Type of footings and design method

April 6, 2021
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OVERVIEW

 

The footing is a structural member that safely transmits the load of the upper building to the ground, and is the first construction member of the building. The basic method of footing design is to distribute the load so that the size per area of ​​the load transmitted from the upper part of the building is less than the strength that the ground can support, that is, the bearing capacity. With this function, the footing is a structure installed between the ground and the column or wall immediately above it, and the important point in the basic design is to reduce the total amount of settlement and prevent the occurrence of immobile settlement.

In a general building, the footing design is to determine the area of ​​the footing, the thickness of the footing and the amount of required rebar.

In this topic, we deal with starting from finding out the Footing Type necessary for a building and and proceeding with the Footing Design step by step in nGen.

 

1. Footing Type

1-1. Supporting Scope for Footing

midas nGen supports modeling, analysis and design of various footing systems.

   - Enables to design of members connected to the footing.

   - Provides tools to quickly model files.

Supporting Scope for Footing-2<Supporting Scope for Footing>

 

1-2. Footing Type

Footing can be categorized by Member and Design type. There are three types of footing member.

- Mat Footing, Strip Footing, Isolated Footing

There are two types of footing design.

- Rigid Design : Footing is assumed to be infinitely rigid. The bearing pressure against the bottom of the footing follows a planar distribution where the centroid of the bearing pressure coincides with the line of action of the resultant force of all loads acting on the footing. 

- FEA Design : Footing is designed according to the results of a finite element analysis. Since it is designed based on an analysis, more accurate analysis results can be obtained.

Footingtype<Supporting Scope for Footing>

1-3. Support for Footing

Soil Spring and Pile Spring is supported in midas nGen.

- Supported Soil Spring Property : Linear Elastic, Compression Only

- Supported Pile Spring Property : Linear Elastic, Tension Only, Compression Only, General

 

Support for footing-1<Support For Footing>

 

2. Footing Design

2-1. How to Design Isolated Footing with Rigid method

An isolated mat footing transfers the loads from a single column to the supporting soil. The size of the footing is determined by the allowable soil bearing pressure. The footing is designed for flexure, punching or two-way shear and one-way shear. The depth of the footing is generally governed by punching shear.

 

The calculation of soil pressure in serviceability state and ultimate state as below.

rigidmethod<Calculation of Pressure Distributions by each status>

 

2-2. How to calculate a design bending moment

The value of design bending moment at the column face is applied to the calculation.

BendingMoment-2<Calculation of bending moment>

 

2-3. How to calculate a design shear force

The soil pressure inside the shear face or shear perimeter is not considered in the design shear force calculation, assuming the load is transferred to the column.

ShearForce-2

<Calculation of shear force>

 

2-4. Supporting Design Items for Rigid Method

 

Ultimate Status

Flexural design

All

1-way shear design

All

Punching shear design

All

Serviceability Status

Stress limitation check

Only Eurocode-2

Crack width check

Only Eurocode-2

Linear creep check

Only Eurocode-2
Stability

Sliding

All

Bearing capacity of Soil

All

Bearing capacity of Pile

All

Overturning

All
Sliding All

 

For a full description of the basics or to know the type and design of footing, you can click to the following link > Footing - Type & Design

 

In this topic, we will cover how to build a foundation of building.

 

1. Create Footing 

1-1. Isolated Footing

Isolated footings (also known as Pad or Spread footings) are commonly used for shallow foundations in order to carry and spread concentrated loads, caused for example by columns or pillars. Isolated footings can consist either of reinforced or non-reinforced material.

IsolatedFooting-2<Create Isolated Footing>

  1) Select a member Set
  2) Select Story set.
  3) Select Thickness of footing.
  4) Select Analysis type. (Refer to “Footing Type” guide.)
  5) Select a target columns or pedestals.
  6) Select a footing shape.
  7) Enter a shape information of footing according to the footing shape
  8) Click to turn on/off a offset option. (Footing is placed considering a offset Info. of column.)
  9) Click “Apply”
 

1-2. Strip Footing 

Strip foundations (or strip footings) are a type of shallow foundation that are used to provide a continuous, level (or sometimes stepped) strip of support to a linear structure such as a wall or closely-spaced rows of columns built centrally above them.

Strip Footing-1

<Create Isolated Footing> 
 
  1) Select a member Set
  2) Select Story set.
  3) Select Thickness of footing.
  4) Select Analysis type. (Refer to “Footing Type” guide.)
  5) Select a target columns or pedestals.
  6) If you select “By Drawing”, you can create a footing by manual.
  7) Enter a width value.
  8) Click to turn on/off a offset option. (Footing is placed considering a offset Info. of wall.)
  9) Click “Apply”
 
 1-3. Footing Girder
A grade beam or grade beam footing is a component of a building's foundation. It consists of a reinforced concrete beam that transmits the load from a bearing wall into spaced foundations such as pile caps or caissons. It is used in conditions where the surface soil’s load-bearing capacity is less than the anticipated design loads.

FootingGirder<Create Footing Girder> 

  1) Select a member Set
  2) Select Story set.
  3) Select a Section Name
  4) Click the creating method.

     - “By Select” : by selecting two vertical elements.

     - “By Drawing” : by clicking two points.

  5) Click 1st column or pedestal to create a footing girder (under “By Select”).
  6) Click 2nd column or pedestal to create a footing girder (under “By Select”).
  7) Enter a height to create a footing girder.
  8) Click “Apply”.

 

 1-4. Pedestal

A concrete pedestal is a compression element provided to carry the loads from supported elements like columns, statues etc. to footing below the ground. It is generally provided below the metal columns. In general pedestal width is greater than its height.

 

Pedestal-1

<Create Footing Girder> 

  1) Select a member Set
  2) Select Story set.
  3) Select a Section Name
  4) Click the creating method.

     - “By Select” : by selecting two vertical elements.

     - “By Drawing” : by clicking two points.

  5) Click  a column to create a pedestal (under “By Select”).
  6) Enter a height of pedestal.
  7) Click to turn on/off “Offset” (Pedestal is placed considering a offset Info. of target Column.

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Young-il Seo
Young-il Seo | Principal Structural Engineer

Young-il has over 13+ years of experience in building design, especially high-rise buildings with column reduction analysis, plant structures, pushover analysis, health monitoring, and vibration control projects. Since 2016, he is planning and providing technical supports for midas building products such as midas Gen, nGen, and Design+.

E-BOOK Practical Design Guide for
Footings and
Basement Walls

This Design Guide provides basic concepts to design the footing and basement walls.
It is helpful in understanding a variety of member types which are composed of
the footing and basement walls.

Read all