Columns come in many different shapes, sizes and orientations. There is a specific kind of column which holds a special place in the hearts of Architects and Structural Engineers alike, the “Walking Column”. So what is a walking column? This article lifts the curtain on what a walking column is, how to approach the analysis of a walking column and why Architects, Structural Engineers and Builders like them…
A walking column is a type of column used to facilitate a change in plan location of a column from one floor to the next in a multi-storey building. Walking columns can be used instead of transfer beams, large depth transfer walls or raking columns for instances where a transfer column is required. A transfer column being a column whose centroid is not consistent from foundation level to roof level.
To begin our journey of discovery on what a walking column is, we need to first understand the principle of transfer structures and transfer columns (however for those less patient and want to cut straight to the chase, use THIS jump link to head straight to the explanation on walking columns) .
What is a Transfer Column and Transfer Structure
In a perfect world, all structural columns in a building would line up perfectly from foundation level all the way to roof…
Unfortunately the perfect world does not exist. The structure of a building needs to satisfy two primary roles:
- Be adequately strong and serviceable to ensure life safety of its occupants under the given loading applied to it (dead, live, wind, earthquake and more!).
- Not adversely impact the actual usage of the building itself. i.e. the structure shouldn’t get in the way of the look, feel and purpose of the building.
The first point above is easily quantifiable. Structural Engineers have codes and design standards to adhere to which ensures life safety of the buildings they design. The second point is a little bit more intangible and often open to interpretation. The intent for the structure to not get in the way of a buildings purpose is the key trigger for the requirement for a transfer column.
A building may have multiple different uses from floor to floor. Different uses have different space and layout requirements. If a building has multiple uses over different levels, this can often generate the requirement for a transfer column to be introduced. Some example uses within a single building may include:
- Car parking in a basement level
- Entry and reception lobby at ground floor
- Office floor
What may be an economical location for a column in an apartment tower, may not work so great for its ground floor lobby. The ground floor lobby column grid may equally not work well with its basement car park layout. Enter the transfer column…
The transfer column requires a transfer structure to “transfer” the load laterally from a discontinuous column to one or more columns at the location of transfer. This is best visualised with an image. Here is a transfer column in its most common and simple from. The base of the transfer column terminates at the transfer level, the load is carried laterally to two adjacent columns through the assistance of a “transfer beam”…
A transfer structure which carries a transfer columns load laterally through bending and shear can take the form of:
- A simple spanning beam (for further reading on transfer beams, take a look at THIS article)
- A full depth wall taking up a full storey height
- A wall whose depth spans over multiple floors.
But what if the transfer column and the transfer structure were one and the same? This can unlock a more efficient outcome to solve the transfer dilemma. This type of transfer arrangement can take the form of either:
- A Raking Column
- A Walking Column
Lets first take a look at the raking column…
Philosophy of the Raking Column
Load within a structure is like electricity. It finds the most direct path from point of application straight down to footing level.
The most direct load path is therefore through compression within a column. Load paths which are required to be transmitted through members undergoing bending and shear are less direct and efficient, although in a large amount of cases unavoidable. With this in mind, lets take our original transfer column and turn it into a “Raking Column”…
As you can see, the raking column provides a much more direct and therefore efficient load path for the column transfer. Rather than an abrupt 90 degree transfer of loading provided by one single transfer beam, the load is progressively carried across over two levels, a more gently transition.
Raking columns may also be called angled columns, slanted columns or strut columns.
This form of column is not always desirable however. This leads us finally to the Walking Column…
What is a Walking Column
A raking column raises a few of issues depending on the project in question:
- The airspace above and below the raking column is not entirely “usable” form an architectural perspective
- Constructing a slanting column requires more complex formwork and construction compared to a regular column
- From a building user perspective, seeing a single slanted column on a floor plate without the context of knowing why its there, especially when all other columns are conventional, can look quite odd and disconcerting
The walking column largely solves most of the dot points above. The difference between a walking column and a raking column being that a walking column is a series of conventionally constructed columns which shift incrementally from floor to floor, rather than an angled or raking column arrangement.
Here is our example transfer column again, this time with the walking column overlaid above the raking column…
The load path assumptions between a raking column and a walking column are very much the same. In simple terms, the “raking column” is hidden or embedded in a larger concrete mass which is the walking column. There is however some addition concrete mass for the walking column which needs to be considered in your load assessments.
This brings us to unique design considerations which need to be taken into account when adopting the walking column…
Design Considerations for a Walking Column
There is no such thing as a free lunch. And Structural Engineering is no exception. Even though the walking column arrangement is a relatively direct load path, there is still an eccentricity of the global system which needs to be considered and resolved.
To ensure equilibrium at the nodes where the load path transitions from vertical to slanted and from slanted to vertical again a horizontal net force is generated.
This horizontal net force (a thrust or a tension force depending on where your lateral supporting element is) needs to be be accounted for.
The theory of node equilibrium is the same used for simple truss analysis and vector theory. For a tutorial on this, take a look at THIS article.
Here is our example walking column arrangement now with the resultant “push-pull” loads indicated to show this arrangement…
This horizontal thrust load needs to be accounted for within the effected floor plate. The floor plate is usually required to act as a diaphragm and transmit this lateral load to the buildings inherent stability system. (for more on what stability systems are in buildings, have a read of THIS article). I often see Structural Engineers pray this load away without proper consideration to the load path and the overall system (scary!!).
Why do Structural Engineers Like a Walking Column
Structural Engineers like walking columns because it provides a much more direct load path compared to transfer beams or walls.
Transfer beams can provide a more “springy” support to the column which can result in increased deflections to the subsequent floors above. The walking column arrangement does not have the same issue.
Considering the main responsibility of a Structural Engineer is to ensure life safety to the building occupants, we generally lean towards more direct and simple load paths to better ensure this outcome.
There is a limit to everything however and too much of anything can become problematic. Take this example below of a very extreme case of a series of walking columns facilitating a very elaborate an complex building form. I’m sure this design would have been the cause for many sleepless nights for the Structural Engineering design team involved…
Why do Builders Like a Walking Column
Builders like walking columns due to the simplified formwork and construction detailing.
It is superior to a raking column as a raking column often requires a loss-formwork steel tube to allow support pouring of the wet-concrete concrete.
It can also be superior compared to a transfer beam as transfer beams can require significant back-propping to support the wet-weight of concrete during construction. They can also require significant reinforcement which can result in congestion and add complexities to both the installation of the reinforcing and the placement of the concrete.