Questions and answers from Dr. Alirezaei:
Thank you for explaining about the two cases A and B from paragraph 9-23-3-5.
Is it true that it is said that ETABS16 is not able to consider clause b in the design of the damping for the medium column and the combination of 2 times the earthquake load should be introduced?
According to ACI318-14 we have:
– In the columns, the cross-sectional shear strength ϕVn should not be considered less than the smallest two values (a) and (b) below:
a) The shear force created in the column due to the factored gravity loads and the shear force corresponding to the nominal bending moment in the end sections with two-way bending curvature, in each extension. The factored axial load should be selected from the column loading combination that results in the highest corresponding nominal anchor.
b) The maximum shear obtained from the coefficient loading combination including the earthquake, in which instead of the shear caused by the earthquake, E is replaced by the value of Ω0E.
18.4.3.1 ϕVn shall be at least the lesser of (a) and (b):
(a) The shear associated with development of nominal moment strengths of the column at each restrained end of the unsupported length due to reverse curvature bending. Column exural strength shall be calculated for the factored axial force, consistent with the direction of the lateral forces considered, resulting in the highest exural strength
(b) The maximum shear obtained from factored load combinations that include E, with ΩoE substituted for E
In the 9th issue of the 92nd edition, an additional resistance value of 2 is considered.
The ETABS program in the cross-sectional design of the columns designs them for both perpendicular extensions.
The shear reinforcement is designed for each design combination in the major and minor directions of the column.
Then, factored loads are determined in Pu and Vu sections. In the calculation of Vu, the effect of gravity is also considered.
Determine the factored forces acting on the section, Pu and Vu. Note that Pu is needed for the calculation of Vc.
Finally, the amount of shear reinforcement is determined for the excess shear force.
Calculate the reinforcement steel required to carry the balance.
In the program guide, the shear design method of medium bending frames is as follows, which apparently controls the requirements of the regulations in this section, and the user must set the value of System Design Omega0 equal to 2 in the regulations settings section.
For Intermediate Moment Frames (seismic design), the shear design of the columns is based on the smaller of the following two conditions:
a) The shear associated with the development of nominal moment strengths of the columns at each restrained end of the unsupported length (ACI 18.4.3.1a),
b) The maximum shear obtained from design load combinations that include earthquake load (E), with E increased by a factor of Ωo (ACI 18.4.3.1b).
But in the output of the program and in the Design Forces section, it equals the shear force to omega zero, but does not use it to calculate the shear.
Structure Design Question Group link
(Assembly of Designers of Tabriz)
@Tabrizporsemancivil
This post is written by AminNajafgholizadeh