Prestressed Concrete Design Examples

Beams

Example 1 - Calculating stresses for a simply supported beam subjected to dead loading, live loading, and compression due to pretension

For the simply supported beam shown below, find the maximum stresses at midspan due to the beam self weight and the following loading and prestressing scenarios:

 

Uniform live load = 800 lbs/ft

 

Uniform live load = 800 lbs/ft , Longitudinal compressive force due to prestressing = 259.2 kips Eccentricity of compressive force from centroid of cross-section = 0 inches

 

Uniform live load = 2000 lbs/ft, Longitudinal compressive force due to prestressing = 259.2 kips Eccentricity of compressive force from centroid of cross-section = 4 inches

 

Uniform live load = 2700 lbs/ft Longitudinal compressive force due to prestressing = 259.2 kips Eccentricity of compressive force from centroid of cross-section = 6 inches

 

Normal weight concrete

Compressive strength of concrete = 4500 psi

Allowable compressive strength of concrete = 2050 psi

Example 2 - Calculating the design flexural strength of a prestressed concrete beam

Calculate the design flexural strength of the prestressed bonded beam shown below given the following parameters:

 

Specified Yield Strength of Prestressing Steel, fpy = 240 ksi

Specified Tensile Strength of Prestressing Steel, fpu = 275 ksi

Compressive Design Strength of Concrete, f'c = 5 ksi

 

Example 3 - Calculating the deflection of a pretensioned concrete beam

For the pretensioned rectangular beam shown below, calculate the deflection at midspan immediately after the cables are cut given the following parameters:

 

Straight cables with initial stresses of 175 ksi and final stresses after losses of 140 ksi

Modulus of elasticity of concrete, Ec = 4000 ksi

Assume that the concrete has not cracked