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Tuesday, December 18, 2018

Propriety of Bending Schedules for Construction Purposes

Structural engineers usually issue bar bending schedule to clients/contractors after completing design and detailing of reinforced concrete structures. Bar bending schedules usually show the cut length, shape, and quantities of reinforcements needed for a particular phase/element of a project.

The bar bending schedule issued by structural engineers are usually very exact. In other words, they provide the exact quantity of steel that is needed to produce what they have shown in their detailing, without further considerations.

The primary challenge faced by contractors on site is the issue of off cuts, since rebars are usually supplied in 12 m length. Off cuts are not usually accounted for in the schedule issued by the structural engineers, and practically, not all off cuts are reusable.

In this case, there are usually two things;

(1) The contractor usually buys more reinforcement than as given in the bill, even though he gets to keep the off cuts, and perhaps resell them for recycling.

(2) If a contractor purchases reinforcements based on bar bending schedule only, he risks requesting for supply twice, which will add to his expenditure due to transportation cost.

Case Study
To make this point clearer, let us consider this practical scenario below. The ground floor of a 3 storey commercial development is 4.0 m high, and the details of the column reinforcement are as shown below;

CT1 - Column Type 1 (230 x 230)mm - 2 Nos
Main bars: 4Y16
Links: Y10-200 c/c

CT2 - Column Type 2 (450 x 230)mm - 4 Nos
Main bars: 6Y16
Links: Y10-200 c/c

CT3 - Column Type 3 (600 x 230)mm - 2 Nos
Main bars: 8Y20
Links: Y10-250 c/c

CT4 - Column Type 4 (450 x 450)mm - 1 No
Main bars: 12Y20
Links: Y10-300 c/c

At a lap length of 45ϕ (45 x diameter of bar), the typical bending schedule prepared by the structural engineer for the main bars of the ground floor to first floor columns is as follows:

From the above bending schedule, the quantity of steel to be purchased (per 12 m length) for the main bars are as follows;

Y16: [242.28/(1.579 x 12)] = 12.78 (say 13 lengths of Y16mm)
Y20: [343.51/(2.466 x 12)] = 11.61 (say 12 lengths of Y20mm)

As you can see, these quantities are easily verifiable from the bending schedule, and could have possibly been used in the preparation of the bill. It is the exact quantity required as given in the drawing.

However, let us go to site and see how it will possibly play out:

Reinforcements are supplied in 12m length.

The total quantity of bar mark 01 that will need to be cut by the iron bender is 32 pcs (kindly verify), and the cut length of each bar is 4795 mm. What this means is that we can only obtain 2 pcs of bar mark 01 from each length of reinforcement, an an off cut of 2410 mm. This off cut cannot be used anywhere in the column, except perhaps it gets useful in the beams or other places. Since we need 32 pcs of bar mark 01, we will need 16 lengths of Y16 mm against the 13 lengths calculated in the bending schedule. We will therefore have 16 pcs of 2410 mm as offcut!

Likewise, the total quantity of bar mark 02 that will need to be cut by the iron bender is 28 pcs, and the cut length of each bar is 4975 mm. This also means that we can only obtain 2 pcs of bar mark 02 from each length of reinforcement, with an off cut of 2050 mm. In effect, we will need 14 lengths of Y20 mm against the 12 lengths calculated in the bending schedule.

If the contractor did not make this consideration before requesting for supply, he will likely make his request twice, at the detriment of extra cost. Therefore, the effects of off cuts should be verified by the contractor before he can place his order for reinforcement. This is usually very common especially for someone who has not experienced it before.

So thank you for visiting Structville today, and God bless you.

Do you need help in knowing the exact quantity of reinforcement that you will need to complete your project without needing to go to the market twice? We are here to help you with that. When you send your drawings to us, we will assess all reinforcements and bar marks independently, and tell you where all off cuts can be reused. We achieve this by issuing a special bending schedule and cutting scheme that takes into account re-usage of off cuts for foundations, beams, slabs, and columns. We will also issue a cutting sequence to the iron bender such that wastage can be minimised as practically as possible. This can save you a reasonable amount of money for your projects, and we do this for a very low fee that is based on the quantity of off cuts that we saved for you. Try us today by sending an e-mail to;info@structville.comor call +2348060307054


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