2014 - In ETABS, shell or area element has two types of stiffnesses i.e. inplane stiffness refers as f11, f22 and f12 and out-of-plane stiffness refer...
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Excel sheet for pile design
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Excel sheet for pile design
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Analysis & Design using ETABS_Sections Modifiers
20140126
Sections Modifiers for ultimate limit state analysis For Line Objects
Colum
I22 = I33 = 0.7
Beam
I22 = I33 = 0.35
Notes:
The modification factors are multiplied by the section properties specified for a frame element to obtain the final analysis section properties used for the frame element. Note that these modification factors o nly affect the analysis properties. They do not affect the design properties. These modifiers are based on ACI318-08 clause 10.10.4.1 and will be used in arriving the following results: Structural design of all elements. o Pile loads. o Building drift / story drift under seismic loads only. o The Service limit state analysis shall be carried out with the augmented section modifiers as per ACI 318-08 clause 10.10.4.1 and its commentary that allows multiplying the above section modifiers by 1.43. This analysis will be used to check : Wind drift (overall and story drift). o Acceleration. o Torsion constant modifier in beams may reduced (recommended), practical practical value for torsional constant modifier can be 0.20. When torsion is compatibility torsion, the reduction can occur due to redistribution of internal forces upon cracking. Torsional stiffness modifier does not change equilibrium torsion magnitude. This recommendation based on ACI318-08 clause 11.5.2.2 and its commentary. To ensure that the stiffness modifiers are assigned to all the elements, it is generally recommended to assign the stiffness modifiers after the completion of the model and prior to analysis using the "select by object type" option in ETABS. This not only relives the laborious task of defining the stiffness modifiers separately for each frame section, but also provide a quick, yet reliable way to change these modifiers in no time.
Sections Modifiers for ultimate limit state analysis For Area Objects
f 11= f 22 = f 12 = m11 = m22 = m12 = 0.7, for un-cracked walls Pier shear wall f 11= f 22 = f 12 = m11 = m22 = m12 = 0.35, for cracked walls
Spandrel shear wall (Coupling beam)
f 22 = f 11 = f 12 = m11 = m22 = m12 = 0.35
Slab
f 11= f 22 = f 12 = m11 = m22 = m12 = 0.25 (f 11, f 22, f 12 modifiers are not important if rigid diaphragms is assigned)
Notes:
The above modifiers applied for shell elements with default orientation of local axis. In ETABS, the default is to have the 1 axis horizontal and the 2 vertical. When local axes correspond with default settings, modifiers and their associated properties in ETABS are as follows: f 11 controls flexure through EI. o f 22 controls axial behavior through EA. o f 12 controls shear behavior through the GA component. o Designer can start the analysis assuming un-cracked walls and then check the axial stresses against the modulus of rupture to see if the walls are un-cracked or cracked. For those portions of the wall that are cracked, reassign the modifiers as shown above. Consider "Sections Modifiers for ultimate limit state analysis For Line Objects" notes. If designer want the shear walls to have no out of plane stiffness and act as a classical in-plane resisting element (recommended) then the m11, m22 & m12 modifiers should be assigned as 0.10. See Notes of Sections Modifiers for ultimate limit state analysis For Line Objects .
