To better represent cracked behaviour in concrete, a stiffness modifier constitutes a modification component utilized in the parameters of cross-sections, such as the time of inertia, torsion, etc. Only concrete buildings, which break under stress, need the stiffness modifier. Forces that vary with the stiffness of a component are often used to analyze structures resting on other structural elements.
A concrete must be inflexible or resistant to deformation when subjected to an applied force to be stiff. Compression and tension in the building material fibres originate from the fracture of the member due to the internal pressures exerted by these loads on structures. In this blog, we will talk about the stiffness modifiers in detail.
The stiffnesses f11, f22, and f12 represent the in-plane stiffness of a shell or area element, whereas the stiffnesses m11, m22, and m12 represent the out-of-plane stiffness. Depending on the direction of the local axis, f11 or f22 modify the flexural and axial behaviour of the shear wall, whereas f12 regulates the shear behaviour. Modifications to EI or EA would map to column f11 or f22 in terms of codes, whereas changes to GAhear would map to column f12. Section 10.10 of the ACI 318 code suggests changing EI (equivalent to f11 or f22 or shear walls) due to slenderness effects when flexural deformations rule.
To be accurate in their models, however, some of our customers additionally apply modifications to f12, where they anticipate a loss of shear stiffness. The preceding explanation is valid if the shear wall region object’s local axes 2 and 1 are vertical. The user decides how this works. By default, ETABS positions one axis horizontally & the two axes vertically when drawing. This indicates that f22 should be used for wall piers while f11 should be used for spandrels when calculating the flex modifier for EI. The findings are little altered if the modifier is applied to f11 and f22. Modifiers m11, m22, and m12 are needed to describe cracking behaviour for slabs when bending is always within the out-of-plane direction.
The Role of the Modifier for Stiffness
When the inertia of each structural member is calculated in full for typical frame systems. As a result, the structural parts become more rigid and gravitationally attract seismic forces. Tension zone cracking caused by these stresses would reduce the member’s stiffness and cross-sectional area. The result would be a smaller moment of inertia than the gross length of inertia.
To make the structure more flexible, we may define the structural components as cracked sections and give a stiffness modifier on them. Once the members have cracked, the needed capacity may be calculated using stiffness modifiers.
Stiffness modifiers in etabs is the force needed to produce a certain amount of motion. Since columns will experience more axial compression than beams, their stiffness modifier value is greater than the beams. As a result, fractures in the columns would be less severe than those in the beams. Therefore, columns have a greater stiffness modifier than beams do.