Modified Compression-field theory
MODIFIED COMPRESSION-FIELD THEORY
Based on the shortcomings of the shear design methods found in current design codes, Dr.techn.Olav Olsen was motivated to implement a design method that enables a consistent and rational shear design in ShellDesign. The new method had to include in-plane forces and transverse shear forces in the non-linear sectional response and it was concluded that the "Modified Compression Field Theory" (MCFT) was the most suitable model for implementation.
MCFT has been developed at the University of Toronto and is a method capable of predicting the shear response of cracked reinforced and prestressed concrete. MCFT is based on principal stresses and strains. Tension stiffening is included through average stresses by adding post-cracking tensile stresses to the stress-strain curve of the concrete. Modelling tension stiffening by averaged stresses requires the inclusion of a “crack check”. The crack check ensures that no concrete tensile stresses are necessary to obtain equilibrium at the location of a fully developed crack.
The implementation of MCFT in SD is mostly based on the work of Evan Bentz [LINK]. The implementation of the MCFT has already been done by Bentz for both 2D and 3D through the Response-2000 and Shell-2000 programs. The technique has been successfully applied in ShellDesign, both for 2D and 3D, and it is possible to display shear stress profiles through the section depth. The verification of MCFT in ShellDesign is performed against the existing analysis program Shell-2000, two of the design codes used today, NS 3473 and Eurocode 2, and experimental results.
The Figure below shows that NS 3473 underestimates the shear capacity of sections subjected to moderate and high levels of axial tension, when compared to MCFT. For low levels of axial tension, the shear capacity of NS 3473 is somewhat higher than that predicted by MCFT.
MCFT in ShellDesign reflects a consistent and rational shear design and will, in addition to providing a more correct answer, give engineers a tool that increases the understanding of the structure's behaviour.