Table 3 Minimum effective prestress (fse,min) to satisfy the tensile stress limit (Δfps,allow).
For the partial PSC members with 550 MPa reinforcements, the minimum effective prestress (fse,min) is 0.60f pu to meet the Δf ps limit of 250 MPa.
As shown in Fig. 10, in the case of the T-shaped sections, the minimum effective prestress (fse,min) can be determined as 0.5 f pu for all the full PSC members and the partial PSC members except the partial PSC members with 550 MPa reinforcements.
If the minimum required effective prestress condition is not satisfied, i.e., f se < fse,min, however, detailed check should be performed through the cracked section analysis, or the section should be redesigned, if necessary.
If the minimum required effective prestress condition is satisfied, i.e., f se ≥ fse,min, Eq. (17) can be used to calculate Δfps,ACI, and if Δfps,ACI does not exceed 140 MPa, it is not necessary to check the spacing of the tension reinforcements for the crack control design.
In that case, the minimum magnitude of the effective prestress (fse,min) can be 0.5 f pu for all the partial PSC members reinforced with 550 MPa nonprestressed steel.
Table 3 shows the minimum magnitude of the effective prestress (fse,min) required to satisfy 250 MPa stress limit (Δfps,allow) specified in the ACI318 code based on the parametric study results shown in Figs. 6, 9, and 10.
Open image in new window Fig. 1 Class of prestressed concrete sections and effect of effective prestress on ∆f ps.
After performing an elastic analysis for the PSC section, if the stress in the precompressed extreme tension fiber at the service loads (f b ) belongs to the Class C category, the design effective prestress (f se ) is then compared with the minimum required effective prestress (fse,min) presented in Table 3.
The factors examined in this study include the content of non-prestressed steel, tendon eccentricities, tendon area, effective prestress, span-to-height ratio, concrete strength, CFRP modulus of elasticity and load type.
The resulting effective prestress was 1,144 MPa.
