Technical
NSC 27
May 20
almost linear, from 8.07 to 5.93. This reduced value would require
the design lateral loads to be increased by 20% to allow for
second-order effects. Increasing the rafter by one serial size gives
αcr = 6.2 and a lateral load increase of 19%.
An unverified estimate of initial stiffness for the specific
elements in the example found a value of about 60 MNm/radian
for a joint with a moment resistance of 622 kNm. This gives
αcr = 6.6 and a support moment of 603 kNm. The value of MEd/MRd
is therefore 0.97 and μ = 2.6 approximately. This value of μ
corresponds to a lower joint stiffness which reduces the support
moment to about 508 kNm. Iteration indicates a joint stiffness of
about 29 MNm/radian giving a support moment of 538 kNm and
μ ≈ 2.05. The corresponding value of αcr is about 5.3.
3 Conclusions
The above example illustrates the effect of joint stiffness on
frame behaviour, in terms of the design bending moments, the
deflections and the global stability and second-order effects. The
sequencing of analysis and design steps is also affected as the
designer must either have a preliminary idea of joint details
when setting up the analysis model or iteration will be necessary.
The presence of the resistance ratio μ in the stiffness
calculation potentially introduces difficulties where the frame
and joints are designed by different parties. The designer could
specify design moments 50% larger than those determined in the
analysis, in the hope of the joint remaining elastic. The steelwork
contractor could well find it challenging and expensive to satisfy
such a requirement.
The UK National Annex to BS EN 1993-1-8:2005 states in clause
NA.2.6 that connections designed in accordance with the
principles given in the SCI publication P2071 may be classified on
the basis of the guidance given in section 2.5 of the same
publication. SCI publication P3982, the successor to P207 contains
the advice that well-proportioned connections that follow the
recommendations for standardisation given in P398 and
designed for strength alone can generally be assumed to be rigid
for joints in braced frames and single-storey portal frames.
1 SCI P207, Joints in Steel Construction –
Moment Connections
2 SCI P398 Joints in Steel Construction –
Moment Resisting Joints to Eurocode 3
10.00
9.00
8.00
7.00
6.00
5.00
40.00
30.00
0.6 0.7 0.8 0.9 1 1.1
MEd/MRd
alpha cr
Figure 2.2 Elastic critical load factor
/Steel_section_sizes
/Portal_frames#In-plane_frame_stability
/Design_codes_and_standards#National_Annexes
/The_Green_Books#Moment_resisting_connections
/Braced_frames
/Portal_frames