Introduction
Modern buildings are complex assemblies of diverse components, each fulfilling a specific function. Their form and the material they are made from provides the functionality of the component. The built environment which is assembled from these components is designed to accommodate the activities of its occupants. The assembly of components used in this built space responds in different ways. One key characteristic is movement. The extent and manner in which these components — and ultimately the building — shift depend on various factors.
 
Understanding Building Movement
Several factors can induce movement in buildings:

• Type of Construction: The fundamental construction method influences movement patterns.
• Component Dimensions: Larger spans in floors and beams increase the potential for deflection.
• Wind Loadings: Wind forces exert dynamic pressure on the structure.
• Dynamic Loadings: Variable loads from occupancy and equipment contribute to movement.
• Changes in Humidity: Fluctuations in humidity cause materials to expand and contract.
• Ground Movement: Soil settlement or seismic activity can induce structural shifts.
• Temperature Changes: Thermal expansion and contraction are significant drivers of movement

 
Accommodating building movement is essential to prevent damage to the structure and its components by minimising stress on adjacent elements. A prime example is the gap at slab edges behind curtain walling, which allows for differential movement between the glass/aluminium façade and the concrete structure.


Fig 1. Hilti CFS SP WB Slab edge joint seal
 
When movement accommodation joints coincide with fire compartmentation lines, these joints must provide both movement capability and fire resistance. This ensures that compartmentation is maintained, and fire spread is effectively mitigated. Figure 1 illustrates a typical slab edge joint seal designed for this purpose (Hilti CFS SP WB Slab edge joint seal).
 
The Challenge of Deflecting Slabs
The increasing trend toward longer structural spans in modern buildings leads to greater slab deflection. Where walls divide the space beneath these deflecting slabs, a suitable gap must be provided at the wall head. This gap allows the slab to deflect freely and, more importantly, prevents the transfer of load to the walls.

Traditionally, deflection heads have been used on walls to mitigate this issue, particularly in non-loadbearing walls. However, when these walls are part of a fire-rated compartment, the deflection head joints must also maintain fire resistance. Figures 2 and 3 depict fire rated partition deflection heads (Hilti TTS) and wall head joint seals (Hilti CFS SP WB), respectively.
 
   
Fig. 2 Partition deflection head with Hilti TTS


 Fig. 3 Hilti CFS SP WB Wall head Joint seal
 
The Impact on MEP Services
Mechanical, electrical, and plumbing (MEP) services are typically suspended from the concrete slabs. Consequently, these services move in conjunction with slab deflection. When these services penetrate fire-rated walls, the seals around the penetrations must be fire-resistant and capable of accommodating the movement of the services. This requirement mirrors the needs of slab edge joints and wall head joints. Figure 4 shows typical service runs supported from a slab.


Fig. 4 Service runs supported from slab

Limitations of Traditional Firestopping Methods
Traditional firestop seals, such as ablative batt systems, often lack the necessary flexibility to accommodate service movement. As these type of seals are considered "rigid" in terms of movement accommodation, these systems can exhibit cracking and flaking around services due to insufficient flexibility. In extreme cases, the stone wool core can also be damaged. This damage often goes unnoticed because service runs are commonly concealed above suspended ceilings.

While some manufacturers have developed more flexible firestopping materials, the degree of deflection now required in modern construction is pushing the limits of even these solutions. Some products are approved for use in seismic zones, but these approvals typically address one-time crisis events rather than the ongoing deflection that occurs during the normal service life of a building. Seismic approvals often emphasise high intumescent performance to compensate for earthquake-induced gaps, whereas the focus here is on accommodating continuous building movement, whether from dynamic loads or long-term dead load deflection.

A Systems-Based Approach to Firestopping Deflecting Services
When selecting a firestop solution for deflecting services, it's crucial to remember that the firestop seal is just one component of a larger system. This system includes the firestop material, the service penetrant, the substrate, and the service supports. A holistic approach is essential for ensuring both fire performance and movement accommodation. Figure 5 illustrates the components of a typical service penetration.


Fig. 5 Components of a service penetration

As the slab deflects, the services move in unison. Sufficient clearance must be provided within the penetration opening to prevent the service from "bottoming out." The size of this clearance should account for the movement accommodation capacity of the seal material, as over-compression can damage the seal. In movement joint design, over-compression is generally more detrimental than over-extension. Modern elastomeric materials can usually withstand stretching, but over-compression leaves the material with nowhere to go, resulting in damage.

Considerations for Service and Substrate
In addition to the firestop seal, the effect of deflection on the service itself must be considered. Deflection could induce "hogging", bending the service over the wall. This can create concentrated loading, potentially damaging both the firestop material and the opening edge of the partition, especially in flexible partition systems. While a rigid substrate might resist this damage, it could subject the penetrating service to increased stress.

The Role of Service Supports
Service supports, in tension carrying the weight of the service, could be subjected to compression forces as the slab deflects if the service is prevented from deflecting due to insufficient clearance or a rigid firestopping material. Slender components like threaded rods are not designed for these compressive loads and may buckle under extreme conditions. Calculations indicate that even a relatively short M8 threaded rod can buckle under a load of just 0.4 kN. Damaged supports can lead to inadequately supported services, which is particularly critical during a fire when the load-bearing capacity of steel and anchors is significantly reduced. This highlights the importance of adhering to standards like BESA TR50.

Performance and Testing Standards
By design, movement joints are supposed to accommodate various movements induced by the reasons mentioned at the beginning of this article. EN 1366-4 Fire testing standard for linear gap seals began to acknowledge this impact and added testing methods for movement capabilities of linear gap seals. This is signified by the letter "M" in the classification. Figure 6 provides an example of a firestop linear gap seal classification.

Fig. 6 Example of a firestop linear gap seal classification
 
Penetration systems testing standard EN 1366-3 assumes static conditions, rigid with no movement. As of writing, movement for a firestop penetration seal would be the subject of ad hoc testing with subsequent assessment. In 2016 ASTM issued a movement testing standard ASTM E307 ‘Standard Test Method for Measuring Relative Movement Capabilities of Through-Penetration Firestop Systems’. While a manufacturer has approvals to this standard, it is not part of EN testing and prevents it being classified accordingly. That and some ad hoc testing does provide practical support for use of those products tested and approved.

Installation Considerations
As with all construction products, proper installation is critical for achieving the required performance. Movement firestopping seals require a high level of installation quality due to the technically demanding nature of the application. Simplifying product solutions reduces the potential for errors. Currently, movement firestop seals are labour- and skill-intensive, highlighting the need for simplified, easy-to-install products. Intensive research and development efforts are underway to meet this market need.

Conclusion
Specifying firestopping for deflecting services requires a comprehensive understanding of building movement, fire safety principles, and the limitations of traditional firestopping methods. A systems-based approach, considering all components of the penetration, is essential for ensuring long-term performance and safety. As building designs continue to evolve, so too must firestopping solutions, with a focus on flexible, reliable, and easy-to-install products that can accommodate the dynamic nature of modern structures.

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