ASDIP CONCRETE is a structural software that includes the design if biaxial slender columns, multi-span continuous beams, and concrete / block bearing walls exposed to out-of-plane loads. Now ASDIP CONCRETE complies with ACI 318-19. Load combinations per ASCE 7-16. Following are the most important changes affecting the software.
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Tension-controlled limit in columns
Concrete columns are elements mostly subject to compression and bending stresses. Under the presence of axial loads higher than the balanced condition, the column will fail in compression, therefore this is the compression-controlled zone. For axial loads smaller than a certain limit, the column will fail in tension, and this is called the tension-controlled zone. Between these two areas there is a transition zone where the type of failure is not well defined.
In previous editions of ACI 318, the steel strain that defined the tension-controlled limit was εt = 0.005. In ACI 318-19 the tension-controlled limit is εt = fy/Es + 0.003. The under-strength factor ϕ varies depending on the different regions. The left image below shows at left the strain diagram of a concrete section for the tension-controlled limit condition. The right image below shows the variation of ϕ for the different regions. This change mostly affects columns with high strength reinforcing steel.
Calculation of development length Ld
Now the ACI 318-19 includes a new factor in the calculation of the development length Ld, called the grade factor. This factor accounts for the use of high strength reinforcing bars. The higher the steel grade the higher the factor, up to 1.3. For steel grade 60 (fy = 60 ksi) the grade factor is 1.0, for steel grade 80 the grade factor is 1.15, and for steel grade 100 the grade factor is 1.3, as shown below.
This change mostly affects beams with high strength reinforcing steel.
Calculation of concrete shear strength Vc
The shear strength is composed of two parts: one is the strength of the concrete Vc, and two is the strength of the reinforcing steel Vs. Therefore the applied shear factored load must be less than the shear strength, or Vu < ϕ(Vc + Vs). The calculation of the concrete shear strength per ACI 318-19 is more involved than in previous editions of ACI 318. The calculation depends on both the shear reinforcement and the flexural reinforcement.
For Av > Av min the calculation per option (a) below is similar to previous editions of ACI 318. However, when Av < Av min the calculation per option (c) below includes the flexural reinforcement ratio ρw, and the new shape factor λs as shown below. This factor takes into account the fact that the relationship between shear strength and section depth is not linear.
This change mostly affects the stirrups design in beams. The calculation of Vc per option (c) below gives smaller results, and therefore the design may need more stirrups to comply with ACI 318-19.
Now ASDIP CONCRETE complies with ACI 318-19. The new provisions may affect the design of the concrete members designed with the software, particularly the stirrups design in beams, and members with high strength reinforcing steel.
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Javier Encinas, PE
ASDIP Structural Software