Fundamental period of timber structures in the event of seismic activity M
Based on interpreted ambient vibration tests (AVT), these guidelines aim to help planners realistically determine and validate the fundamental period of timber and timber-hybrid STRUCTURES for seismic design.
Factsheet
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Schools involved
School of Architecture, Wood and Civil Engineering
School of Architecture, Wood and Civil Engineering - Institute(s) Institute for Timber Construction IHB
- Research unit(s) Timber Structures group FGH
- Funding organisation Schweizerische Eidgenossenschaft (Bundesverwaltung)
- Duration (planned) 01.08.2022 - 31.12.2026
- Head of project Lukas Furrer
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Project staff
Lukas Furrer
Urs Theodor Oberbach
Kylian Maître
Lukas Kramer -
Partner
Bundesamt für Umwelt BAFU
BBL
armasuisse
Swiss Timber Engineers STE - Keywords seismic engineering, fundamental period, ambient vibration tests
Situation
The fundamental period is a key parameter for determining seismic actions. However, its measurement in timber structures is associated with considerable uncertainties. While the building’s mass can be determined comparatively reliably, the effective horizontal stiffness can only be modelled to a limited extent in simplified structural models. In particular, non-load-bearing or secondary seismic components and other effects can significantly influence stiffness. Consequently, the fundamental period is frequently overestimated in computational models, resulting in an underestimation of seismic actions in force-based methods. Current normative approaches only partially take this issue into account. Hence, practical guidelines are required for realistically determining and validating the plausibility of the fundamental period of timber and timber-hybrid structures.
Course of action
The dataset is based on ambient vibration tests (AVT) conducted on a large number of timber and timber-hybrid structures. Measurements from the research project, from other BFH projects and from international specialist literature are taken into account. Since the fundamental periods determined using AVT cannot be used directly for seismic design, the measurements and spatial structural models for 24 selected structures were compared in detail. In particular, the study examined the differences in mass, vibration amplitude and effective stiffness. Conversion factors were derived from this comparison, which can be used to interpret further AVT measurements and convert them into fundamental periods relevant for design purposes. The resulting dataset is categorised by bracing systems and analysed using regression analysis.
Result
The studies show that simplified structural models often underestimate the effective horizontal stiffness of timber structures, resulting in comparatively long fundamental periods. By comparing the measurements with the model, it is possible to systematically take into account the effects of mass, vibration amplitude and additional stiffness contributions. Regression analyses are carried out for different bracing systems on the basis of the interpreted AVT measurements. Approximations are then derived to determine the design-relevant fundamental period as a function of the structure’s height. The relevant coefficients vary according to the type of bracing system. The results thus provide an empirically and analytically grounded basis for determining and verifying the fundamental period of timber and timber-hybrid structures.
Looking ahead
The project findings will be compiled into a practical guide, which will enable planners to estimate the fundamental period of timber and timber-hybrid structures with simple means and to validate results from spatial structural models. This is intended, in particular, to avoid the risk of underestimating seismic actions due to unrealistically long calculated fundamental periods.
