Deflection limits are a powerful tool in modern structural design. They help to control the amount of deflection in structural members so as to provide comfort to users and prevent extensive cracks in finish materials. Whether you are designing steel or concrete structures, deflection limits are important.
As a design criterion, deflection limits are dependent on the type of structural elements ( beams, columns and slabs) under consideration. These limits will also vary depending on the material under consideration. A proper understanding of deflection limits can help you ensure that their designs have integrity and functionality under loading without experiencing excessive deflection.
It is worth mentioning that the deflection limits can vary based on the intended purpose of the structure. In cases where vibration control is of utmost importance, such as in laboratories or high-tech facilities, more stringent deflection limits may be prescribed to ensure minimal disruption.
In the following section, we are going to discuss the deflection limits of different structural elements and materials:
1. Beam
In the case of beams, the allowable deflection is based on the span of the beam, and the type of load. The allowable deflection can be expressed as a fraction of the beam span (L) such as L/360. It is important to ensure that beams stay within the required deflection limits to maintain their shape and function.
The nature of a beam’s support can also influence its deflection limits. For example, a cantilever beam is allowed for deflection compared to a simply supported beam. The table below some deflection limits for beams based on Eurocode and British Standards:
| Member | Vertical deflection limit |
|---|---|
| Cantilevers | L/180 |
| Beams carrying plaster or other brittle finish | L/360 |
| Other beams (except purlins and sheeting rails) | L/200 |
2. Slabs
Similar to beams, the deflection limits of a slab are also based on the span, but they are also influenced by the slab thickness. When addressing deflection in slabs, it is important to take into account factors such as the type of slab (e.g., one-way or two-way), the material properties of the slab, the span of the slab, and the applied loads.
To control deflection in slabs as an engineer, you may employ various techniques such as increasing the slab thickness, using reinforcing bars or mesh or adding more support beams or columns. These measures help to enhance the stiffness and load-carrying capacity of the slab, thereby reducing deflection.
For more about controlling deflection in slabs, you can read our article on the design of reinforced concrete slabs.
3. Columns
Deflection limits in columns refer to the maximum allowable displacement or bending that a column can undergo under a given load. The allowable deflection for a column is influenced by its height and the nature of loading.
To address deflection limits in columns as a structural engineer, you should consider factors such as the column’s material properties, cross-sectional dimensions, height, and the applied loads. Utilizing mathematical models and calculations, you can examine the behaviour of the column and establish the threshold for the maximum permissible deflection. For more about column design, you can read our article on the design of reinforced concrete columns.
Conclusion
Deflection limits play a crucial role in the design and construction by ensuring structural integrity and functionality. By striking the right balance between performance and deflection, you can design structures which are safe, functional, and visually appealing as well as meet the needs of the occupants.
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