Building Design and Construction Handbook - Merritt - Ventech!

7.76 SECTION SEVEN
7.22 WEB OR FLANGE LOAD-BEARING
STIFFENERS
Members subject to large concentrated loads within their length or large end reactions
should be proportioned so that the forces on the web or flange cannot cause
local failure or the webs or flanges should be stiffened to carry the concentrated
loads. Both ASD and LRFD procedures include design criteria.
7.22.1 ASD for Load-Bearing Stiffeners
Webs of rolled beams and plate girders should be so proportioned that the compressive
stress, ksi, at the web toe of the fillets does not exceed
F � 0.66F (7.45)
a y
where F y � specified minimum yield stress, ksi.
Web failure probably would be in the form of buckling caused by concentrated
loading, either at an interior load or at the supports. The capacity of the web to
transmit the forces safely should be checked.
FIGURE 7.35 Web crippling in a simple
beam. The critical web section is assumed to
occur at the fillet.
Load Distribution. Loads are resisted
not only by the part of the web directly
under them but also by the parts immediately
adjacent. A 45� distribution
usually is assumed, as indicated in Fig.
7.35 for two common conditions. The
distance k is determined by the point
where the fillet of the flange joins the
web; it is tabulated in the beam tables
of the AISC ‘‘Manual of Steel Construction.’’
F a is applicable to the horizontal
web strip of length b � k at the end
support or b � 2k under an interior load.
Bearing stiffeners are required when F a
is exceeded.
Bearing atop Webs. The sum of the compression stresses resulting from loads
bearing directly on or through a flange on the compression edge of a plate-girder
web should not exceed the following:
When the flange is restrained against rotation, the allowable compressive stress,
ksi, is
� �
4 10,000
Fa � 5.5 � (7.46)
2 2
(a/h) (h/t)
When the flange is not restrained against rotation,
� �
4 10,000
Fa � 2 � (7.47)
2 2
(a/h) (h/t)