The future of mass timber floors is on shaky ground without CALMFLOOR

The appeal of mass timber floors in construction is growing fast, but without the successful management of the inevitable floor vibration problems this exciting material offers, its future is on distinctly shaky ground. In this article, Alex Pavic explains why CALMFLOOR active vibration control is the only solution that addresses and resolves the design and operational uncertainties of long-span, open-plan mass timber floors and unlocks their incredible structural and architectural potential.

Mass timber floors are engineered wood flooring systems designed largely for use in multi-story buildings. A sustainable, structurally sound high-performance alternative to traditional steel and concrete, mass timber floors are typically constructed using cross-laminated timber (CLT), glued laminated timber (Glulam, GLT), nail-laminated timber (NLT) or dowel-laminated timber (DLT).

These are all new solutions that provide superior structural integrity compared to the traditional light-frame wood construction that humans have relied upon for hundreds of years.

Over the last decade, global adoption of mass timber floors has surged and the rate of newly constructed floor areas continues to rise exponentially. Some estimates suggest that at least 20 million m² of new mass timber floors are constructed worldwide each year. Investors, owners, users, architects and engineers are wholeheartedly embracing the environmental and structural benefits of mass timber floors relative to their concrete and composite steel-concrete counterparts.

Here’s why we have to welcome this shift…

Mass timber floors: the pros

The environmental benefits of mass timber are well documented; key to this is its carbon sequestration ability, which is certainly unique for a construction material. The most obvious differences in the material properties of concrete, steel and mass timber – along with the potential structural benefits of mass timber – are shown on the tables below.

Table 1 serves as a useful reminder of how soft timber really is. Its stiffness, expressed through the modulus of elasticity, is several times lower than that of concrete and steel, making timber much more deformable than these materials.

Table 1: Mass timber floors have a far lower modulus of elasticity (E) compared to steel or concrete.

As for the floor’s structural mass, this is typically expressed in kilograms per square metre (kg/m²) or pounds per square foot (lbs/ft²). Table 2 shows the mass of mass timber is 2 to 10 times lower compared to standard concrete and composite floors, meaning mass timber floors are far lighter than concrete and steel floors.

Table 2: Mass per square meter of different structural floor systems.

However, due to its strength in both tension and compression, which is comparable to that of concrete (20-50 MPa), mass timber boasts the best ‘specific strength’ or strength-to-mass (or weight) ratio of the three materials, as shown in Table 3.

Table 3: Strength-to-weight ratio for three types of floor materials.

Compare the data in Table 1 and Table 3 for a valuable reminder that stiffness and strength (Table 3) are two considerably different mechanical properties. Mass timber has much lower stiffness, but the same strength as concrete. Couple that with its low mass and you can see its both its success and downfall. Low weight and great strength are excellent for resisting basic static loading.

Unlike conventional wood flooring, mass timber panels are manufactured by bonding layers of solid wood, significantly enhancing their load-bearing capacity. This is why long-span mass timber floors, exceeding 9m (30 ft), and supporting open-plan spaces that were previously reserved only for concrete and steel structures are becoming more common. Timber-concrete composite (TCC) floors can, in theory, span up to 20m (66 ft) due to their high strength-to-weight ratio. This is colossal.

Mass timber floors: the cons

Despite the desirability and architectural ‘transparency’ of long-span mass timber floors, they have very low stiffness and mass compared to their steel and concrete counterparts. As a result, this structural setup is challenged by the fundamental law of physics: Newton’s Second Law of Motion, which states: force = mass x acceleration (of the mass).

With mass timber floors, their low mass and stiffness lead to increased acceleration for the same dynamic force. This force is due, typically, to in-service human footfalls from human foot traffic, machinery or ground-borne vibrations.

So, the logical question is: how much larger are those accelerations relative to their counterparts when it comes to standard concrete or composite steel-concrete floors?

