Page 393 - Forensic Structural Engineering Handbook

11.24 MATERIAL-SPECIFIC FORENSIC ANALYSES

state for flexure which may be yielding, lateral buckling, local buckling of its flange or web,
or web crippling if concentrated loads are applied to the web such as at support points.
Lateral buckling is dependent on the type and spacing of lateral supports provided to the
beam compression flange. Columns are designed for axial loads if they serve primarily as
compression members. Often they also resist bending as a beam-column. Lateral support and
bracing of the compression member govern its design capacity and are essential to its perfor-
mance. Often beams and columns are assembled into frames for buildings. Their connections
are critical to the frames’ performance. Rigid frames utilize moment resisting connections that
resist sway and deformation of the frame. Depending on the structure and loads, secondary
effects may need to be considered. Braced frames rely on truss-type bracing to resist lateral
loads and deformations. They may use simple, partial, or full moment-resisting connections.
Bridges make extensive use of beams and girders to support the roadway. Girders are sup-
ported by piers and abutments and may have transverse members or diaphragms between the
girders. These provide lateral support to the compression flanges during construction and
before the roadway is installed. The roadway may act compositely with the beams and the gird-
ers after construction. The transverse diaphragms or floor beams play a minor role, providing
lateral support to the top compression flange once the slab has been completed.

Connection Design
Connections are typically designed to transfer the joint forces to which they are subjected.
Generally simple equilibrium models are used to proportion the mechanical or welded connec-
tors and the plate or beam elements used in the connection. 18–20 Design criteria and procedures
are fully explained in these references. The connectors generally used for steel structures are
either high-strength bolts or welds. Bolts are designed based on their nominal shank area, A, for
both tension and shear. For tension, the nominal strength is provided by
F = C F (11.10)
n t u
where F is the minimum specified tensile strength and C = 0.75, which is the ratio of the
t
u
stress area to the nominal shank area. For shear, the nominal strength of a single bolt is pro-
vided by:
F = C F 11.11)
v
s u
The coefficient C = 0.62 for a single bolt when the threads are excluded from the shear
s
plane. This coefficient is reduced to C = 0.5 to account for connection lengths up to 50 in
s
parallel to the line of force. When threads are not excluded from the shear plane, the coef-
ficient C is reduced by 20 percent to 0.496 and 0.4, respectively. Bolts transfer forces from
s
one component into another by bearing and shear of the fastener. This can result in net sec-
tion failure of the connected member. Another failure mode is block shear, which combines
the tension and shear resistance of the connected elements. An example of a combination
of two of these failure modes can be seen in Fig. 11.10. This shows four angles of a tension
chord member framing into a gusset plate. Two angles have caused the connecting bolts to
fail in double shear. The other two angles show failure on the net section. This indicates
that all elements were at their design load capacity. The commonly used weld connectors
are either groove welds or fillet welds. Groove welds are designed for the same basic capac-
ity as the connected base metal. Fillet welds, on the other hand, are designed for the weld
throat area, which is the effective cross-sectional area of the weld. The nominal strength of
a linear weld group loaded in-plane through the center of gravity is 32–35
+
15
.
.
F = 06 F ( 10 05sin θ ) (11.12)
.
.
w EXX