RKALC Learning Centre

pad footing design on rock and otherwise

A practical design guide showing when a reinforced-concrete pad footing may be treated as a conventional flexural member and when its proportions require a strut-and-tie model, with worked AS 3600 checks and RKALC calculation outputs.

Start the walkthrough Preview the engineering guide
Project overview

One footing type, two different structural behaviours

The bearing material and resulting footing proportions determine whether load transfer is governed by ordinary flexure or by a deep-region compression field.

Introduction

Pad footings founded on normal soil are often wide and relatively shallow. Where the projection from the column face is sufficiently large compared with the effective depth, the footing can be designed using conventional flexural, one-way shear and punching-shear checks.

A footing founded on competent rock may be much smaller in plan because of the higher allowable bearing pressure. The same column load can then produce a deep, compact footing with a short load path between the column and the founding surface. In that case, ordinary beam theory may no longer represent the behaviour and a strut-and-tie model becomes more appropriate.

This worked example develops both approaches, verifies the critical concrete and reinforcement checks, and compares the hand calculations with RKALC-generated reports.

What this learning page covers

  • Selection of footing plan dimensions from bearing capacity.
  • Span-to-effective-depth ratio and behavioural classification.
  • Flexure, punching shear and one-way shear checks.
  • Column bearing and reinforcement development length.
  • Strut-and-tie modelling for a compact footing on rock.
  • CCC and CCT nodal-zone stress verification.
  • Concrete strut capacity and bottle-shaped-strut bursting.
  • Tie reinforcement, bursting steel and anchorage detailing.
  • Comparison with RKALC pad-footing and STM calculation reports.
Pad footing flexural design and strut-and-tie model comparison
Flexural footing behaviour compared with a footing designed using STM.
Guide contents

Pad footing design workflow

Move directly to a specific part of the worked example.

01 · Structural behaviour

Flexural member or deep region?

The footing proportions should be checked before selecting the design model.

A practical classification is made by comparing the clear projection from the column face with the footing effective depth. Where the relevant span-to-depth ratio exceeds about 1.5, the footing may be treated as a flexural member. Where the ratio is below about 1.5, deep-region behaviour becomes significant and an STM representation should be considered.

High soil pressure does not automatically make a footing unsafe. It changes the required plan area and can shorten the internal load path. The designer must then use a model that follows the actual compression field between the column, footing and founding surface.

Flexural behaviourSpan/depth generally above 1.5
Deep-region behaviourSpan/depth generally below 1.5
Normal soilLarger plan area, longer cantilever
Competent rockCompact plan, direct compression path

02 · Conventional design

Worked flexural footing example

A rectangular footing is sized for bearing and then checked as a reinforced-concrete flexural member.

The worked example uses a 300 × 1000 mm column, 40 MPa concrete, 500 MPa reinforcement, a service bearing capacity of 700 kPa and an ultimate column load of 2400 kN. The adopted footing is 1400 × 2100 mm in plan and 450 mm deep.

Column300 × 1000 mm
Footing1400 × 2100 × 450 mm
Ultimate load2400 kN
Effective depthApproximately 366 mm

The effective cantilever projection is approximately 550 mm in the critical direction, giving a span-to-effective-depth ratio of about 1.50. The guide therefore proceeds with conventional flexural design and provides N16 bars at 200 mm centres each way, subject to confirmation of minimum reinforcement and project detailing requirements.

Pad footing geometry, loading and flexural design calculation
Adopted dimensions, bearing pressure and flexural reinforcement calculation.
03 · Strength checks

Punching, one-way shear and column bearing

Flexural reinforcement is only one part of the footing design.

Check 01

Punching shear

Define the critical perimeter, deduct the soil reaction inside it and compare the resulting stress with the concrete punching capacity.

Check 02

One-way shear

Check the critical section at the appropriate distance from the column face in each footing direction.

Check 03

Column bearing

Verify the loaded area, effective supporting area and permissible concrete bearing stress.

Check 04

Development

Confirm the column reinforcement can be developed within the footing depth and nodal region.

Pad footing punching and one-way shear checks
Critical punching perimeter and punching shear check.
RKALC pad footing calculation report
RKALC calculation output for the conventional footing example.
04 · Footing on rock

Building the strut-and-tie model

The compact footing transfers load through direct concrete compression and a bottom tension tie.

The second example uses a 300 × 1000 mm column with a service load of 4950 kN and an ultimate load of 6300 kN. With a rock bearing capacity of 3000 kPa, the adopted footing is only 1100 × 1800 mm in plan and 700 mm deep.

Rock bearing capacity3000 kPa
Footing1100 × 1800 × 700 mm
Ultimate load6300 kN
Span/depth ratioApproximately 0.65

The footing is represented by two inclined concrete struts carrying compression from the column into the rock reactions, a bottom horizontal tie, top CCC nodes beneath the column and bottom CCT nodes at the footing-rock interface.

