Guide to Foundations

7/28/2019 Guide to Foundations

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Advance Design America (VisualDesign)TUTORIAL 13

Foundation DesignFootings

Soil-Structure Interaction&Piles

May 2008

CivilDesign Inc1995-2008


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I N T R O D U C T I O N

CivilDesign Inc. 13-1

Project Configuration Foundation

tabSelect the Foundation tab in the Project Configuration dialog box (Filemenu) and configure the foundation parameters.

Among other parameters, users must specify the global resistance factor forshallow and deep foundations. When that analysis is completed, this factormust be multiplied to the ultimate bearing capacity of foundation.

SoilsMake sure that types of soils that you will be using in your project are includedin Advance Design America's soils spreadsheets, namely: Cohesive Soils,Granular Soils and Rocks. They are located in the Common/Soils menu.To add a soil in the database, select the appropriate spreadsheet, insert a rowand complete the required parameters.

Introduction


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A D A T U T O R I A L 1 3

13-2 CivilDesign Inc

Important Parameters

The following parameters are required:

Calculation of BearingCapacity

Calculation of Settlements

COHESIVE SOILS Name of the soilUndrained shear resistance (notnecessary if qult is known)Saturated and humid unit weights

Young's modulusPoisson's ratio

GRANULARSOILS Name of the soilEffective angle of internal

frictionSaturated and humid unit weights

Young's modulusPoisson's ratio

Stratigraphic Profile

Before defining any foundation model, create the stratigraphic profile that issupporting the structure. Enter data in the StratigraphicProfiles spreadsheet(Structure menu).

A geotechnical study may show that more than one profile is required. Foreach stratigraphic profile, you must indicate, the elevation of natural groundand water table.

Soil Layers

Usually, a stratigraphic profile is composed of many soil layers. The Layers taballows you defining soil layers for each stratigraphic profile.

Specifications

A standard or building code must be specified in foundation specificationsspreadsheets (deep or shallow), located in Structure / Specifications, alongwith the type of analysis (design or check). Then, this specification is selectedin each foundation model through Foundation Models, deep or shallow.

Shallow Foundations

Enter the footing maximum dimensions if you plan to design the footings. Fora check of foundations, enter current dimensions.

Deep FoundationsDeep foundations, which are piles, cannot be designed. They can be checkedonly. A steel specification must be assigned to piles for structural verification inthe Deep Foundation Specifications spreadsheet.


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I N T R O D U C T I O N


Definition of Foundation Models

The selection of a foundation model must be done according to theoretical andempirical calculation models. To learn more about these, refer to Online HelpChapter 6 Foundation Design.

Spreadsheets and Dialog Boxes

Data can be entered in the Shallow Foundations spreadsheet and the DeepFoundations spreadsheet, which are located in Structure / FoundationModels. Define as many models as you think there will be in the project.

We recommend that you enter data in the dialog box because it contains moreinformation and it is much easier this way. To open the dialog box, click anycell (Shallow Foundations or Deep Foundations spreadsheets), right click,and select Detailsin the context menu.

In the dialog box, data are divided into several tabs: Foundation Model, Footing,

Column, Design, Piles, and Piles Layout.The Foundation Modeling WizardCreate foundation models in a quick way using the Wizard. Users have tospecify lesser parameters when this Wizard is used. Advance DesignAmericacreates a stratigraphic profile and a specification for the user. Whenthe foundation model is completed, data are transferred in appropriatespreadsheets (Shallow or Deep Foundation spreadsheet) and can bemodified afterwards. There is no limit to the number of models created withtheWizard.

Assigning Foundation Models to supportsA foundation model is assigned to a support through the Support tab ofNode Characteristics dialog box.

A foundation model can be assigned to one support or more. Usually, the samemodel is assigned to supports that carry approximately the same load to thefoundation.

Soil/Structure Interaction

S H A L L O W F O U N D A T I O N S O N L Y

Soil/structure interaction can be considered so that internal forces created byfooting settlements are redistributed into the structure. An Interaction type ofload case must be defined and serviceability load combinations must beincluded. The program performs iterations until it reaches convergencebetween calculated settlements and calculated forces to attain force/settlementcompatibility.


