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Tuesday, May 22, 2018

Design and Calculation of Formwork Requirement of Slabs

Formworks are structures to used to support and hold fresh concrete in place in order to obtain the desired shape prior to setting, curing, and hardening. Formwork is usually temporary (struck from concrete after curing) or permanent. Normally, formwork is designed to support load from fresh concrete (including static pressure), their self weight, live load from working personnels, and load from other equipment.

A typical formwork configuration for floor slabs is shown in the figure below;

In Nigeria, the standard given below works for normally proportioned reinforced concrete slabs.

Sheathing - 20mm thick marine plywood
Joists - 200mm deep wooden H-beams spaced at 600 mm c/c
Stringers - 200mm deep wooden H-beams spaced at 1000 mm c/c
Shores - Steel props spaced at 1000 mm c/c

For low cost construction, the following can be used;

Sheating - 25mm thick wooden planks
Joists - 2" x 3" wood (50mm x 75mm) spaced at 400mm c/c
Stringers - 2" x 4" wood (50mm x 100mm) spaced at 600 mm c/c
Shores - Bamboo wood spaced at 600 mm c/c

It is usually interesting to compare the cost of using these two alternatives, but that will be a post for another day.

Now having known how to arrange formwork for floor slab, the next phase is to determine how to place your order of materials. We will use a typical example to show how it is done.

The first floor plan of a building is as shown below. We are to determine the formwork requirement of the floor slab. We will be neglecting the floor beams in this calculation.

On studying the drawing,
Length of longer side of building = 16.120 m
Length of shorter side of building = 12.530 m
Therefore, gross area of building =  201.9836 m2

Area of openings (lift and staircase) = 18.21 m2

Therefore net area of slab = 201.9836 - 18.21 = 183.7736 m2

Marine Plywood Requirement
Area of each marine plywood = 2.4m x 1.2m = 2.88 m2

Number of marine plywood required = 183.7736/2.88 = 63.8
Therefore, supply 64 pieces of (2.4m x 1.2m) marine plywood (no allowance for wastage)

If plank were to be used;
Area of one plank = 3.6m x 0.3m = 1.08 m2
Therefore, supply 183.7736/1.08 = 171 pcs of (1" x 12" x 12' plank)

Floor Joist Requirement
Length of wooden H-beam = We normally have variety of 3.9 m or 2.9 m
Considering the longer side of the building;

Number of H-beam required per line = 16.12/3.9 = 4.13 No
We can therefore say, provide 4 No of 3.9m H-beam and 1 No of 2.9m H-beam per line (there will be projections though, and appropriate considerations should be made on site)

Spacing = 600mm

Therefore number of lines required = 12.53/0.6 + 1 = 22 lines

Hence provide;
3.9m H-beam = 4 x 22 = 88 pieces
2.9m H-beam = 1 x 22 = 22 pieces

If it were to be that 2" x 3" wood will be used;
Supply length of 2" x 3" wood in Nigeria = 3.6m

Number of 2" x 3" wood required per line = 16.12/3.6 = 4.47 pcs
At a spacing of 400 mm, we have 12.53/0.4 + 1 = 33 lines

Therefore number of 2" x 3" wood required for floor joists = 4.47 x 33 = 148 pieces

Stringer Requirement
We will also be using wooden H-beams for stringers. This will run parallel to the shorter side of the building;

Number of H-beam required per line = 12.53/3.9 = 3.21
We can therefore say, provide 3 No of 3.9m H-beam and 1 No of 2.9m H-beam per line

Spacing = 1000mm

Therefore number of lines required = 16.12/1.0 + 1 = 17 lines

Summarily provide;
3.9m H-beam = 3 x 17 = 51 pieces
2.9m H-beam = 1 x 17 = 17 pieces

Shoring Requirement
Steel acrow props will be used for the shoring;
Spacing = 1m c/c

Number required on the longer side = 16.12/1 + 1 = 17 pcs
Number required on the shorter side = 12.53/1 + 1 = 14 pcs

Acrow props required = 17 x 14 = 238 pieces

Number of props required in the lift and staircase area
Length of lift area and staircase area = 5.055m (6 props)
Width of lift and staircase area = 3.60m (5 props)
Number of props that would have been in lift area = 5 x 6 = 30 pieces

Therefore, the total number of acrow props required = 238 - 30 = 208 pieces

Formwork summary for floor slab;

  • Marine ply wood = 64 pieces
  • 3.9m H-beam = 88 + 51 = 139 pieces
  • 2.9m H-beam = 17 + 22 = 39 pieces
  • Acrow Props = 208 pieces

Note that all these materials can be hired because they are very reusable. This is one of the advantages. For instance, marine plywood can be used 8 times before it gets damaged, and wooden H-beams (joists) are very durable for a long period of time provided they are well handled and protected from long exposure to moisture.

I hope you found this piece of information helpful. Thank you for visiting Structville today and God bless you. Kindly contact and find out how we can be of help to you.

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