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Bar Bending Schedule (BBS) is used to calculate the total steel required for building construction. It organizes reinforcement bars by structural element and provides details like bar location, marking, size, quantity, and shape. Using a bar bending schedule makes it easy for site engineers to check and approve the length of the bent and cut bars during an inspection before the concrete is poured. It helps with better quality control. It makes it easy and quick for clients and contractors to produce bills for construction work. Accurate placement of bars reduces material wastage and minimizes rework.
IMPORTANT POINTS TO UNDERSTAND BEFORE CREATING REBAR SCHEDULE
1. Location of bar. The placement and orientation of the reinforcement bars within the structural element.
2. Type of bar. The specific type of reinforcement bar used in the construction project.
3. Size of bar. The diameter of the reinforcement bar.
4. Cutting length of bar. The length of the reinforcement bar required for a specific structural element.
5. Number of bars. The total number of reinforcement bars needed for a particular structural element.
6. Bending detail of bars. the specific angles and shapes required for bending the reinforcement bars to match the structural design.
7. Total quantity of steel. The overall amount of reinforcement steel required for the entire project.
WEIGHT OF STEEL BARS
The unit weight of steel is important for many applications. Steel is frequently used in construction, and the weight of the steel must be known to calculate the loads on the structure. The unit weight of steel is also used in shipping containers and other applications where the weight of the steel must be known.
Formula:
W = D2/162Kg/m
Where:
W = Weight of bars
L = Length of bars in meters
D = Diameter of the bar
TYPES OF CONCRETE COVERS IN REINFORCED CONCRETE
In reinforced concrete, the concrete cover is the shortest distance between the surface of embedded reinforcement and the outer of the concrete. Types of concrete covers are:
1. Minimum Cover. This is the minimum thickness of concrete cover required to protect the reinforcing steel from corrosion and to ensure adequate bonding. The minimum cover depends on the type of exposure (i.e., whether the structure will be exposed to water, soil, or air) and the size of the reinforcing steel.
2. Nominal Cover. This is the cover specified in the design of the reinforced concrete structure, which is typically greater than the minimum cover to provide additional protection to the reinforcing steel.
3. Effective Cover. This is the actual thickness of concrete cover provide in the construction of the reinforced concrete structure, which may be greater or less than the nominal cover due to variations in construction practices.
Effective Cover= Clear cover + Total Diameter of Stirrup + Diameter of main reinforcement bar / 2
4. Clear Cover. This is the distance from outer surface of concrete to the nearest surface of the reinforcing bar. Clear cover is important for ensuring proper placement of reinforcing steel and preventing congestion.
Clear Cover = Effective Cover - Dia. of Reinforcing Bar
Effective Concrete Cover
1. Grade Slab: 20mm
2. Flat slab: 20mm
3. Beams: 25mm
4. Strap Beam: 50mm
5. Shear Wall: 25mm
6. Staircase: 15mm
7. Column: 40mm
8. Footing: 50mm
9. Raft Foundation Top: 50mm
10. Raft Foundation Bottom/Sides: 50/70mm
11. Retaining Wall: 20/25mm
HOOK LENGTH
Hooks are offered to oppose the seismic movement. To avert concrete crack outward. It averts steel slippage from the concrete. To maintain longitudinal steel bars in place as well as keep steel firmly. The main function of a hook is to develop anchorage.
Why is there a requirement for anchorage?
The tension reinforcement should not be slipped from its position, otherwise, the purpose of providing the reinforcement will not be served. So to develop sufficient bond strength between still and concrete, the development length requirements must be satisfied. Now there may be cases where the required length is not available to meet the requirements of development. In those cases, anchorages may be provided in the form of hooks.
In general, Hooks are added at the two ends of the rebar in stirrups or ties. Generally, hook length is taken as 9d for one side.
Formula:
Hook length = Total length of the Bar + 2 x Hook Length(for two hooks)
= L + 2(9d)
= L + 18d
Where:
L = length of the bar
d = diameter of rebar
BEND LENGTH
Bend is usually used in footing to suit the design of a structure and to ensure proper reinforcement and tension. It also can act as an hook and provide Anchorage at the ends. Bend are also added at the two ends of the rebar for footing. Generally, bend length for footing is taken as 10d to 16d for one side.
Formula:
Cutting length = L + 12d + 12d
Where:
L = length of the bar for the stirrup
d = diameter of rebar
DEVELOPMENT LENGTH
Development length is the amount of reinforcement embedded in a column to achieve the suitable bond strength between the steel and concrete. It has a crucial role to play in the stability of a structure as it helps to prevent joint slippage.
The quantity of the rebar length that is actually required to be embedded into the concrete to create the desired bond strength between steel and concrete and furthermore to produce the required stress for the steel in that area.
Formula:
Development length(Ld) = Φσs/4τbd
Where:
Φ = diameter of rebar
σs = stress in the bar at the section considered as the design load.
τbd = design bond stress
Note: This formula is used to calculate the required development length in millimeter for any given diameter of the bar.
LAP LENGTH
An important term in BBS. Lap length is the overlapping length of two bars side by side, giving the required design length. in the RCC structure, if the length of a bar is not sufficiently available to make design length, lapping is done.
Where:
Lap length for tension members = 40d
Lap length for compression members = 50d
Diameter of bars = d
CRANK BAR(BENT UP BAR) LENGTH
Bars are bent near the support at an angle of 45º. The angle of the bend may also 30º in shallow beams. The purpose of bending near the support is firstly to resist the negative bending moment(hogging moments) which occurs in the region of the support and secondly to resist the shear force which is greater at the support. Crank bars are mostly provided in slabs.
Formula:
Crank length = D/sin45º - D/tan45º
= 1.42D - D
Crank length = 0.42D
CUTTING LENGTH OF STIRRUP
A stirrup is a closed loop of rebars in a reinforced concrete component that keeps the main reinforcement bars together. Stirrups may be of various types depending on the design and shape of the structural members. Stirrups are made of steel rebars wrapped around the top and bottom bars of beams (rectangular shape) or columns (rectangular or circular shape). It is done to avoid shear failure, which occurs in the case of beam cracks and is generally diagonal. The designer should preferably specify the stirrup spacing along the beam.
Shapes of Stirrups:
Rectangular
Square
Triangular
Diamond
Helical
Methods for Calculating Cutting Length of Stirrups
Study the size of a beam or column on drawings
Check the diameter of the bar given for stirrups (Mostly 8mm diameter is used for stirrups)
Subtract the clear cover or concrete cover.
Calculate the total perimeter/outer length for the stirrup by deducting the concrete cover.
Then add the hook length to the outer length of the stirrup.
Now subtract the length of bends.
Formula:
Cutting Length of Stirrup = Perimeter of stirrup shape + Hook Length - Bend Length
Important formulas:
Perimeter of Rectangular = 2(Length + Width)
Perimeter of Square = 4 x Side Length
Circumference/ perimeter of Circle = 2π or πd (Where R = radius, d = Diameter)
Cutting Length of Rectangular Stirrup
The rectangular shape is the most common and generally used for beams and columns in many structures. This shape column and beam have rectangular shape stirrups.
Cutting Length of Square Stirrup
The square shape is also the most common and generally used for beams and columns several structures. This shape column and beam have square shape stirrups.
Cutting Length of Circular Stirrup
Cutting Length of Triangular Stirrup
Cutting Length of Diamond Stirrup
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