Ductile detailing of beams, as outlined in IS 13920:2016, plays a crucial role in the design of reinforced concrete structures in seismic regions. It ensures that structures can withstand large inelastic deformations, thereby absorbing and dissipating seismic energy without significant loss of strength. The Indian Standard IS 13920:2016 provides guidelines for ductile detailing of reinforced concrete structures subjected to seismic forces, focusing on achieving structural resilience and safety. This article discusses the requirements and specifications for ductile detailing of beams as per IS 13920:2016.
Ductile Detailing of Beams Requirement
Size of Beam
Clauses 6.1.1 to 6.1.4 of IS 13920 provide specifications regarding the size requirements for beams when the factored axial compressive stress does not exceed 0.08fck. The size of the beams are:
Clause 6.1.1
A width-to-depth ratio of more than 0.3 should be maintained to achieve a balanced beam design where the beam is not overly narrow relative to its depth. This ratio supports two main objectives:
- Improved Stability and Shear Resistance: A beam with a width of at least 30% of its depth provides better resistance to lateral (sideways) forces and shear forces, which are common during seismic events. Wider beams have a more stable geometry, which can help prevent undesirable buckling and improve the beam’s overall durability.
- Enhanced Ductility: A suitable width-to-depth ratio also contributes to the beam’s ductile behaviour, allowing it to deform more effectively under earthquake-induced stresses without sudden failure.
bw/d > 0.30
Clause 6.1.2
This clause specifies that beams in seismic design should have a minimum width of 200 mm. This minimum width ensures that there is enough space to properly place and arrange the reinforcement, which is crucial for achieving the ductility needed for seismic resilience.
bw > 200 mm
Clause 6.1.3
According to this clause, beams should have a minimum depth (D) of at least 1/4th of the clear span length. This minimum depth helps ensure that the beam has sufficient stiffness and load-bearing capacity, which are necessary for ductile behaviour under seismic forces.
D < 1/4 of the clear span
Clause 6.1.4
This clause specifies that the width of the beam (bw) should be restricted relative to the dimensions of the supporting columns or other members it frames into. This limitation prevents excessive widening of the beam, ensuring a balanced and efficient load transfer to the supporting members. Here’s a breakdown of the conditions:
The beam width (bw) should not exceed: The width of the supporting member (column or wall) plus an additional width on each side of the supporting member equal to the smaller of:
- The width of the supporting member, or
- 0.75 times the width of the supporting member.
Explanation of Each Condition
Width of Supporting Member: This condition means that the beam can extend beyond the supporting member’s edges but not by more than the width of the supporting member itself. For instance, if a column is 300 mm wide, the beam can extend up to 300 mm on either side of it.
0.75 times the Width of the Supporting Member: This condition provides an additional restriction, allowing an extension on each side of the supporting member that is 0.75 times its width. For a column width of 300 mm, the beam extension would be limited to 0.75×300=2250.75 X 300 = 225 mm on each side.
The smaller of these two values determines how much wider the beam can be beyond the supporting member on either side. This requirement ensures that the beam does not excessively overhang the column or support, maintaining the integrity of the joint and improving seismic performance by avoiding undue stress concentrations or torsional effects.
- bw < bc
- bw < 0.75 of bc
where,
- bw is the width of the beam
- bc is the width of the supporting member or the width of the column
Also, read: Types of Cracks in Concrete Beams
Longitudinal Reinforcement
The longitudinal reinforcement for the beam design in the seismic region is discussed in clauses 6.2.1 to 6.2.5 of IS 13920.
Clause 6.2.1: minimum reinforcement
There should be at least 2 numbers of 12 mm diameter bars each at the top and bottom faces or the minimum longitudinal reinforcement ratio \(\rho_{min}\) required on any face at any section is given by:
\( \rho_{min}=0.24\frac{\sqrt{f_{ck}}}{f_y} \)
where,
\(f_{ck}\) is the characteristic compressive strength of the concrete
\(f_{y}\) is the yield stress of steel reinforcement bars
Thus, the minimum reinforcement for the longitudinal bars depends on the characteristic compressive strength of both the concrete and the steel.
For Example,
\(f_{ck}\) = 25N/mm2 and \(f_{y}\) =500N/mm2 and the minimum steel reinforcement ratio is 0.0024
Clause 6.2.2: Maximum reinforcement
The maximum longitudinal reinforcement ratio \(\rho_{max}\) provided at any face of the beam section is 0.025. So, the maximum percentage of longitudinal steel is 2.5%.
