Continuous beams and slabs, and beams in frames - Composite Structures of Steel and Concrete - page 124

well as shear forces, and the bending moments depend on the relative
stiffnesses of the members joined.
If the columns of the framed structure shown in Fig. 3.1 were not
encased in concrete, and the beam-to-column joints were nominal pins,
the columns would be designed to EN 1993. However, the frame still
satisfies the definition of 'composite frame' in EN 1994-1-1:
' composite frame : a framed structure in which some or all of the ele-
ments are composite members and most of the remainder are structural
steel members'.
The columns shown in Fig. 3.1 are composite, and some of the top
reinforcement in each floor slab may be continued into the columns, to
control cracking above the ends of the beams.
In EN 1994-1-1, the definition of 'composite joint' is:
' composite joint : a joint between a composite member and another
composite, steel, or reinforced concrete member, in which reinforce-
ment is taken into account in design for the resistance and the stiff-
ness of the joint'.
It follows that if the top reinforcement is 'taken into account in design',
the joint is composite. It is permitted, however, to ignore light crack-
control reinforcement in design for ultimate limit states, and then to
design the structural steel components of the joint to EN 1993.
The example used in Chapter 5 is the two-bay nine-storey frame
shown in Fig. 5.1. If nominally-pinned joints are used, the member ABC
is designed as a beam with two simply-supported spans. If rigid joints are
used at B, normally it is the column, not the beam, that is continuous
through the joints at B, and the member ABC consists of two beams in a
frame . The bending moments are found from analysis of the frame, or of
a local region of it.
If, however, the line of columns B, E, etc., is replaced by a wall, then
member ABC can be continuous at B, with no transfer of bending moment
to the supporting wall. If nominally-pinned joints are used at A and C, the
model for analysis is a two-span continuous beam , as in the example in
Section 4.6, rather than beams in a frame. Such a beam does not contribute
to the resistance of the frame to horizontal loading; for example, from
wind. In the example in Chapter 5, this resistance is provided by reinforced
concrete walls at each end of the building, to which horizontal loads are
transferred by the floor slabs.
The rest of Chapter 4 refers to 'continuous beams'. In general, it applies
also to 'beams in frames'. For those, the difference lies in the location and