Building Design and Construction Handbook - Merritt - Ventech!

7.72 SECTION SEVEN
2
1 � Cva (a/h)
st � 2�
A � � YDht (7.30)
2 h �1 � (a/h)
where Y � ratio of yield stress of web steel to yield stress of stiffener steel
D � 1.0 for stiffeners in pairs
� 1.8 for single-angle stiffeners
� 2.4 for single-plate stiffeners
When the greatest shear stress ƒ v in a panel is less than F v determined from Eq.
(7.29a), the gross area of the stiffeners may be reduced in the ratio ƒ v/F v.
The moment of inertia of a stiffener or pair of stiffeners, about the web axis,
should be at least (h/50) 4 . The connection of these stiffeners to the web should be
capable of developing shear, in kips per lineal inch of single stiffener or pair, of at
least
3
Fyw � �
ƒvs � h � (7.31)
340
where F yw is the yield stress of the web steel (Table 7.18). This shear also may be
reduced in the ratio ƒ v/F v as above.
TABLE 7.18 Required Shear Capacity of
Intermediate-Stiffener Connections to Girder
Web
F yw, ksi
ƒ vg, kips
per lin in F yw, ksi
ƒ vg, kips
per lin in
36.0 0.034h 60.0 0.074h
42.0 0.043h 65.0 0.084h
45.0 0.048h 90.0 0.136h
50.0 0.056h 100.0 0.160h
55.0 0.065h
Combined Stresses in Web. A check should be made for combined shear and
bending in the web where the tensile bending stress is approximately equal to the
maximum permissible. When ƒ v, the shear force at the section divided by the web
area, is greater than that permitted by Eq. (7.29a), the tensile bending stress in the
web should be limited to no more than 0.6F yw or F yw(0.825 � 0.375ƒ v/F v), where
F v is the allowable web shear given by Eq. (7.29a). For girders with steel flanges
and webs with F y exceeding 65 ksi, when the flange bending stress is more than
75% of the allowable, the allowable shear stress in the web should not exceed that
given by Eq. (7.22).
Also, the compressive stresses in the web should be checked (see Art. 7.22).
7.21.2 LRFD Procedure for Plate Girders
Plate girders are normally proportioned to resist bending on the assumption that
the moment of inertia of the gross section is effective. The web must be propor-