GMC 4000 Series Service Manual page 109

Sec. 2
Page 102
FRAME
be pointed out that dump trucks, as an example,
when operated with the box in a raised position
causes the center of gravity of the load to move
behind the rear axle centerline resulting in a
change of maximum tensile stress location from
the bottom of the lower flanges to the top of the
upper flanges (fig. 4). This information can be very
useful when examining cracks on vehicles used in
dump service, as it would appear that the vehicle
has been operated at excessive speed over rough
terrain while spreading with the dump box in the
raised position or with too long a dump box for
wheelbase selected.
Localized str ess concentration failures may
be the result of bending moment stresses; how­
ever, it must be pointed out that the stress levels
would not be high enough to cause any difficulty
without localized stress concentration points. These
localized stress concentrationpoints may be caused
by poor body or fifth wheel mountings, special
equipment or accessory installation, improper
welding or welding methods, improper reinforce­
ments, loose bolts or rivets and defective material.
They may also occur as a result of high bending
loads, coupled with severe torsional loads as may
be found in off-road service.
The proper installation of fifth wheels or
bodies is covered in the "Body Builder's Book";
however, it should be re-emphasized that the use
of U-bolts for attachment of fifth wheels or bodies
G M C S E R V I C E M A N U A L
Figure 4 — Fram e B e n d in g M om en t (Typical D u m p Truck)
is not an approved installation as high str ess con­
centration may develop. The desired fifth wheel
or body mounting is attached to the frame rail
web section, not through flanges. Heat-treated
rails must not have attachments welded to the
frame. Wood sills should be used between main
rails and sub-frame on body installations to insure
good load distribution.
Special equipment or accessory installation
can cause high stress concentrations due to the
method of attachment or the weight of the equip­
ment. Holes should never be drilled through the
flanges and rapid changes of section modulus should
be avoided. These section modulus changes usually
occur when large mounting plates are added for
supporting special equipment. Heavy equipment
mounted across the flanges 6r on the web of a side
rail may cause enough stress concentration to
cause failuresat the nearest crossm em ber, bracket,
or other frame stiffener or through a nearby hole
in the frame flange.
IMPORTANT
Improper welding or welding methods
are a major cause of stress concentration
points, which may ultimately result in
frame failure. (Refer to "General WeJding
Instructions.")