[TechTalk]: Understanding the concept of Composite Floors

A Civil Engineer, and even the common man now, is encountered with a term named: “Composite Floor” so often these days that it seems just another technical term which keeps on encountering every now and then.

So, what exactly is a composite floor?

To understand this element of civil engineering jargon, you would be surfed through a brief article hereunder so as to familiarize all of you with this terminology which you might come across anywhere in the property/realty bazaar~ be it a multi-storied apartment building, an industrial plant structure, an institutional building or even your new age home~ virtually anywhere and everywhere one may say.

This brief article aims to let you understand the basics behind construction of such composite floors and the purpose for which such floors are mainly used for. So, here we present...

A BRIEF TECHNICAL WRITE-UP ON THE COMPOSITE FLOOR CONSTRUCTION SYSTEM...

General

A Civil Engineer is encountered with a term named: “Composite Floor” so often these days that it seems just another technical term which keeps on encountering every now and then.

So, what exactly is a composite floor? You would be surfed through a brief article hereunder so as to familiarize all of you with this terminology. To start with, let us first understand what purpose such a floor is mainly used for. So, here we go!

Purpose of Providing a Composite Floor

The main purpose of providing a composite floor is to negate the use of the conventional formwork and the necessary props and supports for casting of RCC floor, which need to be kept in place for the requisite number of days, and thereafter to be de-shuttered. Since the said floor is the first main floor of this structure, and at a height of approximately 29m from the ground, casting the floor with conventional shuttering is envisaged to be cumbersome. As such, so as to avoid the time-consuming propping arrangement for such greater height, and to speed up the construction work, this floor shall be cast as a composite floor.

What is a Composite Floor?

A composite floor comprises of a reinforced concrete floor slab and structural steel floor beams acting compositely together.

The scheme mainly consists of profiled steel decking working together with in-situ reinforced concrete. The decking rests over the structural steel floor beams network, which is erected with suitable means. This decking not only acts as permanent formwork to the concrete, but also provides sufficient shear bond with the concrete, so that the two materials act compositely together.

Composite Floor Beams

The composite floor beams (excluding the peripheral beams of the building which shall be RCC beams only) are generally hot-rolled steel sections or suitable built-up steel sections that are erected to make a beam-network, and which act compositely with the slab.

The composite action is normally achieved by welding shear connectors (or studs) through the steel decking and onto the top of the beams before pouring the concrete.

Steel Decking (Deck Sheets)

Decking shall generally comprise of 2mm thick MS folded (profiled) deck plates, with a nominal crest to trough height of approximately 44mm, though the specifications and details may vary from manufacturer to manufacturer. The deck sheets shall be primed with Red oxide Zinc Chromate primer.

The deck sheets are hoisted, placed and erected over the floor steel beam sections by means of welding or studs or other suitable means. The shear connectors also help in keeping the decking in place. The decking is cut to shape making proper slots for openings, bases of column projecting above slab level including providing extra supports wherever required.

Shear Connectors

The shear connectors provide sufficient longitudinal shear connection between the beams and the cured concrete so that they act together compositely.

These can be made of 10, 12 or 16mm in diameter steel rods bent in an anchor-lug like shape, and can be fixed by welding.

Casting of Floor

The designed reinforcement is then placed and fixed over the decking sheets. Suitable formwork is provided for the sides of the floor and around the openings, after which the concrete is placed.

Floor/Beam End Limits

The end limits of floor slab and the building peripheral beams are generally matched by a concrete lip-wall section so as to furnish a flush outer surface. This gives a clean finish from the outside from Architectural point of view.

Concrete Embedments

All steel embedments required in the cast surfaces (viz., insert plates, edge angles, curb angles, etc.) shall be fixed at required positions and levels prior to pouring of concrete over the fixed deck sheets.

Indicative Scheme of Casting

For the purpose of illustration, a sequence proposed can be:

a) Cast the RCC beams leaving the dowels for the lip-wall as indicated in the Tender drawing mentioned above, with the insert plates embedded cast in-situ therein wherever shown in the detailed drawing(s).

b) Cast the lip-wall section by re-arranging the forms upto the soffit of the floor slab (i.e., upto the level of the deck sheet).

c) Erect the floor structural steel members, taking due care not to damage the cast lip-wall sections.

d) Erect and fix the deck sheets followed by fixing of reinforcement and required embedments.

e) Fix forms for sides of the floor slab, and for openings and cutouts as per the detailed drawing.

f) Cast the floor slab ensuring requisite quality control and surface finishes.

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This is post is presented in relation to a query received by us from one of our subscribers Mr. Anil Biswas.
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