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Secant Modulus

Advance Design America calculates footing settlements using the secantmodulus of soils. However, to consider this modulus, the option "Secantmodulus, K" must be selected as degrees of freedom, in the Support tab, foreach foundation support.

You will find an example of such analysis further on.

Static Analysis

Shallow Foundations

In Design mode, the footings are optimized according to the bearing capacity(ultimate limit states) or settlement (serviceability limit states). Eccentricitiescaused by moments or eccentric loads are considered. Eccentricities may belimited as specified. Reinforcement is supplied in a results spreadsheet andreinforcing bars can be edited.

Deep Foundations

Deep foundations (piles) cannot be designed. They are verified using theselected steel shape and maximum length of piles, which is specified in theDeep Foundation Specifications spreadsheet.


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S H A L L O W F O U N D A T I O N S


Shallow foundations will be defined and designed for the following steelbuilding:

Project Configuration

We keep default values listed in the Foundation tab of Project

Configuration dialog box.

Soil Parameters

G E O T E C H N I C A L S T U D Y

A geotechnical study is recommended to obtain the stratigraphic profile belowfoundations. Results can vary much from a type of soil to another.

In our case, the study supplied the following profile:

Design of Shallow Foundations


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Soils spreadsheetsSilty sand and firm clay are composing the stratigraphic profile. They arecohesive soils. Select Soils /Cohesive in the Common menu.

We added these soils in the database by inserting rows at the end of thespreadsheet. Properties are entered according to supplied geotechnical study.

Click OK to save data and exit the spreadsheet.


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Select Structure / Stratigraphic Profiles. Insert a row, give a name to theprofile, and enter elevation of natural ground and water table.

Note. Make sure that ground elevation is compatible with support elevations.

Select the Layer Definition tab and insert two rows in the spreadsheet.Enter the rank of each layer (rank 1 is the first layer below natural ground)and thickness. Double click the Soil Namecell and select the type of soilthat is composing the layer. Click OK.

Specifications for Shallow Foundations

A building code and the type of analysis, design or check, must be specified inthe specification spreadsheet and.

Select Structure / Specifications / Shallow Foundations.

Insert a row and give a name to this specification. Select the buildingcode CAN/CSA-A23.3-04 and a design as type of analysis.


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Enter the footing maximum dimensions, which is 2m x 2m and a type ofoptimization for the footing dimensions (Bx=Bz, Fixed Bx, Fixed Bz orN/a, for a verification).and reinforcing bar material.

Select the reinforcing bar material and the Simplified Methodfor concretedesign.

Close the spreadsheet.This specification will be selected in the shallow foundation model dialog box.

Shallow Foundation Models

Three types of footings will be defined: Corner footings (4), intermediatefootings (4), and the centre one (1).

Select Structure / FoundationModels / Shallow. Insert a row. To open the dialog box, click any cell, right click, and select

the command Detailin context menu.

In the first tab, give a name to this model, choose the concrete material,select the stratigraphic profile, and specify the backfill soil above cornerfootings.


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Select the Column tab and enter the dimensions of concrete column.


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Select the Footing tab. Enter any preliminary footing dimension because it's a design and

dimensions will be calculated. However, the thickness will not bemodified byAdvance Design America.

If bearing capacity is insufficient, maximum dimensions will have to beincreased in the specification and another analysis must be carried on untilbearing capacity is OK.

If the shear stress is too high, the thickness has to be increased too.

Specify the backfill around the footing by checking appropriate boxes.To learn more about backfill, refer to Online Help- Chapter 6.

Select the Design tab. Enter required parameters needed for the design.

Select a method for bearing capacity calculation. For cohesive soils, enter the soil/footing adhesion. If this value is equal

to 0, the footing will slide.

Select the shallow foundation specification, specify concrete covers,reinforcing bar dimension and layout in the footing.


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Click OK.Back in the Shallow Foundation Models spreadsheet, duplicate the firstmodel to create the second and third model. To do this operation quickly,highlight the first row, right click, and select the Duplicate function in contextmenu. Change the names of foundation models only because parameters arethe same as the corner footing.

