Structure Deflection vs Ceiling Deflection

Structure Deflection vs Ceiling Deflection

The ceiling in your home or apartment is generally constructed from plaster lining boards fixed to metal battens or fixed to a metal framing system that is suspended from the structure. Ceilings are decorative in purpose and are not generally considered to be a structural element; they are mainly used to hide services and construction of a roof space or the floor above, and to provide a pleasing finish within the room below.
Key Points:
  • Internal ceilings are generally not designed to withstand significant forces or loads.
  • Applying a load or force to the primary structure will cause it to deflect to some degree.
  • If the primary structure is designed to accommodate considerable movement, then the ceiling system would be expected to move at the same rate as the structure to which it is fixed.
Read time: 4 mins

Most ceiling framing systems consist of a number of lightweight, non-structural metal profiles and clips or brackets to join them together. This framework provides a flat and level surface to which the plaster lining boards can be installed. Internal ceilings are generally not designed to withstand significant forces or loads.

When constructing an internal ceiling system, you need a primary structure to which the ceiling system can be attached. This is generally the building itself: the bottom chord of a roof truss, the joists of a two-storey house, roof purlins in an industrial building or the concrete floor in a multi-storey apartment.

The primary structure is designed by the building engineer to withstand applied forces, such as wind load in the case of a roof truss or roof purlin, or the live load applied by human traffic to joist in a two storey house. Applying a load or force to the primary structure will cause it to deflect to some degree. The deflection characteristics of popular structural elements has long been established and is known to experienced engineers. These limitations are considered and accommodated by the building engineer within the scope of the building design.

However, as the ceiling system is directly connected to the primary structure, it is important that the building engineer considers the impact of the deflection on the ceiling system, and ensures that the limits imposed by the design do not exceed the maximum limits of the ceiling system.

Another factor to consider is movement of the building itself. Ceiling systems are generally designed to be a static building element. That is, it is not expected that you would look up at a ceiling and see it flexing in and out or swaying from side to side when the wind blows outside. However, if the primary structure is designed to accommodate considerable movement, then the ceiling system would be expected to move at the same rate as the structure to which it is fixed.

Such constant movement or excessive deflection has long been proven to cause problems to the ceiling system, including:

  • Elastic deformation of the metal sections that results in creaking noises;
  • Friction and movement between the various ceiling components that results in ticking or straining noises;
  • Hairline cracks in the lining board joints due to movement; and
  • In extreme cases, failure of the steel framing connections or members of the ceiling system.

Let’s look at two common examples to illustrate the problems:

Firstly, a roof purlin on a calm day may be relatively straight, with only 2-3 mm of deflection caused by applied gravity load (see Figure 1A). When the wind blows against the roof, the purlin may bow 25mm under the applied wind load. If the roof has an internal ceiling system fixed to it, then it could be expected that the ceiling would follow the movement of the roof structure (see Figure 1B).

Ceiling and Floor Deflection
Figure 1A
Ceiling and Floor Deflection
Figure 1B

Similarly, a second storey floor joist may be relatively straight when no one is upstairs (see Figure 2A), but when someone walks across the floor, the joists may bow 15mm under the applied load of the person. Again, if the joists have a ceiling system fixed to them, then it would be expected that the ceiling with deflect in concert with the joists (see Figure 2B)

Ceiling and Floor Deflection
Figure 2A
Ceiling and Floor Deflection
Figure 2B

Whilst the joists and purlins in these examples have been designed to withstand the applied forces and deflection, the internal ceiling system has not been designed to accommodate such live loads or amounts of deflection. A common response from the building engineer or manufacturer of the structural elements of the building is that the element is deflecting “within tolerance” or it is “allowable deflection”, and they will often produce design computations to show the exact amount of deflection their designs can allow. A problem arises when they have not considered the impact this movement has on the non-structural building elements that are connected to the primary structure, and internal ceiling systems are often the victim to this misalignment of deflection limits.

Analysing the response of residential floor joists

 

Analysing the response of residential floor joists when used in conjunction with Studco ceiling battens on 2-storey homes,
at Studco’s proprietary Ceiling Research Rig (CRR) in Sydney, Australia.

To solve this problem at the design stage, the building engineer needs to either make the primary structure stiffer or select a stronger ceiling system. Studco’s design engineers can help you select a suitable ceiling system and the correct components to limit the effect of excessive deflection and movement on the internal ceiling, so please get in touch with us in the early design stage of your project to ensure you don’t make these common mistakes and to avoid costly rectification works.

If you have questions about the deflection limits of the primary structure, we recommend you contact the building engineer or the manufacturer of the building element in question.

Studco

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