Geotechnical Design of Isolated Footing According to IBC (ASD) in RFEM 6

Model Description (ASD – Isolated Footing)

In this example, a simple square isolated footing is modeled.
A square reinforced concrete column is placed centrally on a square footing slab.
No stepped footing or special foundation geometry is used, in order to keep the example as simple as possible.

Geometry

• Footing slab: 6.0 ft × 6.0 ft, thickness t = 1.75 ft
• Column: 1.0 ft × 1.0 ft, height 2.0 ft, centrally positioned on the footing slab

3D Model of Rectangular Single Foundation with Central Column in RFEM 6

Material and Soil Parameters

The following material and soil parameters are used for the foundation design.

• Concrete unit weight: γ_c = 150 lb/ft³ = 0.150 kip/ft³
• Soil unit weight: γ_soil = 110 lb/ft³ = 0.110 kip/ft³

Load Cases

Four load cases are defined in the model.

Load Case 1: Dead Load (Self-Weight)

• The self-weight of the model is considered automatically.
• Additionally, a vertical force of 60 kip is applied at the top of the column in the +z-direction (acting downward).

Load Case 2: Live Load

• A vertical force of 25 kip is applied at the top of the column in the +z-direction (acting downward).

Load Case 3: Wind Load (W+)

• A member load of w = 2 kip/ft acting in the positive x-direction is applied to the upper half of the column (1 ft of the column height).
• Additionally, a moment of -25 kipft about the y-axis is applied in the same rotational direction as the member load.

Load Case 4: Wind Load (W−)

• A member load of w = 2 kip/ft acting in the negative x-direction is applied to the upper half of the column (1 ft of the column height).
• Additionally, a moment of 25 kipft about the y-axis is applied in the same rotational direction as the member load.

Load Combinations (ASD)

For the verification of the isolated footing, the load combinations according to the Allowable Stress Design (ASD) provisions are used.
The numbering shown in the first column corresponds to the ASD load combination numbers defined in the standard.

The following load cases are defined in the model:

• LC1 – D: Dead load (self-weight and applied axial load)
• LC2 – L: Live load
• LC3 – W+: Wind load acting in positive x-direction, including an additional moment about the y-axis acting in the same rotational direction as the resulting bending from the horizontal load
• LC4 – W−: Wind load acting in negative x-direction, including an additional moment about the y-axis acting in the same rotational direction as the resulting bending from the horizontal load

Based on these load cases, the following ASD load combinations are evaluated:

To account for both possible wind directions, the wind load is defined with two load cases (W+ and W−).

All ASD load combinations are evaluated for the foundation checks.
For each verification (soil bearing pressure, sliding, overturning, uplift, and highly eccentric loading), the governing load combination with the highest utilization ratio is determined.

Determination of Vertical Dead Load (Load Case 1)

Load Case 1 contains the total vertical dead load acting on the foundation.
This includes the applied column load, the self-weight of the footing and column, and the weight of the soil cover above the footing.

Self-weight of footing

W_f = 6.0 ft · 6.0 ft · 1.75 ft · 0.150 kip/ft³ = 9.45 kip

Self-weight of column

W_c = 1.0 ft · 1.0 ft · 2.0 ft · 0.150 kip/ft³ = 0.30 kip

Weight of soil cover

G_soil = (6.0 ft · 6.0 ft − 1.0 ft · 1.0 ft) · 1.0 ft · 0.110 kip/ft³ = 3.85 kip

Total vertical load in Load Case 1

P_LC1 = 60.0 kip + 9.45 kip + 0.30 kip + 3.85 kip = 73.60 kip

Geotechnical Design Configuration

The geotechnical design parameters for the foundation checks are defined in the Geotechnical Design Configuration.

For the present example, the following settings are used:

• Allowable soil bearing capacity: σ_a = 2.979 ksf
• Coefficient of friction: μ = 0.40
• Limit safety factor for sliding: Φ_SF,lim = 1.50
• Limit safety factor for overturning: Φ_SF,lim = 1.50
• Passive earth pressure resistance: R_passive = 3.620 kip for all foundation sides (±x, ±y)

All parameters shown above can be defined and adjusted by the user in the Geotechnical Design Configuration.
This allows full control over the design assumptions for sliding, overturning, uplift, and bearing checks.

Alternatively, RFEM can determine the passive earth pressure automatically according to Coulomb’s theory.

All settings related to soil bearing pressure, sliding, overturning, uplift, and loads with large eccentricities are available in the Geotechnical Design Configuration of the Concrete Foundations add-on.

The following figure shows the geotechnical design configuration in RFEM 6.
All relevant parameters for the geotechnical design checks can be defined directly by the user.

Geotechnical Design Configuration in RFEM 6 for Concrete Foundations

Geotechnical Design Checks

The isolated footing is verified according to the ASD approach defined in IBC.
RFEM automatically evaluates all relevant load combinations and determines the governing combination for each verification.

