FIR Note Explicative -Civil Engineering_part 1

8/15/2019 FIR Note Explicative -Civil Engineering_part 1

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FOUNDATIONS FOR COLUMNS (SPREAD FOOTING) DESIGN

4. Foundations for cast in situ reinforced concrete columns

Types of foundation for reinforced concrete columns reinforced concrete foundation – elastic foundation fig. 4.1. a

stepped foundation (plain concrete block and reinforced concrete block) - stiff

foundation fig. 4.1. b

TBF

> 2 0 c m

NAS

> 2 0 c m

NAS

Cuzinet de beton armat

Bloc de beton simplu

TBF

a. b.

Fig. 4. 1. a) Elastic foundation; b) stiff foundation

Criteria:

a) If the bearing layer is encountered at the surface we choose the first variant

b) Groundwater level : if is high – elastic foundations


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c) Load’s magnitude

High loads – elastic foundations

Low loads (


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Fig. 4. 3. Stiff foundation for one column

a. Determine the plain concrete block dimensions

The dimensions in plane of the foundation bottom are determined in order to

undertake the effect of the design actions, which acts on the foundation bottom, to be less or

equal with the design resistance of the bearing layer:

Vd ≤ Rd (4.1)

In order to evaluate the bearing capacity of the bearing layer, beside the geotechnical

parameters we need the values of the surface in plane of the foundation bottom (B x L). For

this reason, in order to verify the condition (4.1) it is imposed to know the estimative

dimension of the surface in plane for the foundation bottom. These can be adopted for one of

the following variant:

- Adopt some dimensions based on the experience or similar projects;

- Aproximative calculation, of initial sizing, based on acceptable pressures. For

this stage we can assume the base conventional pressures or plastic pressures

from STAS 3300/2 – 85 or NP 112 – 04.

Fig. 4. 4.


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B L p

N B

undede p B

N p

B L B

L

b

a

B

L

conv

Ed

conv Ed

ef

,8.0

2.1

,8.02.1

4.12.1

2

Ed fd Ed d

conv

N G N V

p

2.1

8.0

acc

accd

ef

p

pBxL

Vp

The following steps should be done for initial sizing:

a. Initial sizing

(4.4)

(4.5)

where: Gfd – design weight of the foundation, and the sustained soil

( f md fd D L BG )

It is considered a reduced value for the acceptable pressures of the bearing layer (0,8

conv p ) in order to take into account the corrections effect (depth and width).

(4.6)

(4.7)

(4.8)

The height of the concrete block is provided by the condition that at any point of the

dangerous sections of the plain concrete block there should be no tensile stress. This

condition is satisfied by adopting the height of the concrete block according to fig. 4.6.

The following relation should be satisfied:

tg≥tgadm. (4.9)

Values for tgadm are given in table 4.5. This relation should also be satisfied for

stepped foundation (fig.4.6.). The condition should be satisfied in all directions.

Tab. 4.5. Values for tgadm

For intermediate values of acceptable pressures, the corresponding value of the

closest superior effective pressure is to be used.

pacc (kPa)tgadm

C4/5 C8/10

200 1,15 1,05

250 1,30 1,15

300 1,40 1,30

350 1,50 1,40

400 1,60 1,50

600 2,00 1,85


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Fig. 4. 5. Stepped foundation

Construction requirements:

Minimum height 400mm – block with one step ;

Where we have more than 2, max. 3 steps, whose height is 300mm; minimum height for

the lower step should be 400mm;

Concrete class is minimum C8/10; if in the concrete block there is any reinforcement used

to fix the reinforced block , the minimum concrete class should be C12/15;

The forces transmitted on the superior part of the foundation are equal with the forces

encountered at the column. For this reason the concrete class used in the reinforced block

should be the same class used for the column. A lower class can be used only if the

punching verification it is done locally.

b. Initial sizing for reinforced block

Normally the reinforced block shape is prismatic. In case where the console length (l)

is higher than 400 mm, it can be rectangular.

Minimum class C8/10 results from the condition of local compressive strength of

concrete in the bearing column section. (Rc RC block ≥0.7 Rc colou;n);

One step block:

65.05.0 B

b

L

l cc (4.10)

2-3 step block:

5.04.0 B

b

L

l cc.

