It strikes fear into the hearts of Structural Engineers. It goes against all logical load path theory. It is a counter intuitive way to transmit vertical load from your building to your foundations. But in some cases it is a necessity. I am of course talking about the transfer beam. But what is a transfer beam and what considerations do Structural Engineers and contractors need to take in the design and construction of transfer beams? Lets take a look…
A transfer beam in Structural Engineering, is a horizontally spanning beam which supports a load bearing wall or column above it. The beams structural purpose is to transmit the vertical force from the supported column laterally through bending and shear to two or more columns beneath.
Load Paths in Relation to Transfer Beams
Load transmission in a building is very similar to electricity. In an electrical circuit the current wants to take the path of least resistance straight to ground or earth. When lightning strikes an object, the current quickly finds the most efficient path to reach terra firma.
This is very similar to how loads within a building flow from a slab, to beams, to columns then finally foundations. If we consider an idealised square slab bay arrangement and apply a point load directly in the middle, the load share by the four support columns will be equal as indicated in this isometric view…
Once the load has “entered” the column it will tend to transmit directly to the foundations as it travels down past subsequent lower storeys. This is because the load support provided through compression within the columns is much stiffer and direct than the load support of an adjacent slab or beam in bending. This is why it is very rare for loading to exit a column and enter a floor system once its found its way into the column in the first place… unless however the column does not continue all the way down to foundation level, which is where the introduction of a transfer beam comes into play.
Why do we Need a Transfer Beam in Buildings
Transfer beams are sometimes necessary to allow the function of a building to work for its intended use.
The very point of designing and constructing a new building is to provide a desirable space for a specific use. 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 beam to be introduced. Some example uses within a building may include:
- Car parking in a basement level
- Entry and reception lobby at ground floor
- Office floor
The main goal of any good Structural Engineer is to work with the architect and the wider design team to conceive the most efficient structural design while at the same time facilitate and not impede the usage of its spaces.
For example, a given column location in a high-rise apartment building may fit perfectly within the architectural layout within an apartment dividing wall for the majority of the building height. However if that same column location lies directly above a car park traffic aisle then a transfer may need to be introduced which straddles the car park aisle. Usually the best outcome however is to find a common column location which fits ideally in the same location for the buildings full height, without the need for a transfer. This however is not always feasible or practical.
What is a Transfer Column
Simply, a transfer column is a column which does not reach foundation level but rather terminates one or more storeys above. because of this, whatever load the column supports at its base needs to be “transferred” (laterally) to other adjacent columns. This is achieved via the transfer beam.
Design Considerations for a Transfer Beam vs a Regular Beam
A transfer beam and a conventional beam behave in a very similar manner structurally… they transmit the load that they support in a lateral direction via bending and shear to vertical support elements below it.
However there are a couple of special design considerations that need to be addressed for transfer beams specifically.
Transfer beams are often required to support higher loading than conventional beams. For this reason the effective structural depth is an order of magnitude larger for transfer beams. This can result in a span to depth ratio causing the transfer beam to act under a strut-and-tie model rather than conventional bending (often called Euler-Bernoulli bending).
In the Australian concrete design code (AS3600), the strut-and-tie model should be used for beams with a span to depth ratio of 3 for simply supported beams. A strut and tie model is a load path assumption within a deep transfer beam that resembles a truss comprised of compression struts and tension ties. For a deep dive look at how to design a truss from start to finish take a look at THIS article.
Another special consideration for the design of transfer beams is the deflection. Generally the maximum allowable deflection for transfer beams are more strict than those for a conventional beam. This is because the deflection of the transfer beam has an impact on the transfer column it supports and therefore the floors which are supported by the transfer column.
The Australian concrete code AS3600 specifies a recommended limit of between span/500 to span/1000. Whereas conventional beams have a recommended deflection limit of span/250.
Construction Considerations for a Transfer Beam
Transfer beams have additional complexities associated with them from a construction perspective. Due to the high loads which they are required to support, they can quickly become over congested with reinforcement. This means that both the Structural Engineer and the builder need to employ adequate planning and detailing to ensure that the construction runs smoothly.
The other complexity for transfer beams is associated with the sheer mass of concrete which is required for their construction. This often catches construction planners off-guard as usually significantly more levels of propping are required to support the wet weight of concrete during construction.
This can delay fitout of lower levels and hold up the construction programme overall.
The propping requirement can be reduced if the transfer beam is constructed in stages. This requires that a horizontal construction joint (cold joint) be introduced at some point within the beams depth. (for a more in-depth discussion on construction joints in slabs and how to analyse them, take a look at THIS article).
By adopting this approach, the first lower half of the transfer beam is more manageable to back prop when it is poured due to its reduced thickness. When the concrete has set for the bottom half it then provides propping support for the wet concrete weight of the top half of the transfer beam.
The trade-off being that additional shear ligatures are required to ensure that the longitudinal shear interface between the two pours is adequately reinforced against shear failure. Incidentally, shear failure between construction joints was a primary reason why the FIU pedestrian bridge collapsed in Florida. Take a look at THIS article for a detailed look into the engineering numbers behind this catastrophic event.
There is possibly an alternative solution to the transfer beam. A more cost effective and efficient outcome for both construction and load path. That is the Walking Column. For a detailed article on what a walking column is and considerations you need to make for its design, take a look at THIS article.