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Restraint Effects in Concrete Bridges : A Study of Cracking due to Thermal Actions and Shrinkage in Portal Frame Bridges

Gottsäter, Erik LU (2019)
Abstract
Thermal actions and shrinkage cause volume changes in concrete structures. In structures which are restrained from movements, the restraint effects can cause cracking. However, when cracking occurs, the structure can deform to some extent, which reduces the restraint stresses and thus also the crack widths. As the exact crack development is hard to predict beforehand, the actual restraint and thus also the crack widths are hard to estimate. This makes it difficult to determine the reinforcement required for limiting crack widths to acceptable values. The common design approach does not include the reduction of restraint due to cracking. Combined with an unclear formulation of the thermal load case for temperature differences between... (More)
Thermal actions and shrinkage cause volume changes in concrete structures. In structures which are restrained from movements, the restraint effects can cause cracking. However, when cracking occurs, the structure can deform to some extent, which reduces the restraint stresses and thus also the crack widths. As the exact crack development is hard to predict beforehand, the actual restraint and thus also the crack widths are hard to estimate. This makes it difficult to determine the reinforcement required for limiting crack widths to acceptable values. The common design approach does not include the reduction of restraint due to cracking. Combined with an unclear formulation of the thermal load case for temperature differences between structural parts in bridges, the overestimation of restraint effects that this results in can lead to inefficient use of reinforcement.

In this study, temperature differences between structural parts of portal frame bridges were investigated, in order to suggest a more detailed load case to be used in design of this bridge type. Temperature simulations of portal frame bridge cross sections and surrounding soil were carried out using a finite element model. The model was validated by measuring and simulating temperature in a bridge during a one-year period. Thereafter, it was used to determine new load values, using climate data from different locations in Sweden. Also, crack widths due to thermal actions and shrinkage were investigated using a finite element model with a non-linear material behavior of concrete. In these analyses, the reduction of stiffness due to cracking was included. The model used was validated using test results from previous research.

The thermal load case suggested in this study describes the temperature distribution in portal frame bridges in a more detailed way than the current load case. It also presents values both for quasi-permanent and characteristic load cases. In the non-linear analyses, cracking due to the investigated restraint effects was shown to be unlikely in bridge decks of portal frame bridges, but cracks might form both in the top and in the bottom of abutments. The largest cracks were found in the bottom of the abutments, where the effects of the spatial temperature difference and shrinkage were coinciding.

When comparing the non-linear finite element analysis results with results obtained without including the stress reduction due to cracking, significantly smaller reinforcement amounts were needed for crack width limitation in the non-linear analyses. However, the minimum reinforcement amount was insufficient in order to limit crack widths also in the non-linear analyses. As it was also found that the reinforcement amount had a small impact on the crack widths in restraint situations, limiting the initial degree of restraint by e.g. changing the bridge geometry could be a more effective way to reduce the crack widths than to add extra reinforcement.
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Abstract (Swedish)
Temperaturändringar och krympning påverkar betongkonstruktioners volym, och i konstruktioner som hindras från av röra sig kan sådana volymändringar leda till så kallade tvångseffekter, som i sin tur kan orsaka sprickbildning. När detta sker kan emellertid konstruktionen deformeras till viss del, vilket gör att tvångsspänningarna och därmed också sprickvidderna minskar. Uppsprickningsförloppet är svårt att förutsäga, och därför blir också tvångskrafterna och sprickvidderna svåra att uppskatta på förhand. Detta gör det svårt att fastslå vilken armeringsmängd som krävs för att begränsa sprickvidderna till acceptabla värden, och i dagens dimensioneringsmetod finns det inget etablerat sätt för hur en minskning av tvångskrafter på grund av... (More)
Temperaturändringar och krympning påverkar betongkonstruktioners volym, och i konstruktioner som hindras från av röra sig kan sådana volymändringar leda till så kallade tvångseffekter, som i sin tur kan orsaka sprickbildning. När detta sker kan emellertid konstruktionen deformeras till viss del, vilket gör att tvångsspänningarna och därmed också sprickvidderna minskar. Uppsprickningsförloppet är svårt att förutsäga, och därför blir också tvångskrafterna och sprickvidderna svåra att uppskatta på förhand. Detta gör det svårt att fastslå vilken armeringsmängd som krävs för att begränsa sprickvidderna till acceptabla värden, och i dagens dimensioneringsmetod finns det inget etablerat sätt för hur en minskning av tvångskrafter på grund av uppsprickning ska göras. Kombinerat med en otydlig formulering i lastfall för temperaturskillnader i broar kan detta göra att armeringen i broarna inte utnyttjas optimalt.

I den här studien undersöktes temperaturskillnader mellan konstruktionsdelar i plattrambroar, för att ett mer detaljerat lastfall skulle kunna föreslås för den aktuella konstruktionstypen. Temperatursimuleringar gjordes med en finita-elementmodell av plattrambroar samt intilliggande jord och fyllnadsmaterial. Modellen validerades genom att temperaturen mättes och simulerades i ett givet brotvärsnitt under ett års tid. Därefter användes modellen för att ta fram nya lastvärden med klimatdata från olika platser i Sverige. Dessutom undersöktes sprickvidder orsakade av tvångseffekter med en finita-elementmodell med ett olinjärt materialsamband för dragen betong. Modellen kan beskriva den minskning av tvång som uppsprickningen ger upphov till, och har validerats med hjälp av försök som genomförts i tidigare forskning.

