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Climate Related Thermal Actions for Reliable Design of Concrete Structures

Larsson, Oskar LU (2012)
Abstract (Swedish)
Popular Abstract in Swedish

De flesta av Sveriges cirka 15000 större broar är byggda i betong. Ett problem som kan uppkomma i betongbroar är sprickor, vilka kan bidra till att minska broarnas beständighet och säkerhet. I en del broar har fler sprickor uppkommit på sydsidan än på nordsidan vilket antyder att solen har stor påverkan på dessa broar, eftersom det är den enda faktor som skiljer sig väsentligt mellan sidorna. På grund av denna sprickrisk är det viktigt att kunna hantera temperaturförändringar i byggda anläggningar som utsätts för väder och vind. Säsongsmässiga variationer i temperatur gör att t.ex. broar förlängs under sommaren och förkortas under vintern. Broar måste därför byggas på ett sådant sätt att rörelser... (More)
Popular Abstract in Swedish

De flesta av Sveriges cirka 15000 större broar är byggda i betong. Ett problem som kan uppkomma i betongbroar är sprickor, vilka kan bidra till att minska broarnas beständighet och säkerhet. I en del broar har fler sprickor uppkommit på sydsidan än på nordsidan vilket antyder att solen har stor påverkan på dessa broar, eftersom det är den enda faktor som skiljer sig väsentligt mellan sidorna. På grund av denna sprickrisk är det viktigt att kunna hantera temperaturförändringar i byggda anläggningar som utsätts för väder och vind. Säsongsmässiga variationer i temperatur gör att t.ex. broar förlängs under sommaren och förkortas under vintern. Broar måste därför byggas på ett sådant sätt att rörelser kan äga rum utan att säkerheten äventyras. Temperaturen varierar också mellan brons olika delar. Vid kraftig solstrålning kommer bron dels att bli varmare på ovansidan än på undersidan, vilket gör att bron vill kröka sig uppåt, och dels bli varmare på utsidan än insidan i en lådbro med ett inre hålrum, vilket medför att väggarna vill kröka sig. I många fall kan sådana rörelser inte äga rum fritt. Då genereras stora krafter, som kan medföra att konstruktionen spricker och i vissa fall kan bärförmågan påverkas väsentligt.



För att kunna hantera dessa belastningar på grund av temperaturvariationer har statistiska värden beräknats som är baserade på flera års indata från SMHI. Resultaten visar att de nuvarande rekommendationerna inte stämmer med svenska förhållanden. Vi får högre temperaturskillnader för plattbroar i Sverige jämfört med värdena i den europeiska byggnormen. De resulterande spänningar som kan orsaka sprickor i en lådbro är relativt höga jämfört med betongens hållfasthet. Om det rekommenderade temperaturvärdet från byggnormen används kommer spänningen att överskattas, vilket kan medföra att onödig armering läggs in. Det är bättre att direkt använda klimatdata från en kort period för att beräkna spänningarna eftersom det ger en mer realistisk nivå än de givna normvärdena.



I projektet har temperaturfördelningar simulerats i konstruktioner med datormetoder på basis av klimatdata från SMHI, med syfte att definiera extrema scenarier som kan användas för säker projektering av nya broar och utvärdering av existerande broar. Fältförsök har utförts där resultaten använts för att bekräfta att datorsimuleringarna ger korrekt resultat, vilket visade sig vara fallet. Ett av fältförsöken utfördes på den nya Svinesundsbron mellan Sverige och Norge, där temperaturen inne i betongen har uppmätts. Kompletterande mätningar av lufttemperatur, vindhastighet, solstrålning och värmestrålning har också gjorts vid bron. Dessa data ligger till grund för beräkningarna av temperaturer. Från klimatdata har sedan spänningar på grund av rörelser från temperaturskillnader beräknats. (Less)
Abstract
When the temperature changes in the environment the temperature will also change in a structure. Complex interactions of climate factors affect the temperature changes. When the temperature is varied in a material it will expand or contract, and movements will be produced. If the movements are prevented, stresses may be induced which can contribute to cracks in the structure. Severe cracks have been found in box-girder bridges in Sweden, with the most cracks on the south side. This indicates a large impact of solar radiation.



Thermal actions or climate situations used in design and analysis of temperature loads in concrete structures are often based on approximations or generalisations of climate data which are more or... (More)
When the temperature changes in the environment the temperature will also change in a structure. Complex interactions of climate factors affect the temperature changes. When the temperature is varied in a material it will expand or contract, and movements will be produced. If the movements are prevented, stresses may be induced which can contribute to cracks in the structure. Severe cracks have been found in box-girder bridges in Sweden, with the most cracks on the south side. This indicates a large impact of solar radiation.



Thermal actions or climate situations used in design and analysis of temperature loads in concrete structures are often based on approximations or generalisations of climate data which are more or less accurate compared to real temperature distributions. Assumptions concerning the independence of the actions during certain time periods have been made; in the European code three day intervals were assumed to be mutually independent. The thermal actions stated in the code are based on climate data from Germany, which may differ from the climate situation in other parts of Europe. Not much information and guidelines are given concerning thermal actions at cross-sectional level, where significant stresses may occur due to these effects. The climate situations used for analyses gives the largest thermal actions, but may or may not give the most unfavourable stress field. Large differentials are assumed to occur either during a day with a large amount of incident solar radiation and a large difference in daily minimum and maximum air temperature or during a night with no cloud-cover and a similar but reversed difference in air temperature.