Click OK to exit the spreadsheet.


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Assign Foundation Models to Supports

Corner Footings

Activate the Support icon on Elements toolbar and click each cornersupport while keeping the [Ctrl] key down. Select the Properties function

to open the Node Characteristics dialog box.

Select the name of foundation model in the drop-down list and click OK.

Do the same to assign other foundation models to supports.View Options

Footing Dimensions and Models

To display footings on screen, open theView Options dialog box and activatethe Foundationbox in theViewtab. Footing dimensions and names can also bedisplayed. Click OK.


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Display the Stratigraphic Profile

The stratigraphic profile that corresponds to a support can be displayedthrough the Supports spreadsheet (Structure menu).

Click a cell in the row corresponding to the support that interests you, rightclick, and select the function Detailsin the context menu.

The stratigraphic profile will be displayed in a dialog box, as follows. Double-click the "Legend" box and move it elsewhere.


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Static Analysis or Design

Launch the static analysis by clicking icon on Tools toolbar. AdvanceDesign Americawill iterate until an optimum solution is found for each

load combination and each support.

If you own a Design module (Steel, Concrete, or Timber), launch the design by

clicking the icon. The foundation design will be part of the design process.


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Graphical Results

View Options

Open theView Options dialog box and check options "Dimensions" and"Name of Model" in theViewtab.

The designed footings are as follows:

Structural and Geotechnical Design Load Rates

Activate the Design Resultsmode and open theView Options dialog box. Selectthe Results tab and activate the geotechnical or structural design load.


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The table below includes structural and geotechnical design load obtainedthrough the coloured legend.

Model Structural Design Load Geotechnical Design Load

Corner Between 40% and 60% Between 20% and 90%

Intermediate Between 70% and 80% Between 40% and 90%

Centre 100% and + Between 90% and 100%

Numerical Results

Exact computed design loads are available in Foundation Resultsspreadsheets (Results/ Foundations):

Select Results / Foundations /Footings and look at calculated bearingcapacities and geotechnical design loads.

Select Footing Reinforcement to obtain structural design loads,minimum thickness, and calculated reinforcing bars.


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Bearing Capacity and Geotechnical Design Loads Select Results / Foundations / Footings. The Shallow Foundation

Results spreadsheet presents geotechnical results for each support andload combination. It includes many results: Stability (sliding andoverturning), reactions, bending moments, and much more.

Note. Some columns have been masked.

Bearing capacities, qr, and geotechnical design loads are OK. The centralfooting reached the highest geotechnical design load of 98.24%.

Footing ReinforcementSelect Results / Foundations / Footing Reinforcement. The spreadsheetsupplies the required number of reinforcing bars, length and spacing, for eachfooting model. Look at footing shear forces and resistances (Qf and Qr),punching shear (vf and vc), and minimum required thickness (Hmin).

Yellow columns are editable. Double click to modify the reinforcing bardimension. Then, Advance Design America automatically recalculatesrequired spacing and new resistances in the spreadsheet.


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The central footing is not adequate. The punching shear, vf, exceeds the shearresistance, vr. The structural design load is 115.53%. We can see that theminimum required thickness for this footing is 460mm.

Go back to the Shallow Foundation Models spreadsheet and specify athickness of 435mm for the central footing. Launch another static analysis.

Final Results

The central footing has increased to 1.3mx1.3m but the punching shear is OK.


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S O I L - S T R U C T U R E I N T E R A C T I O N

CivilDesign Inc 13-19

Overview

When the soil-structure interaction is considered, the program performsiterations until it reaches compatibility between footing settlements, stresses inthe structure and support reactions.

Soil-structure interaction is applicable to shallow foundations only. AnInteraction type of load must be defined in the Loads Definition spreadsheetand included in appropriate load combinations.

To obtain settlements, foundation supports must be modeled using the optionSecant modulus K, which is available as degrees of freedom in the Support tab ofNode Characteristics dialog box. With this option, Advance DesignAmericacalculates settlements using the secant modulus of soils that are partof the stratigraphic profile, for each footing. The calculated soil stiffness issupplied in the Support tab when a foundation model is assigned to afoundation support.

