hollowblock&ribbedslabs

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Under supervision of

Dr . Mohamed Nabil

This report has been prepared by :

Mohamed Mohsen Mohamed Hussein

Mohamed Nabil Ali Kamel

Mohamed Youssef Mahmoud Youssef

Ahmed Hamdy Mohamed Hassan

Hollow block & Ribbed slabs


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Outlines

Introduction.

Comparison between different codes.

Solved example by different codes.

One & two way slab solved example.

Effect of cross ribs on deflection.

Modeling.

Case study.

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Introduction

Hollow block and Ribbed slabs are formed by placing blocks on the slaband concrete ribs.

In case of hollow block slab voided blocks are placed to reduce the total

weight of the slab .

In case of ribbed slab the blocks are not permanent.

The blocks are considered as non-structural element as they do not

contribute to strength of the slab.

Blocks may be made concrete with lightweight aggregate or other material

as polystyrene ( foam ).

Types hollow block slab are one way and two way slabs .

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Introduction

The advantages of Ribbed and Hollow block slabs are as follows:

They provide an economical, versatile lightweight monolithic slab system and

this its main advantage reduction of weight by removing the part of the concrete

below the neutral axis.

Components are relatively light and no mechanical handling is necessary there

is ease of construction.

Economic for spans more than 5m with light or moderate live loads, such as

hospitals, office or residential buildings.

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ACI 318 ECP 203

BS 8110 Euro code 2

Comparison between different codes

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Net distance between ribs "e"

shall not exceed 700 mm.

Web width "b" shall not be less

than 100 mm or one the third ofdepth "t", whichever is greater.

Compression slab thickness "ts"

shall not be less than 50 mm or

one Tenth 1/10 of distance "e",whichever is greater.


Egyptian code (ECP 203)

Limitations of Hollow block slabs :


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The minimum value of rib width (b) isn't less than 1/4 slab thickness

(t) or 100 mm whichever greater, with taking into account

requirements of concrete cover, distance between bars and fire

requirements

Egyptian code (ECP 203)

Distance between ribs axes can be increased up to 1.5 m .

Thickness of upper slab is determined by a value not less than e/12 or

50 mm whichever greater.

Limitations of Ribbed slabs:


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British code (BS 8110)


Clear distance between ribs (e) not more than 500 mm , jointed

in cement : sand mortar tsis 25 mm.

Clear distance between ribs (e) not more than 500 mm not

jointed in cement : sand mortar tsis 30 mm.

All other slabs with permanent blocks tsis 40 or one-tenth

1/10 of clear distance between ribs, whichever is great


Clear distance between ribs not more than 1.5 m tsis 50 mm or one-

tenth of clear distance between ribs, whichever is greater.

tb

e

b

t

b t/4


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Euro code (En2)

The rib spacing does not exceed 1500 mm.

The depth of the rib below the flange does

not exceed 4 times its width.

The depth of the flange is at least 1/10 of

the clear distance between ribs or 50 mm,

whichever is the greater.

Transverse ribs are provided at a clear

spacing not exceeding 10 times the overall

depth of the slab.

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One limitation will change from ribbed slab The minimum flange

thickness of 50 mm may be reduced to 40 mm.


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American Concrete Institute (ACI)

When permanent filling materialhaving a unit compressive strength at

least equal to fc in the joists are used ts

shall be not less than 1/12 the clear

distance between ribs, nor less than 40

mm.

Otherwise tsis not to be less than 1/12

the clear distance between ribs, nor less

than 5.0 cm

Ribs are not to be less than 10 cmin

width, and a depth of not more than 3.5

times the minimum web width.

Clear spacing between ribs is not to

exceed 75.0 cm


Limitations of Hollow block slabs & Ribbed slab:


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Simply sup. L/20 L/20

-----------

L/16

1 end cont. L/25 L/20.8 L/18.5

2 end cont. L/28 ----- L/21

cantilever L/8 L/5.6 L/8

ts / ts / ts / ts /

e 700 mm e 1500 mm e 1500 mm e 750 mm

b

/ b t/4 b t/4 b 100 mm

L.L 3

KN/m2 , Ls >

5

L.L > 3

KN/m2

, Ls (4:7)

L.L >

7m.

