Page 229 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch05.qxd 9/29/05 5:12 PM Page 5.9

CRITERIA FOR BUILDING DESIGN

CRITERIA FOR BUILDING DESIGN 5.9

5.3 DESIGN OF TENSION MEMBERS

For tension member design, the limit states of tensile yielding in the gross section and tensile rup-
ture in the net section must both be considered. Also, it is important to meet these limit states in both
the body of the member and at connections. The design tensile strength (LRFD) φ t P n and the allow-
able tensile strength (ASD) P n /Ω t is the lower of these two limit states.
For yielding in the gross section,

(5.12)
P n = F y A g
φ t = 0.90 (LRFD) Ω t = 1.67 (ASD)
For rupture in the net section,

(5.13)
P n = F u A e
φ t = 0.75 (LRFD) Ω t = 2.00 (ASD)

2
2
where A e = effective net cross-sectional area, in (mm )
2
2
A g = gross cross-sectional area of member, in (mm )
F y = specified minimum yield stress, ksi (MPa)
F u = specified minimum tensile strength, ksi (MPa)
When members without holes are fully connected by welds, the effective net area is as defined in
Art. 5.3.2. When holes are present in a member with welded-end connections, and in the case of plug
or slot welds, use the net area through the holes.

5.3.1 Net Area

The net area A n of a member is the sum of the products of the thickness and the net width of each
1
element. In net-width calculations for tension and shear, take the width of a bolt hole as / 16-in
(2 mm) greater than the nominal dimension of the hole. For a chain of holes extending across an ele-
ment in a diagonal or zigzag line, the net width of the part is obtained by deducting from the gross
width the sum of the diameters or slot dimensions of all holes in the chain, and adding, for each gage
2
space in the chain, the quantity s /4g, where s = longitudinal center-to-center spacing (pitch) of any
two consecutive holes, in (mm), g = transverse center-to-center spacing (gage) between fastener gage
lines, in (mm). For angles, the gage for holes in opposite adjacent legs is taken as the sum of the
gages from the back of the angles less the thickness. In the design of splice plates for connections,
A n is limited to 0.85A g .

5.3.2 Effective Net Area
Because of a phenomenon known as shear lag, stresses are not distributed uniformly over the cross
section at a connection when each element of the cross section is not attached. In such cases, because
the net area is not fully effective in transferring tensile forces, an effective net area of tension mem-
bers must be determined:

A e = A n U (5.14)
where U is the shear lag factor, A n , Table 5.2. The AISC Specification requires that connections for
members such as angles (single or double) and WT sections be proportioned such that U is no less
than 0.60, unless the members are designed for the effect of eccentricity.

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