Discussion on Seismic Detailing Concept of RC Structures (IS:13920-1993)

Collapse Mechanism in RCC members can be in the following category: 

Bond Failure: Brittle

Shear Failure: Brittle

Flexural Failure

Brittle: if over-reinforced section (compression failure)

Ductile: if under-reinforced section (tension failure)

Hence, We need to ensure that bond failure does not take place. Shearfailure does not precede flexural yielding. Beam is under-reinforced. IS:13920 code has stipulated this condition. In Clause-6.2.2, it says- the maximum steel ratio on any face of a flexural member should not exceed  row, max=0.025

Failure of RC Section : This may be in the following form: 

Yielding of tension bars- this type of failure is having the following category-

• Ductile
• Tension failure
• Under-reinforced section

Crushing of compression concrete this-type of failure is having the following category- 

• Brittle
• Compression failure
• Over-reinforced section failure

Tension failure more likely if:

• Less tension reinforcement
• Morecompression reinforcement
• Higher grade of concrete
• Lowergrade of steel
• Lower value of axial compression

Section ductility increases as- 

• Grade of concrete improves
• Grade of steel reduces
• Tension steel reduces
• Compression steel increases
• Axial compression  force reduces 

Generally, columns are less ductile than beams.

Capacity Design Concept:

• The chain has both ductile and brittle elements.
• To ensure ductile failure, we must ensure that the ductile link yields before any of the brittle links fails.

For instance, in a RC member:    

 
Shear failure is brittle      
Flexural failure can be made ductile      
Element must yield in flexure and not fail in shear

Capacity Design of Frames:  

We need to consider the following: 

• Choose yield mechanism
• Locate desirable hinge locations
• Estimate reasonable design seismic force on the building
• Design the members at hinge locations (upper bound type)
• Assess the member forces at other locations under the action of “capacity” force
• Design other locations for that force; need not detail these for high ductility

Materials in RC Members:  

Concrete and steel have very different characteristics

Steel ductile: strain capacity: ~12% to 25%

Concrete brittle: strain capacity: ~0.35%

Confinement of concrete:

It considerably improves its strain capacity:

Main Steps:

• Weak Girder – Strong Column Philosophy
• Shear Failure Prevented by Special Calculations (Capacity Design Method)
• Good Development Length
• Regions Likely to have Hinges Confined with Closely-spaced and Closed Stirrups

To ensure ductility: 

• Correct collapse mechanism
• Adequate ductility at locations likely to form hinge in collapse mechanism
• Need sufficient member ductility to ensure adequate structural ductility.
• Prevent brittle failure mechanisms to take place prior to ductile yielding

Storey collapse Mechanism: 

• Columns require too much ductility
• Columns are difficult to make ductile
• Collapse in column may lead to failure of whole structure.

Beam –Hinge Mechanism (Sway Mechanism):

Preferred mechanism

Ensure that beams yield before columns do

Strong Column –Weak Beam Design