10 design of geometry of spread footing

8/13/2019 10 Design of Geometry of Spread Footing

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Engineering manuals for GEO5 programs - Part 1 www.finesoftware.eu

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Chapter 10. Design of geometry of spread footingIn this chapter, we are going to show you how to design spread footing easily and effectively.

Assignment:

Using EN 1997-1 (EC 7-1, DA1) standards, design the dimensions of a concentric spread

footing. Forces from columns act on the top of foundation. Input forces are: y x y x M M H H N ,,,, . The

terrain behind the structure is horizontal; foundation soil consists of S-F – Sand with trace of fines,

medium dense soil. At 6,0 m is Slightly weathered slate. The GWT is also at a depth of 6,0 m. The

depth of foundation is 2,5 m below the original terrain.

Scheme of the assignment – analysis of bearing capacity of spread footing

Solution

For solving this problem, we will use the GEO5 program – Spread footing. Firstly, we input all

the data in each frame, except “Geometry”. In the Geometry frame, we will then design the spread

footing.

Basic input

In the frame “Settings”, click on “Select” and then choose nr. 3 – “Standard – EN 1997 – DA1”.




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Frame “Settings list”

Also select an analysis method – in this case “Analysis for drained conditions”. We will not

analyze settlement.

Frame “Settings”

Note: Usually, spread footings are analyzed for drained conditions= using the effective parameters of

soil ( ef ef c, ). Analysis for undrained conditions is performed for cohesive soils and short-term

performance using total parameters of soil ( uu c,ϕ ). According to EN 1997 total friction considered is

always 0=u .

In the next step enter the geological profile, soil parameters and assign them to the profile.

Table with the soil parameter

Soil, rock

(classification)

Profile

[ ]m

Unit weight

3mkN γ

Angle of internalfriction [ ]°

ef

Cohesionof soil

[ ]kPac ef

S-F – Sand with trace offines, medium dense soil

0,0 – 6,0 17,5 29,5 0,0

Slightly weathered slate from 6,0 22,5 23,0 50,0




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In the next step, open up the frame „Foundation“. As a type of foundation, choose „Centric spread

footing“ and fill in the dimensions such as depth from the original grade, depth of footing bottom,

thickness of foundation and inclination of finished grade. Also, input the weight of overburden, which is

the backfill of spread footing after construction.

Frame „Foundation“

Note: The depth of the footing bottom depends on many different factors such as natural and climatic

factors, hydrogeology of the construction site and geological conditions. In the Czech Republic the

depth of footing bottom is recommended to be at least 0,8 meters beneath the surface due to freezing.

For clays it is recommended that the depth be greater, such as 1,6 meters. When analyzing the bearing

capacity of a foundation, the depth of the foundation is considered as the minimal vertical distance

between the footing bottom and the finished grade.

In the frame „Load“ enter the forces and moments acting on the upper part of foundation:

y x y x M M H H N ,,,, . These values we obtained from a structural analysis program and we can import

them to our analysis by clicking on „Import“.

Frame „Load“

Note: For design of dimensions of spread footing, generally the design load is the deciding load. ,

However, in this case we are using the analysis settings EN 1997-1 - DA1, and you must enter the value

of service load too, because the analysis requires two design combinations.




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Dialog window „Edit load“

In the frame “Material”, input the material characteristics of the foundation.

Skip the frame “Surcharge”, as there is no surcharge near the foundation.

Note: Surcharge around the foundation influences the analysis for settlement and rotation of the

foundation, but not bearing capacity. In the case of vertical bearing capacity it always acts favorably

and no theoretical knowledge leads us to analyze this influence.

In the frame „Water“ enter the ground water depth as 6,0 meters.

We are not going to enter a sand gravel bed because we are considering permeable cohesionless

soil at the of footing bottom.

Then open up the frame „Stage settings“ and select „permanent“ as the design situation.

Design of dimensions of the foundation

Now, open the frame „Geometry“ and apply the function „Dimensions design“; with which the

program determines the minimum required dimensions of the foundation. These dimensions can be

edited later.

In the dialog window it is possible to input the bearing capacity of foundation soil Rd or select

„Analyze“. We will choose „Analyze“ for now. The program automatically analyzes the foundation

weight and weight of soil below foundation and determines the minimum dimensions of the foundation.




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Dialog window „Foundation dimensions design“

Note: Design of centric and eccentric spread footing is always performed such that that the dimensionsof foundation are as small as they can be and still maintain an adequate vertical bearing capacity. The

option “Input” designs the dimensions of a spread footing based on the entered bearing capacity of the

foundation soil.

We can verify the design in the frame “Bearing cap.”.

Frame „Bearing capacity“

Vertical bearing capacity: 97,7 % 59.53222.545 =>= σ d R [kPa]

SATISFACTORY .

Conclusion:

The bearing capacity of designed foundation (2,0x2,0 m) is satisfactory.

10 design of geometry of spread footing

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