Code References
ACI 318-14
ACI 318-19
Definition
For the purposes of this discussion, one-way (or vertical) shear is defined as the shear force perpendicular to the axis or span of the member. Also, the discussion in this section is limited to precast concrete members that span horizontally and are governed by the ACI 318 Specification. Note, one-way shear is how it is defined in the ACI 318 code, it is also known as beam action shear.
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If the non-composite section is being used for shear resistance, then the non-composite height is used to calculate d and H. If the composite section is being used for shear resistance, then the composite height is use. The user must also make an affirmative response to move the critical section away from the support face.
Member Orientation
When computing shear capacity, it is computed in line with the appeared shear vector. If flexure is computed about a different angle, the 'd' value is computed for the orientation corresponding with the shear force. ACI 318-14 gives no guidance on the direction you should compute it in. ACI 318-19 allows the computation of it about any axis with equations for the interaction. By aligning our orientation with the applied load, we bypass the need to perform any shear interaction.
Calculation of Vc
All values of f’c used in determining Vc are reduced by the appropriate factor if using lightweight concrete. Note that the analysis uses the base area of prestressing strand, ignoring development length, to determine if a member is prestressed or non-prestressed.
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Mild Reinforced (non-prestressed) Members
ACI 318-14
For non-prestressed members, the analysis uses the appropriate equation based on the sign of the of the axial load (compression – 22.5.6, tension – 22.5.7, or none – 22.5.5). Note that both the ‘tension’ and the ‘compression’ equations become the ‘no axial load’ equation when the axial load is equal to zero. For both no axial force and axial compression, both the detailed and the simplified methods are used, and the program selects the greater of the two values (the axial tension section does not include a detailed method for the calculation of Vc).
Beams without any mild reinforcement in tension (that is an undefined d value) will use 0.8h in place of d. This is not supported by the code and typically only happens at the ends of the beam, where shear is not typically checked.
ACI 318-19
For non-prestressed members, the analysis follows section 22.5.5. When following this section, the program behaves in two different methods depending on if the program is computing Av required or if the user entered transverse reinforcement.
Computing Av Required
When the software is computing Av required, we first assume the user is not providing minimum shear reinforcement. Because Av min is not met, we use equation 22.5.5.1c. Once Vc is computed, we see if the user must provide minimum shear reinforcement per chapter 7 or 9 (depending on the member). If minimum shear reinforcement is required, Vc is then recomputed using the minimum of equations 22.5.5.1a and 22.5.5.1b.
Computing ϕVn
When the user inputs their transverse reinforcement, the program computes ϕVn instead of computing Av Required. For these cases, it is just checked if Av provided exceeds Av Min before determining which equations of Table 22.5.5.1 to use.
Prestressed Members
For horizontal prestressed members, the analysis uses the traditional Vci/Vcw equations to determine the shear capacity of the concrete (22.5.8.3). The approximate method for calculating Vc as shown in the equations tabulated in the ACI 318 code (Table 22.5.8.2) is typically reserved for members that span vertically and are therefore unavailable for horizontal members in this analysis. The nominal concrete shear strength determined using this approach includes the upper and lower limits as noted in ACI 318.
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Exceptions and Notes
In the case of single and double tees, when they are simply supported, uniformly loaded, and when they require little or no shear reinforcing as calculated for Avcw, and approximately 0.04 in2/ft or less of shear reinforcing as calculated for Avci, the engineer may omit the leg mesh in the middle 80% (+/-) of the span.
For the case of hollow core slabs, if the untopped section depth is <= 12.5 in., then no minimum shear steel is required. If the untopped depth is > 12.5 in., then minimum shear reinforcement is not required where Vu <= 0.5φVcw. Currently, the analysis does not take this provision into account.
Note that required shear reinforcement does not include any suspension steel that may be required for ledges. A separate ledge analysis is required.
Critical section for shear. The critical section for shear is based on the depth of the resisting element, and assumes that the loads on the beam (including self-weight) between the critical section and the support go directly into the support without the mechanism of beam action (acting kind of like a small column). The depth used to calculate the critical section should be based on the cross section used to resist shear. If there is an applied concentrated load between the critical section and the support, then the critical section is the face of the support.
References
ACI Committee 318, Building Code Requirements for Structural Concrete (ACI 318-14), American Concrete Institute, Detroit, 2014.
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