...
| Tip |
|---|
|
...
Notation
Ag = gross area of concrete section, in2
As = area of nonprestressed longitudinal tension reinforcement, in2
Ast = total area of nonprestressed longitudinal reinforcement, in2
α = depth of equivalent rectangular stress block, in.
bw = width of compression face of member, in.
c = distance from extreme compression fiber to neutral axis, in.
Cc = concretecompressive force, lb
Cs = reinforcement tension force, lb
fc'= specified compressive strength of concrete, psi
fy = specified yield strength for nonprestressed reinforcement, psi
Mn = nominal flexural strength at section, in.-lb
Pn = nominal axial compressive strength of member, lb
Pn,max = maximum nominal axial compressive strength of a member, lb
Pnt = nominal axial tensile strength of member, lb
Pnt,max = maximum nominal axial tensile strength of member, lb
Po = nominal axial strength at zero eccentricity, lb
ϕ = strength reduction factor
εt = net tensile strain in extreme layer of longitudinal tension reinforcement at nominal strength, excluding strains due to effective prestress, creep, shrinkage, and temperature
β1 = factor relating depth of equivalent rectangular compressive stress block to depth of neutral axis
...
Nominal axial compressive strength Pn is limited to a value of Pn,max =0.8Po for nonprestressed members.
For nonprestressed concrete members, Po shall be calculated by:
| Mathinline | ||||
|---|---|---|---|---|
|
...
Nominal axial tensile strength Pnt is limited to a value of Pnt,max in ACI 22.4.3.1.
| Mathinline | ||||
|---|---|---|---|---|
|
...
Strength reduction factors ϕ is determined according to using ACI Table 21.2.2. Since the section is tension controlled, a ϕ factor for tension control is used.
Strain, εt | Section Classification | ϕ |
|---|---|---|
εt ≤ εty | Compression Controlled Moment | 0.65 |
εty < εt < (εty + 0.003) | Transition region | 0.65 + 0.25[(εt - εty)/0.003] |
εt ≥ (εty + 0.003) | Compression Controlled Moment | 0.90 |
...
Nominal flexural strength Mn and axial strength are calculated based on the following Design Assumptions. While finding the flexural design strength, combined with axial force ϕMn, it should be found in which control zone the cross section is. When the section is tension controlled, a ϕ factor for tension control is used. When the section is compression controlled, a ϕ factor for compression control is used. When the section is within the transition region, ϕ is linearly interpolated between the two limit values.
...
Maximum strain at the extreme concrete compression fiber is assumed equal to 0.003.
Tensile strength of concrete is neglected.
The relationship between concrete compressive stress and strain is represented by equivalent rectangular concrete stress distribution method.
Concrete stress of 0.85fc' is assumed uniformly distributed over. Equivalent rectangular concrete stress zone bounded by edges of the cross section and a line parallel to the neutral axis located a distance α from the fiber of maximum compressive strain, as calculated by:
...
The distance between the fiber of maximum compressive stress and the neutral axis, c, is perpendicular to the neutral axis.
The value of β1 is determined using ACI Table 22.2.2.4.3.
fc' , psi | β1 |
|---|---|
2500 ≤ fc'≤ 4000 | 0.85 |
4000 < fc'< 8000 | 0.85 - 0.05(fc' -4000)/1000 |
fc' ≥ 8000 | 0.65 |
...
Next Topic
...