Page 244 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch05.qxd 9/29/05 5:12 PM Page 5.24

CRITERIA FOR BUILDING DESIGN

5.24 CHAPTER FIVE

For unstiffened girders, h/t w must not exceed 260, and the ratio of the web area to the compres-
sion flange area must not exceed 10.

5.5.10 Cover Plates and Built-up Beams

Where flexural strength requirements vary along the length of a built-up member, it is often more
cost effective to vary the flange thickness or width by splicing a series of plates than to use cover
plates. With rolled shapes, it may be more effective just to use a stronger section. Nevertheless,
where cover plates are used, certain proportioning rules should be followed.
The total cross-sectional area of cover plates of bolted girders should not exceed 70% of the total
flange area. High-strength bolts or welds connecting flange to web, or cover plate to flange, must be
proportioned to resist the total horizontal shear resulting from bending. The longitudinal distribution
of bolts or intermittent welds should be in proportion to the shear. Also, the spacing should not
exceed the maximum permitted for compression or tension members (see Arts. 5.3.3 and 5.4.3).
Bolts or welds connecting flange to web must also be designed to transmit to the web any loads
applied directly to the flange, except when the loads are transferred by direct bearing.
Partial-length cover plates should extend beyond the theoretical cutoff point and the extended
portion attached by high-strength bolts in a slip-critical connection (see Art. 5.9.8) or fillet welds.
The attachment must develop the cover plate’s portion of the flexural design strength in the beam or
girder at the theoretical cutoff point.
For welded cover plates, the welds connecting the cover plate termination to the beam or girder
should have continuous welds along both edges of the cover plate in the length a′, defined by
Eqs. (5.85) to (5.87) in terms of the cover plate width w, and be adequate to develop the cover plate’s
portion of the available strength in the beam or girder at the distance a′ from the end of the cover plate.
When there is a continuous weld equal to or larger than three-fourths of the plate thickness across
the end of the plate,
a′= w (5.85)
When there is a continuous weld smaller than three-fourths of the plate thickness across the end
of the plate,
a′= 1.5w (5.86)
When there is no weld across the end of the plate,
a′= 2w (5.87)

5.6 DESIGN OF MEMBERS FOR SHEAR

This article pertains to the design of webs of singly or doubly symmetric members subject to shear
in the plane of the web, to single angles and HSS, and for shear in the weak direction of singly or
doubly symmetric shapes. For unsymmetric sections and other cases not discussed in this article,
refer to the provisions given by the AISC Specification.
The design shear strength φ v V n and the allowable flexural strength V n /Ω v are determined using
φ v = 0.90 (LRFD) and Ω v = 1.67 (ASD) for all cases except as noted. The nominal shear strength V n
can be calculated by one of two methods. In most cases, such as presented in Art. 5.6.1, it is calcu-
lated without utilizing postbuckling strength. Alternatively, for plate girders that meet certain condi-
tions as presented in Art. 5.6.2, advantage may be taken of such strength, usually referred to as
tension field action.
Note that the effect of any web openings on the nominal shear strength must be determined by a
rational method. Adequate reinforcement must be provided when the required strength exceeds the
available strength of the member at the opening. (See David Darwin, Design of Steel and Composite
Beams with Web Openings, Steel Design Guide Series No. 2, AISC, Chicago, Ill.)

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