1. Umm Al-Qura University Department of Civil & Structural Engineering
Design of reinforced concrete II
Ribbed and Hollow Block Floors
Lecture (4)

2. Ribbed and Hollow Block Floors
The ribbed floor is formed using temporary or
permanent shuttering while the hollow block floor is
generally constructed with block made of clay or
concrete containing lightweight aggregate.
Many design calculations ignore these block
contribution to the slab strength.
Major benefit is the reduction in weight.
They are economical for building with spans  5m
and moderate live load and not suitable for heavy
loads such as garages and warehouses.

3. Near to the support the blocks are stopped off and
the slab is made solid (greater shear strength).
The ribs should be checked for shear at their junction
with the solid slab.
The slabs are usually made solid under partitions and
concentrated loads.
During construction the blocks should be well soaked
in water prior to placing concrete, otherwise shrinkage
cracking of the top concrete flange is liable to occur.

6. The thickness of concrete flange should not be less than:
40 mm or one-tenth of the clear distance between ribs with permanent blocks.
50 mm or one-tenth of the clear distance between ribs without permanent blocks.
The rib width will be governed by (Section 6.1):
Cover
Bar-spacing
Fire resistance

7. The ribs should be spaced no further apart than 1.5m.
Calculation of reinforcement will require evaluation of
effective flange breadths for T-beams (see Chapter 7).
For shear design bw taken as the breadth of the rib.
Span-effective depth ratio is based on the shorter span.
Basic values given in figure 6.3 multiplied by 0.8.

8. Light reinforcing mesh in the topping flange can give
extra strength and durability.
This can resist cracking due to shrinkage or thermal
movement.
Area of mesh equivalent to 0.13% of the topping flange
will be adequate.