------- If L 10th.b -------

One X rib One X rib 3 X ribs



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Fcu= 25 N/mm2

Fy= 360 N/mm2

F.C= 1.5 KN/m2

L.L= 3.0 KN/m2

Solved examples by different codes


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th.b 250 mm 250 mm 250 mm 300

tS 50 mm 50 mm 50 mm 50 mm

h 200 200 200 250

Mu (

Kn.m/rib)19.187 19.187 17.75 23.5

AS 293.30 mm2 255.72 mm2 322.005 mm2 226.67 mm2

Main ribs dim. 100 200 mm 100 200 mm 100 200 mm 100 250 mm

Cross ribs No cross ribs No cross ribs One cross rib No cross ribs

Solid part Short dir. long dir. Short dir. long dir. Short dir. long dir. Short dir. long dir.0.3 m. 0.3 m. 0.3 m. 0.3 m. 0.25 m. 0.3 m. 0.3 m. 0.3 m.

Conc.

Quantities2.806 m3 2.806 m3 2.934 m3 3.07 m3

Solved examples by different codes


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One & two way slab Solved examples

One wayslab

Two way

slabFcu= 25 N/mm2

Fy= 360 N/mm2

F.C= 1.5 KN/m2

L.L= 3.0 KN/m

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th.b. 320 mm 250 mm

h 250 mm 200 mm

ts 70 mm 50 mm

Mu(KN.m/rib) 45.64 dir. dir.

15.68 15.68

As 529.255 mm2/ rib

239.68 mm2/ rib. For dir.

And 251.09 mm2/ rib. for dir.

Ribs dim. 0.12 * 0.25 m 0.1 * 0.2 m

NO. of main ribs 11 12

No. of cross ribs 1 12

Conc. quantities 5.6854 m3 5.234 m3

RFT 1017.79 mm2 /m 981.54 mm2/m

One & two way slab Solved examples

It is obvious that the RFT and concrete quantities in the two way hollow

block slab is more economic however the execution of the two way hollow

block slab is harder than the one way hollow block slab.


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Effect of cross ribs on deflection

In The previous example the effect of cross ribs on deflection will be

discussed on sap program.

At first one cross rib is put in the mid span of the slab.

Then two cross ribs are put at 1/3 of the span from each side.

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Case of one cross rib:

Effect of cross ribs


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Case of two cross rib:

Effect of cross ribs

The two cases have the same deflection value so it is more economic to use one

cross rib at mid span


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Modeling Introduction

Different models of hollow block slabs on computer programs (Sap 2000)

are illustrated.

The Example:

F.C =1.5 KN/m2,

L.L =3 KN /m2 ,

Block weight = 160 N

Number of blocks is 22 in short direction and 13 in the long direction.

Weight of the blocks on the slab =0.16221357 = 1.3 kN/m2. Wrib = 6.145 (KN/(1.0*s m

2).

Number of ribs in the long direction = 12 rib 21


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Modeling

Model 1

The ribs are frame elements with cross section as T-section and connected

together with a virtual slab with very small thickness.


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The moment on the ribs=19.3675 KN.m


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Modeling

Model 2

The ribs are frame elements with cross section as T-section and connected

together with frame elements with cross section as rectangular section .

The moment on the ribs=19.1793 KN.m


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Modeling

Model 3

The ribs are converted to a thickness on the whole slab and use virtual slab to

connect between elements with very small thickness.


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The moment on the ribs=19.0767KN.m


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Modeling

Model 4

The ribs are represented as frame elements with rectangle section and the

connecting element is frame element with rectangle section.

The moment on the ribs=19.1555KN.m

d li


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This table shows the differences between the models and the manualsolution .

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Manual4321Model

19.187519.155519.076719.179319.3675M (ultimate)

KN .mRectangular

section

Rectangular

section

T-sectionT-sectionCross sectionof ribs

FramesVirtual slabFramesVirtual slabConnecting

elements

W (rib)On slabW (rib)On slabLoads

0101Self weight

multiplier

Modeling


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Case study

Introduction:

Different case study for hollow block slabs and ribbed slabs.

Case 1

29Ordinary one way hollow block slab

C t d


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Case study

C t d


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Case study

Case 2

Cantilever hollow block slabs

Case study


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Case study

Case 3

Two way hollow block slabs

Case study


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Ribbed slabs one way and two way ( waffle ) and how the blocks are

placed .

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Case study

Case 4

Case study


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Case study

Case study


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Case study

Case 5

In this case foam blocks are used

Case study


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Clay blocks are used in this case.

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Case study

Case 6


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Questions

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