Det temperaturlastfall som föreslås i denna studie beskriver temperaturfördelningen i plattrambroar på ett mer detaljerat sätt än vad som görs i det nuvarande lastfallet, och lastvärden föreslås för både kvasipermanent och karakteristisk lastkombination. De olinjära finita element-analyserna visade att uppsprickning på grund av de undersökta tvångseffekterna är osannolik i broarnas farbaneplattor, men att sprickor däremot kan uppstå både i rambenens ovankant och i nederkant. De största sprickvidderna påträffades vid rambenets nederkant, eftersom effekterna av temperaturlasten och krympningen sammanfaller i detta område.

När resultaten från de olinjära analyserna jämfördes med resultat som togs fram utan hänsyn till den spänningsminskning som uppstår vid uppsprickning sågs stora skillnader i sprickvidd, där sprickvidderna i de olinjära analyserna blev betydligt mindre. Samtidigt var minimiarmeringsmängden otillräcklig för att begränsa sprickvidderna även i de olinjära beräkningarna. Då resultaten också visade att armeringsmängden har en liten påverkan på sprickvidderna i tvångssituationer, kan det vara mer effektivt att minska den ursprungliga tvångsgraden genom att t.ex. ändra brons geometri, än att lägga in extra armering.
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Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Adjunct Professor. Hallgren, Mikael, KTH, Stockholm.
organization
alternative title
Tvångseffekter i betongbroar : En studie av uppsprickning på grund av temperaturlaster och krympning i plattrambroar
publishing date
type
Thesis
publication status
published
subject
keywords
Tvång, Betong, Uppsprickning, Temperaturlast, Plattrambro, FE-analys, Restraint, Concrete, Cracking, Thermal action, Portal frame bridge, FE-analysis
pages
96 pages
publisher
Lunds University, Building Construction
defense location
Lecture hall V:B, building V, John Ericssons väg 1, Faculty of Engineering LTH, Lund
defense date
2019-11-29 13:15:00
ISBN
978-91-87993-14-5
language
English
LU publication?
yes
id
0ed0dfd6-364b-4021-b309-e1f83eb3dec4
date added to LUP
2019-10-29 10:47:34
date last changed
2019-11-05 14:07:57
@phdthesis{0ed0dfd6-364b-4021-b309-e1f83eb3dec4,
  abstract     = {Thermal actions and shrinkage cause volume changes in concrete structures. In structures which are restrained from movements, the restraint effects can cause cracking. However, when cracking occurs, the structure can deform to some extent, which reduces the restraint stresses and thus also the crack widths. As the exact crack development is hard to predict beforehand, the actual restraint and thus also the crack widths are hard to estimate. This makes it difficult to determine the reinforcement required for limiting crack widths to acceptable values. The common design approach does not include the reduction of restraint due to cracking. Combined with an unclear formulation of the thermal load case for temperature differences between structural parts in bridges, the overestimation of restraint effects that this results in can lead to inefficient use of reinforcement. <br/><br/>In this study, temperature differences between structural parts of portal frame bridges were investigated, in order to suggest a more detailed load case to be used in design of this bridge type. Temperature simulations of portal frame bridge cross sections and surrounding soil were carried out using a finite element model. The model was validated by measuring and simulating temperature in a bridge during a one-year period. Thereafter, it was used to determine new load values, using climate data from different locations in Sweden. Also, crack widths due to thermal actions and shrinkage were investigated using a finite element model with a non-linear material behavior of concrete. In these analyses, the reduction of stiffness due to cracking was included. The model used was validated using test results from previous research. <br/><br/>The thermal load case suggested in this study describes the temperature distribution in portal frame bridges in a more detailed way than the current load case. It also presents values both for quasi-permanent and characteristic load cases. In the non-linear analyses, cracking due to the investigated restraint effects was shown to be unlikely in bridge decks of portal frame bridges, but cracks might form both in the top and in the bottom of abutments. The largest cracks were found in the bottom of the abutments, where the effects of the spatial temperature difference and shrinkage were coinciding. <br/><br/>When comparing the non-linear finite element analysis results with results obtained without including the stress reduction due to cracking, significantly smaller reinforcement amounts were needed for crack width limitation in the non-linear analyses. However, the minimum reinforcement amount was insufficient in order to limit crack widths also in the non-linear analyses. As it was also found that the reinforcement amount had a small impact on the crack widths in restraint situations, limiting the initial degree of restraint by e.g. changing the bridge geometry could be a more effective way to reduce the crack widths than to add extra reinforcement.<br/>},
  author       = {Gottsäter, Erik},
  isbn         = {978-91-87993-14-5},
  language     = {eng},
  month        = {10},
  publisher    = {Lunds University, Building Construction},
  school       = {Lund University},
  title        = {Restraint Effects in Concrete Bridges : A Study of Cracking due to Thermal Actions and Shrinkage in Portal Frame Bridges},
  url          = {https://lup.lub.lu.se/search/ws/files/71205707/Dissertation_Erik_Gotts_ter.pdf},
  year         = {2019},
}