Slabs. Types and Design.
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Transcript of Slabs. Types and Design.
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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
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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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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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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
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- 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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--simplesup
portedonallsides
--simplesupporte
donthreesidesandfixedona
side
-simplesupportedontwooppositesides
andfixedontheotherstwo,
--simplesupportedontwosidesandfixed
ontheo
therstwo
,
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-simplesu
pportedononesideandfixed
on
theothersthree,
--fixedonallsupportsts,
Example:
The reinforcements for
a continuous slab with
equal panels on two
rows:
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41
42
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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