Page 109 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch03.qxd 9/29/05 5:05 PM Page 3.41

CONNECTIONS

CONNECTIONS 3.41

Though contrary to historic procedures, recent tests indicate that it is acceptable to use the net
plastic section modulus instead of the more traditional elastic section modulus. This more accurately
represents the ultimate capacity of the element in bending.
Compression. The capacity of an element in bending can also be governed by buckling of the ele-
ment in the compression zone. See Art. 3.3.5.

3.3.4 Localized Limit States
Connections are often subjected to, or subject main members to, localized stresses that are usually
not considered in typical main-member design.
Bearing at Bolt Holes. As loads are transferred from one element to another through the bolts,
large localized compression stresses can occur where the shank of the bolt bears on the connected
material. The design strength at these locations, when deformation at the bolt hole at service load is
a design consideration, is
(3.28a)
φR n =φ2.4d b tF u
where φ= 0.75
d b = bolt diameter
t = thickness of material
Alternatively, if deformation at the bolt hole under service loads is not a design consideration, the
bearing strength can be determined as
(3.29a)
φR n =φ3.0d b tF u
In some cases, such as a single-plate shear connection, deformation at the bolt holes is desired for
ductility and to relieve eccentricities.
Bolt Tear-out. An additional bearing check introduced in recent specifications actually represents
a shear fracture failure mode in which the bolt tears out through the material. Although this tear-out
requirement is included as a bearing check, it is not a bearing limit state but rather a shear fracture
limit state. It modifies Eqs. (3.28a) and (3.29a) to the following:
(3.28b)
φR n =φ1.2L c tF u ≤φ2.4d b tF u
(3.29b)
φR n =φ1.5L c tF u ≤φ3.0d b tF u
This requirement complicates the checking of bolt shear, bearing, and bolt tear-out, by tying one to
the other. At each bolt there are five possible limit states: bolt shear, bearing on the main material at
the bolt, bearing on the connection material at the bolt, bolt tear-out through the main material, and
bolt tear-out through the connection material. For each bolt in the bolt group, the minimum of these
five limit states must be determined in order to calculate the available strength of the connection.
As an example, consider a tension strut connected to a support by a plate as shown in Fig. 3.27.
3
The W8 × 15 tension strut is A570 Grade 50 steel with F u = 65 ksi and the / 8-in-thick plate is A36
steel with F u = 58 ksi. Calculations are as follows.
7
Design shear strength of / 8-in-diameter A490X bolt:
φR n =φF n A b = 0.75 × 75 × 0.601 = 33.8 kips/bolt
Design bearing strength of the strut web [Eq. (3.28a)]:
φR n = 0.75(2.4)(0.875)(0.245)(65) = 25.1 kips/bolt

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