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Modeling of moisture-induced deformations and stresses in cross-laminated timber and timber-concrete composite floors

Johansson, Marcus LU (2021) In TVSM-5000 VSMM01 20201
Structural Mechanics
Department of Construction Sciences
Abstract (Swedish)
Korslimmat trä är ett tämligen nytt material, bestående av korslagda lager av brädor. Produkten tillåter nya byggmetoder och kan ses som ett grönare alternativ till stål och betong. Dess uppbyggnad gör det mer robust och mindre beroende av träets varierande egenskaper för olika geometriska dimensioner. Som ett nytt byggnadsmaterial är dess beteende och karaktär inte lika välstuderat. Med trä som ingående material blir fukt en viktig faktor att beakta. Deformationer och påföljande spänningar är i högsta grad ett resultat av förändrad fuktkvot och relativ fuktighet. De glipor som uppstår mellan bräderna vid uttorkning är missgynnsamma och leder till bland annat ett påskyndat brandförlopp.
I detta examensarbete utsätts olika uppsättningar... (More)
Korslimmat trä är ett tämligen nytt material, bestående av korslagda lager av brädor. Produkten tillåter nya byggmetoder och kan ses som ett grönare alternativ till stål och betong. Dess uppbyggnad gör det mer robust och mindre beroende av träets varierande egenskaper för olika geometriska dimensioner. Som ett nytt byggnadsmaterial är dess beteende och karaktär inte lika välstuderat. Med trä som ingående material blir fukt en viktig faktor att beakta. Deformationer och påföljande spänningar är i högsta grad ett resultat av förändrad fuktkvot och relativ fuktighet. De glipor som uppstår mellan bräderna vid uttorkning är missgynnsamma och leder till bland annat ett påskyndat brandförlopp.
I detta examensarbete utsätts olika uppsättningar av KL-träelement för ensidig och dubbelsidig fuktbelastning. Elementets deformerade form och glipbredder som uppstår som ett resultat av uttorkning studeras. I samtliga fall studeras KL-trä utan kantlimning.
Uttorkningen av elementen studeras med hjälp av parameterstudier. Här varieras brädernas tjocklek, brädernas bredd, antalet skikt och varierande tjocklek mellan de ingående skikten. Samverkansbjälklag studeras också: KL-träelement med nygjuten betong ovanpå och den ökade fuktighet i träet som följer.
Studien utförs med ett kommersiellt finita element-program (Abaqus). En viktig del av arbetet syftar till att bredda kunskapen om hur sådana modeller kan sättas upp.
En huvudslutsats kopplad till de glipor som fuktdeformationer ger upphov till är att en minsta glipbredd erhålls när de ingående bräderna är av minsta möjliga storlek, både sett till dess tjocklek och bredd. Att variera antalet lager eller att ge olika tjocklek mellan de ingående skikten ger ingen märkbar effekt på glipbredden, bortsett från vad som kan förväntas av att brädernas enskilda tjocklek ändras. En annan slutsats är att gliporna för elementens yttre skikt blir dubbelt så stora som för de inre. Gliporna är större för tvåsidig uttorkning än för ensidig. Trots att gliporna blir mindre för smalare brädor så blir antalet glipor desto fler, vilket leder till att en större total area exponeras. Vid en procents ändring av fuktkvoten blir glipornas bredd vanligen kring 0,3 mm. Elementet böjs 0,18 mm/m när det utsätts för en ensidig fuktbelastning motsvarande en procent skillnad i fuktkvot mellan över- och underkant.
Samverkansbjälklagen behåller en RF över 75 procent för de två övre skikten, även ett år efter pågjutningen. Det är främst den övre halvan av KL-träelementet som når en högre RF. Skiktens geometriska riktning i förhållande till urtaget för betongen påverkar maxvärdet för RF och när kulmen inträffar. Spänningar större än 1 MPa förväntas för det översta skiktets gränsyta. Mellan skikten förväntas rullskjuvningen nå hälften av det maximalt tillåtna värdet. (Less)
Abstract
Cross-laminated timber is a fairly new material, composed of layers of timber boards cross-placed. The product allows for new construction techniques and can be seen as a more sustainable alternative to steel and concrete. Its structure makes it more robust and less dependent on the variation in properties of wood for different geometrical dimensions. As a new material, the CLT is not well studied, and its behavior not as well known. As it is composed of timber, moisture becomes a factor important to consider. Deformations, and resulting stresses, are largely affected by change in moisture content and relative humidity. The gaps that appear between the boards from drying leads to disadvantages, mainly concerning a higher burning rate.
In... (More)
Cross-laminated timber is a fairly new material, composed of layers of timber boards cross-placed. The product allows for new construction techniques and can be seen as a more sustainable alternative to steel and concrete. Its structure makes it more robust and less dependent on the variation in properties of wood for different geometrical dimensions. As a new material, the CLT is not well studied, and its behavior not as well known. As it is composed of timber, moisture becomes a factor important to consider. Deformations, and resulting stresses, are largely affected by change in moisture content and relative humidity. The gaps that appear between the boards from drying leads to disadvantages, mainly concerning a higher burning rate.
