Structural Concrete - Hassoun

11.4 Safety Provisions 361
11.4 SAFETY PROVISIONS
The safety provisions for load factors were discussed earlier in Section 3.5. For columns, the safety
provisions may be summarized as follows:
1. Load factors for gravity and wind loads are
U = 1.4D
U = 1.2D + 1.6L
U = 1.2D +(1.0L or 0.5W)
U = 1.2D + 1.0L + 1.0W
U = 0.9D + 1.0W
The most critical factored load should be used.
2. The strength reduction factor, φ, to be used for columns may vary according to the following
cases:
a. When P u = φP n ≥ 0.1f cA ′ g , φ is 0.65 for tied columns and 0.75 for spirally reinforced
columns. This case occurs generally when compression failure is expected. Area A g is the
gross area of the concrete section.
b. The sections in which the net tensile strain, ε t , at the extreme tension steel, at nominal
strength, is between 0.005 and 0.002 (transition region) φ varies linearly between 0.90
and 0.65 (or 0.75), respectively (Fig. 11.4). Refer to Section 3.7. For spiral sections,
[
1
φ = 0.75 +(ε t − 0.002)(50) or φ = 0.75 + 0.15 − 5 ]
(11.1)
c∕d t 3
0.90
= 0.75 + ( ε t - 0.002)(50)
0.75
0.65
Spiral
Other
= 0.65 + ( ε t - 0.002)(250/3)
Compression
controlled
Transition
Tension
controlled
ε t = 0.002
ε t = 0.005
c
= 0.600
c
= 0.375
d t
d t
Interpolation on
c
d t
:
Spiral = 0.75 + 0.15
Other
= 0.65 + 0.25
1 5
c/d
-
t 3
1 5
c/d
-
t
3
Figure 11.4
Variation in φ with NTS for grade 60 steel 7. Courtesy of ACI.