Research
•Vibration modelling for timber structures
When it comes to modelling floor vibration response factors for prospective designs, current guidance is based on concrete and steel structures. But with the growing popularity of timber construction, the guidance may be overly conservative, causing overspecification and overspending on projects – both financially and in terms of embodied carbon.
Having delivered over 70 timber buildings, Heyne Tillett Steel has comprehensive experience in hybrid and mass timber structures, which extends to modelling vibration response factors. In our research, we’ve found that predicted vibration response factors for timber floors don’t necessarily correlate with real world results.
To explore this potential disparity between modelled vs. real-world timber vibration response factors, we collaborated with the University of Sheffield to conduct physical tests on our own completed projects to measure vibration responses. Our ambitions were twofold: firstly, to test the accuracy of our computational analyses compared to real world data; and secondly, to see if we could use the results to enhance the efficiency of our designs.
Current guidance
Vibrations are a key issue for clients and tenants, impacting the perceived quality and comfort levels of a building. Current British Council for Offices (BCO) specifications require a response factor of 8 or below, with this dropping to about 1 or less for specialist buildings containing labs or surgical theatres. While current footfall guidance is based on steel and concrete structures, guidance on modelling and measuring vibration responses for timber floors is gradually being developed, with a new Eurocode 5 (EC5) standard currently in the works.
The Gramophone Works
Technique
16 Chart Street
Our research
We carried out tests in three of our buildings: The Gramophone Works, Technique and our London office, 16 Chart Street. Each of these projects features a single or double-storey CLT extension atop an existing building, and we conducted tests on both the raised access floors and directly on the timber slabs. Through these three buildings, we wanted to understand the vibration response factors when working with different structures involving timber: from mass timber to hybrid structures using concrete or steel.
Vibration testing with a modal shaker at The Gramophone Works
We conducted manual testing with people walking and running across the floors, as well as with a modal shaker – a small machine that generates vibrations across the floor – which allowed us to detect the natural frequency of the floor, the dampening response (how long the vibrations take to dissipate), and the vibration response factors.
Results
All projects fell well within the BCO guidance, with response factors ranging between about 2 and 6. However, we noted disparities between our digitally modelled predictions compared to the physical test results.
What’s significant about these findings is the impact they could have on material consumption and project costs. Vibration response factor governs the design of most of the steel beams on the project – with more accurate modelling, we can reduce the overall steel tonnage, thereby saving on cost and carbon.
Modelled analysis predictions vs physical test results for The Gramophone Works (GW), Technique (TQ) and 16 Chart Street (CS)
Next steps
It’s important to note that there are many variables which can influence the vibration response factor, such as the properties of the CLT used, the stiffness of the joints in the building, and the structural grid spacing. Different walking routes also make a huge difference. We know that modelling can be inaccurate, with shrinking and swelling in timber causing inconsistencies. As such, the digital modelling results were conservative, and all tested floors performed better than expected. To drill further into these findings and draw more conclusive results, more focused testing would be required to explore the effects of different walking paths on the vibration performance of the floors.
Timber’s low weight makes it more susceptible to higher footfall response. In mass timber buildings with Glulam or LVL timber beams, the shorter spans help to alleviate issues with vibration. In hybrid structures where CLT floors are combined with long spanning steel beams (>9m), the vibration response becomes the governing criteria for the design of those elements.
Our research has shown that the measured response factors are lower than those predicted in computational models. Our research shows that, through a deeper understanding of the many variables that influence the performance of real timber floor, the gap between modelled vs. real-world timber vibration response factors could be reduced, enabling us to reduce the material- and carbon-intensity of structural designs whilst still complying with BCO guidelines.
More research is needed, but we’re interested to see how the industry responds to the new EC5 when it is implemented by 2027, and whether future guidance will be tailored for each of the major structural materials to create more specific and efficient models.
Professional photography: Dirk Lindner (The Gramophone Works), Jack Hobhouse (Technique) and Edmund Sumner (16 Chart Street)
