Online Manuals RFEM 6 / RSTAB 9 | Concrete Foundation

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006219

Ann-Kathrin Dannwerth, B.Sc.

2026-07-22

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Overview

Add-on Concrete Foundation

Theoretical Principles

Design Specifications

Design Properties of Single Foundations

Design

Results

Documentation

Bending

IBC 2021 / ACI 318-19 (LRFD): Bending design

Purpose of check

The bending design check verifies whether the existing longitudinal reinforcement can resist the bending moments resulting from the design loads. The check is performed separately for:

Top and bottom reinforcement
Both foundation directions (x and y)

Determination of Design Moment

The design moment for the respective reinforcement layer results from the superposition of the moment from the compressive stress block (soil pressure) and the moment from additional foundation loads (self-weight, earth covering), related to the design section at the column face:

M_{f, (Achse), (top), u} = \frac{\min (M_{C, + x, (top)} + M_{A, + x, (top)}; M_{C, - x, (top)} + M_{A, - x, (top)}; 0)}{Φ}

M_C	moment from the compression stress block (soil pressure) in positive/negative axis direction
M_A	moment from additional foundation loads in positive/negative axis direction
Φ	Reduction factor (see below)

Design moment – upper reinforcement

M_{f, (Achse), (bottom), u} = \frac{\max (M_{C, + x, (bottom)} + M_{A, + x, (bottom)}; M_{C, - x, (bottom)} + M_{A, - x, (bottom)}; 0)}{Φ}

M_C	Moment from the compressive stress block (soil pressure) in positive/negative axis direction
M_A	Moment from additional foundation loads in positive/negative axis direction
Φ	reduction factor (see below)

Design moment – bottom reinforcement

Additionally, for interior columns, the moment from the unsymmetrical shear stress distribution during punching (moment transfer via the critical perimeter) is considered:

M_{2 w, u} = \frac{\sqrt{{(γ_{f, 1} \cdot M_{sc, 1})}^{2} + {(γ_{f, 2} \cdot M_{sc, 2})}^{2}}}{Φ}

Moment from moment transfer in punching

The governing design moment is the larger value from both components:

M_{(Achse), (Seite), u} = \max (M_{f, (Achse), (Seite), u}; M_{2 w, u})

Design moment

Reduction Factor Φ

The reduction factor depends on the strain condition of the cross-section (ACI 21.2, Tab. 21.2.1):

ε_{t, y} = \frac{f_{y}}{E_{s}}

Yield strain of the reinforcement

ε_t,t = 0.003 – concrete strain at crushing (ACI 318-19, 22.2.2.1), as a constant addition to the yield strain for determining the limit strain of tension-controlled cross-sections

Φ = Φ_tens = 0.90, if the cross-section is tension-controlled: ε_t > ε_t,y + ε_t,t
Φ = Φ_compr = 0.65, if the cross-section is compression-controlled: ε_t ≤ ε_t,y

RFEM automatically checks this strain condition for each reinforcement layer – in contrast to simplified calculations. This can lead to deviating, more conservative results for the top (compression-controlled) reinforcement.

Required Reinforcement

From the design moment, the statically required reinforcement area is determined, additionally considering the contribution from the minimum reinforcement:

A_{s, stat} = \max (A_{s, stat, t}; A_{s, stat, p})

A_s,stat,t	required reinforcement for flexural tension (design moment)
A_s,stat,p	required reinforcement from soil pressure / minimum reinforcement (ACI 8.6.1.1)

Static Required Reinforcement

Check / Utilization Ratio

A_{s, prov} \geq A_{s, stat}

A_s,prov

existing (inserted) reinforcement area [in²]

Verification condition – Bending design

η = \frac{A_{s, stat}}{A_{s, prov}}

Utilization ratio – Bending design

Software Configuration

The existing reinforcement is taken from the defined foundation parameters. The material properties are taken from the assigned concrete and reinforcement materials. The design moment as well as the strain condition (and thus Φ) are determined automatically for each reinforcement layer and each design section from the governing load combinations.

Parent Chapter

Internal Stability - Reinforced Concrete Design