4.2 - VSL

8. Execution of the calculations
8.1. Flow diagram
8.2. Calculation example
8.2.1. Bases
- Type of structure: commercial building
- Geometry: see Fig. 59
- Loadings:
Live load p = 2.5 kN/m2
Floor finishes gB = 1.OkN/m2 Walls gw = 1.5 kN/m2
q = 5.0 kN/m2 20
- Material properties:
Concrete f28 = 35 N/mm2 fcd = 0.6 . f28 = 21 N/mm 2
Prestressing steel Monostrands ∅ 15 mm (0.6")
Ap = 146 mm2 fpy = 1570 N/mm2 c
c
= 1770 N/mm2
f pu
Ep = 1.95 ⋅ 10 5
N/mm2 very low relaxation (3%)
Admissible stresses:
- at stressing: 0.75 fpu - after wedge draw-in: max. 0.70 fpu Friction coefficients: µ=0.06
k = 0.0005/m
Reinforcing steel fsy = 460 N/mm 2
- Concrete cover:
Prestressing steel c p = 30 mm
Reinforcing steel c s = 15 mm
- Long-term losses (incl. relaxation): assumed to be 10% (see Chapter 4.3.2.)
8.2.2. Preliminary design
⎯ Determination of slab thickness:
Assumption: I/h = 35
→ h = 8.40 = 0.24 m
35
g = 0.24 ⋅ 25 = 6 kN/m 2
q = 5 kN/m 2
11 kN/m 2
g+q 11
= = 1.83; hence from Fig. 57
g 6
→I/h = 36
h = 8.40 = 0.233 m
36
chosen: h=0.24 m
⎯ Determination of prestress:
a) Longitudinal direction:
g+q = 1.83;κ = 0.24 ⋅ 1000 = 0.136;
g 8.40 2
⋅ 25
hence from Fig. 58
→ u = 1.39; u = 1.39 . 6 = 8.34 kN/m2 g
P u . I
= 2
8 . hp h 4.20
p = 0.144 . 2
= 0.178 m (Fig. 60)
3.78 2
P 8.34 ⋅ 8.40
= 2
= 413 kN/m
8 . 0.178
on 7.80 m width: P = 7.80 - 413 = 3221 kN
per strand: PL= 146 .1770 . 0.7 . 10-3 = 181 kN
Number of strands:np= 3221 = 17.8
181
→ 18 monostrands ∅ 15 mm on 7.80 m width
on 7.40 m width: n 7.40
p= . 17.8= 16.9
7.80
→ 17 monostrands ∅ 15 mm on 7.40 m width