In selecting a trial value of Ab for frames with rigid connections, the
minimum brace size will be controlled by slenderness requirements. In
addition, in each particular application, there will be a limiting value of
Ab beyond which there will be no substantial weight savings in the frame
members since there are maximum slenderness requirements for the frame
members. In general, values of Ab of about 2 square inches will result
in a substantial increase in the overall resistance for frames with rigid
connections. Hence, an assumed brace area in this range is recommended as
a starting point. The design of the beams and colunms of the frames follow
the procedures previously presented.
a. Dynamic Design Stresses for Connections. As stated in Chapter 6 of
Design of Steel Structures to Resist the Effects of HE Explosions, by
Healey, the connections in a steel structure designed in accordance with
plastic design concepts must fulfill their functions up to the ultimate load
capacity of the structure. The AISI Specification states that bolts, rivets
and welds shall be proportioned to resist the maximum forces, using stresses
equal to 1.7 times those given in Part I of the same Specification.
Additionally, these stresses are increased by the dynamic increase factor
specified in paragraph 2.a of this section. Hence:
Fdy = 1.7cFs
Fdy = the maximum dynamic yield stress
c = the dynamic increase factor
Fs = allowable static design stress
Rather than compiling new tables for maximum dynamic loads for the
various types of connections, the designer will find it
advantageous to divide the forces being considered by the factor
1.7c and then refer to the allowable load tables.
b. Requirements for Panel Connections. Panel connections generally
involve two types, namely: the attachment of light-gage materials to each
other and the attachment of sheet panels to heavier cross-section.
The most common type of fastener for decking and steel wall panels is the
self-tapping screw with or without washer. The screw fastener has been the
source of local failure for both conventional loads and blast pressures.
Special care should therefore be exercised in designing these connections to
reduce the probability of failure by using oversized washers or increasing
the material thickness at the connection. Certain panel connectors are
shown in Figure 47.