The use of prestressed tie beams in a reinforced concrete truss considerably reduces cross sectional dimensions of bottom chord member, which is subjected to high degree of tension in case of large span trusses.

After insitu concrete has hardened whole section is assumed to be monolithic and stresses that develop due to subsequent live loads are computed using properties of composite sections.

The most common type of composite construction consists of a number of precast prestressed inverted T beams placed side by side and connected by a continuous top slab of insitu concrete and this type of construction is widely used in construction of bridge decks.

The average horizontal design shear stress is computed by dividing horizontal shear force by area which is obtained as product of contact width of bear and its length between points of zero moments.

vu = ultimate shearing force, ? = v_us/ln, s= (350x30x95)mm³, b = 80mm, ? = 0.6n/mm², v_u = (0.6x2487x10⁵x80/350x30x95) = 12000n = 12kn.

Z_t = 225�10, Z_b = 128�10⁴mm³, moment due to self weight = 1200nmm
Stresses due to this moment in composite section:
At top = (1200000/225�10⁴) = 0.53n/mm² (compression), At bottom = (1200000/128�10⁴) = 0.94n/mm² (tension).

The maximum eccentricity for composite sections is given in step 8 is
e = Z_tZ_b(P_t-P_b) / A(P_bZ_t+P_tZ_b) where Z_t = section modulus of top fiber, Z_b section modulus of bottom fiber, minimum prestressing force is given as W = A(P_tZ_b+P_bZ_t) / Z_t+Z_b.

The higher value of compressive stress is permissible only in composite sections with stipulation that failure of section is due to excessive elongation of steel and this requirement is to safe guard against explosive compressive failure of concrete at limit state of collapse.

When modulus of elasticity of precast and insitu cast concrete are different flexural rigidity is worked out by computing equivalent second moment of area of composite sections using modular ratio and deflections computed under service loads should not exceed limiting values prescribed in codes.

The dead load stress developed in precast prestressed units can be minimized by Propping m while casting concrete in situ this method of construction is termed as propped construction and if precast units are not propped while placing insitu concrete, stresses are developed in unit due to self weight of member and dead weight of insitu concrete and this method of construction is referred as unpropped construction.