In this master thesis, CLT elements of different configurations are investigated as regards uniform and non-uniform moisture loading. By non-uniform drying is meant a drying of the CLT resulting in a moisture gradient over the thickness of the CLT. The deformed shape and the width of the opening of the gaps that results from the moisture loading are examined, assuming no edge bonding.
Parametric studies were conducted for the drying of CLT elements, varying the board thickness, board width, number of layers and relation of thickness between layers. A timber-concrete composite structure is also studied, looking at a CLT slab with newly poured wet concrete on top of the CLT element and the resulting increased humidity of the timber that follows.
The study is performed by the use of a commercial finite element software, Abaqus. An important part of the work is to investigate different modeling approaches.
A main result regarding the gap openings that moisture induced deformations lead to is that to minimize their width, the boards should have as small dimensions as possible, both considering their thickness and width. The number of layers and a varying thickness between the layers had no significant effect on the gap widths apart from the varying thickness each board received. Another conclusion is that the gap openings that appear on the outer face of the CLT become twice as wide as those in the inner parts of the CLT. The gap widths are larger for the case of uniform drying than for non-uniform. Even though the gaps are smaller for smaller widths, the number of gaps are higher, and the exposed area is larger. The gap width for one percent change in moisture content were commonly around 0.3 mm. The element would displace 0.18 mm/m when subjected to non-uniform loading, corresponding to a difference of one percent MC between both sides.
The TCC structures keep a RH above 75 percent for the two top layers, even a year after casting. It is mostly the top half of the CLT element that reaches a higher humidity. The orientation of the board layers relative to the concrete notch has a slight effect on the top layer’s maximal value for the RH and when that value peaks. Stresses of more than 1 MPa are expected at the top layer interface. Between the layers, the rolling shear stress is likely to reach half of its maximum strength. (Less)
Please use this url to cite or link to this publication:
author
Johansson, Marcus LU
supervisor
organization
alternative title
Modellering av fukt och deformationer i korslimmat trä och samverkansbjälklag
course
VSMM01 20201
year
type
H3 - Professional qualifications (4 Years - )
subject
keywords
abaqus, numerical modeling, moisture content, relative humidity, cross-laminated timber, timber-concrete composite, finite element
publication/series
TVSM-5000
report number
TVSM-5249
ISSN
0281-6679
language
English
id
9063158
date added to LUP
2021-08-18 09:56:02
date last changed
2021-08-18 09:56:02
@misc{9063158,
  abstract     = {{Cross-laminated timber is a fairly new material, composed of layers of timber boards cross-placed. The product allows for new construction techniques and can be seen as a more sustainable alternative to steel and concrete. Its structure makes it more robust and less dependent on the variation in properties of wood for different geometrical dimensions. As a new material, the CLT is not well studied, and its behavior not as well known. As it is composed of timber, moisture becomes a factor important to consider. Deformations, and resulting stresses, are largely affected by change in moisture content and relative humidity. The gaps that appear between the boards from drying leads to disadvantages, mainly concerning a higher burning rate. 
In this master thesis, CLT elements of different configurations are investigated as regards uniform and non-uniform moisture loading. By non-uniform drying is meant a drying of the CLT resulting in a moisture gradient over the thickness of the CLT. The deformed shape and the width of the opening of the gaps that results from the moisture loading are examined, assuming no edge bonding. 
Parametric studies were conducted for the drying of CLT elements, varying the board thickness, board width, number of layers and relation of thickness between layers. A timber-concrete composite structure is also studied, looking at a CLT slab with newly poured wet concrete on top of the CLT element and the resulting increased humidity of the timber that follows. 
The study is performed by the use of a commercial finite element software, Abaqus. An important part of the work is to investigate different modeling approaches. 
A main result regarding the gap openings that moisture induced deformations lead to is that to minimize their width, the boards should have as small dimensions as possible, both considering their thickness and width. The number of layers and a varying thickness between the layers had no significant effect on the gap widths apart from the varying thickness each board received. Another conclusion is that the gap openings that appear on the outer face of the CLT become twice as wide as those in the inner parts of the CLT. The gap widths are larger for the case of uniform drying than for non-uniform. Even though the gaps are smaller for smaller widths, the number of gaps are higher, and the exposed area is larger. The gap width for one percent change in moisture content were commonly around 0.3 mm. The element would displace 0.18 mm/m when subjected to non-uniform loading, corresponding to a difference of one percent MC between both sides. 
The TCC structures keep a RH above 75 percent for the two top layers, even a year after casting. It is mostly the top half of the CLT element that reaches a higher humidity. The orientation of the board layers relative to the concrete notch has a slight effect on the top layer’s maximal value for the RH and when that value peaks. Stresses of more than 1 MPa are expected at the top layer interface. Between the layers, the rolling shear stress is likely to reach half of its maximum strength.}},
  author       = {{Johansson, Marcus}},
  issn         = {{0281-6679}},
  language     = {{eng}},
  note         = {{Student Paper}},
  series       = {{TVSM-5000}},
  title        = {{Modeling of moisture-induced deformations and stresses in cross-laminated timber and timber-concrete composite floors}},
  year         = {{2021}},
}