Why Strip Footing Width Should be Three (3) times its Depth on Firm Soil

One of the things young engineers learn on-site as part of necessary site experience is that when setting out a foundation for walls, the width of the foundation should be three (3) times the width of the block. The reason usually cited is to give masons enough space to work. Using a 225 mm thick block, the footing width should be (3 x 225) = 675 mm. Using a 150 mm thick block, the footing width should be (3 x 150) = 450 mm. Some site supervisors usually consider these spaces as waste. Some may use 600 mm width for a 225 mm block and 400 mm width for a 150 mm block.

When casting concrete for strip foundation footing on firm soil for a storey building, 225 mm is usually the thickness of concrete adopted for strip footing whereas, for bungalows, it is usually 150 mm. Recall that a 225 mm thick block is normally used for storey buildings while a 150 mm block is normally used for bungalows.

It is expected for the sake of economy and safety that a strip footing for a storey building using a 225 mm thick block should have a footing width, B of (3 x 225) = 675 mm and footing depth, D of 225 mm. For a bungalow, using a 150 mm thick block should have a footing width, B of (3 x 150) = 450 mm, and footing depth, D of 150 mm.

The reason for this is the shear plane but this is strictly applicable to soils that have good bearing capacity (≥75 kNm^-2 for bungalows and ≥100 kNm^-2 for 2-story to medium-rise buildings. Figures 1 to 3 show the different shear planes on the footing of a block wall produced by a 225 mm thick block. Figure 1 shows when B = 3D. Figure 2 shows when B ˂ 3D while Figure 3 shows when B ˃ 3D

Due to the fact that the shear plane of concrete is at 45⁰, it is very important that Figure 1 should be strictly adopted if one does not intend to reinforce the footing. From Figure 1 we can see that the shear plane of the block load would not have an effect on the footing as the area under the shear plane is adequate to resist the shear force. Figure 2 should not be adopted as it may not be economical. Figure 3 should not be used if one would not reinforce the footing. Using Figure 3 without reinforcing the footing would cause the footing to fail in shear due to inadequate area to resist shear force. Even though one may initially adopt Figure 1, if one does not supervise manual excavation properly, one may get Figures 2 and 3. It is necessary that foundation excavations are properly controlled. Likewise, the casting of footing concrete should be properly monitored to ensure that the expected thickness is achieved.