Truss members generally are designed to carry only axial forces. At panel points, where members intersect, it is desirable that the forces be concurrent, to avoid bending moments.
Hence the gravity axes of the members should be made to intersect at a point if practicable.
When this cannot be done, the connections should be designed for eccentricity present. Also, groups of fasteners or welds at the ends of each member should have their centers of gravity on or near the gravity axis of the member. Otherwise, the member must be designed for eccentric loading. In building trusses not subject to repeated variation in stress, however, eccentricity of welds and fasteners may be neglected at the ends of single-angle, doubleangle, and similar members.
At panel points, several types of connections may be used. Members may be pinconnected to each other (Art. 5.6), or they may be connected through gusset plates, or they may be welded directly to each other. In bridges, fasteners should be symmetrical with the axis of each member as far as practicable. If possible, the design should fully develop the elements of the member.
Design of connections at gusset plates is similar to that for gusset-plate connections illustrated in Art. 5.25. In all cases, gusset plates should be trimmed to minimize material required and for good appearance. When cuts are made, minimum edge distances for fasteners and seats for welds should be maintained. Reentrant cuts should be avoided.
At columns, if the reactions of trusses on opposite sides of a support differ by only a small amount (up to 20%), the gravity axes of end diagonal and chord may be allowed to intersect at or near the column face (Fig. 5.62). The connections need be designed only for the truss reactions. If there is a large difference in reactions of trusses on opposite sides of the support, or if only one truss frames into a column, the gravity axes of diagonal and chord should be made to intersect on the column gravity axis. Shear and moment in the column flange should be considered in design of the end connection.
If the connection of bottom chord to column is rigid, deflection of the truss will induce bending in the column. Unless the bottom chord serves as part of a bracing system, the connection to the column should be flexible. When the connection must be rigid, bending in the column can be reduced by attaching the bottom chord after dead-load deflection has occurred.
Splices in truss chords should be located as near panel points as practicable and preferably on the side where the smaller stress occurs. Compression chords should have ends in close contact at bolted splices. When such members are fabricated and erected with close inspection and detailed with milled ends in full-contact bearing, the splices may be held in place with high-strength bolts proportioned for at least 50% of the lower allowable stress of the sections spliced.
In other cases, compression and tension chords should be designed for not less than the average of the calculated stress at the point of splice and the strength of the member at that point but not less than 75% of the strength of the member (Arts. 5.26 and 5.27). Tension and compression chords may be spliced with complete-penetration groove welds without splice plates.
Gusset Plates. These should be sized to resist all loads imposed. The design of gussets and the connection of members to the gusset is based on statics and yield strength, which are the basic ingredients of the lower-bound theorem of limit analysis. Basically, this theorem states that if equilibrium is satisfied in the structure (or connection) and yield strength is nowhere exceeded, the applied load will, at most, be equal to the load required to fail the connection. In other words, the connection will be safe. In addition to yield strength, gussets and their associated connections also must be checked to ensure that they do not fail by fracture (lack of ductility) and by instability (buckling).
AASHTO has a stability criterion for the free edges of gusset plates that requires that an edge be stiffened when its length exceeds plate thickness times 347/Fy, where Fy is the specified yield stress (ksi). This criterion is intended to prevent a gusset from flexing when the structure deforms and the angles between the members change. Because of repeated loading, this flexing can cause fatigue cracks in gussets.
For statically loaded structures, which include structures subjected to wind and seismic loads, this flexing is not detrimental to structural safety and may even be desirable in seismic design because it allows more energy absorption in the members of the structure and it reduces premature fracture in the connections. For buildings, the AISC seismic specification has detailing requirements to allow gussets to flex in certain situations. AISC ASD and LRFD manuals include requirements regarding gusset stability. It is important that gusset buckling be controlled to prevent changes in structure geometry that could render the structure unserviceable
or cause catastrophic collapse (see also Art. 5.36).
Fracture in gusset-plate connections also must be prevented not only when connections are made with fillet welds, which have limited ductility in their transverse direction, but also with bolts, because of the possibility of tearout fracture and nonuniform distribution of tension and shear to the bolts.
