If you are learning BBS, stirrup cutting length is usually the first thing that confuses you. A straight bar is easy. But a stirrup has to be bent, and the steel stretches a little at every bend. That is why simply adding up the sides does not give the right answer.

Let’s take one column and work it out from start to finish.

The formula

Cutting length = 2a + 2b + (2 × hook) − bend deductions

• a and b: the width and depth of the stirrup. Take the member size and subtract the cover on both sides.
• Hook: 10d on each side, where d is the stirrup diameter. There are two hooks, so the total is 20d.
• 90° bend: deduct 2d. A stirrup has three of these.
• 135° bend: deduct 3d. A stirrup has two of these.

Put it all together and the formula gets shorter:

Cutting length = 2a + 2b + 20d − 6d − 6d = 2a + 2b + 8d

Example: 300 × 450 mm column

• Column size: 300 mm × 450 mm
• Clear cover: 40 mm
• Stirrup: 8 mm dia at 150 mm c/c
• Column height: 3000 mm

Step 1: Find a and b

a = 300 − 40 − 40 = 220 mm
b = 450 − 40 − 40 = 370 mm

Step 2: Use the formula

2a + 2b = (2 × 220) + (2 × 370) = 1180 mm
Hooks = 2 × 10 × 8 = 160 mm
90° deduction = 3 × 2 × 8 = 48 mm
135° deduction = 2 × 3 × 8 = 48 mm

Cutting length = 1180 + 160 − 48 − 48 = 1244 mm, or 1.244 m.

Step 3: Find the number of stirrups

Number = (3000 / 150) + 1 = 21 nos

Step 4: Find the total steel

Total length = 21 × 1.244 = 26.12 m
Unit weight of 8 mm bar = 8² / 162 = 0.395 kg/m
Total weight = 26.12 × 0.395 = 10.32 kg

Common mistakes on site

Forgetting the cover. If you use the member size as a and b, the stirrup comes out too big. It will not fit inside the shuttering, or you will lose your cover.

Skipping the bend deduction. The difference looks small, only 96 mm per stirrup. But across 2000 stirrups, that is 192 m of steel.

Using one spacing for the whole column. Check the drawing. Many columns have closer spacing near the ends, such as 100 mm, and wider spacing in the middle. Count the stirrups for each zone separately.

Cutting everything at once. Make one sample stirrup first and check it on the column cage. If it fits, cut the rest.

One important note

The hook and bend values used here are common site practice. Some projects give different values in the structural drawing or specification. If they differ, follow the drawing.

Try this yourself

Work out the stirrup for a 230 × 450 mm beam, with 25 mm cover and 8 mm dia. Your answer should be 1224 mm.

Want to learn more? Read our BBS guide for a slab example. The BBS E-Book explains BBS components and formats, and the Automatic BBS Excel sheets save you from doing these calculations by hand.

Choosing your next book? See our guide to the best construction e-books for civil engineers.