Design of Aluminum Structure

Design of member in Tension

Strength reduction factor and safety factor

LRFD design:

Strength reduction factors-building type structures

f = 0.75 for tensile rupture

f = 0.75 for tensile yielding

ASD design for building-type structures

W = 1.95 for tensile rupture

W = 1.65 for tensile yielding

ASD design for bridge-type structures

W = 2.20 for tensile rupture

W = 1.85 for tensile yielding

Unwelded members

Normal strength of tension yielding in gross section

P_n = F_ty A_g                [1]

F_ty: Tensile yield strength.

A_g: Gross section area

Normal strength of tension rupture in the net section

P_n = F_tu A_e/k_t               [2]

F_tu: Tensile ultimate strength.

A_e: Effective section area. 

k_t: Tension Coefficient in Table A3.3 of Aluminum Design Manual

k_t = 1.25 for 2014-T6, -T651,-T6510,-T6511, Alclad 2014-T6, -T651. 6005-T6, 6105-T5.

k_t = 1.1 for 6066-T6,-T6510,-T6511, 6070-T6, -T62.

k_t = 1.0 for all others.

Note: See Aluminum Design Manual, Commentary D.3 for calculation of effective net area.

Member with longitudinal welds

Normal strength of tension yielding in gross section

P_n = F_ty (A_g-A_wz)+F_tywA_wz              [3]

F_tyw: Tensile yield strength of welded member

A_wz: Welded area

Normal strength of tension rupture in the net section

P_n = F_tu (A_e-A_wz)/k_t+F_tuwA_wz              [4]

F_tuw: Tensile ultimate strength of weld member.

Example:

Situation: A aluminum strap plate member in tension with two bolt holes in a building type structure.

Design data:

Plate width: 4 inch

Thickness: 0.25 in

Bolt hole diameter: 0.25 in

Material: 6061-T6

Tensile yield strength: 35 ksi

Tensile ultimate strength: 38 ksi

Requirement: determine allowable tensile strength of the strap.

Solution:

Gross area:  A_g=(4)(0.25) = 1 in². 

Nominal strength based on gross area:

P_n = F_ty A_g = (35)(1) = 35 kips

Safety factor, W = 1.65

Allowable tensile strength: Pa = 35/1.65 = 21.2 kips

Net area: A_e = 1 - (0.25)(0.25)(2) = 0.875 in².

Tension coefficient, k_t = 1

Nominal strength based on net area:

P_n = F_tu A_e/k_t  = (38)(0.875)/1 = 33.2 kips

Safety factor, W = 1.95

Allowable tensile strength: Pa = 35/1.95 = 17.1 kips  (Govern)