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Wind-Induced Vibrations in Timber Buildings-Parameter Study of Cross-Laminated Timber Residential Structures
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Industrilized and sustainable construction.ORCID iD: 0000-0002-0675-6875
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Industrilized and sustainable construction.ORCID iD: 0000-0002-5907-7788
2017 (English)In: Structural Engineering International, ISSN 1016-8664, E-ISSN 1683-0350, Vol. 27, no 2, p. 205-216Article in journal (Refereed) Published
Abstract [en]

A current trend (2016) to construct high-rise timber buildings is seen. In order to understand the limitations posed by the timber material, wind-induced dynamic behaviour causing vibrations in the serviceability limit state has to be studied. The aim of this research is to calculate the natural frequency and acceleration levels of timber buildings having a cross-laminated timber structure to further the understanding of its behaviour and how a change in parameters affects building performance as reflected against comfort criteria. The results were calculated through finite element modelling using commercial software and by performing a modal analysis. The parameters under scrutiny were material stiffness, wall density, damping ratio, building height, and building footprint. The results show that even at moderate building heights (12-14 storeys), the comfort criteria are not fulfilled. Furthermore, the interaction between stiffness and mass for timber buildings needs to be explored further. And since the change of building footprint has a strong influence on the dynamic behaviour, the interplay between architectural and structural design becomes more important. Finally, more data on measurements of damping in timber buildings need to be collected to further validate simulation models.

Place, publisher, year, edition, pages
International Association for Bridge and Structural Engineering, 2017. Vol. 27, no 2, p. 205-216
National Category
Building Technologies
Research subject
Timber Structures
Identifiers
URN: urn:nbn:se:ltu:diva-63686DOI: 10.2749/101686617X14881932435619ISI: 000400418500008Scopus ID: 2-s2.0-85021183724OAI: oai:DiVA.org:ltu-63686DiVA, id: diva2:1105073
Note

Validerad;2017;Nivå 2;2017-06-02 (andbra)

Available from: 2017-06-02 Created: 2017-06-02 Last updated: 2018-11-22Bibliographically approved
In thesis
1. Modal Analysis, Dynamic Properties and Horizontal Stabilisation of Timber Buildings
Open this publication in new window or tab >>Modal Analysis, Dynamic Properties and Horizontal Stabilisation of Timber Buildings
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Modalanalys, dynamiska egenskaper och horisontalstablisering i träbyggnader
Abstract [en]

Engineers face new challenges as taller timber buildings are constructed. According to Eurocode 1-4, both horizontal deformations from static wind and acceleration levels shall be limited. Due to the low self-weight of wood, dynamic vibrations and acceleration levels cancause problems. The current knowledge in the field is limited and there is a need for increasing the understanding of dynamic properties in tall timber buildings. This research project has been a collaboration between Luleå Technical University and Sweco Structures AB, where the author has gained practical experience as a designer in parallel to the research studies.

The purpose of this research is to understand and describe the dynamic behaviour of tall timber buildings using FE-simulations , studying their dynamic properties, and comparing acceleration levels to comfort criteria. By varying different parameters, dynamic properties have been studied and compared with assumptions and recommendations in Eurocode 1-4.

In this study, buildings with cross-laminated timber panels (CLT) have been studied but also post-and-beam systems with trusses. Depending on the shape, layout and materials of the building, the dynamic properties of the building will vary: natural frequency, mode shape, modal mass, and modal stiffness. To assess the comfort of the building, the standard ISO 10137 has been used evaluating the natural frequency of the building and its peak acceleration. Simulations have been performed using finite element (FE) software where modal analyses have been performed. Over 250 simulations have been performed in this study.

Adding mass reduces the natural frequency and the acceleration level of the building, which is an appropriate measure if the building has a frequency below 1 Hz. Increased stiffness increases the natural frequency and reduces the acceleration level, which is suitable for buildings with a natural frequency over 2 Hz.

The empirical expression f = 46 / h should be used with caution as it is based onmeasurements of concrete and steel buildings. The recommendation is to perform FEsimulations until the empirical knowledge base is sufficient for timber buildings.

The placement of the stabilizing system is important for creating a balanced (symmetrical) system resulting in pure translation modes. Eurocode 1-4 presupposes 2D modes in the plane while asymmetry can create diagonal and even torsional modes, which Eurocode 1-4 cannot handle. Openings and asymmetry in the floor plan affect the dynamic properties of the building. The assumption that the building can be modelled as a homogeneous beam where the mass is evenly distributed can result in an over- or underestimation of the equivalent mass, which in turn can lead to an underestimation of the acceleration level, around 20% - 30%. It is recommended that the equivalent mass is calculated from FE generated modal mass and mode shapes.