Essentially, they’re considerably larger because the mass is considerably smaller. Comparing like-for-like long floor spans, a mass timber floor typically weighing a fraction (20-25%) of the standard floor (see Table 2) will inherently have acceleration several times higher than its steel or concrete equivalents. So, it should be unsurprising that preventing excessive vibration has become the governing design criterion for mass timber floors; successful management of this feature will unlock all the benefits mass timber floors deliver.

There has been a recent proliferation of design guidelines, standards and draft standards that are pertinent, specifically to vertical vibration of mass timber floors. The key takeaways are:

1. There’s an increased uncertainty in the predicted vibration behaviour of mass timber floors, relative to standard steel and concrete floors.
2. There’s a tendency to recalibrate expectations and normalise the greater level of vibration that can be expected in mass timber floors.

Mass timber sensitivity analysis: why uncertainty is so high

Every floor vibration serviceability problem can be rationalised into three key considerations:

• Vibration source (e.g. human walking)
• Vibration path (i.e. mass, stiffness and damping of the floor structure)
• Vibration receiver (effects of vibration typically on humans present or vibration-sensitive processes on the floor)

In civil structural engineering, ‘sensitivity analysis’ is a euphemism for dealing with uncertain structural modelling assumptions. In Woodworks* excellent 100-page, state-of-the-art guide, published in 2023, the need for sensitivity analysis was mentioned frequently. This was done in conjunction with mass timber floor modelling of:
(1) weight and mass, (2) damping and (3) dynamic excitation. In addition, the implied considerable “variation in each person’s tolerance for vibration and uncertainty in the excitation” was mentioned.

Therefore, as with any floor vibration serviceability assessment, uncertainties exist with all three key considerations. However, they’re considerably higher in mass timber floors than in standard floor construction, such as concrete or composite steel-concrete floors.

Here are some examples:

1. Wood is an organic material with a naturally imperfect structure and is more variable, making it more difficult to predict mechanical properties. Crucially, these depend on the moisture content in wood, something that varies and doesn’t feature so strongly in concrete and steel. This directly affects the prediction of floor stiffness.
2. Connections in wood are not as standardised as they are in steel; this also directly affects stiffness and, to some extent, damping.
3. The lower mass of mass timber floors makes inevitable errors in estimating floor imposed permanent and live loads (i.e. the mass stemming from these) much more consequential. A 20kg/m² (only 0.2kN/m²) error is much bigger when the floor is 100kg/m² than when it is 500kg/m².
4. Similarly, the lower stiffness of mass timber floors makes the inevitable and difficult-to-predict contribution of non-structural elements (such as false floors and full height partitions) more consequential.
5. Mass timber floors have a far more non-linear behaviour compared with standard floors. Consequently, they exhibit a significant amplitude-dependent behaviour, which means key dynamic properties, such as damping and natural frequency, will depend on the level of vibration response. This may explain the wide variation of as-built measured damping values of mass timber floors reported compared with standard floors. As most mass timber floors are low frequency and can be excited in resonance by a walking harmonic modal force of amplitude P, the resonant peak vibration response of a of an offending resonant mode is inversely proportional to the modal damping ζ, and modal mass m, as shown in the formula below:

a=P/(2ζm)

The modal acceleration is directly proportional to the physical acceleration which means that two-times more modal damping will yield half the physical acceleration response. The results are therefore quite sensitive to the highly uncertain assumption about damping.

We could continue listing the increased uncertainties specific to mass timber floors, but anyone who has assessed floor vibrations knows that even a full awareness of these uncertainties – along with ‘sensitivity analysis’ to explore them – only goes so far. It cannot account for the wide variations in assumptions and outcomes. This is especially true when one set of perfectly reasonable assumptions results in a mass timber floor ‘passing’ the adopted vibration serviceability criterion (typically a pass-fail threshold), while another equally reasonable set leads to ‘failing.’ This is the elephant in the room when addressing the vibration serviceability of mass timber floors.

Normalising expected high vibration response levels in floors

Historically, there has been a tendency for lighter types of floor to attract more relaxed vibration criteria. Needless to say, more relaxed vibration criteria will allow floors to vibrate more and, naturally, be lighter, more economical and less damaging to the planet. In principle, there’s no problem with that.

