Longitudinal Reinforcement

Longitudinal reinforcement is used in the analysis of several limit state conditions including:

• Ultimate moment capacity
• Longitudinal Reinforcement for Shear Capacity
• Allowable tensile stress for temporary conditions

Each of these conditions is described in more detail in the sections below.

Ultimate Moment Capacity

Moment Capacity is computing using the strain compatibility method. If longitudinal rebar exists in a section, and the option to use girder longitudinal rebar for moment capacity is enabled, this reinforcement will be utilized when computing the ultimate positive moment capacity only. The effectiveness of partially developed reinforcement will be reduced according to LRFD 5.10.8.2.1 (pre-2017: 5.11.2.1).

NOTE: The value of shear depth, d_v, which is used extensively in shear capacity computations, is computed as part of the ultimate moment capacity analysis. Hence, shear capacity can be affected indirectly by the existence of longitudinal reinforcement. We have noted several cases where longitudinal steel reduces d_v, which will result in a reduction of vertical shear capacity and may reduce longitudinal reinforcement for shear capacity. This behavior can be quite unexpected.

TIP: The option to use longitudinal reinforcement must be enabled on the Project Criteria - Moment Capacity tab.

Longitudinal Reinforcement for Shear

Provision 5.7.3.5 (pre-2017: 5.8.3.5) of the AASHTO LRFD Specifications states that, for sections not subject to torsion, longitudinal reinforcement shall be proportioned so that at each section the following equation (5.7.3.5-1) is satisfied:

Where:

A_s = Area of non-prestressed tension reinforcement
f_y = Specified minimum yield strength of reinforcing bars
A_ps = Area of prestressing steel on the tension side of the member, reduced for any lack of full development
f_ps = Average stress in prestressing steel at the time for which the nominal resistance of member is required
M_u = Factored moment at the section
d_v = Effective shear depth
f_m = Resistance factor for moment
N_u = Factored axial force at the section
f_a = Resistance factor for axial force
V_u = Factored shear force at section
f_v = Resistance factor for shear force
V_s = Shear resistance provided by shear reinforcement
V_p = Shear resistance provided by vertical component of prestressing
q = Angle of inclination of diagonal compressive stresses

Tip: The option to use longitudinal reinforcement must be enabled on the Project Criteria library - Shear Capacity tab

Allowable Tensile Stress for Temporary Conditions

LRFD 5.9.2.3.1b (pre-2017: 5.9.4.1.2), "Temporary Tensile Stress Limits in Prestressed Concrete before Losses, Fully Prestressed Components", allows tensile stress limits for temporary conditions to be increased if sufficient bonded reinforcing bars and/or prestressing steel are provided to resist the tensile force in the concrete computed assuming an uncracked section. The higher allowable limits may be specified for the following cases:

• release in the casting yard (See Project Criteria - Spec. Checking and Design tab).
• during lifting (See Project Criterial - Lifting tab);
• during hauling (See Project Criterial - Hauling tab);

AASHTO suggests that the required tensile steel can be computed per the figure below:

In addition to the basic approach descrbed in LRFD 5.9.2.3.1b (pre-2017: 5.9.4.1.2), the following assumptions are made:

1. The actual area of the tension zone is computed based on the location of the neutral (bending) axis and the geometry of the girder cross section.
2. Sufficiency of reinforcement in the tension zone is determined without consideration of lateral bending (only primary bending about the strong axis is considered).
3. Longitudinal reinforcement will only be utilized if it lies on the tension side of the neutral axis.
4. Reinforcement is only considered at sections where it is fully developed.
5. The area of prestressing strands will not be considered. It is assumed that the stress capacity of prestressing steel is already fully utilized after jacking and losses at release. Hence, only mild steel reinforcement is considered when computing A_s for this requirement.
6. If the reinforcement is sufficient to resist the tensile force for primary bending, the alternative (higher) tensile stress limit is utilized for primary bending stresses and stresses that include lateral bending effects.

TIP: A common source of confusion is adding reinforcement to your model and finding that the tension limit did not increase. The most common reason for this is that the reinforcement is on the compression side of the beam. Use the stress diagram shown above to compute "x". If your reinforcement is below "x" it is on the compression side and does not contribute to the tensile resistance and therefore does not constitute bonded reinforcement sufficient to resist the tensile force in the concrete computed assuming an uncracked section .