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Lintel beams are constructed on the top of openings in the wall like windows and Doors. These are designed based on the dimensions of the openings above which it is built. Among different types of lintels, RCC lintel is the most commonly used lintel due to its ease of design and construction. RCC lintels can be designed for any combination of loads and also for any span.
The article explains the general design steps of an RCC lintel based on limit state method of design.
Contents:
Loads on RCC Lintel Beams
The different types of loads taken into consideration for the design of RCC lintel beam are:
- Self-weight or the dead-load of the lintel
- The dead load of the wall above the lintel
- The dead load and the live load transferred from the roof or the floor slabs above.
The figure-1 below shows the illustration of how load acts on the lintel beam. It is similar to a simply supported beam subjected to a triangular load.
Now, when the height from the lintel beam to the top of the wall, H is greater than 1.25h, then the load is transferred to the lintel beam by arch action. Here, 'h' is the height of the triangle formed.
Where, for arch action H>= 1.25h
h= l x (sin i), l is the effective span of the lintel beam and 'i' is the angle made by the triangular load with the lintel beam.
In arching action, the masonry distributes the load so that there is no load coming on the lintel. In this case, the lintel beam is designed for the self-weight of the lintel beam and the triangular area load coming over the lintel. The angle made by the triangular load with the lintel beam varies from 45 to 60 degrees.
Here, the design procedure of a lintel beam subjected to a triangular load by arch action is explained.
Design Steps for an RCC Lintel Beam
The basic steps involved in the design of an RCC lintel beam are:
1. Determination of Effective depth and Span of RCC lintel
- Take effective depth of lintel, d = l/10; [IS:456-2000, Cl.23.2.1]
- Overall depth D = Effective depth (d) + Effective cover
- Width of the lintel, B = Thickness of the masonry wall
- Effective Span,l = Least of ([ Clearspan + Bearing] , [Clean Span + Effective Depth])
- The minimum bearing width of a lintel beam is taken to be 150mm. It is bedded over a PCC of 50mm thick.
2. Determination of Loads and Moments on RCC Lintel Beam
The triangular design load coming over the lintel beam can be calculated by the formula:
Here, lef is the effective span; the thickness of the masonry is given by 't', and 'rho' is the density of masonry, i.e. 19.2kN/m2.
- Self-weight of the lintel per metre length (kN/m) w = ( 1 x b x D ) x Density of RCC
- Moment due to self-weight of Lintel, M1 = wl2/8
- Moment due to triangular load of masonry wall above the lintel, M2 = Wl/6
- Total Bending Moment M = M1 + M2
- Factored Ultimate moment = Mu = 1.5M
3. Check for Effective depth of Lintel Beam
As per Indian Standard IS:456-2000, the moment of resistance for RCC beam is given by:
- For Fe250 bar steel, Mu = 0.149fckbd2
- For Fe415 steel, Mu = 0.138fckbd2
The depth obtained from the above formula is the depth required for the given lintel beam (dreq).
- If d < dreq the design is not safe. Revise the design for an increased depth
- If d > dreq the design is safe.
4. Design of Steel Reinforcement for Lintel Beam
Tension Reinforcement (Ast) for Lintel Beam
The formula for moment of resistance is given by,
From the above equation, determine the value of Ast. Choose the diameter of bars (8mm, 10mm, 12mm..).
Number of bars for tension reinforcement = Ast/ (Area of one bar)
Shear Reinforcement (Ash)
- Shear Force V = (wl/2 + W/2);
- Factored shear force Vu = V x 1.5;
- The ultimate nominal Shear Stress is given by :
4. Percentage Steel = 100[Ast/bd]
From the value of tv and percentage steel, Refer Table-19 of IS: 456-2000 and by interpolation, determine the value of design shear strength tc. The entire value of shear is taken by concrete.
If,
1. tv < tc, Then no shear reinforcement is required. But nominal stirrups are provided with a minimum spacing Sv) is determined by the Asv/(b.Sv) = 0.4(0.87fy)
2. tv > tc, Provide shear reinforcement in the form of vertical stirrups, bent up bars, or inclined stirrups. The design for shear is similar to the design of the RCC beam.
The spacing selected must be Minimum of (Sv,0.75d,300mm). The shear reinforcement is designed similar to the Design of a Rectangular R.C.C Beam.
5. Reinforcement Detailing of RCC Lintel Beam
The reinforcement detailing of the RCC lintel beam is similar to that of an RCC beam, with main reinforcement, top distribution reinforcement and stirrups for shear.
From the above figure, the main bottom reinforcement is provided by 10mm diameter bars in 4 numbers. Out of which, 2 bars are bent up bars. The top reinforcement is also provided by 10mm dia bars in 2 numbers. The vertical stirrups provided are two-legged -6mm diameter bars at a spacing of 125mm c/c.
Frequently Asked Questions
The formula below can calculate the triangular design load coming over the lintel beam:
Here, lef is the effective span; the thickness of the masonry is given by 't,' and 'rho' is the density of masonry, i.e., 19.2kN/m2.
The different types of loads taken into consideration for the design of R.C.C lintel beam are:
a. Self-weight or the dead-load of the lintel
b. The dead load of the wall above the lintel
c. The dead load and the live load transferred from the roof or the floor slabs above.
The minimum bearing width of a lintel beam is taken to be 150mm. It is bedded over a P.C.C of 50mm thick.
When the height of the wall above the lintel is H, then H must be greater than equal to 1.25h, where h is the height of the triangular load.
h= l .sin 60, l is the effective span of the lintel beam.
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