between the two? The implications can be significant:
· Columns require as much as eight times more vertical steel as a wall (1% versus 0.12%).
· Columns require closed ties; walls often have a single curtain of horizontal steel.
· The minimum dimension of a column for a given fire rating can be significantly more than the minimum thickness for a wall (e.g. 12″ versus 6.25″ on a project of mine).
An example of bearing walls I designed in a high-rise building
The ACI 318 code is silent on the subject, although it does clarify when a pedestal becomes a column. The Concrete Q&A in the October 2003 issue of Concrete International addressed this question directly. In short, the author confirms that the behavior and design of the section does not depend on the terminology: the equations for axial and flexural design are identical, including the slenderness checks. Ties are required in walls if the reinforcement ratio exceeds 1% (i.e., if it looks like a column) or if the steel can yield in compression (Section 14.3.6 in ACI 318-11 and older codes). In summary: it doesn’t matter much for structural design.
What about fire ratings? Neither IBC nor ACI have apparently addressed the fire rating issue, as demonstrated by this thread at Eng-Tips.com. In these cases, my advice is to use engineering judgment and the “believability” test: if a disinterested observer won’t believe that it’s a wall (say, because it’s square in plan), it shouldn’t be a called a wall. My personal threshold is an aspect ratio of around 4:1.
Agree? Disagree? Please feel free to add your own thoughts via the comments below.
Many thanks to my new friend Triadi, who prompted this post with an email.