Civil Engineering Reference
In-Depth Information
that flexible beam framing end connections in beams (typically assumed in the design
of stringers and floorbeams) and girders be designed for 125% of the calculated shear
force. Connections considered as semirigid or rigid may be designed for the combined
bending moment and shear force applied at the joint. Rotational end restraint may
be modeled for analysis using rotational springs with spring stiffness,
k
φ
=
,at
the ends of the beam. Knowledge of
k
φ
, from analytical and experimental research,
enables the determination of the end bending moment,
M
r
, for connection design.
Secondary and bracing member connections must be designed for the lesser of the
allowable strength of, or 150% of the calculated maximum force in, the member. The
connections in members used as struts and ties to reduce the unsupported member
length of other members should be designed for 2.5% of the force in the member
being braced.
M
r
/
φ
9.2 WELDED CONNECTIONS
Weldingisthemetallurgicalfusionofsteelcomponentsormembersthroughanatomic
bond. The steel must be melted to effect the coalescence and, therefore, a relatively
large quantity and concentration of heat energy is required. The heat energy is usually
supplied by an electric arc created between a metal electrode and the base metal
during the welding of structural steel components and members.
There are many electric arc welding processes in use. SMAW, SAW, and FCAW
are the most commonly used processes for railway superstructure fabrication. Several
passes are often required during the welding process to ensure fine-grain metallurgy
of the deposited weld metal. The maximum size of weld placed in a single pass
depends on position of the weld and is specified in the American Welding Society
Bridge Welding Code (ANSI/AASHTO/AWS D1.5, 2005).
Residual stresses (already introduced into rolled plates and members at steel mills)
combinedwithweldingheatcyclescancausedistortionstooccur.Residual(orlocked-
in) stresses may be increased if the distortions are restrained. The distortions and/or
residual stresses occur when welds contract more than the base metal along the weld
longitudinalaxisand/orduetothetransversecontractionofweldmetal(whichtendsto
pull plates together and may involve a transverse angular distortion). Weld balancing
and multipass welding procedures can often mitigate distortion or excessive residual
stress. However, when residual stresses are inevitable, such as in thick butt welded
plates, supplementary heat treatments are often required to “stress relieve” the weld
and base metal adjacent to the weld.
Lamellar tearing can occur in thick plates with welded joints where tensile stresses
are directed through the plate thickness by weld shrinkage (usually not of concern
for plates less than about 1
2
in. thick). Joint preparation and welding sequence are
important to mitigate lamellar tearing.
Therefore, it is of critical importance that welding processes and procedures pro-
duce quality welds with proper profiles, good penetration, complete contact with the
base metal at all surfaces, no cracks, porosity, and/or inclusions.
There are many weld and joint types used for welded connections in railway
superstructure steel fabrication (see
Sections 9.2.2
and
9.2.3,
respectively). Fillet
and groove welds are the most prevalent weld types for steel railway superstructure
