Designing for Deflection: Why do we need a DHT anyways?

Designing for Deflection: Why do we need a DHT

It’s a fair question, and one that’s often asked on site when looking at a 250mm thick concrete slab overhead. It seems impossible that such a massive, rigid structure will ever move enough to justify the additional cost of a deflection head detail. But the reality is that all buildings move, and the Deflection Head Track (DHT) is the critical component that manages this movement.

The traditional and most common reason for a DHT is to accommodate vertical deflection. Every floor and roof structure is designed to flex under load. These forces include "dead loads"—the permanent weight of the building's own materials—and "live loads," which are the transient forces from occupants, furniture, or even heavy rainfall. While a lightweight steel stud partition is strong, it has nowhere near the capacity to support the immense weight of a concrete slab. If the partition is built "tight" to the structure above, the downward deflection of the slab—even just 15-20mm—will transfer directly onto the wall framing, causing studs to buckle and linings to crack. The DHT creates a telescopic joint, an engineered gap that allows the primary structure to move without imposing its load on the partition below.

This movement isn't always slow settlement. Consider a full-height wall in a warehouse or large retail space attached to roof purlins. These structures are designed for a great deal of movement. Wind action can create significant upward suction on the roof, while snow or ponding water can impose heavy downward loads. A partition fixed rigidly in this environment would be pulled apart or crushed. A DHT is essential to handle this large, bi-directional movement.

Figure 1 - Common DHT details. Note the difference in the location of the lining screw fixing for slotted and standard DHT

When specifying a deflection head track, you can get slotted or standard details. It’s important to understand the difference between these details, and when they are used. The most common detail is the standard head track, where the wall studs sit up into the DHT, but are not connected to the DHT. This allows for vertical and in-plane wall movement. If walls are subject high out of plane loads, either seismic or wind loads, then a slotted DHT can be detailed. The slotted DHT is connected directly to the wall studs, which stops the wall studs from coming free of the DHT under heavy loads, at the expense of in-plane wall movement.

However, vertical deflection is only the start of the story. Buildings also move horizontally. This lateral movement, known as interstory drift, is caused by wind and, most critically for New Zealand, seismic action.  This introduces a whole new dimension to the problem, demanding even more sophisticated solutions.

Figure 2 - Building loads that can create deflections (LHS - Dead and Live loads, Middle - Wind loads, RHS - Earthquake loads)

An evolving story - how much damage is too much?

The lessons learned from the Canterbury and Kaikōura earthquakes have fundamentally shifted the industry’s focus from basic life-safety to resilience, and this is an area of significant current focus for the industry and regulators. This has given rise to Low Damage Seismic Design (LDSD), a philosophy aimed at ensuring buildings remain operational and asset damage is minimised after a significant seismic event.

This shift is being formalised through the Ministry of Business, Innovation and Employment (MBIE) Low Damage Seismic Design programme, a multi-volume set of technical resources developed to guide the industry towards greater resilience.  Volume one, which covers the benefits and options, is available now, with volumes two (Performance framework) and three (Technical guidance) nearing completion and expected later in 2025^[1].

For partitions, this means designing for much greater movement. Modern standards like NZS 1170.5 can require designs to accommodate inter-story drifts of 2.0% to 2.5% or more. This creates a massive performance gap as a standard plasterboard partition can start to show damage at drifts as low as 0.5%. When a structure is designed to move 2.5%, a partition built with traditional methods will fail, compromising fire and acoustic ratings and potentially creating life-safety hazards. Importance Level 4 (IL4) buildings, like hospitals or police stations, are where post-earthquake functionality is expected and even repairable damage can be looked at as too expensive or disruptive. For contractors, this isn't a design problem to be solved on-site; it's a critical risk to be identified and priced at the tender stage.

