A strip footing is a continuous, elongated concrete foundation that supports linear structural loads, typically beneath load-bearing walls. It transfers the load from the wall to a soil layer located relatively close to the ground surface. This footing type spreads concentrated loads from the superstructure over a wider area of soil, reducing pressure and mitigating settlement risks. The soil layer must have adequate bearing capacity and sufficient depth to prevent frost heave and other environmental issues.

Strip footings are best suited for structures with walls, such as residential buildings, schools, and light commercial facilities. They are ideal when the soil has adequate bearing capacity at shallow depths, and the imposed loads are moderate and uniformly distributed. Common applications include supporting masonry or concrete walls, continuous columns, and situations where isolated pad footings are impractical due to wall geometry or load distribution.

Typically, two types of strip footings are used:

• Plain Concrete Strip Footings - which are ideal for lighter structures and low-rise buildings with stable bearing soils.
• Reinforced Concrete Strip Footings - which are used for heavier loads or when increased durability is required due to environmental conditions. These are suitable for heavier structures where the soil bearing capacity is relatively low.

Flexural reinforcement is typically placed at the bottom of the footing, perpendicular to the face of the wall. In the transverse direction, shrinkage and temperature reinforcement should be provided parallel to the length of the wall.

Strip footings usually support linear loads beneath load-bearing walls. However, in some cases, a line of closely spaced columns may also be supported by a strip footing.

Strip footing failure modes can genrally be classified into three categories: soil bearing failures, stability failures, and structural failures. These are illustrated in the following figure.

Figure 1: Strip Footing Failure Modes

The design of strip footings involves several steps due to the various parameters and variables that affect the final dimensions and characteristics.

Step 1: Geotechnical Investigation and Considerations

The design of foundations generally requires determining the behavior and stress-related deformability of the soil under the foundation. To achieve this, the geotechnical properties of the soil should be determined. These properties include the grain-size distribution, soil classification, plasticity, compressibility, and shear strength. The investigation aims to determine the suitability of different foundation types and the soil's bearing capacity. This process normally includes performing the ultimate bearing capacity calculation and a settlement analysis. These steps determine the allowable bearing pressure (q_a) to avoid soil bearing failures. If a strip foundation is suitable, the engineer can then proceed to the next step.

Step 2: Stability Checks

Ensure the foundation system is safe against overturning, sliding, and avoid excessive uplift due to eccentricities.

Step 3: Define the Base Area

In the US, this is determined using the allowable stress and the service load combinations. The presumptive load-bearing values (IBC Table 1806.2) may also be used if permitted. The allowable stress is normally included in the geotechnical report, considering the bearing capacity and possible settlements. In a strip footing, the soil stress for a footing with an axial load (P) and moment (M) can be calculated as shown in Figure 2.

Figure 2: Soil Stress Calcualtions in Strip Footing

Step 4: Define Base Thickness and Calculate Bending Reinforcement

This is normally done by a trial-and-error procedure to avoid any structural failure. In this case, a footing thickness is adopted, and then it is checked for the bending and shear strength. In this step, the footing must be designed for bending moments, one-way shear (two-way shear is not applicable for strip footings) caused by the soil pressure due to factored loads. A minimum depth of 6 in should be considered (ACI 318-19 c13.3.1.2) and a minimum concrete cover equal to 3 in for concrete cast against and permanently in contact with ground (ACI 318-19 c20.5.1.3.2). It is also important to consider the minimum footing thickness based on the development of the bars that start from the footing to the wall for concrete walls.

If the bending moment diagram is analyzed (see Figure 2), it appears that the maximum moment in the strip footing occurs under the middle of the wall, but tests have shown that this is not correct because of the rigidity of the walls. ACI code suggests (ACI 318-19, c13.2.7.1) computing it at the face of the wall for reinforced concrete walls or at a section halfway from the face of the wall to its center for masonry walls. In the calculations, it is only required to consider the upward pressure caused by the external loads applied to the footing. The self-weight and overburden soil weight should be neglected. Only the net pressures over the footing should be used for the structural design.

Figure 3: Shear and Moment Diagrams for a Wall Footing with Uniform Soil Pressures

If the wall footing is loaded until it fails in shear, the failure will not occur on a vertical plane at the wall face but rather at an angle approximately 45° with the wall face, therefore the critical section for shear is calculated at a distance “d” from the face (ACI 318-19c13.2.7.2), being “d” the effective depth, see Figure 3. The effective depth is calculated as:

where h is the footing slab thickness, c is the cover, and d_b is the bar diameter.

Once the maximum bending moment (M_u) at the critical section has been determined, the required area of reinforcement (A_s) is determined in the same way as any flexural member. Although a footing is not a beam, it is desirable that it is ductile for flexure, and this can be done by limiting the net tensile strain in the tension reinforcement (ε_t) to a value larger than ε_ty + 0.003 (ACI 318-19 c21.2.2, ε_ty is equal to f_y/E_s).

