Structural Concrete - Hassoun

3.6 Strength Reduction Factor φ 93
It is to noted that
1. The load factor L in Eqs. 3.1c, 3.1e, 3.2a, 3.3b, 3.3c, and 3.3d shall be permitted to be reduced
to 0.5L, except for garages, areas occupied as places of public assembly, and all areas where
theliveloadL is greater than 100 pounds per square foot (psf).
2. Where W is based on service wind loads, 1.6W shall be used in place of 1.0W in Eqs. 3.1c,
3.1d, and 3.3c and 0.8W shall be used in place of 0.5W in Eqs. 3.1e and Eqs. 3.3b.
3. If the service level of the seismic load E is used, 1.4E shall be used in place of 1.0E in Eqs. 3.2a
and b and 3.3d.
4. In a flood zone area, the flood load or load combinations of ASCE shall be used.
5. Impact effects shall be included with the live load L.
The ACI Code does not specify a value for impact, but AASHTO specifications give a simple
factor for impact, I, as a percentage of the live load L as follows:
50
I = ≤ 30% (3.6)
125 + S
where I is the percentage of impact, S is the part of the span loaded, and live load including impact
is L(1 + I).
When a better estimation is known from experiments or experience, the adjusted value shall
be used.
3.6 STRENGTH REDUCTION FACTOR φ
The nominal strength of a section, say M n , for flexural members, calculated in accordance with the
requirements of the ACI Code provisions must be multiplied by the strength reduction factor, φ,
which is always less than 1. The strength reduction factor has several purposes:
1. To allow for the probability of understrength sections due to variations in dimensions, material
properties, and inaccuracies in the design equations.
2. To reflect the importance of the member in the structure.
3. To reflect the degree of ductility and required reliability under the applied loads.
The ACI Code, Table 21.2.1, specifies the following values to be used:
For tension-controlled sections φ = 0.90
For compression-controlled sections
a. with spiral reinforcement φ = 0.75
b. other reinforced members φ = 0.65
For plain concrete φ = 0.60
For shear and torsion φ = 0.75
For bearing on concrete φ = 0.65
For strut and tie models φ = 0.75
A higher φ factor is used for tension-controlled sections than for compression-controlled
sections, because the latter sections have less ductility and they are more sensitive to variations in
concrete strength. Also, spirally reinforced compression members have a φ value of 0.75 compared
to 0.65 for tied compression members; this variation reflects the greater ductility behavior of spirally
reinforced concrete members under the applied loads. In the ACI Code provisions, the φ factor is