Composite structures

I. INTRODUCTION

Composite structures can be defined as the Structures in which composite sections made up of two different types of materials such as steel and concrete are used for beams, and columns. This paper include comparative

study of R.C.C. with Steel Concrete Composite (G+12, G+16, G+20, G+24) story buildings which situated in

Nagpur earthquake zone II and wind speed 44m/s. Equivalent Static Method of Analysis is used. For modeling of Composite & R.C.C. structures, STAAD-Pro software is used and the results are compared. Comparative study includes deflection, axial force and shear force, bending moment in column and beam, cost. It is found that composite structure is more economical and speedy than R.C.C structure.

II. COPOSITE MULTISTORIED BUILDINGS

The primary structural components use in composite construction consists of the following elements.

1. Composite deck slab

2. Composite beam

3. Composite column

4. Shear connector

2.1. COMPOSITE DECK SLAB

Composite floor system consists of steel beams, metal decking and concrete. They are combined in a very

efficient way so that the best properties of each material can be used to optimize construction techniques. The most common arrangement found in composite floor systems is a rolled or built-up steel beam connected to a formed steel deck and concrete slab. The metal deck typically spans unsupported between steel members, while also providing a working platform for concreting work. The composite floor system produces a rigid horizontal diaphragm, providing stability to the overall building system, while distributing wind and seismic shears to the lateral load-resisting systemsComposite action increases the load carrying capacity and stiffness by factors of around 2 and 3.5 respectively. The concrete forms the compression flange – the steel provides the tension component and shear connectors ensure that the section behaves compositely. Beam spans of 6 to 12 m can be created giving maximum flexibility and division of the internal space. Composite slabs use steel decking of 46 to 80 mm depth that can span 3 to 4.5 m without temporary propping. Slab thicknesses are normally in the range 100 mm to 250 mm for shallow decking, and in the range 280 mm to 320 mm for deep

decking. Composite slabs are usually designed as simply supported members in the normal condition,

with no account taken of the continuity offered by any reinforcement at the supports.

 COMPOSITE ACTION IN BEAMS :- Composite action increases the load carrying capacity and stiffness by factors of around 2 and 3.5 respectively. The concrete forms the compression flange – the steel provides the tension component and shear connectors ensure that the section behaves compositely. Beam spans of 6 to 12 m can be created giving maximum flexibility and division of the internal space. Composite slabs use steel decking of

46 to 80 mm depth that can span 3 to 4.5 m without temporary propping. Slab thicknesses are normally in

the range 100 mm to 250 mm for shallow decking, and in the range 280 mm to 320 mm for deep decking. Composite slabs are usually designed as simply supported members in the normal condition, with no account taken of the continuity offered by any reinforcement at the supports. zero at mid-span and maximum at the support of the simply supported beam subjected to uniformly distributed load. Hence, shear is less in connectors located near the centre and maximum in connectors located near the support.Composite beams are often designed under the assumption that the steel beam supports the weight of the structural steel or wet

concrete plus construction loads.This approach results in considerably less number of connectors than they are required to enable the maximum bending resistance of the composite beam to be reached. However the use ofsuch partial shear connection results in reduced resistance and stiffness.

2.2 ADVANTAGES OF COMPOSITE BEAMS

1. Keeping the span and loading unaltered, more economical steel section in terms of depth and weight) is adequate in composite construction compared with conventional non-composite construction.

2. Encased steel beam sections have improved fire resistance and corrosion.

3. It satisfied requirement of long span construction a modern trend in architectural design.

4. Composite construction is amenable to fast track construction because of use of rolled steel sections.

5. Composite sections have higher stiffness than the corresponding steel sections and thus the deflection is lesser.

6. Permits easy structural repairs/ modification.

7. Provides considerable flexibility in design and ease of fabrication.

8. Enables easy construction scheduling in congested sites.

9. Reduction in overall weight of the structure and there by reduction in foundation cost.

10. Suitable to resist repeated earthquake loading which requires high amount of resistance and

ductility.

THE ADVANTAGES OF COMPOSITE COLUMNS ARE

1) Increased strength for a given cross sectional dimension.

2) Increased stiffness, leading to reduced slenderness and increased bulking resistance.

3) Good fire resistance in the case of concrete encased columns.

4) Corrosion protection in encased columns.

5) Significant economic advantages over either pure structural steel or reinforced concrete alternatives.

6) Identical cross sections with different load and moment resistances can be produced by varying steel thickness, the concrete strength and reinforcement. This allows the outer dimensions of a column to be held constant over a number of floors in a building, thus simplifying the construction and architectural detailing.

7) Erection of high rise building in an extremely efficient manner.

8) Formwork is not required for concrete filled tubular sections.