The iterative process is stopped when the number of iterations exceeds themaximum specified. Then, a message could be displayed on screen, such as:"The solution is acceptable but footing dimensions could not be optimized."The iterative process rarely requires more than five iterations. If more than fiveiterations are required, the solution cannot be optimized. However, results arevalid since calculated capacities are greater than 100%.

Modeling the Soil/Structure Interaction

Soil-structure interaction is considered in the next example. The structuralmodel that we previously analysed will be used. Foundation parameters andfooting models are already defined.

The following data need to be modified:

Foundation supports with elastic behavior, using the Secant modulus ofsoils,

An Interaction type of load must be added in the Load Definitionspreadsheet

Load combinations will be generated to include the Interaction load caseand serviceability load combinations.

Soil-Structure Interaction


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Foundation Supports - Secant Modulus, K

Use the option Secant modulus K for appropriate degrees of freedom tomodel elastic supports under the footings and automatically obtain the soil

stiffness. This soil stiffness is required for the calculation of footingsettlements.

Note. In this example, foundation models are already assigned to foundationsupports.

Activate the Support icon, select all supports and use the short cut keys[Ctrl]+H to open the Supports spreadsheet. Use the Replace function ofcontext menu and modify degrees of freedom to Secant modulus K.

Stiffness (linear) Kx, Ky, and Kz are automatically calculated and indicated inthe Elastic tab.

Close the spreadsheet.InteractionLoad Case Select the Loads Definition spreadsheet and insert a row. Give a number

(name) to this load case, double-click the "Type" column, expand theappropriate root, and select Interaction.


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P I L E F O U N D A T I O N S


Close the spreadsheet.Load Combinations

Serviceability load combinations are required to consider soil/structureinteraction. Use the Load Combination Generation Wizard to generate loadcombinations according to the Canadian National Building Code.


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The following load combinations were generated:

Static Analysis or Design

Launch the static analysis or design.Results

Footing DimensionsFooting dimensions have decreased in size.

Envelope Results Activate the Envelope mode and select the Serviceability Limit States

Envelope.

Select foundation supports and select Results/Envelope/NodeDisplacements.


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Settlements (Footings) and Geotechnical Design Loads Select Results / Foundations / Footings. Settlements are calculated at

each support, for serviceability load combinations:

Bearing capacities are OK.

At the far right of the spreadsheet, you will find geotechnical design loads andothers, including the settlement ratio according to allowable settlement(25mm).


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Footing Reinforcements and Structural Design Loads Select Results / Foundations / Footing Reinforcements.

The design is OK.


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Piles will be added below footings that were designed in the previous exampleon shallow foundation design (shallow foundations, page 13-5). Footingdimensions are increased to 3m x 3m and thickness is fixed to 500mm because

of punching shear and shear forces that could develop around pile heads. Thestratigraphic profile is also modified.

The procedure for defining pile foundations is the same as for shallowfoundations except that the Deep Foundation Models dialog box includestwo more tabs.


Open the Stratigraphic Profiles spreadsheet and enter the following data.

Specifications for Pile Foundations

A specification must be defined and assigned to foundation support beforerunning the verification. (Piles cannot be structurally designed. They areverified according to chosen steel shape, maximum length, and steelspecification.)

Select Structure / Specifications / DeepFoundations. Insert a row and give a name to the specification. Select the steel

specification that is used for the structural verification.

Pile Foundations


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Close the spreadsheet.Definition of Foundation Model

Select Structure / Foundation Models / Deep. Insert a row in thespreadsheet and open the dialog box. (Select a cell, right click and chooseDetailin the context menu.)

The Definition tab

Give a name to the model, select the concrete material, stratigraphicprofile, and backfill soil among lists box.

The Column tab

Select the Column tab and enter the concrete column dimensions andheight according to global axis system.


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The Footing tab

Select the Footing tab and enter the footing dimensions and thickness.Specify the location of backfill over the footing.

