Cross bracing
NSC 23
Shortening Incremental Stiness
Technical Digest 2019
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This is the bow at which the extreme fibre at the point of maximum bow
(and bending moment) reaches yield stress due to combined axial load
and bending. The bow is about 15% less than that in the flat bar. As the
frame deflects and load on the member is increased, the bow increases,
the member shortens more and more quickly and the stiffness of the
compression member decreases as shown in Figure 2. The member reaches
its buckling load as the frame reaches its maximum sway deflection of
14.6 mm.
Column shortening
If a cross-braced panel with bracing that is intended to behave as tensiononly
has significant axial loads in the columns, the bracing will develop axial
loads which may confuse the unwary. An elastic stick finite element analysis
which includes all the elements in the model with pinned connections
and which makes no provision for members intended to buckle when in
compression, will exhibit compression forces in the bracing and a tension
force in the beams: see Figure 3. The forces may or may not be sufficient to
cause the bracing members to buckle, depending on the magnitude of the
applied forces and the bracing section chosen.
If the braced panel is modelled with pinned joints and only the tension
element present and if only vertical loads are applied, no axial forces will be
developed in the bracing member or beams. The braced panel will deflect
sideways however, to accommodate the bracing member which remains at
its original length.
Lateral stiffness
It is advantageous to mobilise both tension-only bracing members in a
cross-braced panel if this can be achieved, because the increased stiffness
is beneficial to the overall stability of the building. The contribution of
the bracing members to the lateral stiffness is of course doubled and the
magnitude of the αcr value for the building increased, thereby reducing any
amplifier on the lateral loads. A cross bracing system formed of rods, perhaps
adopted for architectural reasons, can be pre-tensioned to prevent the rod
forming the compression diagonal from going slack. In this case, the bracing
members in both diagonals will be effective as the tension force in the
member in the shortening diagonal will be reduced as the bracing resists a
lateral load. There are proprietary systems of rods, rod-ends, turnbuckles and
connecting rings which are designed to achieve this effect 1 .
Tensioned bracing is more difficult to achieve when the bracing members
are a different geometry from rods. In the past it has been standard practice
in some drawing offices to detail the holes in cross bracing members such
that the length of the diagonal is 5 mm “short”. This required the erection
team to lean the columns when making the connections for the first bracing
member to be erected. Installing the second member was much more
difficult as it involved tensioning the first diagonal so as to shorten the
opposing diagonal by enough to make the final connection.
Conclusion
Tension-only bracing members provide a simple means of resisting lateral
loads on a structure but certain features of the behaviour of the bracing need
to be considered:
1) The slack member of flat bar cross bracing can bow significantly which
could possibly damage finishes.
2) If using tubes as cross bracing, the connections must be capable of
resisting a compression force at least equal to the buckling resistance of
the member.
3) A simple stick finite element analysis model of a frame with cross-bracing
will develop compression forces in both bracing members unless steps
are taken in the analysis to avoid this.
4) Mobilising both bracing members (eg by pre-tensioning) increases the
αcr value of the frame and is therefore beneficial.
1. Round bar cross bracing, p21 NSC, September 2015
Figure 2: Member shortening and incremental stiffness
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Axial force (kN) Bow (mm)
Shortening (mm)
Stiness K
Figure 3: Deflection under vertical loads