Page 74 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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CONNECTIONS

3.6 CHAPTER THREE

be 615 ft-kips. The maximum thickness for a 6-in-deep plate (L = 6 in) to facilitate plate yielding
before bolt shearing is

t pmax = 6 M = ( 6 615 ) = 20 . in (3.8)
FL 2 50 () 6 2
y

3.1.6 Workpoints and Transfer Forces
Main-member design is usually performed with the members represented and analyzed as one-
dimensional elements. Members are usually arranged so that the axial forces act concentrically at a
point, thereby eliminating the need to consider additional moments in the member design. In practice,
however, connecting multiple members to a single point can be difficult, if not impossible. Also, the
need to support other elements of the structure, such as a floor slab or cladding, may force the members
to move from their assumed concentric positions into an eccentric configuration. One common
condition occurs when beams of different depths are required to transfer an axial load across a joint.
Typically these beams are assumed to share a common mid-depth elevation during analysis, but in
reality they will be positioned to a common top of steel elevation. This situation will result in moments
being transferred to the main members, regardless of the approach used to design the connections.
Transfer forces are forces that are transmitted across joints in a structure. Such forces can occur
in horizontal and vertical bracing systems, trusses, and even in beams that are not connected directly
to braces. Both lateral and gravity loads can induce transfer forces. When lateral loads are delivered
from a diaphragm system, such as a floor slab, into a skeletal system, such as a vertical bracing system,
the beams in the unbraced bays, which transfer load into the braced bays, are sometimes referred to
as drag beams or collector beams, denoting the fact that these beams collect or drag forces from one
system and deliver it to another.

Transfer Force Example 1. It is often thought that the maximum transfer force can be determined
from the maximum member forces in a system. This is not always correct, as can be shown using the
relatively simple case of a roof truss subjected to uniform snow and snow drift loads, Fig. 3.2. In
Case I (uniform snow load), the vertical transfer force (from the gusset to the chord at point A) is
obviously 10.0 kips, but in Case II (snow drift load) the vertical transfer force is 0 kips, even though
the member forces are larger (the maximums) for the second load case. Forces at point A are sum-
marized below. This analysis becomes much more complex for larger structures subjected to both
lateral and gravity loads.

Force in vertical, Force in diagonal, Vertical transfer
Case kips kips force, kips
I 25 (C) 21.2 (T) 10
II 25 (C) 35.4 (T) 0
C = compression
T = tension

Since the path that lateral loads take through the structure is often complex and involves numer-
ous systems, both skeletal and diaphragm, and encompassing multiple load cases, determination of
the required transfer forces at each joint can be cumbersome. A common mistake is to confuse trans-
fer forces with member forces within a vertical bracing system. The transfer forces must be trans-
ferred through the beam-to-column connection from one bay to the next, while the member forces
remain within a single bay.

Transfer Force Example 2. A typical bracing connection is shown in Fig. 3.3a. For the bracing
along line 2 in Fig. 3.3b, the only transfer forces that may exist are those at the edge of the structure.

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