09-06-2010, 06:20 PM
@george85
Thin concrete shell roofs are nowadays mostly calculated by the use of finite elements methods and particularly shell elements. Most of the current software however will allow for the use of orthotropic element properties (ie different behaviour in different axes). In the case of unstiffened thin shell concrete roofs isotropic conditions may be assumed while for stiffened (ribbed) concrete shells orthotropic conditions should be accounted for.
In the past with less advanced or even lack of personal computers engineers would traditionally analyze such structures with the use of grillages which you can say they are the equivalent of the beam element space frame analysis. This method is still used for the assessment of such structures especially by more experienced engineers.
Nowadays, it is often quicker to model space frames with the use of shell elements. This is done by assigning the elements the equivalent stiffness of the space frame (upper and lower chords contribute towards the bending and axial stiffness while diagonals contribute towards the shear stiffness of the structure).
However, a word of caution, modelling space frames by shell elements will only assess you in the macroscopic behaviour of the structure (ie order of magnitude of deflections, overall buckling, initial sizing of the members etc.). In order to perform the final/detailed design I would think that you cannot avoid the use of a space frame model.
For concrete shell structures though, with the exception of high stress concentrations, you can get away without having to model it by the use of shell elements.
Hope this helps.
Thin concrete shell roofs are nowadays mostly calculated by the use of finite elements methods and particularly shell elements. Most of the current software however will allow for the use of orthotropic element properties (ie different behaviour in different axes). In the case of unstiffened thin shell concrete roofs isotropic conditions may be assumed while for stiffened (ribbed) concrete shells orthotropic conditions should be accounted for.
In the past with less advanced or even lack of personal computers engineers would traditionally analyze such structures with the use of grillages which you can say they are the equivalent of the beam element space frame analysis. This method is still used for the assessment of such structures especially by more experienced engineers.
Nowadays, it is often quicker to model space frames with the use of shell elements. This is done by assigning the elements the equivalent stiffness of the space frame (upper and lower chords contribute towards the bending and axial stiffness while diagonals contribute towards the shear stiffness of the structure).
However, a word of caution, modelling space frames by shell elements will only assess you in the macroscopic behaviour of the structure (ie order of magnitude of deflections, overall buckling, initial sizing of the members etc.). In order to perform the final/detailed design I would think that you cannot avoid the use of a space frame model.
For concrete shell structures though, with the exception of high stress concentrations, you can get away without having to model it by the use of shell elements.
Hope this helps.