This isn’t a simple task, as the definition of damage is evolving along with the identification of the need for Low Damage Seismic Design. For example, check out the following table from ER95 - Code of Practice for the Seismic Performance of Non-Structural Elements^[2], which defines different performance states, or levels of damage. These definitions are not easy to understand, and could easily lead to contract disagreements later on. For example, what is a reasonable “cost of repair significantly less than replacement cost”?

Table 1 - Performance States for Partitions (from Table B-1, ER95)

The Tender Trap: What to look out for

The move towards LDSD, especially in IL4 facilities, means that the simple line item for "partitions" in a tender document can hide enormous complexity and cost. A specification note requiring an LDSD limit state fundamentally changes the scope of work. It’s no longer a matter of installing standard stud and track; it’s about engineering and constructing a high-performance system capable of accommodating significant multi-axis movement.

When reviewing tender documents, look out for these new performance requirements. Key questions to ask include:

• What is the specified inter-story drift? Anything above 1% should be a red flag that standard partition details are insufficient.
• Is this an IL4 building? If so, expect stringent requirements for the seismic restraint of all non-structural elements to ensure operational continuity.
• Who is responsible for the building movement design? The BRANZ research report ER95, for example, establishes a 'Code of Practice for the Seismic Performance of Non-Structural Elements' to help streamline specification and quality assurance. This reinforces the need to clarify at the tender stage exactly who is carrying the design risk. If there is no Building Movement Strategy yet, be wary that LDSD may be imposed post-tender.

Failing to identify and price these complexities upfront can expose a contractor to significant financial and liability risks.

Compounding Challenges: Fire, Squeaks, and Cracks

Vertical and Lateral deflection limits is not the end of the story, there are other factors to consider as well:

Passive Fire Protection: Firestopping a dynamic joint is a major challenge. Traditional fire-rated mastics are often tested for minimal movement. A 100mm seismic gap that needs to compress and extend requires a specialised, tested system. These can include flexible fire-rated blankets, high-movement sprays, or pre-formed solutions designed specifically for large, dynamic joints, like the Hilti Top Track Seal. These systems are specialist applications and can carry a significant cost premium that must be factored into the tender.

The "Squeaky" Building: Another emerging issue is building noise generated by the movement itself. In Australia, where high-rise construction has pushed deflection detailing for years, "stick-slip" noise has become a recognised problem. As the structure moves, friction between the studs and the deflection track can cause audible creaking, groaning, and popping sounds. This is particularly problematic in residential apartments and hotels. The solution lies in holistic building movement design coupled with specifying systems that are designed for smooth, low-friction movement.

A Toolkit of Solutions: From Slotted Tracks to the "Box-in-Box"

Fortunately, manufacturers and designers around the world have developed a range of solutions to meet these challenges. The key is selecting the right system for the specified level of movement.

• Standard Deflection Tracks: For moderate drift requirements, the workhorse solution is the standard head track detail. Perfectly capable of taking vertical deflection requirements and suitable for low levels of lateral deflections or non-critical applications.
• Advanced Systems: As drift demands increase, the industry is looking to more advanced, internationally-proven systems. These include rocking partition designs, seismically slotted deflection tracks and bracketed systems that use specialised clips to isolate the framing and allow for large, controlled movements without damaging the linings or passive fire ratings.
• The Premium Solution: "Box-in-Box" Construction: For the highest level of performance, particularly in IL4 facilities or critical spaces, the ultimate solution is "box-in-box" construction. This method involves building a structurally independent room inside the primary structure, attached to the primary structure floor only. While the main building sways and moves during a seismic event, the inner room remains protected, ensuring that critical services—like a hospital operating theatre or a data centre—can remain fully functional.

For the NZ market, below is a summary of four common proprietary DHT solutions you can ask your favorite supplier about:

Table 2 - Common Proprietary Wall System DHT Solutions

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[1] https://www.building.govt.nz/getting-started/seismic-work-programme/seismic-risk-series/low-damage-seismic-design

[2] https://www.branz.co.nz/pubs/research-reports/er95/