Page 54 - Handbook of Structural Steel Connection Design and Details

Design of Connections for Axial, Moment, and Shear Forces

Design of Connections for Axial, Moment, and Shear Forces 39

2.1.1 Philosophy
Connection design is both an art and a science. The science involves equi-
librium, limit states, load paths, and the lower bound theorem of limit
analysis. The art involves the determination of the most efficient load
paths for the connection, and this is necessary because most connections
are statically indeterminate.
The lower bound theorem of limit analysis states: If a distribution of
forces within a structure (or connection, which is a localized structure) can
be found, which is in equilibrium with the external load and which sat-
isfies the limit states, then the externally applied load is less than or at
most equal to the load that would cause connection failure. In other words,
any solution for a connection that satisfies equilibrium and the limit
states yields a safe connection. This is the science of connection design.
The art involves finding the internal force distribution (or load paths) that
maximizes the external load at which a connection fails. This maximized
external load is also the true failure load when the internal force distri-
bution results in satisfaction of compatibility (no gaps and tears) within
the connection in addition to satisfying equilibrium and the limit states.
It should be noted that, strictly speaking, the lower bound theorem
applies only to yield limit states in structures that are ductile. Therefore,
in applying it to connections, limit states involving stability and fracture
(lack of ductility) must be considered to preclude these modes of failure.

2.1.2 General procedure
Determine the external (applied) loads, also called required strengths, and
their lines of action. Make a preliminary layout, preferably to scale. The
connection should be as compact as possible to conserve material and to
minimize interferences with utilities, equipment, and access, and to facil-
itate shipping and handling. Decide on where bolts and welds will be used
and select bolt type and size. Decide on a load path through the connection.
For a statically determinate connection, there is only one possibility, but
for indeterminate connections, there are many possibilities. Use judgment,
experience, and published information to arrive at the best load path. Now
provide sufficient strength, stiffness, and ductility, using the limit states
identified for each part of the load path, to give the connection sufficient
design strength, that is, to make the connection adequate to carry the given
loads. Complete the preliminary layout, check specification-required spac-
ings, and finally check to ensure that the connection can be fabricated and
erected. The examples of this chapter will demonstrate this procedure.

2.1.3 Economic considerations
For any given connection situation, it is usually possible to arrive at
more than one satisfactory solution. Where there is a possibility of using

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