Bridges
U-frames in bridges
Bridge designers will be familiar with compression flanges restrained by u-frames.
David Brown of the SCI introduces the concept and illustrates the same principle
commonly found in the design of portal frames.
Engineers are always concerned with the buckling of elements in
compression and how restraint might be provided. In bridge construction
and (for example) a twin truss span, it may be possible to brace between
compression chords, as shown in Figure 1, to form an enclosed box.
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Technical Digest 2018
If bracing between the
compression chords is to be avoided,
some other means of restraining the
compression chord (or compression
flange, if the member is a beam)
must be found. There are many
examples of older footbridges
where a horizontal cross member
is extended laterally at deck level,
and a diagonal brace provided to
restrain the compression flange, as
shown in Figure 2. People without
an engineering background often
think the metalwork was provided to
support pipework (and it was often
used for this), but the arrangement
has a much more important
function.
With so-called “half-through” bridges, such as that shown in Figure 3,
clearly no bracing is possible between the compression flanges. In this form
of construction, the compression flanges are restrained by intermediate
u-frames.
A typical cross section at a u-frame location is shown in Figure 4. A u-frame
consists of a horizontal member (usually part of the deck steelwork) and
vertical members. The connection between the horizontal member and the
vertical member is continuous or semi-continuous forming a u-shaped stiff
frame to provide restraint to the compression elements.
Figure 4: “Half-through” bridge typical cross section
Bridge design codes such as BS 5400-3 or BS EN 1993-2 allow designers
to calculate an effective buckling length of the compression flange. The
effective length primarily depends on the stiffness of the vertical members,
the stiffness of the horizontal member and the stiffness of the connection
between the members. Increased flexibility in the members or at the
connections will lead to a longer buckling length. Detailed information on
the design of half-through bridges, including the effect of u-frames, may be
found on steelconstruction.info.
U-frames can also be seen in the footbridge pictured in Figure 5. In this
form of construction, the compression flanges of the main girders are formed
of square hollow sections, orientated as a diamond. Restraint to these
compression flanges is provided by external u-frames fabricated from plate,
which wrap around the bridge cross section at intervals along the span.
Application in buildings
Although u-frames are associated with bridge construction, the same
principle is found in portal frames, when the inside flanges of the members
are restrained by bracing back to the purlins or side rails, as shown in
Figure 6.
Some authorities (notably in other parts of Europe) consider this restraint
system results in axial loads in the secondary steelwork, and that the restraint
is only effective if purlins (or rails) assumed to provide restraint intersect with
a node on the bracing (typically in the end bay). In the UK, there is no such
Figure 1: Truss bridge with bracing
between the compression chords
Figure 2: Bridges with external bracing to restrain the compression flange
Figure 5: Footbridge with u-frames; cross section
Figure 3: “Half-through” bridge
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