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Why the scheduled cut length differs from the sum total of the input parameters.

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PROKON’s detailing programs are based on the guidelines, set out within the various detailing codes. All calculations and assumptions made, are therefore, code dependent. This article is based on the South African detailing code, SANS282-2004, and will include an example of a shape code 55.

In majority of cases, the value of the last input parameter of your shape code, will form part of the redundant dimension within your bending schedule. The scheduled cut length will differ from the sum of the input parameters, for various shape codes, as the cut length is calculated from a specific equation, discussed throughout this article.

A shape code’s main dimensions are defined as orthogonal – meaning, the dimensions are given as horizontal, or vertical, as seen below:

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Figure 1 : Shape code 55 layout

However, when starting to bend a straight bar, the bend radii will come into play and affect the calculation of the cut lengths. The calculation of a shape code 55’s cut length is set out within Table 1 of the code (SANS282-2004, Section 4), as seen below:

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Figure 2 : Table 1 of SANS282-2004

As seen within Figure 2 above, the equation used to calculate a shape code 55’s cut length is as follow:

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Figure 3 : Cut length equation for shape code 55

If you draw this shape code 55 out, with specific dimensions, your total perimeter length (from A-E), will be 1450mm.

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Figure 4 : Drawn out perimeter of shape code 55

The loss in length between the calculated perimeter and the scheduled cut length, is due to the bend radii, and diameters of the bars, as described within the equation seen within Figure 3 above. If a straight bar is bent to a certain specification, the calculation set out within the detailing code is used to calculate the required cutting length.

If you calculate the cutting length manually and use the guidelines within Figure 3 from the detailing code (SANS282-2004, Section4), for the correct values of radii and diameters, you can calculate your shape code’s cutting length, as seen below:

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Figure 5 : Figure 3 of SANS282-2004
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Figure 6 : Manual calculation of cutting length

The manual calculation of the cutting length can be compared to the calculated and generated cutting length within the bending schedule of Padds, as well as Probar 2D.

image 10
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Figure 7 : Padds bending schedule
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Figure 8 : Probar 2D bending schedule

The calculated and scheduled cut length within Padds and Probar 2D, equates to exactly the same length, as what was calculated manually, as seen within Figures 6-8 above.

As seen below, from the snip taken from Table 1 of the design code (SANS282-2004, Section 4), the calculated cut length, is an approximate value, which involves a slight positive difference, as one would typically experience. Therefore, the calculated cutting length will be more than adequate.

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Figure 9 : Extract from SANS282-2004, Table 1

It can be seen that PROKON’s calculation of the cutting length, is accurate and will lead to the most economic lengths for the various shape code cutting lengths. It is also clear that the cut lengths calculated by PROKON’s detailing programs, conforms to code requirements.

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