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Modal Analysis, Dynamic Properties and Horizontal Stabilisation of Timber Buildings
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Industrilized and sustainable construction.ORCID iD: 0000-0002-0675-6875
2019 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Modalanalys, dynamiska egenskaper och horisontalstablisering i träbyggnader (Swedish)
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: urn:nbn:se:ltu:diva-71703ISBN: 978-91-7790-276-8 (print)ISBN: 978-91-7790-277-5 (electronic)OAI: oai:DiVA.org:ltu-71703DiVA, id: diva2:1265054
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
List of papers
1. Wind-Induced Vibrations in Timber Buildings-Parameter Study of Cross-Laminated Timber Residential Structures
Open this publication in new window or tab >>Wind-Induced Vibrations in Timber Buildings-Parameter Study of Cross-Laminated Timber Residential Structures
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
National Category
Building Technologies
Research subject
Timber Structures
Identifiers
urn:nbn:se:ltu:diva-63686 (URN)10.2749/101686617X14881932435619 (DOI)000400418500008 ()2-s2.0-85021183724 (Scopus ID)
Note

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

Available from: 2017-06-02 Created: 2017-06-02 Last updated: 2018-11-22Bibliographically approved
2. Dynamic Properties of Cross-Laminated Timber and Timber Truss Building Systems
Open this publication in new window or tab >>Dynamic Properties of Cross-Laminated Timber and Timber Truss Building Systems
2019 (English)In: Engineering structures, ISSN 0141-0296, E-ISSN 1873-7323, Vol. 186, p. 525-535Article in journal (Refereed) Published
Abstract [en]

Incorrect prediction of dynamic properties of tall buildings can lead to discomfort for humans. It is therefore important to understand the dynamic characteristics such as natural frequency, mode shape and damping and the influence they have on acceleration levels. The aim of this study is to compare two timber building system, one with cross laminated panels and one with post-and-beam elements with diagonals for stabilisation. Empirical formulae for predicting the natural frequency and mode shape are compared to measured and numerical results. Tall building assumptions such as ‘line-like’ behaviour and lumped mass at certain points were evaluated for both systems. The post-and-beam system showed a stiffer behaviour than the cross laminated system where more shear deformation occurred. Empirical formulae should be used with care until more data is collected.  For the post-and-beam systems an assumption of linearity may be appropriate, but for cross laminated systems the approximation can give results on the unsafe side. Finally, the relationship between stiffness and mass for cross laminated timber systems and its effect on dynamic properties needs to be further investigated.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
serviceability limit state, tall building, wind-induced vibration, mode shape, natural frequency, modal analysis, modal mass
National Category
Construction Management
Research subject
Construction Management and Building Technology
Identifiers
urn:nbn:se:ltu:diva-71699 (URN)10.1016/j.engstruct.2019.01.136 (DOI)000462104900042 ()2-s2.0-85062095214 (Scopus ID)
Projects
BioInnovation - Framtidens biobaserade byggande och boende
Note

Validerad;2019;Nivå 2;2019-03-07 (inah)

Available from: 2018-11-22 Created: 2018-11-22 Last updated: 2019-04-24Bibliographically approved
3. Dynamic Properties of Timber Buildings – the Effects of Openings
Open this publication in new window or tab >>Dynamic Properties of Timber Buildings – the Effects of Openings
(English)Manuscript (preprint) (Other academic)
Keywords
Modal analysis, high-rise timber buildings, openings, modal mass, modal stiffness, mode shape, natural frequencies
National Category
Building Technologies
Research subject
Timber Structures; Construction Management and Building Technology
Identifiers
urn:nbn:se:ltu:diva-71701 (URN)
Projects
BioInnovation - Framtidens biobaserade byggande och boende
Available from: 2018-11-22 Created: 2018-11-22 Last updated: 2018-11-22
4. Dynamic Properties of Timber Buildings – the Effects of Asymmetrical Layout
Open this publication in new window or tab >>Dynamic Properties of Timber Buildings – the Effects of Asymmetrical Layout
(English)Manuscript (preprint) (Other academic)
National Category
Building Technologies
Research subject
Timber Structures; Construction Management and Building Technology
Identifiers
urn:nbn:se:ltu:diva-71702 (URN)
Projects
BioInnovation - Framtidens biobaserade byggande och boende
Available from: 2018-11-22 Created: 2018-11-22 Last updated: 2018-11-22
5. Horizontal Displacements in Medium-Rise Timber Buildings: Basic FE Modeling in Serviceability Limit State
Open this publication in new window or tab >>Horizontal Displacements in Medium-Rise Timber Buildings: Basic FE Modeling in Serviceability Limit State
2014 (English)In: Materials and Joints in Timber Structures: Recent Developments of Technology, Dordrecht: Encyclopedia of Global Archaeology/Springer Verlag, 2014, p. 3-12Chapter in book (Refereed)
Abstract [en]

Higher and larger timber buildings are built today. The building is exposed to static lateral wind loads that cause displacements that might lead to discomfort and non-function of the building. To determine the size and behavior of horizontal displacements, two timber systems has been studied, a light frame system with shear walls and a post and beam system with diagonal bracing. The stabilizing wall segments have been analyzed with a FE model and subjected to static lateral wind load and vertical dead load in serviceability limit state. Both plywood and particle board were used as sheet materials. To reduce flanking transmission Sylomer® is applied in the light frame system. The total lateral displacement varies between 4 to 125 mm in the light frame system and 2 to 5 mm in the post and beam system. Removing the Sylomer® damping material from the light-frame system would decrease the lateral displacements with 1.5-3 times, which needs to be further investigated

Place, publisher, year, edition, pages
Dordrecht: Encyclopedia of Global Archaeology/Springer Verlag, 2014
Series
R I L E M Bookseries, ISSN 2211-0844 ; 9
National Category
Building Technologies
Research subject
Timber Structures
Identifiers
urn:nbn:se:ltu:diva-20926 (URN)10.1007/978-94-007-7811-5_1 (DOI)000329970600001 ()2-s2.0-84885452053 (Scopus ID)946836ed-4994-4855-b465-3ee9e8a3cb50 (Local ID)978-94-007-7810-8 (ISBN)978-94-007-7811-5 (ISBN)946836ed-4994-4855-b465-3ee9e8a3cb50 (Archive number)946836ed-4994-4855-b465-3ee9e8a3cb50 (OAI)
Note
Godkänd; 2014; 20131030 (andbra)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-11-22Bibliographically approved
6. Stiffness of sheathing-to-framing connections in timber shear walls: In serviceability limit state
Open this publication in new window or tab >>Stiffness of sheathing-to-framing connections in timber shear walls: In serviceability limit state
2014 (English)Conference paper, Oral presentation only (Refereed)
National Category
Building Technologies
Research subject
Timber Structures
Identifiers
urn:nbn:se:ltu:diva-38309 (URN)ca7a5b4c-a073-4363-981d-477f087206b8 (Local ID)ca7a5b4c-a073-4363-981d-477f087206b8 (Archive number)ca7a5b4c-a073-4363-981d-477f087206b8 (OAI)
Conference
World Conference on Timber Engineering : 10/08/2014 - 14/08/2014
Note
Godkänd; 2014; 20150327 (andbra)Available from: 2016-10-03 Created: 2016-10-03 Last updated: 2018-11-22Bibliographically approved

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