Why is RCDC taking wind load shears to do ductile detailing for earthquake loads?


Applies To
Product(s):STAAD Advanced Concrete Design
Version(s):Connect Edition
Environment:
Area:Concrete Design
Subarea:Beam Design
Original Author:Aaradhya Rahate, Bentley Technical Support Group

Why is RCDC taking wind load shears to do ductile detailing for earthquake loads? It should take hogging and sagging moments of receiving frame beams for the earthquake loads and calculate the sway shears for comparison of shear force with the static analysis shear force.

Reply:

As mentioned in 13920-2016 code, ductility criteria is to be applied to beams which are resisting the Seismic forces. Below image for reference:

As per above sentence all the beams on which seismic forces are applied or the beams that behaves as a part of seismic force resisting system needs to be designed for ductility. But the critical load combination can be any one; which imposes Max Flexure or Max shear on the beam.

So, based on same, when Seismic forces are applied in the analysis file and also when Seismic load cases are selected for design of beams in RCDC, ductility criteria is applied.

Also, there is an option in RCDC where user can select the beams which needs to be designed for Ductility. By default, when ‘Ductile Design’ option is selected from Design settings, RCDC selects all Primary column to column beams to be designed for Ductility criteria.

Going ahead, for shear design of Beams, RCDC follows below mentioned steps:

  1. RCDC identifies the ‘Max Shear’ value for the considered beam from all the available load combinations. This happens for Left Mid and Right zone of the beam.
    1. To this max shear, the additional shear due to torsion is added:

Vut = Vu (from analysis) + Vtu (Shear induced due to torsion)

  1. Once the Max shear is determined, RCDC will trace the load combination from where Max Shear is found and take it as critical load combination.
  2. The sway shear (Vu sway) is calculated as below (as per IS 13920 – 2016).

The simply supported shear is calculated as below for the DL and LL cases independently and added when calculating ductile shear –


V-left = Shear at left end from analysis

V-left-ss (Shear at left end as per simply-supported action) =

= V-left – M-left/L + M-right/L

Calculation of ductile-shear at any section from left to right can be worked out as below -

V-x = Shear at ‘x’ left end from analysis

V-x-ductile = Maximum of (V-x – M-left/L + M-right/L – Sway shear-right), (V-x – M-left/L + M-right/L + Sway shear-left)

  1. The final Design Shear i.e. Vud = Max (Vut, Vusway) is used for the shear design of stirrups.