Prestressed concrete - The Design of Prestressed Concrete Bridges - page 82

Figure 4.6 Tied arch
4.2.2 Reinforced concrete option
As the concrete cannot be relied upon to provide a safe tension member, all the tension
in the tie must be carried by steel reinforcement. The regulations in force in most
countries require that reinforced concrete members are designed for the ultimate load
(that is at the ULS), and are then checked for their performance at working load (at the
SLS). Thus using reinforcing bars with an ultimate strength of 460 MPa, and a material
factor of 1.15, the area of steel required is 1.9 × 10 6 × 1.15 / 460 = 4,750 mm 2 . This
may be provided by 8 No 25 mm bars and 1 No 32 mm bar giving a steel area of
4,731 mm 2 , marginally short. The strength of the reinforced concrete tension member
is 4,731 × 460 × 10 -6 / 1.15 = 1.89 MN. This area of reinforcing steel represents
3.9 per cent of the concrete cross-section area of the tie, Figure 4.6 (b), which is close
to the maximum practicable, above all as 20 m long bars are not available, and the bars
have to be lapped or coupled.