Foundations - vutbr.cz

FoundationsFoundations – footings – terms

Commonly foundation is the part of

buildings which transfers the load of the

whole building structure to the ground soil

or rock.

As the English language is wide expanded

in the entire world, the terms vary a little

bit. For explanation of basic and frequent

terms see fig. on this page.

Superstructure.

Foundation.

Footings.

Acc. to Eurocode 7 there is not defined

difference between terms foundation and

footing no big importance of these

terms for BL005. Notice it for life, only.

E

In the case of our multi-storey/bay frames are

as foundation/footing usually used RC pads.

Situation can be simplified acc. to enclosed

fig.

By all means foundations/footings are an important part of each structure. Secondary failures of a superstructure due to primary failure of foundations/footings are very frequent. The reasons normally are:• Imperfect knowledge about

geotechnical conditions.• Change of geotechnical

conditions during the service life of a structure e.g. watering / dewatering of the soil.

• Underestimated design of foundation’s project.

By design of footings we have two tasks, standardly:

G= geotechnical task = To design footing for

safe transfer of the whole structure’s load to

the soil. With dependence to an actual load

and soil properties. It really means the

design of footing’s dimensions for contact

with a soil. Important part of G can be

verifying of structural stability.

S=structural task it means design of cross-

sections, concrete grade and mostly (for RC

footings) reinforcements to protect the

footing from failure of itself.

From the above you can clearly recognize, the design of footings is an interdisciplinary case of Geotechnical and Structural (concrete) engineering.

In both parts (G and S) the design has to be done with respect to actual knowledge and valid codes. (In both cases Eurocodes – see next slide). As the BL005 subject is a part of concrete structures theory we will be focused mostly on structural design. Prior this we must softly touch the geotechnics to be able to design main dimensions of footing.

EDesign of foundations

EP

EP

Foundations are frequently categorized according to their kind of load transfer into soil. Types of foundation which transverse (spreads) the load through relative large contact area are called „spread foundations“ and the rest is called „deep foundations“ or the „special“ ones. By deep is the load transfer secured mainly with s.c. „mantle friction“. Next frequently used term is shallow foundations: They caught their name acc.to not very deep position under terrain. Indispputable fact is that footing pad in the same tine spread

E

and shallow foundation. As the BL005 subject is not the geotechnical one, we will deal with basic types of spread foundation case , only.

Mini-homework: identify for me and for yourself which scheme represents which kind of a real type of foundation.

Common sequence of geotechnical footing design

1. Analysis of the conditions at building place.

2. Decision about “geotechnical category”.

3. Additional findings according to “geotechnical category”.

4. preliminary design (dimensions) according to ULS (usually STR

limit state)

5. Verification (Review) according to all needed ULS and SLS

EP

E

Possible ULS – STR limit cases by geo design of structures

In the circle is “Sinking of a

spread footing” the only ULS -

STR solved in BL005. The rest

of ULS-STR schemes is

informative, only.

EP

Other possible ULSs, in the circle “overturning” the only ULS-EQI

solved by BL005 (footing pad)

EP

Possible types of SLS

{service limit states)

for BL005 it is

informative, only

EP

Ed ≤ Rd

For the loading action-effect we will use in the soil induced stress so-

called so contact (bearing], pressure szd, preferably in [kPa].

For the measure of the soil resistance we will use the soil strength Rd in

[kPa], too.

Used indexes “d” means actually we have ULS case, therefore both

quantities are in their design values, so usage off load and material

factors coefficients is necessary.

+ Recommended formula for loads combination (EN 1990 number 6.10)

is:

EThe basic condition of reliability is well known (hopefully) and in this specific

case can be formulated as: The max. probable E (effect of the loading) should

be below (or equal) to the max probable resistance of the soil R.

EThere are three possible sequences (approaches) for footings design in the basic EN 1997. In the Czech NA is for spread foundation (like a pad)recommended approach No. 3. which is based on coefficients presented in the tab. 1 and tab.2.

For determining of the deciding value of contact pressure correct calculation of

loads and knowledge of its distribution in contact area (shape of distribution) is

necessary. Let us discuss the shape:

There is a lot of possibilities there. The shape depends on:

Rigidity or Flexibility of the footing. The relative rigidity footing/soil is of a great

importance, too.

Elasticity or plasticity of the actual soil.

Cohesion or not cohesion of the soil.

And others.

Shape in ideal cases:

e.g. sand e.g. clay

grains of the sand can move outside of the ground plan area

E

Influence of footing’s flexibility on elastic soil E

In real life influences shown on previous two slides combines in various way together and the real shape is hard to predict. Therefore are in the most cases for design of simple footing structures (like a pad) following simplified presumption used:

Another shape influences

In words: The contact pressure sz is distributed uniformly on a part of contact area called „effective contact area“ and marked - Aef We determine the size and position of Aef from the principle: The centroid of Aef is identical with resulting force application point.In formulas and figure:

Vd is resulting vertical force

Vd= Nd + Zd

Zd Self-weight of the pad + backfill + relevant part of the floor slab

Nd Axial force from the superstructure (more loading cases possible)

Md Bending moment in the foot of actual column (more loading cases possible)

Hd Horizontal force in the foot of actual column (more loading cases possible)

ed eccentricity of Vd

ed (e)= ( Md + Hd . H)/ (Nd + Zd)

Aef = (Nd + Zd) / (L-2 ed) . B

E

Once more with more details. The principleis the same by geotechnical and structuraldesign. Moreover, the principle is near to the same by pad and strip under a wall(SUW). More in the next lecture.Vd is resulting vertical force of Nd , Zd

Vd= Nd + Zd

Zd Self-weight of the pad + backfill + relevant part of the permanent load + variable load from the floor slab (If actual).

Nd Axial force from the superstructure (more loading cases possible).

Md Bending moment in the foot of the actual column from superstructure (more loading cases possible).

Hd Horizontal (shear) force in the foot of the actual column (more loading cases possible).Aef Effective contact area

ed = ( Md + Hd . H)/ (Nd + Zd)

Aef = (L-2 ed) . B !attantion! B=B or B= 1m Use the equivalent values for action and resistance! By SUW always B= 1m!

End of foundations part 1Link for 2 Educative and 2 interesting videos:

https://www.edisk.cz/stahni/72505/ev5___ev6.zip_48.8MB.html/