In order to determine the reinforced concrete block, a particular condition should be

satisfied: the reinforced concrete block should not shear on the perimetral contour of the

column. The real behavior of the reinforced concrete block is better than the one resulted


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from the above condition, because the reinforced block cannot shear, if the block on which is

placed do no shear. For this reason the following condition should be satisfied:

- Hc≥300mm,

- 25,0c

c

l

H (4.11)

- tg≥2/3.

If: tg≥1 the reinforced concrete block does not require the shear check;

- Anchorage length should be satisfied lancoraj+250mm, where la is taken

according to SR EN 1992.

c. Check

Bearing resistance check is done in ultimate limit state GEO in design approach (CP2).

Loads on foundation bottom:

Vd=NEd+Gf , (4.12)

Gf = BxLxDxmed (4.13)

med=20…22kN/m3,

NEd – design value of actions at column base.

The following condition should be satisfied:

Vd≤Rd (4.14)

Where: Vd – design load on foundation bottom

Rd – bearing resistance of the soil.

Bearing capacity value of the soil:

' A

R p d ,

R

d

d A

R

A

R

'

' , 1 R (for CP 3) (4.15)

''' L B A Error! Objects cannot be created from editing field codes.; Error! Objects

cannot be created from editing field codes. (4.16)Characteristic eccentricities:

6/ BekB ; 6/ LekL Where:

A’ – reduced area for the surface in plane of the foundation ( '' B L ), p – design critical pressure, according to SR EN 1997-1 (STAS 3300/2-85 or NP

112-04)

The design value of the pressure is determined according to the site conditions anddrained or undrained conditions. Eccentricities are limited to maximum 1/6 from the

foundation bottom dimensions or 0.20 from the radius in the case of circular foundations.


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Values of critical pressure for bearing layer: :- Undrained conditions

')2('/ qi sbc A R cccud (4.17)Where:b- coefficient depending on the foundation base inclination;

s- coefficient depending on the foundation shape;

i - coefficient depending on the inclination of the load caused by the horizontalload H ;- Drained conditions :

i sb N Bi sb N qi sb N c A R qqqqccccd ''5.0'''/ (4.18)

d. Settlement calculation

This check is done for the serviceability limit state (SLS)

The value of partial safety factor for this limit state is equal to 1.

Verification according to EC7:Ed≤ Cd

The soil is divided into 3 elementary layers with the height h≤0.4B.

hi= thickness of one elementary layer

ad ef s s

i

i

med

zi

E

h s

100 ; 8.0

2

inf sup

zi zimed

zi

n z p 0 (4.19)

),(0 B

L

B

z f

gz z 2.0 (active area)

)( i f ii gz Dh

e. Reinforcement

The strength reinforcement at the bottom of the reinforced block it determined in order

to overcame the bending moment determined in the sections surrounding the column, by

loading the surface at the bottom of the reinforced block with the pressure ’s diagram on the

bottom of the block with the exterior loadings.

a. Strength reinforcement on the reinforced block bottom, type 1 and 2 are parallel to the

block sides (fig. 4.7)

Conditions to be satisfied:


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ømin≥10mm,

dmin≥100mm / dmax≤250mm (4.20)

pmin≥0,01% (OB37)

pmin≥0,075 (PC52), on each direction.

b. Superior reinforcement (fig. 4.8), type 3, assures the connection of the reinforced

block with the plain concrete block

Fig. 4. 6. Reinforcement for reinforced concrete block

If at the reinforced concrete block in the bottom part there is only compressive

stresses – is not necessary to calculate or to dispose any anchorage reinforcement

On the other side, when tensile stress appears, it is imposed to dispose the

anchorage reinforcement in order to overcome the tensile stresses.

Fig. 4. 7. Superior reinforcement


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cbaT 12

1

It is disposed from the eccentric compression check of the reinforced section on the

contact surface between the reinforced and plain concrete block.

Design concrete strength :

2

,2

cc

cuz cap st calc

l b

M R

(4.21)

where bc – width of reinforced block

- the reinforcement is disposed uniformly on the block sides , and the

reinforcement sides are prolonged with the minimum length of 15Φ.

In the case where large loads appear, the pressure distribution it is done according to

the figure, and in this case the reinforcement type 3 is going to overcome the tensile

tensions.

(4.22)

In this case an additional check is to be done, namely the check of negative moment

(M-) for the reinforced concrete block, which is loaded with the forces developed in the

anchorage reinforcement.