With the former assumption, the required reinforcement area may be calculated with the following equations

b is the section width, f_’c is the specified compressive strength of concrete, f_y is the specified yield strength of the reinforcement, and E_s is the modulus of elasticity of the steel reinforcement.

The shear strength is normally calculated only considering the contribution of the concrete. It is not advisable to use shear reinforcement due to increased costs. Therefore, the shear calculated at the critical shear section should be larger than the strength resisted by the concrete. It is calculated using the equation given in Table 22.5.5.1(c) (ACI 318-19 c22.5.51)

Where ρ_w is the reinforcement ratio equal to A_s/(b×d), λ is the modification factor to reflect the reduced mechanical properties of lightweight concrete, and ϕ is the shear reduction factor.

Once the thickness of the footing is confirmed to resist flexure and one-way shear, and the adopted reinforcement is larger than the one required, we can continue with the following step.

Step 5: Calculate the Transfer Forces

The vertical and horizontal forces transferred to the footing by bearing of the concrete or a combination of bearing and interface reinforcement should be checked. This requirement is detailed in Section 22.8 of the ACI 318-19:

Where A₁ is the loaded area, A₂ is the area of the lower base of the largest frustum of a pyramid, cone. The sides of the pyramid, cone, or tapered wedge shall be sloped 1 vertical to 2 horizontal. And ϕ is a reduction factor.

Step 5: Detailing Checks

The last step is devoted to the reinforcement details as minimum and maximum spacing, development length to critical sections. The details are given in Chapter 25 of ACI 318-19.

Wall footings are essentially a subset of strip footings and are often used interchangeably, as both describe a continuous, narrow footing that supports linear loads. However, strip footings have a broader definition and may also support a line of closely spaced columns, accepting point loads arranged in a row. In terms of reinforcement, both types are similar.

Strip footings are closely related to spread footings, as both are types of shallow foundations commonly used in small to medium structures due to their low cost. Strip footings are typically long and rectangular, while pad footings may be square, rectangular, or circular. Strip footings generally support linear loads, whereas pad footings support concentrated loads. In design, all checks performed for strip footings should also be applied to spread or pad footings, with additional checks such as the two-way shear (punching) check.

The wall footing tool works with a try-and-error philosophy. The user can modify the input data until all checks will pass. Normally when there are failures, the solution involves enlarging the footing or incrementing the reinforcement. In any case, the tool also checks minimum and maximum conditions that help avoid excessive reinforcement. It is suggested to enlarge the height for shear failures, enlarge the width for stability failures, and increment the reinforcement area for bending failures when the footing height and shear checks are OK.

• Strip footings are economical and widely used for shallow foundations.
• Proper geotechnical investigation is essential.
• Design must address soil bearing, settlement, structural strength, and stability.
• Follow ACI 318-19 for all checks and detailing.

• ACI 318-19: Building Code Requirements for Structural Concrete
• IBC Table 1806.2: Presumptive Load-Bearing Values
• CRSI, Design Guide on the ACI 318 Building Code Requirements for Structural Concrete, CRSI (2020).
• Reinforced Concrete: Mechanics and Design 6th Edition by James K. Wight, James G. MacGregor.

What foundation-soil friction angle should be used?

This angle is normally between one-half and two-thirds of the soil friction angle. ("Principles of Foundation Engineering" from Braja M. Das)

Which reduction factors are used in the strength verifications?

The ACI strip footing calculator uses φ = 0.75 for shear, φ = 0.90 for bending (for reinforced concrete assuming a tension-controlled condition involving a reinforcement area that is less the maximum limit for this condition), φ = 0.60 for plain concrete bending, and φ = 0.65 for bearing.

What value is used for concrete unit weight ?

The default value used is 150 lb/ft3 as suggested by the standard for normal-weight concrete.

What value of soil unit weight can be used?

The common values are between 90 to 130 lb/ft3. It is advised to use the humid value suggested in the geotechnical report of the project.

Is the wall reinforcement used for in calculations?

It is not used, only for drawing purposes. The wall dowels, however, are used in the load forces transfer check.

Why does my footing have a small maximum spacing?

The ACI 318-19 Section 24.3.2 specifies quite low values considering the values used for the concrete cover (normally around 3 inches). Some references (Avoiding Problematic Uses of Slabs on Ground, Jan, 2021 Structure Magazine By Alexander Newman, P.E., F.) mention that the ACI should consider exempting these provisions for footings and slabs on ground, but, as of now, they still apply, and therefore they are included in the program.

• SkyCiv Quick Design
• Bearing Capacity Calculator
• ACI 360 Slab on Grade Design Calculator
• Spread Footing Calculator
• Screw Pile Design Calculator
• AS 2870 Slab on Grade Design Calculator
• Lateral Pile Stability Calculator
• Reinforced Concrete Column Design Calculator

SkyCiv offers a wide range of Cloud Structural Analysis and Design Software for engineers. As a constantly evolving tech company, we're committed to innovating and challenging existing workflows to save engineers time in their work processes and designs.