Acceleration levels vary over the building height depending on the mode shape. Timber buildings with a slenderness <3.9 have more or less a pure shear mode and with increasing height it shifts to a linear mode. For timber buildings, it is recommended to use the generated mode shape from FE simulations, and not those prescribed in Eurocode 1-4 as these can underestimate the acceleration levels, around 30 %.

Abstract [sv]

Ingenjörer möter nya utmaningar i takt med att allt högre trähus byggs. Enligt Eurokod 1-4 ska både horisontell deformation från statisk vind och accelerationsnivåer begränsas. På grund av träets låga vikt kan dynamiska svängningar och accelerationsnivåer skapa problem. Den nuvarande kunskapen på området är begränsad och det finns ett behov av att öka förståelsen för dynamiska egenskaper i höga trähus. Detta forskningsprojekt har varit ett samarbete mellan Luleå Tekniska Universitet och Sweco Structures AB där författaren har fått praktisk erfarenhet som konstruktör parallellt med forskarstudierna.

Syftet med forskningen är att förstå och beskriva det dynamiska beteendet hos flervåningshus i trä genom simulering, studera trähusens dynamiska egenskaper och jämföra accelerationsnivåer mot komfortkrav. Genom att variera olika parametrar har dynamiska egenskaper studerats och jämförts med antaganden och rekommendationer i Eurokod 1-4.

I denna studie har byggnader med korslimmade skivor (KL-trä) studerats men också pelar-balksystem med fackverk. Beroende på byggnadens form, planlösning och material kommer byggnadens dynamiska egenskaper variera: egenfrekvens, modform, modalmassa och modalstyvhet. För att bedöma byggnadens komfort har standarden ISO 10137 använts som utgår från byggnadens egenfrekvens och maxacceleration. Simuleringar har utförts med hjälp av finita element (FE) program där modalanalys har använts. Över 250 simuleringar har utförts i denna studie.

Att addera massa reducerar byggnadens egenfrekvens och accelerationsnivå vilket är en lämplig åtgärd om byggnaden har en lägsta egenfrekvens under 1 Hz. Ökad styvhet ökar byggnadens egenfrekvens och reducerar accelerationsnivån vilket lämpar sig för byggnader med en egenfrekvens över 2 Hz.

Den empiriska formeln f =46/h ska användas med försiktighet då den baseras på mätningar av betong- och stålbyggnader. Rekommendationen är att utföra FE-simuleringar tills den empiriska kunskapsbasen är tillräckligt stor för trähus.

Placeringen av det stabiliserande systemet är viktigt för att skapa ett balanserat (symmetriskt) system med rena translationsmoder. Eurokod 1-4 förutsätter 2D moder i planet medan asymmetri i byggnaden kan skapa diagonala och även vridmoder, vilket Eurokod 1-4 ej kan hantera. Öppningar och asymmetri i planlösningen påverkar byggnadens dynamiska egenskaper. Antagandet i Eurokod 1-4 om att byggnaden kan ses som en homogen balk där massan är jämnt fördelad kan ge över- eller underskattning av den ekvivalenta massan, vilket i sin tur kan leda till en underskattning av accelerationsnivåerna, omkring 20%-30%.

Accelerationsnivåerna varierar över byggnadens höjd beroende på modformen. Trähus med en slankhet < 3.9 har mer eller mindre skjuvmod och med ökad höjd övergår den till en linjär modform. För trähus rekommenderas att använda en genererad modform från FE-simulering, inte de modformer som finns föreskrivna i Eurokod 1-4 då dessa kan leda till en underskattning av accelerationsnivåerna omkring 30%.

Place, publisher, year, edition, pages
Luleå: Luleå tekniska universitet, 2019
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Building Technologies
Research subject
Timber Structures
Identifiers
urn:nbn:se:ltu:diva-71703 (URN)978-91-7790-276-8 (ISBN)978-91-7790-277-5 (ISBN)
Public defence
2019-01-25, F231, Luleå, 10:00 (English)
Opponent
Supervisors
Available from: 2018-11-22 Created: 2018-11-22 Last updated: 2019-01-07Bibliographically approved

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