Why do we avoid Ductile Detailing in Secondary Beams?
Since secondary beams are not connected directly to columns they are not getting rotated due to column moments. Under the assumption that the diaphragm is very rigid, in simply supported secondary beam, the ends do not rotate significantly when the incoming earthquake induces rotations on Column-Primary Beam Joint.
Primary Beam-Column joints rotate under Earthquake. If rotation is too much due to very high seismic load, the rotation causes plastic hinge formation in the Column-Primary Beam junction (a distance away from the face, of course!). Before the first plastic hinge is formed, the deflection of the building is elastic. Once the plastic hinge is formed, many subsequent plastic hinges keep on getting formed leading to inelastic drifts/deflection.
Now, why shear stirrups are extra at Beam-Column Joints? That is because Beam Column Joint cores are subjected to very high shear stresses due to Earthquake. The reversal of this earthquake causes the cracks to open and close and hence the shear capacity of the concrete cores drops significantly. To compensate for this loss, extra shear stirrups are added in the hinge area to confine the core as well as to make up for the loss of shear strength. In the new IS-1893, we have said that let's neglect the cracked core and lets take all the shear to be absorbed by shear stirrups.
In secondary beams this shear stress demand is not that significant (in primary beam to secondary beam joint) and that is why we avoid ductile detailing in secondary beams (simply supported and continuous secondary beams both)!
Project Engineer at Thornton Tomasetti
10 个月My question is what should be done for the beams connected to framed columns at one end and to primary beams at the other end. Whether to do ductile detailing near the beam to column end (ideal case) or should we remove that beam from lateral system as it does not have complete frame action and follow gravity detail which is more preferable(in my opinion).
Senior Construction Engineer
11 个月Abhishek Singh Thank You Dear Eng. Abhishek for sharing your valuable insights based on "Ductile Detailing of Secondary Beams". Referring to your post, you are treating the secondary beams as "Gravity only members". However, gravity only members must be able to support gravity loads up to "design level lateral displacement" as per the requirements of Clause 4.2.2 of EN 1998-1 code as well as complying with the requirements of Clause 18.14.2.1 of ACI 318-19 code. Now the question arises : Are the induced moments and shears under the action of design level lateral displacement (drift/sway during seismic event) lesser than the moment and shear strength of the member ??? If No, then Ductile Detailing seems to be necessary for secondary beams.
Structural Engineer | M.Eng | B.Tech ( Civil )
11 个月Thank you for the knowledge, you always give us important facts and information.