D1.G.2 Allowable Stress per AITC Code
Explanation of terms and symbols used in this section
| Symbols | Description |
|---|---|
| fa | Actual compression or tension stress (in PSI). For tension, the axial load is divided by net sectional area (i.e, NSF x X-area). |
| FA | Allowable design value for compression or tension (in PSI) modified with applicable modifiers or calculated based on slenderness in case of compression. |
|
fbz fby |
Actual bending stresses about local Z and Y axis (in PSI). |
|
FBZ FBY |
Allowable design values for bending stresses about local Z and Y axis (in PSI) modified by the applicable modifiers. |
|
JZ JY |
Modifier for P-DELTA effect about the Z and Y axis respectively as explained in formula 5-18 of TCM. |
|
fvz fvy |
Actual horizontal shear stresses. |
|
FVZ FVY |
Allowable horizontal shear stresses. |
|
VZ VY |
Shear in local Z and local Y direction. |
|
ZD YD |
Depth of section in local Z and Y axis. |
|
EZ EY |
Minimum eccentricity along Z and Y axis. |
|
CFZ CFY |
Values of the size factors in the z-axis and y-axis, respectively. |
|
CLZ CLY |
Represent the factors of lateral stability for beams about the z-axis and y-axis, respectively. |
| RATIO | Permissible ratio of stresses. The default value is 1.0. |
D1.G.2.1 Combined Bending and Axial Tension
The following interaction formulae are checked :
fa/FA + fbz/(FBZ x CFZ) + fby/(FBY x CFY) ≤ RATIO
Lateral stability check with Net compressive stress:
fa/FA + fbz/(FBZ x CLZ) + fby/(FBY x CLY) ≤ RATIO
D1.G.2.2 Combined Bending and Axial Compression
fa/FA + fbz/(FBZ-JZ x fa) + fby/(FBY-JY x fa) ≤ RATIO
Applicability of the size factor:
-
When CF < 1.00,
if fa > FBZ x (1-CFZ), FBZ is not modified with CFZ. if fa > FBY x (1-CFY), FBY is not modified with CFY.
if fa < FBZ x (1-CFZ) FBZ is taken as FBZ x CFZ + fa but shall not exceed FBZ x CLZ
if fa < FBY x (1-CFY) FBY is taken as FBY x CFY + fa but shall not exceed FBY x CLY
- When CF ≥ 1.00, the effect of CF and CL are cumulative FBZ is taken as FBZ x CFZ x CLZ FBY is taken as FBY x CFY x CLY
D1.G.2.3 Minimum Eccentricity
The program checks against min. eccentricity in following cases:
- The member is a FRAME member and not a truss member and under compression.
- The value of actual axial compressive stress does not exceed 30% of the allowable compressive stress.
- The actual moments about both axes are less than moments that would be caused due to min. eccentricity. In this approach, the moment due to min. eccentricity is taken as the compressive load times an eccentricity of 1 in. or 0.1 x depth whichever is larger.
In case of min. eccentricity,
| fbz = fa x (6+1.5 x JZ)/(EZ/ZD) |
| fby = fa x (6+1.5 x JY)/ (EY/YD) |
the following conditions are checked :
| fa/FA + fbz/(FBZ-JZ x fa) ≤ RATIO |
| fa/FA + fby/(FBY-JY x fa) ≤ RATIO |