Locating Welds at Zero Stress Points
Figure 6–80 (a) shows a
cross-supporting beam that must have minimum deflection and be
removable. One way to do this is to weld the I-beam to plates and bolt the end
plates to the structure. While this will work, it may be difficult to remove,
depending on the rigidity of the surrounding structure. Is there a better way
to do this?
Yes, Figure
6–80 (c) shows the bending moment (or force) on a beam with a concentrated load
at its center and fixed ends. If we splice the beam at these two zero-stress
points where there is no bending force on them, the connection will only have
to support the vertical shear load. A simple welded clip on the bottom of the
end beam stubs and a bolt in the middle of the beam web will support the load
as rigidly as Figure 6–80 (b) design, yet be easier to remove.
Figure
6–80. Placing welds at zero-stress points.
The vertical welds on the angle
iron assembly shown in Figure 6–81 (upper) keep failing when subjected to the
indicated cyclic load. What is going on and how can this problem be fixed?
As the cyclic
load is applied, the steel angle flexes and puts the two vertical welds into
horizontal shear pulling against each other. These welds carry the horizontal
shear loads from the stretching of the beam in addition to the vertical
shear load. If we replace these two vertical welds with a single horizontal
weld, these horizontal shearing forces will be reduced, Figure 6–81 (lower).
Also, the closer the weld is to the thicker part of the angle iron, in this
case the top section, the smaller the stress on the weld. This is because the
additional metal in the top of the angle reduces the amount of stretching in
the angle iron.
Figure
6–81. Replacing two welds in shear with a single weld along the neutral axis.
Copyright © 2002
Industrial Press Inc.