Composite columns and frames - Composite Structures of Steel and Concrete - page 175

may also occur. The check on bolt fracture may need to allow for prying
action (increase of bolt tension caused by compressive force where the
edges of the end plate bear against the column flange). The tensile resist-
ance of the top bolts F T,Rd is given by the weakest of these types of
deformation.
The longitudinal reinforcement in the slab is assumed to be at yield in
tension, so the force F t,s,Rd is known.
Assuming that any axial force in the beam is negligible, the compress-
ive force at the bottom of the joint cannot exceed F T,Rd F t,s,Rd . Failure

could occur by buckling of the column web at E, so this resistance is
found next, allowing for the axial compression in the column. If buckling
governs, a stiffener can be added, but this is rarely necessary. The com-
pressive force to cause yielding of the bottom flange, F a,fl,Rd , is then found.
If it is less than the total tensile force, an area of web is assumed also to
yield, such that

+

F a,fl,Rd + F a,web,Rd = F T,Rd + F t,s,Rd

The lines of action of these four forces are known, so the bending
resistance of the joint, M j,Rd , is found.
For beams of unequal depth, or at an external column, checks are also
needed on the resistance of the column web to shear and the transfer to
the column of the unbalanced tensile force in the slab reinforcement. The
methods can also allow for concrete encasement to the column and/or the
webs of the beams.
The resistance of the joint to vertical shear is normally provided by the
bolts at B and C, and may be limited by the bearing strength of the end
plate or column flange. The allocation of some of the shear to the bolts at
A would reduce their design resistance to tension.

Moment-rotation curve for an end-plate joint
The other information needed for design is a curve of hogging bending
moment against rotation of the joint,

. This is defined as the rotation
additional to that which would occur if the joint were rigid and the beam
continued to its intersection with the centre-line of the column, as shown
in Fig. 5.3. For steel connections, methods are given in EN 1993-1-1 [15]
for the prediction of this curve. They are applicable also to composite
joints, with modifications given in Annex A of EN 1994-1-1. These allow
for the effect of slip of the shear connection on the longitudinal stiffness
of the top reinforcement, and are explained in Reference 17. The elastic
properties of the components give the initial elastic stiffness, S j,ini . This is
assumed to be applicable for bending moments M j,Ed up to 2 M j,Rd /3, where
M j,Rd is the bending resistance of the joint.

φ