Hollow block and ribbed slabs

1

الرحيم الرحمن الله بسم

2

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

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.

3

Introduction

Hollow block and Ribbed slabs are formed by placing blocks on the slab

and 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 .

4

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.

5

ACI 318 ECP 203

BS 8110 Euro code 2

Comparison between different codes

6

7

■ Net distance between ribs "e" shall not exceed 700 mm.

■ Web width "b" shall not be less than 100 mm or one the third of

depth "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 :

8

■ 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:

9


British code (BS 8110)


■ Clear distance between ribs (e) not more than 500 mm , jointed in cement : sand mortar ts is 25 mm.

■ Clear distance between ribs (e) not more than 500 mm not jointed in cement : sand mortar ts is 30 mm.

■ All other slabs with permanent blocks ts is 40 or one-tenth 1/10 of clear distance between ribs, whichever is great

Limitations of Ribbed slabs: Clear distance between ribs not more than 1.5 m ts is 50 mm or one-

tenth of clear distance between ribs, whichever is greater.

t b

e

b

t

b ≤ t/4

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.

10



Limitations of Ribbed slabs:

■ One limitation will change from ribbed slab The minimum flange thickness of 50 mm may be reduced to 40 mm.

11

American Concrete Institute (ACI)

■ When permanent filling material having 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 ts is 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 cm in 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:

12

Code

Limits

Egyptian code British code Euro code American code

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

b b ≥ b ≤ t/4 b ≤ t/4 b ≥ 100 mm

Cross ribs

L.L ≤ 3 KN/m2 , Ls >

5

L.L > 3 KN/m2

, Ls →(4:7)

L.L > 7m.

------- If L ≥ 10 th.b -------

One X rib One X rib 3 X ribs


13

( 5 m * 7m )

USING ECP

USING BS

USING EURO CODE

USINGACI

( 7m * 7m )

AS ONE WAY HOLLOW

BLOCK SLAB

AS TWO WAY HOLLOW

BLOCK SLAB

14

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

15

Code

prop.Egyptian code British code Euro code American code

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 partShort 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. Quantities

2.806 m3 2.806 m3 2.934 m3 3.07 m3

Solved examples by different codes

16

One & two way slab Solved examples

One way slab

Two way slab

Fcu= 25 N/mm2

Fy= 360 N/mm2

F.C= 1.5 KN/m2

L.L= 3.0 KN/m2

17

SLAB TYPE

PROP.

One way hollow blocks slab Two way hollow blocks slab

th.b. 320 mm 250 mm

h 250 mm 200 mmts 70 mm 50 mm

Mu (KN.m/rib) 45.64 α dir. β dir.

15.68 15.68

As 529.255 mm2/ rib 239.68 mm2/ rib. For α dir.And 251.09 mm2/ rib. for β dir.

Ribs dim. 0.12 * 0.25 m 0.1 * 0.2 m

NO. of main ribs 11 12No. of cross ribs 1 12Conc. 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.

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.

18

19

■ Case of one cross rib:

Effect of cross ribs

20

■ 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

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 == 1.3 kN/m2.

Wrib = 6.145 (KN/(1.0*s m2).

Number of ribs in the long direction = 12 rib

21

22

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.

23

The moment on the ribs=19.3675 KN.m

24


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

25


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

to connect between elements with very small thickness.

26


27


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

connecting element is frame element with rectangle section.


This table shows the differences between the models and the manual solution .

28

Manual 4 3 2 1 Model

19.1875 19.1555 19.0767 19.1793 19.3675 M (ultimate)KN .m

Rectangular section

Rectangular section

T-section T-section Cross section of ribs

Frames Virtual slab Frames Virtual slab Connecting elements

W (rib) On slab W (rib) On slab Loads

0 1 0 1 Self weight multiplier

Modeling

Case study

Introduction: Different case study for hollow block slabs and ribbed slabs.■ Case 1

29Ordinary one way hollow block slab

30

Case study

.

31

Case study

■ Case 2

Cantilever hollow block slabs

32

Case study

■ Case 3

Two way hollow block slabs

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

33

Case study

■ Case 4

34

Case study

35

Case study

■ Case 5

In this case foam blocks are used

Clay blocks are used in this case.

36

Case study

■ Case 6

Questions

37

38

Technology

Hollow block and ribbed slabs