To attain composite action precast stem and flange is keyed toger and facilitates in carrying heavier loads effectively between prefabricated beam and cast in situ slab shear connection should be provided in which y act as one unit and resist loads by composite action as in reinforced concrete T beams.

The stresses developed due to self weight of insitu concrete are computed using section modulus of composite section and in all cases live loads stresses are based on composite section.

The precast prestressed I and T beams have been standardized by cement and concrete association for use in construction of bridge decks of span varying from 7 to 36m and standard I and T units are extensively used as highway bridge beams in U.S.A.

Planning of any structure like a building, bridge, marine structures or storage structures requires comprehensive knowledge of various parameters like site conditions availability of skilled labour and materials, transportation facilities, seismic nature of terrain, sub soil water conditions, choice of material like steel, reinforced or prestressed concrete, wear and durability considerations, clients requirements and funds earmarked for structures.

Certain expectations are made regarding high stresses developed at interface of precast and insitu cast elements and in British code BS: 8110 provides for a higher value of compressive stress equal to 0.5f_cu which is 50 percent higher than normally allowable value in prestressed element.

In case of composite members, deflections are computed by taking into account different stages of loading as well as differences in modulus of elasticity of concrete in precast prestressed unit and n insitu cast element.

The ties consisting single bars, multiple leg stirrups or vertical legs of welded face fabric should not have a spacing not exceeding four times least dimension of supported element nor 600mm whichever is less Indian standard code IS: 1343 does not make any specific recommendations regarding shear stresses in composite sections.

The average design shear stress is n distributed in proportion to vertical design shear force diagram to obtain horizontal shear stress at any point along length of members and design shear stresses should not exceed values compiled for different types of surfaces.

The main components of composite beam are prestressed precast stem and a cast insitu flange and prestressed precast stem of higher grade concrete is prepared in a factory while flange having a concrete of lower grade is used for casting is done at site.

The high strength prestressed units are used in tension zone while concrete which is cast in situ of relatively lower compressive strength is used in compression zone of composite members and composite action between two components is achieved by roughening surface of prestressed unit on to which concrete is cast insitu thus giving a better frictional resistance.