End connections of beams to their supports are classified as simple-beam, fully restrained,
and partially restrained connections.
Simple, or conventional, connections are assumed free to rotate under loads. They are designed to carry shear only. The AISC specifications for structural steel buildings require that connections of this type have adequate inelastic rotation capacity to avoid overstressing the fasteners or welds.
Fully restrained (rigid-frame) connections, transmitting bending moment as well as shear, are used to provide complete continuity in a frame (Art. 5.33).
Partially restrained (semirigid) connections provide end restraint intermediate between the rigid and flexible types (Art. 5.33).
For simple connections, design drawings should give the end reactions for each beam. If no information is provided, the detailer may design the connections for one-half the maximum allowable total uniform load on each beam.
Simple connections are of two basic types: framed (Fig. 5.50a) and seated (Fig. 5.50b).
A framed connection transfers the load from a beam to a support through one or two connection angles, or a shear plate attached to the supporting member, or a tee attached to either the supporting or supported member. A seated connection transfers the load through
a seat under the beam bottom flange. A top, or cap, angle should be used with seated connections to provide lateral support. It may be attached to the beam top flange, as shown in Fig. 5.50b, or to the top portion of the web. With both framed and seated connections, the beam end is stopped 1â„2 in short of the face of the supporting member, to allow for inaccuracies in beam length.
These generally are more economical of material than seated connections. For example, in a symmetrical, bolted, framed connection, the fasteners through the web are in double shear.
In a seated connection, the fasteners are in single shear. Hence framed connections are used where erection clearances permit, e.g., for connections to column flanges or to girders with flanges at the same level as the beam flanges. Seated connections, however, usually are more advantageous for connections to column webs because placement of beams between column flanges is easier. Seats also are useful in erection because they provide support for beams while field holes are aligned and fasteners are installed. Furthermore, seated connections may be more economical for deep beams. They require fewer field bolts, though the total number of shop and field fasteners may be larger than those required for a framed connection of the same capacity.
The AISC manual lists capacities and required design checks for beam connections for buildings. Design is facilitated when this information can be used. For cases where such connections are not suitable, beam connections can be designed by the principles and methods given for brackets in Art. 5.31.
Vertical fastener spacing in framed connections is standardized at 3 in. The top gage line also is set 3 in below the beam top, when practicable Closer spacing may be used however, as long as AISC specification restrictions on minimum spacing are met.
To ensure adequate stiffness and stability, the length of the connection material in a framed connection should be at least half the distance T between flange-web fillets of the beam. Distance between inner gage lines of outstanding legs or flange of connection material is standardized at 51â„2 in, but sometimes a shorter spacing is required to meet AISC specification requirements for minimum edge distance.
Thickness of connection material may be determined by shear on a vertical section, availability of material of needed thicknesses, or the bearing value for the nominal fastener diameter.
When a beam frames into a girder with tops of both at the same level, the top of the beam generally is notched, or coped, to remove enough of the flange and web to clear the girder flange. Depth of cut should be sufficient to clear the web fillet (k distance for a rolled section). Length of cope should be sufficient to clear the girder flange by 1â„2 to 3â„4 in. A fillet with smooth transition should be provided at the intersection of the horizontal and vertical cuts forming the cope.
For beams framing into column flanges, most fabricators prefer connections attached to the columns in the shop. Then the beams require punching only. Thus less handling and fewer operations are required in the shop. Furthermore, with connection material attached to the columns, erectors have more flexibility in plumbing the steel before field bolts are tightened or field welds made.
Some of the standardized framed connections in the AISC manual are arranged to permit substitution of welds for bolts. For example, welds A in Fig. 5.51a replace bolts for the web connections. Welds B replace bolts in the outstanding legs (Fig. 5.51b). Angle thickness must be at least the weld size plus 1â„16 in and a minimum of 5â„16. Holes may be provided for erection bolts in legs that are to be field welded. When bolts are used in outstanding legs, the bearing capacity of supporting material should be investigated.
Welds A are eccentrically loaded. They receive the load from the beam web and the connection transmits the load to the support at the back of the outstanding legs. Hence there is an eccentricity of load equal to the distance from the back of the outstanding legs to the center of gravity of welds A. Therefore, when the connections in Fig. 5.51 are used, the combination of vertical shear and moment on welds A should be taken into account in design, unless the tables in the AISC manual are used.
For the connection in Fig. 5.51b, the welds usually are made in the shop. Consequently, the beam bottom must be coped to permit the beam to be inserted between the angles in the field.
Welds B also are eccentrically loaded. The beam reaction is transmitted from the center of web to the welds along the toes of the outstanding legs. This moment, too, should be taken into account in design. To prevent cracking, the vertical welds at the top of the angles should be returned horizontally for a distance of twice the weld size.
Standardized framed connections of the type shown in Fig. 5.51c were developed especially for welding of both the web legs and outstanding legs of the connection angles.
These may be unstiffened, as shown in Fig. 5.50b and Fig. 5.53a, or stiffened, as shown in Fig. 5.52 and Fig. 5.53b. A stiffened seat usually is used when loads to be carried exceed the capacities of the outstanding leg of standardized unstiffened seats. Tables in the AISC manuals facilitate the design of both types of connections.