The Piles tab

Select the Piles tab and complete parameters for the pile verification. Toknow more about data composing this dialog box, use the F1 key to openAdvance Design AmericaOnline Help at this specific topic.


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The Pile Layout tab

Select the Pile Layout tab and define the pile layout under the footing. Todo that quickly, use one of the three buttons posted in the upper part ofthe dialog box. Each button represents a layout and opens a specific dialogbox.

Click the first button, which represents a grid layout. In the Grid Layout dialog box, enter the number of pile rows

(longitudinal) and columns (transverse). Specify the edge distance, whichis from border to centre of pile.


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Click OK to close the Grid Layout dialog box. Corresponding coordinatesare written in the spreadsheet and the pile layout is displayed.


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The Pile Layout spreadsheet includes other parameters: Beta angle of steelsection, Lateral Angleand Vertical Angle, which are the pile slope relatively to xzand xy planes.

Click OK to exit the dialog box and save data.Assigning the Model to Supports

Activate the Supporticon. Select all supports and click the Properties icon.Select the Pilesmodel in the Support tab ofNode Characteristics dialogbox and click OK.

View OptionsSelect the View Options dialog box and display foundation dimensions andname through theViewtab. Click OK.


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Static Analysis

Launch the static analysis by clicking the icon. Advance DesignAmerica iterates until an optimum solution is found for each load

combination and each support. If a design module is installed (steel,

timber, or concrete), click the Design icon.

Results

Piles' Structural and Geotechnical Design Load Verify pile design loads through the coloured legend. Activate the Design

Results mode on Activation toolbar and open the View Options dialogbox. Activate a type of foundation results in the Results tab. Click OK.

We can see that the geotechnical design of the central foundation is not

adequate. We are going to consult the geotechnical results spreadsheet.

Geotechnical Results Select Results / Foundation / Piles.


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Additional Model

A new model will be created for the central foundation. 6 piles will be placedbelow the footing, instead of 4.

Activate the Structure mode and open the Deep Foundation Modelsspreadsheet.

Duplicate the first model using the spreadsheet context menu. Enter aname for the new model.

Open the Deep Foundation Model dialog box using context menu(function Detail).

Select the Pile Layout tab and delete pile coordinates. Click the GridLayout button and place 6 piles under the footing.


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Close the dialog box.Assign the model to the central foundation

Activate the Support element and double click the central support. Select model Piles6in the Support tab.

Close the dialog box.


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Static Analysis

Launch a new static analysis.Graphic Results

Using the View Options, consult geotechnical and structural design loads.Foundations are OK.

Numerical Results Select Results / Foundations / Piles. The maximum design load for the

central foundation has dropped to 78%.

Graphic results are available through this spreadsheet. Select row 43 and clickthe button located at the bottom of the dialog box.


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Ultimate Pile Capacity vs. Embedment

This graph shows the point bearing and compression/friction bearingcapacities of the piles.

Qr = Qbr + Qsr

Qbr = cp * Qb

Qsr = cs * Qs

Where:

Qb is the nominal point bearing capacity and cp, the resistance factor forcompression/point bearing;

Qs is the nominal friction capacity and cs, the resistance factor forcompression/friction bearing;

This graph shows that the total ultimate capacity (Qr) of the pile is 174.1 kN(Place your cursor on the graph to obtain the numerical value at this point.)


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Position of Neutral Plane

The neutral plane corresponds to the depth where negative lateral friction onpile becomes positive, i.e. where the two curves meet.

Qd represents the distribution of dead load according to depth of pile and Qr,the variation of total capacity of pile (Qbu + Qs) with its depth.

Forces and Reactions in Each Pile Select Results / Foundations / Piles Reactions. This spreadsheet

includes results for each pile within a pile group, for each loadcombination.

This part of spreadsheet shows the minimum and maximum axial forces actingon each pile, for each load combinations. "x" is the lateral deflection of eachpile, and "%x", the calculated ratio according to allowable lateral deflection.


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Then, further right, you will find results about the pressure acting on elasticsupports located along the pile, and the resistance of negative earth pressure,Kp.

Finally, you will find structural and geotechnical design loads of each pile.

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