In this thesis a detailed approach for simulating and predicting thermal actions is presented, where all the input factors are real climate data with a high resolution. The developed model has been validated against temperature measurements performed on both a simple slab and the concrete arch of the New Svinesund Bridge. The model is well suited to use for predicting temperature distributions in concrete, since it can capture temperature variations with high accuracy. Long-term climate data from meteorological stations were used with the model to identify extreme events of thermal actions based on annual maxima. The results show that the design values in the European code may be an underestimation of Swedish conditions.



The resulting thermal stresses from the long-term temperature simulations were also calculated and analysed for box cross-sections. The results showed that the recommendation in the Eurocode concerning temperature differentials at cross-sectional level may overestimate the thermal stresses in the transverse directions. An inclusion of the non-linear component of the temperature distribution reduces the calculated tensile stress level. The most important geometrical factor governing the transverse thermal stresses is the wall and slab thickness. The largest tensile stresses will appear in the thinner members, independent of the members being horizontal or vertical. A simulation using a full-scale 3D-model of a bridge confirms the influence of varying thickness, but shows the importance of the boundary conditions when using full-scale analysis. The largest thermal tensile stresses appear on the inside surface on the south web wall, which confirms the impact of solar radiation on the stress field in a structure. A short-term simulation using climate data from just a few days can be useful for estimating the thermal stresses. It is important however to have knowledge of the input data, since the results show that a three or four days intervals cannot always be assumed to be mutually independent events. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Professor Casas, Joan, Universitat Politècnica de Catalunya, Barcelona, Spain
organization
publishing date
type
Thesis
publication status
published
subject
keywords
thermal stress, solar radiation, FE-analysis, climate data, box-girder bridge, KstrConcrete, KstrReliability
pages
158 pages
defense location
Room V:A, V-building, John Ericssons väg 1, Lund University Faculty of Engineering
defense date
2012-04-27 10:15
ISSN
0349-4969
ISBN
978-91-979543-2-7
language
English
LU publication?
yes
id
d46ee7d3-f6da-4da0-8f53-4ca8ecb3baee (old id 2427973)
date added to LUP
2012-04-03 13:26:31
date last changed
2016-09-19 08:45:00
@phdthesis{d46ee7d3-f6da-4da0-8f53-4ca8ecb3baee,
  abstract     = {When the temperature changes in the environment the temperature will also change in a structure. Complex interactions of climate factors affect the temperature changes. When the temperature is varied in a material it will expand or contract, and movements will be produced. If the movements are prevented, stresses may be induced which can contribute to cracks in the structure. Severe cracks have been found in box-girder bridges in Sweden, with the most cracks on the south side. This indicates a large impact of solar radiation.<br/><br>
<br/><br>
Thermal actions or climate situations used in design and analysis of temperature loads in concrete structures are often based on approximations or generalisations of climate data which are more or less accurate compared to real temperature distributions. Assumptions concerning the independence of the actions during certain time periods have been made; in the European code three day intervals were assumed to be mutually independent. The thermal actions stated in the code are based on climate data from Germany, which may differ from the climate situation in other parts of Europe. Not much information and guidelines are given concerning thermal actions at cross-sectional level, where significant stresses may occur due to these effects. The climate situations used for analyses gives the largest thermal actions, but may or may not give the most unfavourable stress field. Large differentials are assumed to occur either during a day with a large amount of incident solar radiation and a large difference in daily minimum and maximum air temperature or during a night with no cloud-cover and a similar but reversed difference in air temperature.<br/><br>
<br/><br>
In this thesis a detailed approach for simulating and predicting thermal actions is presented, where all the input factors are real climate data with a high resolution. The developed model has been validated against temperature measurements performed on both a simple slab and the concrete arch of the New Svinesund Bridge. The model is well suited to use for predicting temperature distributions in concrete, since it can capture temperature variations with high accuracy. Long-term climate data from meteorological stations were used with the model to identify extreme events of thermal actions based on annual maxima. The results show that the design values in the European code may be an underestimation of Swedish conditions. <br/><br>
<br/><br>
The resulting thermal stresses from the long-term temperature simulations were also calculated and analysed for box cross-sections. The results showed that the recommendation in the Eurocode concerning temperature differentials at cross-sectional level may overestimate the thermal stresses in the transverse directions. An inclusion of the non-linear component of the temperature distribution reduces the calculated tensile stress level. The most important geometrical factor governing the transverse thermal stresses is the wall and slab thickness. The largest tensile stresses will appear in the thinner members, independent of the members being horizontal or vertical. A simulation using a full-scale 3D-model of a bridge confirms the influence of varying thickness, but shows the importance of the boundary conditions when using full-scale analysis. The largest thermal tensile stresses appear on the inside surface on the south web wall, which confirms the impact of solar radiation on the stress field in a structure. A short-term simulation using climate data from just a few days can be useful for estimating the thermal stresses. It is important however to have knowledge of the input data, since the results show that a three or four days intervals cannot always be assumed to be mutually independent events.},
  author       = {Larsson, Oskar},
  isbn         = {978-91-979543-2-7},
  issn         = {0349-4969},
  keyword      = {thermal stress,solar radiation,FE-analysis,climate data,box-girder bridge,KstrConcrete,KstrReliability},
  language     = {eng},
  pages        = {158},
  school       = {Lund University},
  title        = {Climate Related Thermal Actions for Reliable Design of Concrete Structures},
  year         = {2012},
}