ForestValue - Dynamic Response of Tall Timber Buildings under Service Load

All high buildings exposed to wind load will to some extent sway and swing. Usually, the movements are small and are not marked by spectators or by people inside the building. Such oscillations do not cause any damage to the building, but persons staying in the building may feel discomfort. The discomfort is similar to car sickness, small, repeated oscillations can lead to a tendency to nausea. Some people detect movements, for example by a pendulum lamp moving, and thereby feel unsafe. There is a great individual difference in how sensitive one is to such vibrations. In the same way as there is differences between suspension and shock absorbers on cars, there is also considerable differences between buildings. Generally, the shape, height, stiffness and mass of a structure determine whether the building tends to cause unpleasant vibrations. The building materials have different weights, stiffness and damping properties, which gives a difference in the vibrating tendency. Lightweight structures are generally easier to put into vibrations than heavy buildings. However, the damping in the building has a lot to say, this works like shock absorbers in the building. High structures in steel or wood are light buildings in this context, so the vibration problem must be given great attention. Horizontal vibrations caused by wind load are perhaps the main challenge in building tall wooden houses today. In the project, several tall wooden buildings, built in Europe over the last ten years, have been investigated for this problem. The buildings have been subjected to controlled oscillations to determine the properties of the buildings regarding the vibration tendency. In parallel with this, typical building components and joints are experimentally characterized with respect to damping properties in laboratories. The goal was to develop a calculation model that can be used to ensure that high timber structures are not designed in a way that can cause unpleasant vibrations. In the project, tests using forced vibrations have been performed on eight buildings in total, located in UK, France, Slovenia, Sweden and Norway. In Norway, forced vibration testing has been performed on the “Mjøstårnet” in Brumunddal. This is probably the largest building in the world which has been testing in this way. The test series was executed in September/October 2021 as a close cooperation between NTNU and University of Exeter, UK. NTNU has also installed ambient vibration monitoring of “Mjøstårnet”over a three years period, ending at 2022. In addition, ambient vibration tests using wind induced loading have been performed on two high rise CLT buildings in Tromsø (10 and 13 stories) with personnel and equipment from NTNU. The test period here was nearly half a year in 2020. In 2022, NTNU performed ambient vibration tests on five approximately identical 9 stories CLT buildings in Trondheim (Moholt 50/50). In Norway the project has collected fundamental frequencies, vibrational modes, and damping properties for eight timber buildings with heights ranging from 9 to 18 stories. Numerical models of the buildings are developed, and the collected experimental data have been used for calibration and verification of the models. Typically, it has been developed two sets of numerical models for the buildings; one set having very detailed representation intended for scientific use, and the other set is simplified and targeted for use in practical design of timber buildings. It is the engineering approach together with the set of parameters to be used, which is the main results of Norwegian part of the project and the results are either published, or under publication in scientific journals. The project results will also be presented at the World Conference on Timber Engineering to be arranged in Oslo June 2023 (WCTE 2023)

Det norske prosjektet er en del av et større internasjonalt «ForestValue» prosjekt med fem partnerland. Omtalen her begrenser seg til den norske delen av prosjektet. En samlet rapport som dekker det internasjonale prosjektet, er vedlagt under «særskilt rapportering». Prosjektet har fjernet det meste av den tidligere usikkerheten rundt vibrasjoner grunnet vind eksponering på høye trehus. I alt 8 høye trehus (9 – 18 etasjer) bygd i Norge har blitt instrument og målt over lengre tid før de innsamlede data har vært etterbehandlet. På denne måten har egenfrekvenser, svingemoder, akselerasjoner og dempningsforhold blitt fastlagt for alle byggene. Alle målte data er, eller vil snart bli, publisert i anerkjente tidsskrifter. Det innsamlede målte grunnlaget har også blitt benyttet som en kilde til utvikling og validering av numeriske modeller for alle byggene. Det har vært lagt vekt på å utvikle enkle FEM modeller som sammen med anbefalte verdier for stivheter, massefordeling og dempningsparametere, er velegnet for praktisk bruk. Det har også vært gjennomført eksperimentelle undersøkelser av store knutepunkter med dybelforbindelser (som benyttet i Mjøstårnet). Hensikten med disse har vært å bestemme stivheter og energidissipasjon ved syklisk belastning. Disse forsøkene er ganske unike, både pga sitt stor parameterområde og protokoll for gjennomføring. Bare om lag halvparten av publiseringene har blitt tilgjengelige så langt, resten er enten under review eller vil bli publisert i forbindelse med World Conference on Timber Engineering 2023, som vil bli arrangert i Oslo 19-22. juni 2023. I alt 19 masterstudenter har utført sin masteroppgave direkte tilknyttet dette prosjektet. Av disse har 8 arbeidet med numeriske modeller, 6 studenter har vært delaktige i instrumentering og behandling av instrumenterte og målte byggverk, mens 5 studenter har gjennomført laboratorieforsøk. Det ligger en betydelig effekt av kompetansespredning i dette, og flere nyutdannede studenter har vært med å gjennomføre evalueringer av svingninger på planlagte byggeprosjekter. PhD student Saule Tulebekova har vært PhD student på dette prosjektet, og vil levere inn sin PhD avhandling og disputere første halvår 2023. Totalt sett mener vi at vi har fått mere ut av prosjektet enn forventet. De målte data fra aktuelle byggverk og gjennomførte laboratorieforsøk vil trolig ha lang levetid og bli hyppig benyttet som referanser. Det har i ettertid vist seg at våre labforsøk også er nyttige i modellering og evaluering av trebruer.

The research hypothesis is that it is possible to create computational models for Tall Timber Buildings, based on system identification and calibration of advanced Finite Element (FE) models. This will be underpinned by data from full-scale tests of a number of representative mid-to-high rise timber buildings in Norway, Sweden, France, Slovenia and UK. The project plan will utilize unique horizontal electro-dynamic sliding shakers from the University of Exeter (UK) and CSTB (France) to perform vibrational tests and use the data to estimate Frequency Response Functions (FRFs). This unique approach will enable thorough identification of the structural systems for TTBs. Additionally, more detailed laboratory testing will be performed on both structural connections, as well as non-structural components, to identify the cause of and quantify the vibration energy dissipation. Finally, both sets of experimental data will be integrated to calibrate FE-models serving as the basis for a new generation of modelling guidelines, to calculate wind sway response of TTBs for their vibration serviceability at the design stage. The in-situ measurements, using forced vibrations, will be preceded by relatively short ambient vibration tests (AVT) to enable optimum shaker positioning within the building. Laboratory experiments will analyse the basis of stiffness and damping characteristics, identifying models of components, connections and sub-assemblies, subsequently combining the characteristics with advanced numerical FE models. The numerical models will be project deliverables, together with the dynamic response characteristics for the tested TTBs. Collectively, this research will enable improved and more reliable designs for TTBs, taking account of the effects of wind and drawing on information contained in the subsequently produced Design Guideline for TTBs.