Moment connections are capable of transferring the forces in beam flanges to the column. This moment transfer, when specified, must be provided for in addition to
FIGURE 7.52 Methods of constructing flexible welded con- nections.
and usually independent of the shear connection needed to support the beam re- action. Framed, seated, and end-plate connections (Arts. 7.35.1 to 7.35.5) are ex- amples of shear connections. Those in Fig. 7.17 (p. 7.32), are moment connections.
In Fig. 7.17a tog, flange stresses are developed independently of the shear con- nections, whereas in h and i, the forces are combined and the entire connection resolved as a unit.
Moment connections may be classified according to their design function: those resisting moment due to lateral forces on the structure, and those needed to develop continuity, with or without resistance to lateral forces.
The connections generally are designed for the computed bending moment, which often is less than the beam’s capacity to resist moment. A maximum con- nection is obtained, however, when the beam flange is developed for its maximum allowable stress.
The ability of a connection to resist moment depends on the elastic behavior of the parts. For example, the light lug angle shown connected to the top flange of the beam in Fig. 7.52bis not designed for moment and accordingly affords negli- gible resistance to rotation. In contrast, full rigidity is expected of the direct welded flange-to-column connection in Fig. 7.52a. The degree of fixity, therefore, is an important factor in design of moment connections.
Fixity of End Connections. Specifications recognize three types of end connec- tions: simple, rigid, and semirigid. The type designated simple (unrestrained) is intended to support beams and girders for shear only and leave the ends free to
rotate under load. The type designatedrigid(known also as rigid-frame, continuous, restrained frame) aims at not only carrying the shear but also providing sufficient rigidity to hold virtually unchanged the original angles between members con- nected.Semirigid,as the name implies, assumes that the connections of beams and girders possess a dependable and known moment capacity intermediate in degree between the simple and rigid types. Figure 7.54 illustrates these three types together with the uniform-load moments obtained with each type.
Although no definite relative rigidities have been established, it is generally conceded that the simple or flexible type could vary from zero to 15% (some researchers recommend 20%) end restraint and that the rigid type could vary from 90 to 100%. The semirigid types lie between 15 and 90%, the precise value assumed in the design being largely dependent on experimental analysis. These percentages of rigidity represent the ratio of the moment developed by the connection, with no column rotation, to the moment developed by a fully rigid connection under the same conditions, multiplied by 100.
Framed and seated connections offer little or no restraint. In addition, several other arrangements come within the scope of simple-type connections, although they appear to offer greater resistance to end rotations. For example, in Fig. 7.52a, a top plate may be used instead of an angle for lateral support, the plate being so designed that plastic deformation may occur in the narrow unwelded portion. Nat- urally, the plate offers greater resistance to beam rotation than a light angle, but it can provide sufficient flexibility that the connection can be classified as a simple type. Plate and welds at both ends are proportional for about 25% of the beam moment capacity. The plate is shaped so that the metal across the least width is at yield stress when the stresses in the wide portion, in the butt welds, and in the fillet welds are at allowable working values. The unwelded length is then made from 20 to 50% greater than the least width to assure ductile yielding. This detail can also be developed as an effective moment-type connection.
Another flexible type is the direct web connection in Fig. 7.52b. Figured for shear loads only, the welds are located on the lower part of the web, where the rotational effect of the beam under load is the least. This is a likely condition when the beam rests on erection seats and the axis of rotation centers about the seat rather then about the neutral axis.
Tests indicate that considerable flexibility also can be obtained with a property proportioned welded top-plate detail as shown in Fig. 7.52c without narrowing it as in Fig. 7.52a. This detail is usually confined to wind-braced simple-beam de- signs. The top plate is designed for the wind moment on the joint, at the increased stresses permitted for wind loads.
The problem of superimposing wind bracing on what is otherwise a clear-cut simple beam with flexible connections is a complex one. Some compromise is usually effected between theory and actual design practice. Two alternatives usually are permitted by building codes:
1. Connections designed to resist assumed wind moments should be adequate to resist the moments induced by the gravity loading and the wind loading, at specified increased unit stresses.
2. Connections designed to resist assumed wind moments should be so designed that larger moments, induced by gravity loading under the actual condition of re- straint, will be relieved by deformation of the connection material.
Obviously, these options envisage some nonelastic, but self-limiting, deformation of the structural-steel parts. Innumerable wind-braced buildings of riveted, bolted,
FIGURE 7.53 Methods of constructing welded rigid connec- tions.
or welded construction have been designed on this assumption of plastic behavior and have proved satisfactory in service.
Fully rigid, bolted beam end connections are not often used because of the awkward, bulky details, which, if not interfering with architectural clearances, are often so costly to design and fabricate as to negate the economy gained by using smaller beam sections. In appearance, they resemble the type shown in Fig. 7.17 for wind bracing; they are developed for the full moment-resisting capacity of the beam.
Much easier to accomplish and more efficient are welded rigid connections (Fig.
7.53). They may be connected simply by butt welding the beam flanges to the columns—the ‘‘direct’’ connection shown in Fig. 7.53aand b. Others may prefer the ‘‘indirect’’ method, with top plates, because this detail permits ordinary mill tolerance for beam length. Welding of plates to stiffen the column flanges, when necessary, is also relatively simple.
In lieu of the erection seat angle in Fig. 7.53b, a patented, forged hook-and-eye device, known as Saxe erection units, may be used. The eye, or seat, is shop welded to the column, and the hook, or clip, is shop welded to the underside of the beam bottom flange. For deep beams, a similar unit may be located on the top flange to prevent accidental turning over of the beams. Saxe units are capable of supporting normal erection loads and deadweight of members; but their contribution to the strength of the connection is ignored in computing resistance to shear.
FIGURE 7.54 Effect of rigidity of connections on end moments.
A comparison of fixities intermediate between full rigidity and zero restrain in Fig. 7.54 reveals an optimum condition attainable with 75% rigidity; end and center- span moments are equal, each beingWL/ 16, or one-half the simple-beam moment.
The saving in weight of beam is quite apparent.
Perhaps the deterrent to a broader usage of semirigid connections has been the proviso contained in specifications: ‘‘permitted only upon evidence that the con- nections to be used are capable of resisting definite moments without overstress of the fasteners.’’ As a safeguard, the proportioning of the beam joined by such con- nections is predicated upon no greater degree of end restraint than the minimum known to be effected by the connection. Suggested practice, based on research with welded connections, is to design the end connections for 75% rigidity but to provide a beam sized for the moment that would result from 50% restraint; i.e., WL/ 12.
(‘‘Report of Tests of Welded Top Plate and Seat Building Connections,’’The Weld- ing Journal,Research Supplement 146S–165S, 1944.) The type of welded connec- tion in Fig. 7.52cwhen designed for the intended rigidity, is generally acceptable.
End-plate connections (Fig. 7.50) are another means of achieving negligible, partial, and full restraint.