Allowable stress


Stress and its limit values:

Reliable use of the products depends on how the designer ensures that the maximum stress in their critical locations is less than its limit value. This inequality must be met with sufficient margin to cover all sources of uncertainty of the relevant quantities; Under these circumstances, violation may occur only in exceptional circumstances. See Equations. below include sources of uncertainty regarding the strength of the component.


Safety factor:

Determination of the corresponding coefficient of safety is a complicated and responsible task. A high coefficient of safety usually results in a safer design, however with a higher weight and thus a higher price and vice versa. It is the basic engineering compromise of "price vs. safety". However, it is the responsibility of the designer to determine such coefficient of safety that ensures corresponding safety at an acceptable price. At the same time, the coefficient of safety can vary within a wide range 1,1 to 5+.


Coefficient according to load:

load[]
Static load1
Unidirectional load, non-impact load0,8
Unidirectional load, with a small impact load0,7
Unidirectional load, with a big impact load0,6
Alternating load, with a small impact load0,45
Alternating load, with a big impact load0,25

Allowable axial stress:

σAall=0,45Rp0,2TSF*Cc

σAall - allowable axial stress - [MPa]

Rp0,2T - the minimum yield strength or 0,2% proof strength at calculation temperature - [MPa]

SF - safety factor - []

Cc - coefficient according to load - []


Allowable bending stress:

σBall=0,6Rp0,2TSF*Cc

σBall - allowable bending stress - [MPa]

Rp0,2T - the minimum yield strength or 0,2% proof strength at calculation temperature - [MPa]

SF - safety factor - []

Cc - coefficient according to load - []


Allowable shear stress:

τall=0,4Rp0,2TSF*Cc

τall - allowable shear stress - [MPa]

Rp0,2T - the minimum yield strength or 0,2% proof strength at calculation temperature - [MPa]

SF - safety factor - []

Cc - coefficient according to load - []


Allowable bearing stress:

σall=0,9Rp0,2TSF*Cc

σall - allowable bearing stress - [MPa]

Rp0,2T - the minimum yield strength or 0,2% proof strength at calculation temperature - [MPa]

SF - safety factor - []

Cc - coefficient according to load - []


Allowable combined stress:

σCall=Rp0,2TSF*Cc

σCall - allowable combined stress - [MPa]

Rp0,2T - the minimum yield strength or 0,2% proof strength at calculation temperature - [MPa]

SF - safety factor - []

Cc - coefficient according to load - []


Literature:

AISC: Specification for structural steel buildings: Allowable Stress design and plastic design 1989

Joseph E. Shigley, Charles R. Mischke, Richard G. Budynas: Konstruování strojních součástí 2010.


Download PDF:

Allowable stress.pdf


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