QS Knowledge - QA/QC Civil Engineering

## Bar Bending Schedule (BBS) for Circular Slab | Quantity Surveying | Cost Estimation

Circular Slab

Today in this post we will be going to establish "Bar Bending Schedule of a Circular Slab".

As the Bar bending Schedule for a circular slab is not that easy as it looks like; We can use AutoCAD to speed up the process; however, we cannot neglect completely the manual calculations as computer is sometimes not available in a limited resource projects.

So this post deals with both the method on how to establish bar bending schedule of a circular slab with manual calculation using Pythagoras theorem and simple triangle solutions.

So let us first go for the manual calculation of the bar bending schedule of Circular Slab:-

## By Manual Calculation :

 General Layout of Reinforcement for a Circular Slab

In the above Figure a slab is shown of 5 meters diameter which have rebar of #5 @ 150 C/C

The total diameter of the slab is 5 meters therefore the no. of rebar for the slab would come as ;-

Sometimes +1 is recommended in above equation just for ensuring sufficient numbers; however, here in order to avoid congestion it is avoided.

 Arrangement of Rebars

The arrangement of these 33 bars are shown in above figure; 1 rebar is at the center / middle of the slab while 16 rebars are upside and 16 are downside of the diagram.
The 16 bars upside and downside are identical.

### Calculation by Using Pythagoras Theorem and Solving Triangle:

 Finding the Chord Length of a Circular Slab

We will use the arrangement as shown in above figure to calculate the length of the chord at any distance from the center. Consider Triangle ABC as shown in above figure;

Thus,

AS x which is the distance of the chord form the center is already known; thus we will solve above equation for L which is half of the chord;

To calculate the length of chord we will multiply above equation with 2;

To calculate the length of rebar we will subtract the cover from both ends which is 25 mm thus 50 mm will be subtracted; in other words the length of rebar at any distance x from the center would be ;

For our base slab; above equation would be;

 Distances of Reinforcement from Center of Circular Slab

For Middle Rebar

For Next rebar upside (using above equation derived)

Taking x = 150

Taking x = 300

Same procedure is adopted to get the length of all the rebars;

### Calculating Unit Weight of Rebar:

For unit weight of #5 diameter rebar;

We can use the above unit weight to calculate the weight of rebar by multiplying it with length.

See the More Examples of Bar Bending Schedules (BBS) of Beam, Slab, Column or Stair (Click Here)

## Bar Bending Schedule, commonly referred to as “BBS” is a comprehensive list that describes the location, mark, type, size, length and number, and bending details of each bar or fabric in a Reinforcement Drawing of a Structure.

This process of listing the location, type and size, number of and all other details is called “Scheduling”. In context of Reinforcement bars, it is called bar scheduling. In short, Bar Bending Schedule is a way of organizing rebars for each structural unit, giving detailed reinforcement requirements.

## General guidelines to be followed in preparing BBS:

• ·         The bars should be grouped together for each structural unit, e.g. beam, column, etc.
• ·         In a building structure, the bars should be listed floor by floor
• ·         For cutting and bending purposes schedules should be provided as separate A4 sheets and not as part of the detailed reinforcement drawings.
• ·         The form of bar and fabric schedule and the shapes of bar used should be in accordance with BS 8666.
• ·         It is preferable that bars should be listed in the schedule in numerical order.
• ·         It is essential that the bar mark reference on the label attached to a bundle of bars refers uniquely to a particular group or set of bars of defined length, size, shape and type used on the job.
• ·         This is imperative as a bar mark reference can then point to a class of bar characteristics. Also, this helps steel fixers and laborers keep track of the type and number of bars needed to complete a certain work.

## Bar Bending Schedule is used by the:

• ·         Detailer
• ·         person checking the drawing
• ·         contractor who orders the reinforcement
• ·         organization responsible for fabricating the reinforcement
• ·         steel fixer
• ·         clerk of works or other inspector
• ·         the quantity surveyor

Quantity surveyor is responsible for estimation and costing operations of a project. This kind of surveying demands a high level of precision. Bar Bending Schedule helps the quantity surveyor to consolidate the number of bars required of each bar type.
This leads to an estimation of the quantity of steel, which translates to the cost requirements for steel work.  Hence, BBS is used by the contractor who orders the reinforcements as well. Unit cost of steel is charged by weight of steel purchased.
Clerk of works and other inspectors refer to the BBS to make sure that the reinforcement work in the site is in tandem with the design requirements as per drawings. It is used as a frame of reference by the steel fixers firsthand. They can easily make note of the number and kind of rebar needed for a structural unit.
In essence, Bar Bending Schedule subsumes all necessary information on reinforcements, used by professionals at various stages of the construction process, right from procurement to finish.

## How to prepare a BBS?

### Necessary Columns

·         Bar number/Bar Mark Reference
·         Bar shape
·         Diameter
·         Spacing
·         Length of bar
·         Cutting Length
·         Number of bars
Most of the information in a BBS can be found in reinforcement drawings of the structural unit. Bar shape, diameter, length and spacing is directly entered in the schedule just by looking at the drawings, which will have detailed dimensioning.

Number of bars:
Suppose the spacing of stirrups is 150 c/c and the length along which they are placed is 6800 mm, we can find the number of bars by the formula below
[ Length / Spacing ] + 1 = number of bars
[ 6800 / 150 ] + 1 =  46.33
In this case, we always round up. Hence, we require 47 stirrups.
Cutting Length:
We must remember than steel is ductile in nature and is subject to elongation. Hence, the length of a bar is increased when bends or hooks are introduced.  Hence, certain deductions are needed to offset this increase in length.
Cutting Length = True Length of a bar – Deductions
For 45 degree
Cutting length  = Total length – 1 x Dia of bar x No. of bends
For 90 degree
Cutting  length  = Total length – 2 x Dia of bar x No. of bends
For stirrups:
90 degree hook:
Length of stirrup = (2A + 2B) + 20 x dia
135 degree hook:
Length of stirrup = (2A + 2B) + 24 x dia

Example BBS of RCC BEAM Calculation: 