#LAP LENGTH OF BARS

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LAP LENGTH:
Development length and lap length are two important terms in reinforcement. But many of us get confused with the difference between development length and lap length. In our previous article we have already discussed what is development length of bars, today we will discuss what is lap length of bars.
During placing the steel in RC structure if the required length of a bar is not sufficiently available to make a design length then lapping is done. Lapping means overlapping of two bars side by side to achieve required design length.
Suppose, we need to build a 100 feet tall column. But practically 100 ft long bar is not available and it is also not possible to cage. Therefore we need to cut the bars in every second story. Now, we need to transfer the tension forces from one bar to the other at the location of discontinuity of bar. So we have to provide the second bar closed to the first bar that is discontinued and overlapping is to be done. The amount of overlapping between two bars is known as lap length.
In case of RCC structure, if the length of reinforcement bars need to be extended, splicing is used to join two reinforcement bars for transferring the forces to the joined bar

LENGTH FORMULA:
LAP LENGTH IN TENSION:
The lap length including anchorage value of hooks shall be
1. For flexural tension – Ld or 30d whichever is greater.
2. For direct tension – 2Ld or 30d whichever is greater.
The straight length of lapping shall not be less than 15d or 20 cm.
LAP LENGTH IN COMPRESSION:
The lap length in compression shall be equivalent to the development length in compression computed but not less than 24d.
FOR DIFFERENT DIAMETER BARS:
In case of bars having different diameter are to be spliced, the lap length is calculated on the basis of smaller diameter bar.
LAP SPLICES:
Lap splices should not be used for the bars having larger dia than 36 mm. In that case, welding should be done. But if welding is not practicable then lapping may be permitted for the bars larger than 36 mm dia. Additional spirals should be provided around the lapped bars.
LAP LENGTH FOR CONCRETE OF 1:2:4 NOMINAL MIX:
Lap length in tension (for plain Grade-1 MS bar) including anchorage value is 58d. So eliminating the anchorage value the lap length = 58 – 2*9d = 40d
where 9d = hook allowance of bars up to 25 mm and k=2
LAP LENGTH FOR M20 CONCRETE:
Columns – 45d
Beams – 60d
Slabs -60d.
So if we need to lap 20 mm dia column bars, we have to provide a minimum lap of 45 * 20 = 900 mm.

METHODS OF CONCRETE CURING

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CONCRETE CURING:

The process of protecting the moisture of concrete surface and enhancing the hydration of cement is known as curing. The physical properties of concrete totally depend on the hydration of cement. If curing is not done properly concrete will be failed to acquire its full strength. Improper concrete curing may also lead the concrete to crack.

METHODS OF CONCRETE CURING:

1. SHADING:

By this method, the evaporation of water is locked in the concrete surface. It also protects the surface from heat, wind etc. In cold climates, it prevents the concrete from freezing by preserving heat of hydration of cement.

2. COVERING THE SURFACE WITH WATERPROOF PAPER:

In this method, the concrete surface is covered with wet gunny bags or waterproof papers to avoid water loss and to protect the concrete from further damage. This method gives satisfactory results for curing of concrete slabs and pavements.

3. SPRINKLING WATER:

In this method, water is sprayed on the concrete with the help of nozzles at proper intervals. This method is not so effective due to the difficulty of keeping the concrete surface be moist all the time.

4. PONDING:

Ponding is the most common method that is adopted for curing of floors, slabs, pavements etc. In this method, concrete surface is first covered with moist wrapper for 24 hours. After that, the covers are then removed and small amount of clay puddles are constructed around all the area. Then water is filled for final curing.

5. MEMBRANE CURING:

In this method concrete surface is covered by waterproof membranes or sealing compounds such as bitumen emulsion, wax, rubber latex emulsion, water repellant, plastic films etc. The membrane protects the water loss from concrete. It is seen membrane curing for 28 days give equivalent strength to two weeks moist curing.

6. STEAM CURING:

Steam curing is done by increasing the temperature of concrete in wet condition. This method allows the concrete to achieve its full strength within a short time, thus curing is also finished within short time. Steam curing is mostly adopted for the production of precast members.

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#One way slab & Two way slab

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One Way Slab

One way slabs are such slabs in which the loads are shared only by the shorter direction of the slab. In above figure (a) represents one way slab. Here ratio of longer span of slab to the shorter span of slab is greater than 2. One way slab being supported on two opposite sides only leads to the structural action in single direction.

Analysis and Design of One-way Slab:

Design steps:

Step 1: Preliminary Design:

Find depth of slab using deflection control criteria.

Take Span/d = 25 for simply supported slab.

= 30 for continuous slab.

D ≥ Dmin = 100mm for normal design.

= 125mm for earthquake resistant design.

Step 2: Analysis of slab:

Analyze the slab considering the slab as beam of 1m width and find respective loads, maximum design bending moment and maximum design shear force.

Step 3: Design of Slab:

Verify depth of slab:

Determine ‘Ast’ and ‘Φ’ and number of bars in shorter direction of slab.

Slab section is normally designed as Singly Reinforced Under Reinforced Rectangular Section.

Ast = Mu/(0.87fy(d-0.416xu)) which should not be less than Astmin.

Check slab for shear.

Step 4: Check slab at limit state of serviceability in deflection.

l/d ≤ αβγδγ

Step 5: Detailing of reinforcement.

Curtail or bent surplus bars.

Check development length at support.

Ld ≤ 1.3Ml / Vu + ld

Provide min. Ast as distribution bars in longer direction of slab.

Two Way Slab

Two way slabs are such slabs in which the loads are shared by both the shorter and longer direction of the slab. In above figure (b) represents one way slab. Here the ratio of longer span of slab to the shorter span of slab is less or equals to 2. Two way slab being supported on four sides the load will be carried in both the directions.

Analysis and Design of Two Way Slab:

Design Steps:

Step 1: Preliminary Design.

Find depth of slab using deflection control criteria.

Take span/d = 28 for simply supported slab and 32 for continuous slab.

D ≥ Dmin = 100mm for normal design and 125mm for earthquake resistant design.

Step 2: Analysis of Slab.

Slab is analyzed by IS code method. (Ref. Annex D-1 and Annex D-2).

Step 3: Design of Slab.

Verify depth of slab:

Determine ‘Ast’ and ‘Φ’ and number of bars in shorter direction of slab.

Slab section is normally designed as Singly Reinforced Under Reinforced Rectangular Section.

Ast = Mu/(0.87fy(d-0.416xu)) which should not be less than Astmin.

Check slab for shear in shorter direction.

Step 4: Check slab at limit state of serviceability in deflection in shorter direction.

l/d ≤ αβγδγ

Step 5: Detailing of reinforcement:

Curtail/bend the surplus bars as per detailing rules of code. (Ref. Annex)

Check development length at support.

Ld ≤ 1.3Ml / Vu + ld

Provide torsion reinforcement at the discontinuous edge of slab as per codal provision.