Hollow block and ribbed slabs

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.
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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 .
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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
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 :

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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 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
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.
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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 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:

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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
e ≤ 700 mm e ≤ 1500 mm e ≤ 1500 mm e ≤ 750 mm
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.
Quantities
2.806 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 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

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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
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 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.

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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19
■ 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
■ 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

25
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.

26

27
Modeling
■ Model 4
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 .
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Manual4321Model
19.187519.155519.076719.179319.3675M (ultimate)
KN .m
Rectangular
section
Rectangular
section
T-sectionT-sectionCross section
of ribs
FramesVirtual slabFramesVirtual slabConnecting
elements
W (rib)On slabW (rib)On slabLoads
0101Self weight
multiplier
Modeling

Case study
 Introduction:
Different case study for hollow block slabs and ribbed slabs.
■ Case 1
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Ordinary one way hollow block slab

.
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Case study
■ Case 2
Cantilever hollow block slabs

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Case study
■ Case 3
Two way hollow block slabs

Ribbed slabs one way and two way ( waffle ) and how the blocks are
placed .
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Case study
■ Case 4

35
Case study
■ Case 5
In this case foam blocks are used

Clay blocks are used in this case.
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Case study
■ Case 6

Hollow block and ribbed slabs

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