slabs and their types

SEPTEMBER 3, 2015

REINFORCED CEMENT CONCRETE SLAB A Reinforced Concrete Slab is the one of the most important component in a building. It

is a structural element of modern buildings. Slabs are supported

on Columns and Beams.

RCC Slabs whose thickness ranges from 10 to 50 centimeters are most often used for

the construction of floors and ceilings.

Thin concrete slabs are also used for exterior paving purpose.

RCC Slab Construction

In many domestic and industrial buildings a thick concrete slab, supported on

foundations or directly on the sub soil, is used to construct the ground floor of a

building.

In high rises buildings and skyscrapers, thinner, pre-cast concrete slabs are slung

between the steel frames to form the floors and ceilings on each level.

While making structural drawings of the reinforced concrete slab, the slabs are

abbreviated to “r.c.slab” or simply “r.c.”.

DESIGN OF VARIOUS TYPES OF SLABS AND THEIR REINFORCEMENT

For a suspended slab, there are a number of designs to improve the strength-to-weight

ratio. In all cases the top surface remains flat, and the underside is modulated:

Corrugated, usually where the concrete is poured into a corrugated steel tray. This

improves strength and prevents the slab bending under its own weight. The

corrugations run across the short dimension, from side to side.

A ribbed slab, giving considerable extra strength on one direction. Ribbed slabs

are made up of wide band beams running between columns with equal depth narrow ribs

spanning the orthogonal direction. A thick top slab completes the system.

A waffle slab, giving added strength in both directions. Waffle slabs tend to be deeper

than the equivalent ribbed slab. Waffle slabs have a thin topping slab and narrow ribs spanning in

both directions between column heads or band beams. The column heads or band beams are the

same depth as the ribs.

Ribbed and waffle slabs benefits:

Flexible Relatively light, therefore less foundation costs and longer spans are

economic Speed of construction  Fairly slim floor depths Robustness Excellent vibration control Thermal mass Good for services integration Durable finishes Fire resistance

REINFORCEMENT DESIGN

A one way slab has structural strength in shortest direction.

  

Figure explains the share of loads on beams in two perpendicular directions depends upon the aspect ratio ly /lx of the slab, lx being the shorter span. For large values of ly, the triangular area is much less than the trapezoidal area. Hence, the share of loads on beams along shorter span will gradually reduce with increasing ratio of ly /lx. In such cases, it may be said that the loads are primarily taken by beams along longer span. The deflection profiles of the slab along both directions . The deflection profile is found to be constant along the longer span except near the edges for the slab panel. These slabs are designated as one-way slabs as they span in one direction (shorter one) only for a large part of the slab when ly /lx > 2.

A two way slab has structural strength in two directions. These slabs could be

cantilevered or Simply Supported Slabs.

in this aspect ratio should be less than 2 .

SLAB SECTION IN ONE WAY AND TWO WAY SLAB

CONSTRUCTION A concrete slab can be cast in two ways: It could either be prefabricated or cast in situ.

Prefabricated concrete slabs are cast in a factory and then transported to the site

ready to be lowered into place between steel or concrete beams.

They may be pre-stressed (in the factory), post-stressed (on site), or unstressed. Care

should be taken to see that the supporting structure is built to the correct dimensions

to avoid trouble with the fitting of slabs over the supporting structure.

In situ concrete slabs are built on the building site using formwork. Formwork is a

box-like setup in which concrete is poured for the construction of slabs.

For reinforced concrete slabs, reinforcing steel bars are placed within the formwork and

then the concrete is poured.

Plastic tipped metal, or plastic bar chairs are used to hold the reinforcing steel bars

away from the bottom and sides of the form-work, so that when the concrete sets it

completely envelops the reinforcement.

Formwork differs with the kind of slab. For a ground slab, the form-work may consist

only of sidewalls pushed into the ground whereas for a suspended slab, the form-work

is shaped like a tray, often supported by a temporary scaffold until the concrete sets.

MATERIALS USED FOR THE FORMWORK The formwork is commonly built from wooden planks and boards, plastic, or steel. On

commercial building sites today, plastic and steel are more common as they save

labour.

On low-budget sites, for instance when laying a concrete garden path, wooden planks

are very common. After the concrete has set the wood may be removed, or left there

permanently.

In some cases formwork is not necessary – for instance, a ground slab surrounded by

brick or block foundation walls, where the walls act as the sides of the tray and

hardcore acts as the base.

SPAN – EFFECTIVE DEPTH RATIOS Excessive deflections of slabs will cause damage to the ceiling, floor finishes and other

architectural details. To avoid this, limits are set on the span-depth ratios.

These limits are exactly the same as those for beams. As a slab is usually a slender

member the restriction on the span-depth ratio becomes more important and this can

often control the depth of slab required in terms of the span – effective depth ratio is

given by,

Minimum effective depth = span/(basic ratio x modification factor)

The modification factor is based on the area of tension steel in the shorter span when a slab

is singly reinforced at midspan, the modification factors for the areas of tensions and

compression steel are as given in the figure 2 and 4 of the code.

SOLID SLAB SPANNING IN TWO DIRECTIONS When a slab is supported on all four of its sides, it effectively spans in both directions,

and it is sometimes more economical to design the slab on this basis. The moment of

bending in each direction will depend on the ratio of the two spans and the conditions

of restraint at each support.

If the slab is square and the restraint is similar along the four sides, then the load will

span equally in both directions. If the slab is rectangular, then more than one-half of the

load will be carried in the shorter direction and lesser load will be imposed on the

longer direction.

If one span is much longer than the other, a large portion of the load will be carried in

the shorter direction and the slab may as well be designed as spanning in only one

direction.

Moments in each direction of span are generally calculated using co-efficients which are

tabulated in the code.

The slab is reinforced with the bars in both directions parallel to the spans with the

steel for the shorter span placed farthest from the natural acis to five the greater

effective depth.

The span-efective depths are based on the shorter span and the percentage of the

reinforcement in that direction.