slabs. types and design

7/25/2019 Slabs. Types and Design.

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Reinforced Concrete Slabs

1. to determine the slab design life,

2. to assess the actions on the slab,

3. to determine which combinations of actions to apply,

4. to determine loading arrangements,

5. to assess durability requirements and determine concrete strength,

6. to check cover requirements for appropriate fire resistance period,

7. to calculate min. cover for durability, fire and bond requirements,

8. to analyse structure to obtain critical moments and shear forces,

9. to design flexural reinforcement,10. to check deflection,

11. to check shear capacity,

12. to check spacing of bars.


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Two-way Flat PlateFlat Slab with Beams

Flat slab wi th banded beams

Two-way Flat Slab with Drop Panels

Two-way waffle slab

(grid slab)

One-way slab and beam

Voided Slab

Ribbed slabs with wide band beamsTroughed slabs

(ribbed slabs with integral beams)

One-way ribbed slab

Precast slabBubbledeck slabSlab with permanent

steel formworks

* A slab is a structural member for wich the minimum panel dimension is not less than five times the overall thickness.


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1. to determine the slab design life,

2. to assess the actions on the slab,3. to determine which combinations of actions to apply,

4. to determine loading arrangements,

5. to assess durability requirements and determine concrete strength,

6. to check cover requirements for appropriate fire resistance period,

7. to calculate min. cover for durability, fire and bond requirements,

8. to analyse structure to obtain critical moments and shear forces,

9. to design flexural reinforcement,

10. to check deflection,

11. to check shear capacity,

12. to check spacing of bars.

Concrete Volume %

Walls 4

Columns 5

Slabs 59

Foundations 22

Other construction elements 10

The reinforced concrete slab may

be supported by:

reinforced concrete beams

(usually cast monolithically), masonry or reinforced

concrete walls,

structural steel members,

directly by columns,

continuously by the ground.

One way slabs:

Two way slabs:

a) cantilever, b) simply supported, c),d),e),f) lx/ly>2

a), b) and c) lx/ly


4/22

I. Major girders

in one direction

load pathII. We need

to shorten

the spans

We need additionalbeams to support

the load from

these new beams

additional beams

We use beams

supported by the

frame girders

Plan

Tributary areas for beams: Building idealisation for

equivalent frame analysis:


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6/22

When the ratio of the long span L to the

short span S is less than 2 (L/S < 2):

the deflected surface of the shaded

area becomes one of double

curvature,

the floor load is carried in both

directions to the four supporting beams

around the panel.

yx aa

xa

ya

Concrete slabs behave primarily as flexural members.

The design is similar to that for beams and in general it is somewhat simpler

because:

the breadth of the slab is already fixed,

a unit breadth of 1 m is used in the calculations,

the shear stresses are usually low in a slab except when there are heavy

concentrated loads,

compression reinforcement is seldom required.


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The minimum slab thickness:

a) two-ways slabs (l2/l12):

- l1/30 simple supported on contour,

- l1/35 embedded on contour.

- l1/25 simple supported on two parallel sides,

- l1/35 embedded on two parallel sides,

- l1/12 for cantilever slab ,

c) slabs without beams:

- l2/30 the colums without capitals,

- l2/32 the columns with simple capitals,

- l2/35 the columns with drop panel,

14

Cazuri de rezemare


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9/22

The minimum slab thickness:

hf>50mm (usually),

the minimum dimensions from

fire resistance conditions,

hf>200mm if the slab has shear

reinforcement.

The minimum diameter

for resistance reinforcement:

slab with beams: 6mm (5mmfor welded steel bars),

slab without beams: 8mm

(10mm on the supports),

foundations and slabs in

corrosive environments: 10mm

(8mm for welded wires).

The maximum diameter

for resistance reinforcement:

max < 0.1hf+ 2mm

The reinforcement

ratio:

< 0.5% ifl2/l1 < 2

< 0.8% ifl2/l1 > 2


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The constructive reinforcements ( l2/l1>2 ):

the constructive reinforcements are disposed perpendicular to the

resistance reinforcements direction,

As,constructive 0.20As,resistance

The maximum distance between reinforcements:

for slabs with concentrated loads

resistance reinforcements: constructive reinforcements:

( welded

wires )

The maximum number of bars on spans or on supports: 12 bars / meter,

One-way concrete slab

reinforced with independent

bars: (small spans)

One-way concrete slabreinforced with independent

bars: (wide spans)

* at least of the tensile

reinforcements from the zone of

positive moments, but not least

than 3 bars/meter, must be

extended beyond the support

edge.


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Reinforced concrete slab

supported on r.c. flange:

Supplementary reinforcements

on the supports, parallel with

the resistance reinforcement:

riders: min. 76mm/m.

Two-way concrete slab

reinforced with independent

bars:


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The one-way slabs

reinforced with welded

wires:

I. Slab simply supported on two opposite sides :

before deformation,

after deformation,

basic (unit) strip,

static model.


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II. Slab double fixed

before deformation,

after deformation,

static model.

III. Slab fixed on a side and simply supported on the other side

before deformation,

after deformation,

static model.


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IV. Cantilever slab

before deformation,

after deformation,

static model.

V. Isolated slab, simply supported. ,

on all four sides :

is designed for a 1m strip, simplysupported at the ends on the short

side.

Example:

Reinforcements for a isolated slab,

simply supported:


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VI. Continuous slab

- if the spans are equal, or the

difference between them is less than

10%, the simplif ied plastic method

can be used:

Simplified Plastic Method:

Example:

The reinforcements for a continuous slab:


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I. Isolated slab, simply supported on all four sides

- the slab can be divided in elementary

strips on both directions,

- in the middle, the deflections on both

directions are equals.

xa

ya

=> if lx < ly then: qx > qy

Denoting: =>

IE

lq

rlSa

cm

xx

I

xx

384

51 42

IE

lq

r

lSa

cm

yy

I

yy

384

514

2

IE

M

r cmI

1

yx aa

qll

lq

yx

yx

44

4

qll

lq

yx

xy

44

4

x

y

l

l

qq xx

41

qqy

41

1


17/22

- from the analysis of values from the table, it can be observed that, if a

rectangular slab is longer, the load distribution on the shorter side increases,

- thus, for example, if the ratio value is 2, on the shorter direction it is distributed

about 94% from the loadq and on the longer direction only 6%,

- the slabs subjected to double bending actions, can be designed as one

way slabs (on the shorter d irect ion) if the rat io of the sides is greater than 2.

- if the sides ratio is smaller than 2, the loads on the two direction are comparable.

- Conclusions -

Example:

The reinforcements for a

isolated slab:


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19/22

--simplesup

portedonallsides

--simplesupporte

donthreesidesandfixedona

side

-simplesupportedontwooppositesides

andfixedontheotherstwo,

--simplesupportedontwosidesandfixed

ontheo

therstwo

,


20/22

-simplesu

pportedononesideandfixed

on

theothersthree,

--fixedonallsupportsts,

Example:

The reinforcements for

a continuous slab with

equal panels on two

rows:


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41

42


22/22

43

bare suplimentarede bordaj

ai2

ci> +lbd

max (a1; a2 ) < 0,2 lx

a1

a2

c1

c2

bare suplimentare

etrieri suplimentari

hf

> 2hf

sus

jos

jos

sus

sus

sus

sus

jos

Treptele

nu se armeaza

> lb,d se va tine cont

de momentul negativ

ls

>lb,d

ls

>lb,d

ls

AF3

1 2

3

2

1

5

5

4

6

4

5 6

jos

sus

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