However, there is little scientific evidence to support the notion that people tolerate greater vibrations on lighter floors. Indeed, why would floor users of office floors in premium buildings know that their composite steel-concrete floor is twice as light than a reinforced concrete counterpart (see Table 2) and tolerate double the vibrations? A premium office floor is a premium office floor and mass timber wants to be in that league.

As stated in the review of the current limits in the Woodworks guidelines, the response factor (R) limits for different types of offices (‘quiet’, ‘general’, ‘busy’, in a commercial building) can be as low as R<2 or as high as R<8, which is a lot of difference for the same application. The main problem is that these different office types and uses are not well defined. The factor of four difference is huge and, to save on cost and reduce environmental impact, there is a natural tendency to adopt higher R-factors without understanding the consequences.

It’s unsurprising then that in a 2015 IStructE survey** of over 100 structural engineering practitioners dealing with vibration serviceability, a quarter staid they had problems with “code compliant structures”. In other words, they followed the guidelines and still had unhappy owners and tenants on floors that were perceived as too wobbly. The survey indicated a considerable concern over the unreliable design process, including the vibration limits for floors that are seemingly not fit for purpose. This was 10 years ago and before the current surge in the design and construction of much lighter and livelier mass timber floors.

Increased movement doesn’t have to be tolerated…

If the full structural potential is to be fully exploited with long, lightweight, open-plan and carbon-negative spans, these floors are expected to be considerably wobblier than standard floors. It’s no surprise the draft of the EC5 Eurocode3 proposes six different vibration limits for various types of office floors. These include three categories – Quality, Base, and Economy – each with five levels (I to V). The Quality Choice at Level III, for example, permits a response factor of R<12, which is 50% higher than the widely recognized R<8 ‘workshop’ criterion in the respected international standard ISO 10137***. Meanwhile, the Economy Choice at Level V relaxes the vibration criterion to an astonishing R<30 – this was formerly a limit for stationary people on footbridges in ISO 10137. What kind of office can move like a footbridge in the past? This highlights the ongoing uncertainty surrounding acceptable vibration levels in structures, which are now expected to move more noticeably than in the past. This, however, does not mean this increased level of movement will be tolerated and accepted.

In principle, defining different design scenarios and adjusting allowable floor vibration accordingly seems like a step in the right direction. However, the issue with this approach is more a philosophical one. In civil structural engineering, we design unique prototypes – each structurally distinct and used differently by rather unpredictable occupants. Trying to categorise these one-of-a-kind structures into broad, general classifications is inherently difficult.

We take pride in designing bespoke building prototypes, yet when it comes to human factors, i.e. how people perceive and interact with these structures, we often default to generalised design rules to save money. In contrast, automotive engineers develop unique car prototypes and rigorously test human factors such as ergonomics and vibration tolerance for each one.

This challenge becomes even riskier when predefined vibration categories are poorly defined, which is almost always the case due to the variables that affect the human perception of floor vibrations. There is also a strong temptation to relax vibration criteria as far as possible, given the substantial environmental and cost benefits of doing so. Based on experience, such as the near disappearance of ‘quiet’ offices requiring R<4, it’s unlikely we’ll see many EC5 ‘Quality Choice-Level I’ mass timber floors designed to meet this stringent requirement. They would be highly uneconomical. Instead, we can expect more ‘Quality Choice-Level III’ floors with R<12, which will lead to an increasing number of mismatched expectations of owners and users and, ultimately, cause vibration serviceability issues. This is a natural consequence of all these factors. Unfortunately, it also risks undermining confidence in mass timber floors.

So, how do we unlock the enormous potential of long-span, open-plan mass timber floors without compromising trust in their inherent flexibility?

CALMFLOOR active mass dampers solve vibration problems in mass timber floors

The way forward is to replace the huge loss of mass in timber floors with an equally huge increase in damping. But how?

