Tension Rod Capacity

[tslewis]

For Design of Carbon Steel Tensin (non-upstand) rods to AISC I understand the following limits to be applicable:

1. Gross Yielding (Chapter D)

a) ASD

Ab - gross area of rod i.e. (pi*d^2)/4 where d is the nominal diameter of the rod

Nominal Strength Fn = Fy*Ab

FS - ASD Factor of safety = 1.67

ASD Allowable Strength = Fn/FS = 0.6*Fy*Ab

b) LRFD

As above but reduction factor phi = 0.9

LRFD Design Strength = phi*Fn = 0.9 *Fy*Ab

2. Threaded Part Chapter J3

a) ASD

Nominal Strength Fn = 0.75*Fu*Ab

FS - ASD Factor of safety = 2

ASD Allowable Strength = Fn/FS = 0.38*Fu*Ab

b) LRFD

As above but reduction factor phi = 0.75

LRFD Design Strength = phi*Fn = 0.563*Fu*Ab

My quesitons is, is can the Chapter D equaiton for fracture in the net section based on the actual thread area Ae be used in lieu of the eqution for the thread capacity from J3?

3. Fracture in the Net Section

a) ASD

Ae - threaded area

Nominal Strength Fn = Fu*Ae

FS - ASD Factor of safety = 2

ASD Allowable Strength = Fn/FS = 0.5*Fu*Ae

b) LRFD

As above but reduction factor phi = 0.75

LRFD Design Strength = phi*Fn = 0.75*Fu*Ae

I ask this as we get more capacity if we use the fracture equation above for a rod of known threaded area Ae vs that of the equation for a threaded part based on the gross area. Is it correct to assume that the fracture equaiton and the threaded part equationa are actually the same, except that the threaded part equation usses an assumption of teh treaded area Ae being equivelent to 0.75*Ab?

Thanks

 

[civilperson]

Two different failure modes are being addressed in these equations. Facture of the net area or yielding of the gross area. This is discussed in AISC D1. Commentary.(5-139)

 

[tslewis]

I understand that there are two different failure mechanisms in tension - AISC Chapter D.

However, there seem to be two possible equations to desicrbie the net fracture failure under tension. The first comes from Chapter D and is based on the actual net area - i.e. the thread area, and the second comes form Chapter J for a threaded part, which appears to be a variation on the net fracture limit of Chapter D but here the nominal strength appears to be based on an approximation of the net area i.e. Ae = 0.75 Ab.

The latest 2005 specifications for Steel building design appears to confirm this in the commentary for Chapter J

 

[civilperson]

What equation in Chapter J are you comparing to Chapter D?

 

[civilperson]

The area of a threaded bolt is defined as the area of the threaded part based on its major diameter.
Tensile stress area= 0.7854(D-.9743/n)^2 n=th'ds per inch

 

[tslewis]

1. Latest ASD/LRFD 2005 Specifications

Chapter J, Table J3.2 and Equation J3-1 for tensile rupture

Nominal strength of threaded part 0.75*Fu*Ab
ASD Factor of safety 2
LRFD Reduciton factor 0.75

ASD Allowable strength 0.375*Fu*Ab

Chapter D, Equation D2-2, Tensile rupture

Nominal strength Fu*Ae
ASD Factor of safety 2
LRFD Reduciton factor 0.75

ASD Allowable strength = 0.5*Fu*Ae

2. AISC ASD Manual 9th Edition

Chapter J, Table J3.2

ASD Allowable Strength = 0.33 Fu*Ab

Chapter D, D1,

ASD Allowable strength on net area = 0.5Fu*Ae

 

[tslewis]

I understand the thread area calculaiton.

The quesiton is

i) Can I ignore the Chapter J equaiton if I use the Actual thread area in the Chpater D equaiton for Net tensile fracture?

Chpater J appears to be an approximation to that of Equaiton D for net tensile fracture. The Chapter J equaiton appers to be very conservative and is used if one is unsure of the actural thread area.

 

[UcfSE]

I would use Chapter J for threaded rods, even though they are acting as tension members. The user note at the beginning of Chapter D of the 2005 spec. suggests you use J3 for threaded rods.

 

[tslewis]

Yes I saw that note too. But both equations are for the same failure mechanism and the commentary to the 2005 specifications for Chapter J appears to say that the nominal strength for the threaded rod is based on the threaded area assumed to be 0.75 times the bolt area.