Page 284 - Handbook of Structural Steel Connection Design and Details

Welded Joint Design and Production

Welded Joint Design and Production 269

should be considered. In recent years, recycling of steel has become a
more predominant method of manufacture. Not only is this environ-
mentally responsible, it is economical. However, in the process, resid-
ual alloys can accumulate in the scrap charge, inadvertently increas-
ing steel strength levels. In the past 20 years, for example, the average
yield strength of ASTM A36 steel has increased approximately 15%.
Testing done with lower-yield-strength steel would be expected to
exhibit different behavior than test specimens made of today’s higher-
strength steels (in spite of the same ASTM designation).
For practical reasons, laboratory specimens tend to be small in size.
Success in small-sized specimens was extrapolated to apply to very
large connection assemblies in actual structures. The design philosophy
that led to a reduction in the number of special moment-resisting
frames throughout a structure necessitated that each of the remain-
ing frames be larger in size. This corresponded to heavier and deeper
beams, and much heavier columns, with an increase in the size of the
weld between the two rolled members. The effect of size on restraint
and triaxial stresses was not evaluated in the laboratory, resulting in
some new discoveries about the behavior of the large-sized assem-
blages during the Northridge earthquake.
There is general agreement throughout the engineering community
that the pre-Northridge connection (as defined above) is no longer
adequate and some modification will be required. Any deviation from
the previous definition constitutes a modification for the purposes of
this discussion.
Minor modifications to the SMRF connection. With the benefit of hind-
sight, several aspects of the pre-Northridge connection detail seem to
be obviously deficient. Weld backing left in place in a connection sub-
ject to both positive and negative moments where the root of the
flange weld can be put into tension is an obvious prescription for
high-stress concentrations that may result in cracking. Failure to
specify minimum toughness levels for weld metal for heavily loaded
connections is another deficiency. The superior performance of the all-
welded web versus the bolted web in past testing draws into question
the assumption of load sharing between welds and bolts. Tighter con-
trol of the strength properties of the beam steel and the relationship
to the column also seem to be obvious requirements.
The amount of testing that controls each of these variables has
been limited to date. Some preliminary results suggest that tightly
controlling all these variables will result in acceptable performance.
At the time of writing, however, the authors know of no test of
unmodified beam-to-column connections where the connection zone
has remained crack-free when acceptable rotation limits were
achieved. It is speculated that for smaller-sized members, this

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