Building Design and Construction Handbook - Merritt - Ventech!

STRUCTURAL STEEL CONSTRUCTION 7.69
TABLE 7.17 Limiting Depth-Thickness Ratios for ASD of Plate-Girder Webs
F y, ksi
h/t
Eq. (7.25)
h/t
Eq. (7.26) F y, ksi
h/t
Eq. (7.25)
h/t
Eq. (7.26)
36.0 322 333 60.0 207 258
42.0 282 309 65.0 192 248
45.0 266 298 90.0 143 211
50.0 243 283 100.0 130 200
55.0 223 270
General Design Method. Plate girders may be proportioned to resist bending on
the assumption that the moment of inertia of the gross cross section is effective.
No deductions need be made for fastener holes, unless the holes reduce the gross
area of either flange by more than 15%. When they do, the excess should be
deducted.
Hybrid girders, which have higher-strength steel in the flanges than in the web,
may also be proportioned by the moment of inertia of the gross section when they
are not subjected to an axial force greater than 15% of the product of yield stress
of the flange steel and the area of the gross section. At any given section, the
flanges must have the same cross-sectional area and be made of the same grade of
steel.
The allowable compressing bending stress Fb for plate girders must be reduced
from that given in Art. 7.20 where h/t exceeds 760/�F b.
For greater values of this
ratio, the allowable compressive bending stress, except for hybrid girders, becomes
� � ��
Awh 760
F�b �Fb 1 � 0.0005 � (7.27)
A t �F
ƒ b
where A w � the web area, in 2 and A ƒ � the compression flange area, in 2 .
For hybrid girders, not only is the allowable compressive bending stress limited
to that given by Eq. (7.24), but also the maximum stress in either flange may not
exceed
3
12 � (A w/A ƒ)(3�
� � )
b b�
12 � 2(A /A ) �
w ƒ
F� � F (7.28)
where a � ratio of web yield stress to flange yield stress.
Flange Limitations. The projecting elements of the compression flange must
comply with the limitations for b/t given in Art. 7.21. The area of cover plates,
where used, should not exceed 0.70 times the total flange area. Partial-length cover
plates (Fig. 7.33b) should extend beyond the theoretical cutoff point a sufficient
distance to develop their share of bending stresses at the cutoff point. Preferably
for welded-plate girders, the flange should consist of a series of plates, which may
differ in thickness and width, joined end to end with complete-penetration groove
welds (Fig. 7.33a).
Bearing Stiffeners. These are required on girder webs at unframed ends. They
may also be needed at concentrated loads, including supports. Set in pairs, bearing
stiffeners may be angles or plates placed on opposite sides of the web, usually
normal to the bending axis. Angles are attached with one leg against the web. Plates