Solution - dm2_080273

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(2) Material strengths
Concrete:
f'c = 5000 psi
Prestressing Steel:
fpu = 270,000 psi
fpy/fpu = 0.85
Welded Wire Fabric:
fy = 65,000 psi
Reinforcing Bars:
fy = 60,000 psi
(3) Dynamic increase factors:
Concrete
flexure:
1.25
1.00
diagonal tension:
direct shear:
1.10
Prestressing Steel:
1.0
Welding Wire Fabric:
1.10
Reinforcing Steel
flexure:
1.10
shear:
1.0
(4) Span length: 40 ft.
Solution:
(1) Select a double tee section.
Try 8DT 24 shown in Figure 69b with section properties:
2
A = 401 in  WWF 12 X 6, Wl.4 X W2.5 in flange
Ig = 20,985 in4
yt = 6.85 in
W = 418 lb/ft
(2) Select Strand Pattern.
Try strand pattern 48-S, two l/2-inch diameter strands in
each tee.
Area of each strand = 0.153 in2
e = 14.15 in
(3) Calculate dynamic strength of materials.
Concrete
- flexure:
f'dc = 1.25 x 5,000 = 6250 psi
- diagonal tension: f'dc = 1.0 x 5,000 = 5000 psi
1.1 x 5,000 = 5500 psi
f'dc =
- direct shear:
f  p u y 1.0 x 270,000 = 270,000 psi
Prestressing Steel:
f  dy = 1.10 x 65,000 = 71,500 psi
Welded Wire Fabric:
Reinforcing Bars
f  dy = 1.10 x 60,000 = 66,000 psi
- flexure:
= 1.0 x 60,000 = 60,000 psi
- shear:
2.08-256