Building Design and Construction Handbook - Merritt - Ventech!

STRUCTURAL STEEL CONSTRUCTION 7.113
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 centerspan
moments are equal, each being WL/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 connections
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 connections
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 Welding
Journal, Research Supplement 146S–165S, 1944.) The type of welded connection
in Fig. 7.52c when designed for the intended rigidity, is generally acceptable.
End-plate connections (Fig. 7.50) are another means of achieving negligible,
partial, and full restraint.
7.36 BEAM SPLICES
These are required in rigid frames, suspended-span construction, and continuous
beams. Such splices are usually located at points of counterflexure or at points
where moments are relatively small. Therefore, splices are of moderate size. Flanges
and web may be spliced with plates or butt welded.