Clause 6.2.5: Exterior Beam-Column Joint
At an exterior beam-column joint, the top and bottom longitudinal bars of the beam shall be provided with anchorage length which extends beyond the inner face of the column. The length of the anchorage is equal to the development length of the bar in tensin plus 10 times the diameter minus allowance of 90° bent. (see figure-2)
\(A_{d}=L_{d}+10×d_{b}−A_{90}\)
Where:
- \(A_{d}\) = total anchorage length of the bar at the beam-column joint,
- \(L_{d}\) = development length of the bar in tension,
- \(d_{b}\) = diameter of the tension bar,
- \(A_{90}\) = allowance or deduction for 90° bends.
Splicing of Longitudinal Bars
The clause 6.2.6.1 provides a guideline on Lap splices and the links that have to be provided at the entire length of the longitudinal bars are spliced. However, links should not be spaced more than 150 mm and the length of the lap should not be less than the development length. The top and bottom longitudinal bars are not allowed to splice more than 50 per cent at any one section of the beam. (see figure-3)
Lap splices shall not be provided
- within a joint;
- within a distance of 2d from the face of the column; and
- within a quarter length of the beam adjoining the location where flexural yielding may occur under earthquake effects.
Also, read: Development Length for Reinforcement Bar: Anchorage Length & Lap Length with Formula
Transverse Reinforcement
Clause 6.3.1: Vertical links
This clause suggests the use of vertical links and strictly disagrees with the use of inclined links in a beam. In typical construction, a link (or stirrup) is a single bent bar that wraps around the main reinforcing bars in a beam to hold them in place and provide shear strength. However, links can also be made using two separate bars instead of one.
- U-Link: One of the bars is shaped like a “U” and has a 135-degree hook at each end. The hooks each have an extension that is at least 6 times the bar’s diameter (but not less than 65 mm) to ensure it stays anchored securely in the core of the concrete. (see Figure 4B)
- Cross Tie: A second bar, called a cross tie, is used in conjunction with the U-link to form a complete link system around the main reinforcement.
Clause 6.3.2: links size
This clause suggests the use of a minimum diameter of 8 mm for vertical links in a beam.
Clause 6.3.5: Close Spacing of Links
This clause specifies the close spacing of links in a beam with the following conditions:
- d/4;
- 8 times the diameter of the smallest longitudinal bar; and
- 100 mm
Clause 6.3.5.1: First link
The first link shall be provided at a distance not more than 50 mm from the outer face of the joint or column.
Clause 6.3.5.2: Spacing of links
In this clause, the close links are to be provided not more than 2d on either side of the longitudinal section of the beam where the flexural yielding may occur under earthquake effect. Also, the remaining length of the beam shall be provided as a spacing not exceeding d/2.
FAQs:
Q: What are the requirements for shear reinforcement in beams under IS 13920:2016?
Answer: The standard specifies closely spaced stirrups, especially near the beam ends and critical regions, to prevent shear failure. The maximum spacing of stirrups should not exceed \(d/4\) (where \(d\) is the effective depth of the beam) or 100 mm, whichever is lesser, near the supports. In non-critical regions, the spacing can be increased to half the effective depth or a maximum of 300 mm.
Q: What is the significance of 135° hooks in stirrups for ductile detailing?
Answer: 135° hooks with an extension of at least 6 times the diameter of the bar (but not less than 65 mm) are required for stirrups. These hooks ensure that the stirrups remain anchored in the concrete, preventing them from opening or slipping out during intense seismic shaking.
Q: Why is additional anchorage required at beam-column joints?
Answer: Additional anchorage at beam-column joints ensures that the longitudinal bars are properly embedded in the joint region, which prevents them from pulling out under seismic forces.
Q: What are the requirements for shear reinforcement in beams under IS 13920:2016?
Answer: The standard specifies closely spaced stirrups, especially near the beam ends and critical regions, to prevent shear failure. The maximum spacing of stirrups should not exceed \(d/4\) (where \(d\) is the effective depth of the beam) or 100 mm, whichever is lesser, near the supports. In non-critical regions, the spacing can be increased to half the effective depth or a maximum of 300 mm.
References:
- Bureau of Indian Standards. (2016). Ductile Design and Detailing of Reinforced Concrete Structures Subjected to Seismic Forces: —Code of Practice (IS 13920: 2016) (First Revision). Bureau of Indian Standard (BIS).
- Bureau of Indian Standards. (2000). Plain and Reinforced Concrete: — Code of Practice (IS 456: 2000) (Fourth Revision). Bureau of Indian Standard (BIS).
