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Dimensionering av en bågbro i trä - jämförelse av handberäkningar och Finita Element Metoden

Tran, Jimmy LU and Tran, Anh Tuan LU (2012) VBK920 20121
Civil Engineering
Division of Structural Engingeering
Abstract
In the past it was common to build in timber but in the last 150 years, steel and concrete in particular have been the dominating material in bridge-buildings. This is mainly due to not having mastered the timber construction to the same extent as steel and concrete. Nowdays the interest in timber bridges have increased and the technical solutions have been greatly developed in the last 15 years. The purpose of this study is to provide an insight of how a design of an arch bridge in timber is according to Eurocode can be done. For the report the design will be checked with hand calculations and with commercial FEM software, SAP2000. When hand calculations have been too time consuming or not possible for the authors to implement FEM have... (More)
In the past it was common to build in timber but in the last 150 years, steel and concrete in particular have been the dominating material in bridge-buildings. This is mainly due to not having mastered the timber construction to the same extent as steel and concrete. Nowdays the interest in timber bridges have increased and the technical solutions have been greatly developed in the last 15 years. The purpose of this study is to provide an insight of how a design of an arch bridge in timber is according to Eurocode can be done. For the report the design will be checked with hand calculations and with commercial FEM software, SAP2000. When hand calculations have been too time consuming or not possible for the authors to implement FEM have been used for the design values.
The case study is based on an existing arch bridge with a stresslaminated timber deck in Norway which was designed by the old Norweigan standards. The results from the hand calculations gave a lower strength compared to methods based on the finite element method. The difference between the two methods were not very large, hand calculation are about 20 % lower than the methods which were calculated by FEM. For out of plane buckling the bracing stiffnes at the attachment points have a great influense on the strength of the arch. In reality the hinged attachment points is not rigid at the transverse direction and should be considered when designing the bridge. The buckling load can vary between 1 and 45 MN but make the greatest contribution before the out of the plane stiffness reaches 10000 N/mm.
Bending stresses in the transverse direction is a big problem for the deck. The material utilization of the deck was calculated to 565 % of the recommended pre-stress level of 1 MPa. Both compression and tension are the same in the deck but with opposite signs, this may be because the modeling is done with shell elements.
When comparising the authors' plate model with Ekström in Chap. 9.6.1 it can be seen that the solid elements provide a different stressdistributions at the top and bottom of the deck. The tensile stresses are less than the compressive stresses in the solid plate member, this is due to the maximum compression stresses that are obtained directly under the load which generates an "disrupted zone" of the stresses. When modeling account, therefor no accurate representation of the results are obtained when modelizing the plate member in SAP2000.
Although the calculations shows that the bridge lacks strength at some points, we must not forget that this is an existing functional bridge in Norway. The bridge is designed for old standards according to Norwegian regulations and has probably passed the requirements. This is a sign that the new Eurocode is much more stricter than previous standards. (Less)
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author
Tran, Jimmy LU and Tran, Anh Tuan LU
supervisor
organization
course
VBK920 20121
year
type
H3 - Professional qualifications (4 Years - )
subject
keywords
Arch bridge, timber bridge, stress laminated timber deck, FEM, hand calculation, plate model, shell element, frame element & solid element.
report number
TVBK-5214
ISSN
0349-4969
language
Swedish
id
3159835
date added to LUP
2012-10-31 14:50:36
date last changed
2012-10-31 14:50:36
@misc{3159835,
  abstract     = {In the past it was common to build in timber but in the last 150 years, steel and concrete in particular have been the dominating material in bridge-buildings. This is mainly due to not having mastered the timber construction to the same extent as steel and concrete. Nowdays the interest in timber bridges have increased and the technical solutions have been greatly developed in the last 15 years. The purpose of this study is to provide an insight of how a design of an arch bridge in timber is according to Eurocode can be done. For the report the design will be checked with hand calculations and with commercial FEM software, SAP2000. When hand calculations have been too time consuming or not possible for the authors to implement FEM have been used for the design values.
The case study is based on an existing arch bridge with a stresslaminated timber deck in Norway which was designed by the old Norweigan standards. The results from the hand calculations gave a lower strength compared to methods based on the finite element method. The difference between the two methods were not very large, hand calculation are about 20 % lower than the methods which were calculated by FEM. For out of plane buckling the bracing stiffnes at the attachment points have a great influense on the strength of the arch. In reality the hinged attachment points is not rigid at the transverse direction and should be considered when designing the bridge. The buckling load can vary between 1 and 45 MN but make the greatest contribution before the out of the plane stiffness reaches 10000 N/mm.
Bending stresses in the transverse direction is a big problem for the deck. The material utilization of the deck was calculated to 565 % of the recommended pre-stress level of 1 MPa. Both compression and tension are the same in the deck but with opposite signs, this may be because the modeling is done with shell elements.
When comparising the authors' plate model with Ekström in Chap. 9.6.1 it can be seen that the solid elements provide a different stressdistributions at the top and bottom of the deck. The tensile stresses are less than the compressive stresses in the solid plate member, this is due to the maximum compression stresses that are obtained directly under the load which generates an "disrupted zone" of the stresses. When modeling account, therefor no accurate representation of the results are obtained when modelizing the plate member in SAP2000.
Although the calculations shows that the bridge lacks strength at some points, we must not forget that this is an existing functional bridge in Norway. The bridge is designed for old standards according to Norwegian regulations and has probably passed the requirements. This is a sign that the new Eurocode is much more stricter than previous standards.},
  author       = {Tran, Jimmy and Tran, Anh Tuan},
  issn         = {0349-4969},
  keyword      = {Arch bridge,timber bridge,stress laminated timber deck,FEM,hand calculation,plate model,shell element,frame element & solid element.},
  language     = {swe},
  note         = {Student Paper},
  title        = {Dimensionering av en bågbro i trä - jämförelse av handberäkningar och Finita Element Metoden},
  year         = {2012},
}