The primary use for seated connections is for beams framing to column webs. In this case, the seat is inside the column flange toes or nearly so, and is not an architectural problem. Its use also avoids the erection safety problem associated with framed connections where the same bolts support beams on both sides of the column web.
When a seat is attached to one side of the column web, the column web is subjected to a local bending pattern because the load from the beam is applied to the seat at some distance eÆ’ from the face of the web. The stiffened seat design table (Table 9.9) in the AISC LRFD manual includes this effect. For unstiffened seats, column web bending also occurs, but its effects are less critical because the loads and eccentricities for unstiffened seats are generally much smaller than for stiffened seats. Fig. 5.54a presents a yield line pattern which can be used to assess the strength of the column web. The nominal capacity of the column web is
Design of a seated connection generally is based on the assumption that the seat carries the full beam reaction. The top, or cap, angle only provides lateral support. Even for large beams, this angle can be small and can be attached with only two bolts in each leg (Fig. 5.52) or a toe weld along each leg (Fig. 5.53).
With the nominal tolerance of 1â„2 in between beam end and face of support, the length of support provided a beam end by a seat angle equals the width of the outstanding leg less 1â„2 in. Thus a typical 4-in-wide angle leg provides 31â„2 in of bearing. Because of the short bearing, the capacity of a seated connection may be controlled by the thickness of the beam web, for resisting web yielding and crippling.
Tables in the AISC manuals list beam reactions, R for ASD and R for LRFD, for 4-in wide outstanding angle legs that are based on a nominal setback a of 1â„2 in and a beam underrun of 1â„4 in. Thus, calculations are based on the beam end being 3â„4 in from the face of the column. The reaction is assumed centered on the bearing length N. Both manuals list additional parameters, R1 through R4 for ASD and R1 through R6 for LRFD, that can be used to determine the web crippling and local web yielding limitations for other bearing lengths.
For unstiffened seats, the bearing length is assumed to extend from the beam end toward midspan. For stiffened seats, the bearing length is assumed to extend from the end of the seat toward the beam end. In design of the seat, however, an eccentricity from the face of the support of 80% of the beam-seat width is used if it is larger than the eccentricity based on the reaction position at the center of N.
Unstiffened Seats. The capacity of the outstanding leg of an unstiffened seat is determined by its resistance to bending. The critical section for bending is assumed to be located at the toe of the fillet of the outstanding leg. When reactions are so large that more than a nominal 31â„2 in of bearing is required, stiffened seats usually are used.
In addition to the capacity of the outstanding leg, the capacity of an unstiffened seat depends also on the bolts or welds used to connect to the column. The small eccentricity of the beam reaction generally is neglected in determining bolt capacities, but is included when calculating weld capacities.
Stiffened Seats. These require that stiffeners be fitted to bear against the underside of the seat. The stiffeners must be sized to provide adequate length of bearing for the beam, to prevent web yielding and crippling. Area of stiffeners must be adequate to carry the beam reaction at the allowable bearing stress.
When bolts are used, the seat and stiffeners usually are angles (Fig. 5.52). A filler with the same thickness as the seat angle is inserted below the seat angle, between the stiffeners and the face of support. For light loads, a single stiffener angle may be used (type B, Fig. 5.52). For heavier loads, two stiffener angles may be required (type A, Fig. 5.52). Outstanding legs of these angles need not be stitched together. To accommodate the gage of fasteners in the supporting member, paired stiffeners may be separated. But the separation must be at least 1 in wide and not more than twice k ts, where k is the distance from outer surface of beam flange to web toe of fillet (in), and ts is the stiffener thickness (in).
For standardized stiffened-seat connections, 3â„8-in-thick seat angles are specified. The outstanding leg is made wide enough to extend beyond the outstanding leg of the stiffener angle. The width of the vertical leg of the seat angle is determined by the type of connection.
In determination of the bearing capacity of a stiffener, the effective width of the outstanding leg of the stiffener generally is taken as 1â„2 in less than the actual width.
When stiffened seats are to be welded, they can be fabricated by welding two plates to form a tee (Fig. 5.53b) or by cutting a T shape from a wide-flange or I beam. When two plates are used, the stiffener should be fitted to bear against the underside of the seat.
Thickness of the seat plate usually equals that of the stiffener but should not be less than 3â„8 in.
The stiffener usually is attached to the face of the support with two fillet welds over the full length L of the stiffener. The welds should be returned a distance of at least 0.2L along the underside of the seat on each side of the stiffener. The welds are subjected to both shear and tension because of the eccentricity of the loading on the seat. Design is much the same as for the bracket in Fig. 5.49 (Art. 5.31).
Size and length of welds between a seat plate and stiffener should be equal to or greater than the corresponding dimensions of the horizontal returns.
Stiffener and seat should be made as narrow as possible while providing required bearing.
This will minimize the eccentricity of the load on the welds. For a channel, however, the seat plate, but not the stiffener, usually is made 6 in wide, to provide space for two erection bolts. In this case the seat projects beyond the stiffener, and length of welds between seat and stiffener cannot exceed the stiffener width.
Determination of stiffener thickness may be influenced by the thickness of the web of the beam to be supported and by the size of weld between seat and support. One recommendation is that stiffener thickness be at least the product of beam- web thickness and the ratio of yield strength of web to yield strength of seat material.