  1. Choose the bearing and column-face nodal dimensions.
  2. Locate the node centroids and establish the strut geometry.
  3. Resolve the vertical reactions and calculate the horizontal tie force.
  4. Check equilibrium before proceeding to capacity verification.
Strut-and-tie model for a pad footing founded on rock
Two compression struts, a bottom tie and the associated CCC and CCT nodes.
05 · STM verification

Nodal stresses, strut capacity and tie steel

Each part of the assumed force path must have sufficient strength and a buildable reinforcement arrangement.

Top CCC nodes

Check the column bearing face and inclined strut face against the permitted compressive stress for a fully compressed node.

Bottom CCT nodes

Include the effect of the anchored tension tie and verify the reduced permitted nodal stress.

Concrete struts

Check the minimum strut width, efficiency factor and compressive stress along the bottle-shaped strut.

Bottom tie

Design the principal footing reinforcement for the calculated tie force and confirm minimum reinforcement.

Pad footing STM nodal stress checks
Stress resolution at the top and bottom STM nodes.
Pad footing concrete strut checks
Strut geometry, effective width and concrete stress verification.
06 · Detailing

Struts, bursting steel and anchorage

The assumed compression field must be supported by transverse reinforcement and properly anchored ties.

The spreading compression field creates transverse tensile forces within the bottle-shaped struts (if user assumes fan-shaped, then this check may not be required). The guide calculates the bursting demand at service and ultimate load and compares it with the available concrete contribution. Where the concrete contribution is insufficient, transverse bursting reinforcement is required.

The worked example also highlights practical ways to moderate the reinforcement demand: increasing footing width, increasing footing depth, changing the node dimensions, improving confinement from the founding material and refining the internal force model where justified.

  • Distribute bursting reinforcement through the strut region.
  • Extend column reinforcement into the footing where required for confinement and anchorage.
  • Anchor the bottom tie beyond the effective nodal zone.
  • Check that bar bends, cover and spacing can be constructed.
  • Keep the reinforcement arrangement consistent with the assumed STM.
Pad footing bursting reinforcement and anchorage detailing
Bursting-force model and a practical footing reinforcement arrangement.
07 · RKALC workflow

Verify both design approaches in one connected workflow

Use the appropriate calculator for the structural behaviour, then review the generated report rather than relying on a pass/fail result alone.

Step 01

Define geometry

Enter the column size, footing dimensions, cover, depth and founding pressure.

Step 02

Enter actions

Provide the service and ultimate column loads used for bearing and strength design.

Step 03

Select behaviour

Use the pad-footing calculator for flexural behaviour or the STM workflow for a deep region.

Step 04

Review the report

Check assumptions, critical sections, utilisation ratios, node geometry and reinforcement demands.

RKALC conventional pad footing report
Conventional footing report covering flexure, shear and bearing.
RKALC pad footing STM report
STM report covering nodes, ties, struts and bursting reinforcement.
Downloadable guide

Pad footings on rock and otherwise

Read the complete worked calculations, diagrams and RKALC report extracts.

Pad Footing Design

The PDF contains the conventional flexural footing example, punching and one-way shear calculations, column bearing, the complete footing-on-rock STM, nodal and strut stress checks, bursting-force calculations, reinforcement detailing and RKALC outputs.

Open PDF guide Open RKALC
RKALC pad footing STM calculation report
Sample RKALC calculation output for the footing-on-rock STM.

The PDF preview may not be supported by every browser. Open the guide in a new tab.

Video walkthrough

Pad Footings on Rock or otherwise using RKALC

walkthrough covering the complete workflow for designing reinforced concrete pad footings

Complete pad footing workflow

  • Explaining differences between fleural and STM footings
  • Solved Example for pad footing on sand.
  • Solved Example for pad footing on 3000 KPa rock.
Engineering perspective
The footing design model should follow the load path created by its actual proportions and founding conditions.

Good footing design starts with geotechnical capacity, identifies whether the concrete behaves as a flexural member or a deep region, and finishes with reinforcement that can carry and anchor the intended tension field while the concrete compression path remains within capacity.

Continue learning

Related RKALC resources

Continue with related strut-and-tie and reinforced-concrete design workflows.

Learning Centre

Browse all engineering tutorials

Explore additional structural analysis and design resources.

View the Learning Centre → STM Calculator

Design reinforced-concrete D-regions

Build the load path, verify nodes and struts, and generate a calculation report.

Open STM Calculator → RKALC RC Calcuator

Design reinforced pad footings (flexural)

Feed geometry, apply loads, and generate a calculation report.

Open RC Calculator → Strut-and-tie modelling

Review a wall transfer D-region

Follow a complete STM example for a reinforced-concrete wall supported on columns.

Open the tutorial →