Ram Concept- Shear Reinforcement

Question

Hi everyone, 
 Why does using 5000 psi concrete strength for a beam design give you a reduced spacing (22.6") shear reinforcement as opposed to using 4000 psi concrete strength (24")? See attached model. Beam (20x 42).zip

Seth Guthrie · Answer

If you look closely at the section audit for a section from each case and compare them side by side it's a little easier to tell what is most significant. Below is a table of just a few lines from the audit to help you, ultimately is a small change in Vc that matters:

5000 psi case 
 4000 psi case

Calculating Vc using Equation 11-9:
(Vu dpt) / Mu = 0.9732
Vc = 486.3 Kips (equation 11-9)
2 root-fc bw d = 95.04 Kips
5 root-fc bw d = 237.6 Kips
Using Vc = 237.6 Kips
Considering cross section as "beam" (not slab)
Maximum (shear) stirrup spacing = 24 inches (for shear, if stirrups required)
Vn Required = 161.1 Kips (from Vu/phi)
Vs Required = 0 Kips (from Vn - Vc)
Minimum stirrups (or more) required
Maximum phi Vn = 463.3 Kips (not considering torsion)
phi Vn taken by torsion = 0 Kips
Available Vn = 617.7 Kips (after torsion considered)
Designing for Vs = 0 Kips
Existing usable Vs = 0 Kips
Av/s = 0 inches (shear only)
Av/s = 0 inches (torsion only)
Av/s = 0 inches (shear + torsion)
Minimum additional design Av/s = 0.01768 inches
Final design Av/s = 0.01768 inches (shear, torsion & minimum)
Total Av/s = 0.01768 inches (shear, torsion & minimum)

Calculating Vc using Equation 11-9:
(Vu dpt) / Mu = 0.9729
Vc = 483.1 Kips (equation 11-9)
2 root-fc bw d = 85 Kips
5 root-fc bw d = 212.5 Kips
Using Vc = 212.5 Kips
Considering cross section as "beam" (not slab)
Maximum (shear) stirrup spacing = 24 inches (for shear, if stirrups required)
Vn Required = 161 Kips (from Vu/phi)
Vs Required = 0 Kips (from Vn - Vc)
Minimum stirrups (or more) required
Maximum phi Vn = 414.4 Kips (not considering torsion)
phi Vn taken by torsion = 0 Kips
Available Vn = 552.5 Kips (after torsion considered)
Designing for Vs = 0 Kips
Existing usable Vs = 0 Kips
Av/s = 0 inches (shear only)
Av/s = 0 inches (torsion only)
Av/s = 0 inches (shear + torsion)
Minimum additional design Av/s = 0.01667 inches
Final design Av/s = 0.01667 inches (shear, torsion & minimum)
Total Av/s = 0.01667 inches (shear, torsion & minimum)