Soil Bearing Pressure

For this example, the governing load combinations are ASD Load Combination 6 (D + 0.75 · L + 0.45 · W) for both wind directions W+ and W−.
Therefore, the vertical load from Load Case 1, the reduced live load from Load Case 2, and the bending moment caused by the reduced wind load from Load Cases 3 and 4 are used in the following calculation.

According to ACI 318-19 §13.2.6.3, the contact pressure below a footing subjected to axial load and bending moment is determined assuming a linear stress distribution over the footing area.

Geometric Properties

Footing area:

A = 6.0 ft · 6.0 ft = 36.0 ft²

Section modulus:

S = (6.0 ft · (6.0 ft)²) / 6 = 36.0 ft³

Vertical Load

P = P_LC1 + 0.75 · P_LC2 = 73.60 kip + 0.75 · 25.00 kip = 92.35 kip

Determination of Bending Moment

The wind load is applied as a distributed member load over the upper half of the column.
In addition, an external moment is applied about the y-axis in the same rotational direction as the bending from the wind load.

Resultant horizontal wind force:

F_W = 2.0 kip/ft · 1.0 ft = 2.0 kip

Moment arm:

a = (2.0 ft − 1.0 ft) + 1.0 ft / 2 + 1.75 ft = 3.25 ft

Moment from distributed wind load:

M_W,dist = 2.0 kip · 3.25 ft = 6.50 kipft

Total wind moment:

M_W = 6.50 kipft + 25.00 kipft = 31.50 kipft

Reduced design moment:

M = 0.45 · 31.50 kipft = 14.175 kipft

Soil Contact Stresses

Using the previously determined vertical load and bending moment, the soil contact stresses result in

q = P / A ± M / S = 92.35 kip / 36.0 ft² ± 14.175 kipft / 36.0 ft³ = 2.565 ksf ± 0.394 ksf

Thus, the edge stresses are:

q_max = 2.959 ksf

q_min = 2.172 ksf

Utilization ratio

η = q_max / σ_a = 2.959 ksf / 2.979 ksf = 0.993 [-]

Since η < 1.0, the allowable soil bearing capacity is not exceeded.

In this example, both wind directions lead to the same governing maximum soil contact pressure due to the symmetric footing geometry and equal load magnitudes.

Design of Allowable Soil Pressure for Single Foundation in RFEM 6

Sliding Check

The sliding check verifies that the horizontal forces acting on the footing do not exceed the available sliding resistance.
For this example, all relevant ASD load combinations with horizontal actions are evaluated.
The governing load combination is ASD Load Combination 7 (0.60D + 0.60W) for both wind directions W+ and W−.

The vertical load is determined from the reduced dead load:

P = 0.60 · P_LC1 = 0.60 · 73.60 kip = 44.16 kip

The horizontal force results from the reduced wind load:

H = 0.60 · F_W = 0.60 · 2.00 kip = 1.20 kip

The friction resistance is calculated as:

R_friction = μ · P = 0.40 · 44.16 kip = 17.664 kip

The passive earth pressure resistance acting opposite to the sliding direction is:

R_passive = 3.620 kip

The safety factor against sliding is therefore

Φ_sliding = (R_friction + R_passive) / H = (17.664 kip + 3.620 kip) / 1.20 kip = 17.737 [-]

The utilization ratio is

η = Φ_sliding,lim / Φ_sliding = 1.50 / 17.737 = 0.085 [-]

Since η < 1.0, the sliding check is satisfied for ASD Load Combination 7 (0.60D + 0.60W).

In this example, both wind directions (W+ and W−) lead to identical results due to the symmetric geometry and equal load magnitudes.
The governing load combination is ASD 7, which produces the highest utilization ratio among all combinations with horizontal loading.

Design of Sliding for Single Foundation in RFEM 6

Overturning Check

The overturning check verifies that the destabilizing moments acting on the footing do not exceed the available stabilizing moments.

In RFEM, the overturning verification is performed edge-based.
This means that the moments are evaluated about the footing edge in the direction of overturning.
For each load combination, the stabilizing and destabilizing moments are determined with respect to the relevant footing edge.

For this example, all relevant ASD load combinations with horizontal actions are evaluated.
The governing load combination is ASD Load Combination 7 (0.60D + 0.60W) for both wind directions W+ and W−.

