Allowable Stresses and Loads

Structure

any object that must support or transmit loads

 

 

Factor of safety, n

the ratio of actual strength to required strength

• generally values from 1 to 10 are used
• structure will presumably fail for n less than 1

n = (Actual strength) / (Required strength)

Photo by Jeff Thomas on March 23, 2002.		Photo by Jeff Thomas on October 22, 2000.

 

Allowable stress

the stress that must not be exceeded anywhere in the structure to satisfy the factor of safety

Allowable stress = (Yield or ultimate strength) / n

Tension:		0.45 σy < σallow < 0.60 σy
Shear:		τallow = 0.40 σy
Bending:		0.60 σy < σallow < 0.75 σy
Bearing:		σallow = 0.90 σy

Mechanical Engineering Design, 5th Ed., by J.E. Shigley and C.R. Michke, McGraw-Hill, 1989, p12.

Design Example - Hay Wagon

 

Load and Resistance Factor Design (LRFD)

an alternative design method commonly used by structural engineers

γ_DP_D + γ_LP_L < φP_U

P_D - dead load - loads associated with the weight of the structure
γ_D - dead load factor - accounts for uncertainties with the dead load
P_L - live load - loads the structure is designed to hold
γ_L - live load factor - accounts for uncertainties with the live load
φ - resistance factor - accounts for uncertainties with the structure
P_U - ultimate load - load at which the structure ceases to be useful

Design Example - Building Construction

 

Margin of safety

an alternative definition to "factor of safety"

• commonly used in the aircraft industry
• structure will presumably fail for margins of safety less than or equal to zero
• usually given as a percent

Margin of safety = (n - 1) x 100

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