The reinforcement type 3 is disposed with a minimum amount of 2 bars. The

constructive conditions that need to be covered by this reinforcement are the same with the

one of calculated reinforcement.

b. Reinforcement for columns type 5, assures the connection of the reinforced concrete

block with the reinforced column (fig. 4.8). They result from the sizing/verification of

column. Have similar position, diameter and number of bars with the column. Minimum

3 stirrups are required for the vertical reinforcement. The longitudinal reinforcement

needs to be prolonged in the foundation on a min. distance of la+250mm.

The connective reinforcement needs to be set in place in the reinforced

concrete block before the concrete is poured.

The overlapping distance between the connective reinforcement and the

longitudinal reinforcement of the column is according to SR EN 1992. For the columns

with 4 reinforcements bars at the corners the overlapping is to be done in the same

section. For the columns that have more than 8 bars, the overlapping is to be done at

least in two sections. If we have high loads, the overlapping is to be done at the

basement level.

Where the overlapping is taking place more stirrups are to be placed. (≤10Φ).

c. Transversal reinforcement is used if tgβ


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4.3.3. Calculation of bending moments, for determining the strengthreinforcement for the bottom of the reinforced concrete block

The positive bending moments in the reinforced concrete block is done considering

fixed console in the section at the face of the column. (fig. 4.9).

It is recommended to use the approximate methods, from the pressures on the contactsurface between the reinforced and plain concrete block (the weight of the reinforced

concrete block is not taken into consideration).

lc

b c

y

y

x x

p1p2

pm

p0

lx

l y

MEdNEd

y

y

My

Fig. 4. 8. Bending moments in the reinforced concrete block

2

2 2

0 1 0 0 1 0

1 2

( )2 2

2 1( )

2 3 2 2 3

2

y y

y c y m c m

x x x x x c x x c

m

l l M l l p l p

l l l l M b l p p p l b p p p

p p p

(4.24) / (4.25)

Bending moments Mx şi My are calculated charging the right console, below the x axis , with

the pressure from the contact surface fig. 4.8.

Observations:

1. If p2


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2. When for high moments, the active area from the contact surface between the

reinforced-plain concrete blocks is lower than 70% from the bottom of R.C. block (lc*bc), the

design bending moments of the reinforcement are the bending moments of the columns on

both directions :

Mx=Mx,st (4.27)

My=My,st.


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f. Foundation beams

To reduce differential rotation of the foundation due to contact pressures present at

the foundation’s extremities; the adjacent foundations can be connected using foundation

beams (fig. 4.11). the solution is recommended in cases where differential rotations of the

foundations are expected, due to non-homogen stratification along the building.

Fig. 4. 9. Calculation scheme for foundation beams

Same solution is to be used for foundations having large eccentricities reported to the

columns. For this particular situation, considering the effect of reducing the eccentricity

of loads from the column do not result in an economical sizing of the foundation , at

eccentric foundation where horizontal forces are not taken by the superstructure. They

can be used to reduce differential settlements between the adjacent foundations,

assuring the absorption of moments induced by the no uniform settlements at the

foundation level.

Design of foundation beams is developed based on two hypothesis:

a. Foundation beams has a high stiffness related to column stiffness

kgr /kst=10…15. In this case the loads coming from the columns are transmitted by the

entire surface of the beam and of the foundation . (fig. 4.34).


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b. The foundation beam has a lower stiffness related to column stiffness

kgr /kst10, loads coming from the columns are transmitted only by the end section of

the beam. Design it is done starting from the calculation scheme fig. 4.13. Startingfrom an “imposed” value of eccentricity “e”, the resultant of reactive pressure for the

foundations is calculated.

,111

L

e N R

L

e N N R 122 (4.29)

By knowing the acceptable pressure of the bearing (pacc), foundation plan will be:

acc

f

p

G R A

11

1

,

acc

f

p

G R A

22

2

, (4.30)

Where Gf – includes foundation weight and a part of beam weight


By knowing the values in plane of the foundation bottom, the dimensions of foundation

can be established, in order to satisfy the initial geometrical dimensions and

eccentricity. In addition, the foundation verification is done according to ultimate limit

state, identically with the case of centric isolated foundation.

If the conditions required in the ULS are not satisfied , a new value for the eccentricity

„e” is chosen , and the calculations are repeated until the conditions for sizing for the in panefoundation surface is satisfied.


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Based on the foundation bottom, the cross section dimensions are to be determined.

Bending moment and shear force diagram is determine for the foundation beam in order to

obtain the required reinforcement quantity. (fig. 4.14). The reinforcement for the two

foundations is done according to the elastic isolated foundation indications.

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