The formula above that calculates the resonant peak acceleration features a product ζm of modal damping and mass in the denominator. This means that, say, three-times reduced mass in timber floors will result in three-times greater acceleration. But if we also manage to increase damping threefold, that will keep the ζm product the same and would have the same effect of not losing any mass at all.

Until recently, increasing floor damping confidently by three times or more was virtually impossible by any commercially available means, including constrained layer damping or tuned mass dampers.

However, since 2022 and the worldwide launch of the pioneering CALMFLOOR active mass dampers (AMDs), several-times-higher mass timber floor damping is now commercially achievable with great certainty and at a minimal cost compared to these alternatives. In fact, the lightweight, long-span mass timber floors can be as good as heavy concrete floors if CALMFLOOR’s vibration-cancelling technology is used to replace the missing mass by damping. Figure 1 below demonstrates that.

Figure 1: Improvement in vibration levels of an actual lightweight office floor: R factor of the lively floor without and with CALMFLOOR. Considerable uncontrolled vibration levels of the kind shown in this figure are to be expected in long-span mass timber floors. CALMFLOOR active mass damper technology reduced this significant level to below all office limits, decimating the probability of adverse comment.

Figure 2 demonstrates the corresponding increase in the damping ratio with the peak of frequency response function (FRF) modulus reducing more than 10 times and indicating the same level of increase in the damping with CALMFLOOR control. This explains the 89% decrease in the actual acceleration response shown in Figure 1.

Figure 2: Modulus of a frequency response function (FRF) measured without and with CALMFLOOR control of a realistic full-scale floor. The modal damping ratio of the fundamental mode of vibration that needed control increased more than 10 times.

All CALMFLOOR units are identical, mass-manufactured, off-the-shelf autonomous vibration reduction devices. They sense floor vibrations and apply damping forces with surgical precision – much like noise-cancelling headphones that detect and suppress unwanted sound. A CALMFLOOR unit seamlessly adapts to the specific dynamics of a lively mass timber floor, continuously controlling and suppressing multiple dominant vibration modes at its location.

Strategically placing CALMFLOOR units at key vibration hotspots enables efficient control without the need for structural intervention. This makes the cost of vibration control per square meter of gross internal area (GIA) significantly lower than more disruptive and risk-prone alternatives, which may not perform as expected once installed. CALMFLOOR is an electromechanical, autonomous solution that works consistently and effectively, as demonstrated in many global installations.

CALMFLOOR is the ideal solution for addressing the design and operational uncertainties of mass timber floors. Its self-adapting control force reduces a wide range of floor vibration uncertainties, including unpredictable dynamic forces from multiple users, structural and non-structural element variations, and material inconsistencies. Weighing just 67 kg (148lbs), AMDs are installed in just hours without structural modification or operational disruption. Over time, they can be repositioned to accommodate changing layouts, targeting only vibration hotspot areas and providing unparalleled flexibility for owners and users.

Like noise-cancelling headphones, CALMFLOOR can be activated only when and where vibration control is needed. Typically, only about a third of a mass timber floors require intervention, with the remaining needing no treatment – these are savings that no other commercial technology can achieve.

CALMFLOOR is the ultimate floor vibration solution for mass timber floors

CALMFLOOR is a game-changing solution for mitigating and managing the unpredictable floor vibration behaviour of mass timber floors. It enables long-span, open-plan designs while ensuring they feel as stable or even better than standard concrete floors – for a fraction of the cost of the alternatives.

*Woodworks. (2023). U.S. Mass Timber Floor Vibration Design Guide. Wood Products Council.

**Pavic, A. (2019) Results of IStructE 2015 Survey of Practitioners on Vibration Serviceability. Society for Earthquake and Civil Engineering Dynamics (SECED) 2019 Conference, 9-10 September, Greenwich, London.

***International Organization for Standardization. (2007). Bases for design of structures – Serviceability of buildings and walkways against vibrations. ISO10173: 2007. Note: the standard was ast reviewed and confirmed in 2024.