The vertical load is determined from the reduced dead load:

P = 0.60 · P_LC1 = 0.60 · 73.60 kip = 44.16 kip

Stabilizing Moment

M_stabilizing = P · B / 2 = 44.16 kip · 6.0 ft / 2 = 132.48 kipft

Determination of Overturning Moment

Resultant horizontal wind force:

F_W = 2.0 kip/ft · 1.0 ft = 2.0 kip

Moment arm:

a = (2.0 ft − 1.0 ft) + 1.0 ft / 2 + 1.75 ft = 3.25 ft

Moment from distributed wind load:

M_W,dist = 2.0 kip · 3.25 ft = 6.50 kipft

Total wind moment:

M_W = 6.50 kipft + 25.00 kipft = 31.50 kipft

Reduced design moment:

M_overturning = 0.60 · 31.50 kipft = 18.90 kipft

Safety Factor Against Overturning

Φ_overturning = M_stabilizing / M_overturning = 132.48 kipft / 18.90 kipft = 7.010 [-]

Utilization Ratio

η = Φ_{overturning,lim} / Φ_overturning = 1.50 / 7.010 = 0.214 [-]

Since η < 1.0, the overturning check is satisfied for ASD Load Combination 7.

In this example, both wind directions (W+ and W−) lead to identical results due to the symmetric geometry and equal load magnitudes.
The governing load combination is ASD Load Combination 7 (0.60D + 0.60W), as it results in the minimum stabilizing moment combined with a significant overturning action.

Lateral-Torsional Stability Analysis of Single Foundation According to ACI 318 with Stabilizing and Destabilizing Moments

Uplift Check

The uplift check verifies that the total vertical force acting on the footing remains positive.
If the sum of all vertical forces becomes negative, uplift occurs and the footing would lose contact with the supporting soil.

For this example, the governing load combinations are ASD Load Combination 7 (0.60D + 0.60W) and ASD Load Combination 10 (0.60D), since they produce the smallest vertical force due to the reduced dead load factor.

The uplift verification is based on the total vertical force acting on the footing:

P_z = sum of all vertical forces acting on the footing

The uplift condition is satisfied if

P_z ≥ 0

Vertical Load Components

P_LC1 = 73.60 kip

Evaluation of Governing Load Combinations

P_z = 0.60 · P_LC1 = 0.60 · 73.60 kip = 44.16 kip

Verification

P_z = 44.16 kip ≥ 0

Since the total vertical force remains positive, no uplift occurs.

In this example, the minimum vertical force acting on the footing is 44.16 kip, occurring in ASD Load Combinations 7 and 10.
Therefore, the footing remains in contact with the supporting soil for all considered load combinations.

Buoyancy Design of Single Foundation According to ACI 318 with Resulting Vertical Force

Loads with Large Eccentricities

The check for loads with large eccentricities verifies whether the resultant vertical load remains within the allowable eccentricity limits of the footing base.

In RFEM, this verification is performed separately for both footing directions.
The eccentricities of the effective vertical load are determined from the bending moments and the vertical load acting on the footing.
These values are then compared with the corresponding limiting eccentricities.

For this example, all ASD load combinations with horizontal actions are evaluated.
The governing load combination is ASD Load Combination 7 (0.60D + 0.60W) for both wind directions W+ and W−.

Vertical Load

P = 0.60 · P_LC1 = 0.60 · 73.60 kip = 44.16 kip

Eccentricity Limits

e_x,max = e_y,max = 6.0 ft / 6 = 1.00 ft

Determination of Bending Moments

Resultant horizontal wind force:

F_W = 2.0 kip/ft · 1.0 ft = 2.0 kip

Moment arm:

a = (2.0 ft − 1.0 ft) + 1.0 ft / 2 + 1.75 ft = 3.25 ft

Moment from distributed wind load:

M_W,dist = 2.0 kip · 3.25 ft = 6.50 kipft

Total wind moment:

M_W = 6.50 kipft + 25.00 kipft = 31.50 kipft

Reduced design moment:

M_y = 0.60 · 31.50 kipft = 18.90 kipft

Eccentricities of the Effective Vertical Load

e_x = M_y / P = 18.90 kipft / 44.16 kip = 0.428 ft

Utilization Ratio

η = max(e_x / e_x,max ; e_y / e_y,max) = max(0.428 / 1.00 ; 0.000) = 0.428 [-]

Since η < 1.0, no highly eccentric loading occurs for ASD Load Combination 7.

Design of Highly Eccentric Loading of Single Foundation According to ACI 318

Conclusion

The example demonstrates the complete geotechnical design workflow for an isolated footing according to IBC (ASD) in RFEM.

All relevant design checks — including bearing capacity, sliding, overturning, uplift, and highly eccentric loading — are performed consistently, based on the selected load combinations and user-defined geotechnical parameters.

The results show that all verification criteria are satisfied for the governing load cases. In particular, the check for highly eccentric loading confirms that no critical stress redistribution occurs, as the utilization ratio remains below the limiting value.

This example highlights how RFEM enables a transparent and efficient evaluation of foundation behavior under combined loading conditions, providing detailed insight into internal forces, soil pressures, and safety factors.

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