Transfer Structures: Design and Analysis

Transfer structures can be described as structural elements that redirect gravity loads (usually from columns or walls) to other structures (such as beams or plates) for distribution to another supporting structure that can resist the load. In other words, transfer structures alter the load path of gravity loads from one alignment to another. The prominent issue in transfer structures is that the load path is unconventional, and this is usually found in high-rise buildings where floor arrangements differ.

I had my first experience in the design of transfer structures in January 2016, when I was given the project to redesign the Cafeteria Hall of Ritman University Community Centre. The original arrangement of the structure is shown in Fig 1. below;

Fig 1: Original Arrangement of the Ground Floor Hall

It was desired to reduce the intermediate columns of the ground floor to a row of single columns as shown below. In this case, the beam to support the two rows of internal columns on the first floor can be described as a transfer beam. The two columns so supported can be described as ‘floating columns‘. The transfer beam will be responsible for altering the load path of the internal columns. In some cases, deep beams can be used as transfer structures.

Fig 2: Modified Arrangement of the Ground Floor Hall

On analysis of the transfer beam, I discovered heavy shear concentration within the intermediate support, which lead to an increase in the depth of the beam, and provision of heavy main and shear reinforcement within the region. This modified arrangement is an example of a transfer structure in its simplest term. The floor beam carrying those upper columns could be described as a transfer beam.

Transfer structures should be designed by experienced engineers, especially those who are very conversant with construction and good understanding of the load path and statics of structures. This is mainly because the design of transfer structures demands that the engineer will manipulate the depth, width, and reinforcements being provided by the design software.

For example, the 5 storey building (G+4) shown below is analysed and designed using Orion Software. The building is a typical transfer structure wherein all the ground floor columns are terminated on the first floor. The rest of the columns started from the first floor and go all the way to the roof. The upper columns are supported on 1.5m long cantilever beams (overhangs) wrapping around the building at the first-floor level (see image below).

For the design of the structure, I started with a trial depth of 600 mm for all the first floor beams and a beamwidth of 230 mm. It was a major disaster in flexure and shear as expected for a 5 storey building. I increased the depth to 750 mm while maintaining the same width of 230 mm. Most of the spans started looking okay (especially for the internal longitudinal/transverse beams), but the cantilever regions were still failing in shear. At this point, I increased the width of the cantilever regions to 300 mm.

Note: The idea is that when you are very sure that the beam is okay in bending (flexure), you can increase the width of the beam in order to reduce the shear stress instead of increasing the depth. However, adequate care must be taken about the detailing requirements.

At the end of the design and manipulation, the depth and width of the beam that satisfied the ultimate limit state requirements was 900 mm and 400 mm respectively, especially for the cantilever regions. The perimeter beams of the first floor were okay at a depth of 600 mm and width of 230 mm, but in order to avoid any awkward appearance of the building, the depth of the perimeter beams was also taken to 900 mm.

In summary, one of the major issues of transfer structures is shear, and more often than not, you will require more than two legs of reinforcement with very close spacing. It is sometimes recommended that the strut-and-tie method is better for the analysis of transfer beams, especially when it is a deep beam.

Furthermore, architects should bear these challenges in mind when designing buildings with transfer structure configuration by increasing the headroom of the building. When the headroom is high enough, there is enough flexibility for proper design by manipulating the width and depth of the beams as appropriate. The suspended ceiling can drop below beams in order to have a flat ceiling finish that will conceal the irregularities of the beam geometry.

Moreover, it is also important that the design engineer keep formwork (cost, construction, reusability) in mind while manipulating section dimensions (remember that I hinted on experience in construction). The more uniform the sections, the better for the contractor/builder.

See the images below for some of the structural details for the beams as produced by Orion (unedited).

Note that for bar mark 59 above, you can change the 7H25 to 2 layers

Note that there will be a need for sidebars since the depth of the beam is 900 mm. Thank you for visiting Structville today and God bless you.

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