Bracing within the span

As explained in Section 5.4.2, restraint to the main beams within the span, to limit slenderness, is considered by Part 3 to be provided in a number of different ways. One of these is direct connection of the slab to the compression flange and one other (continuous U-frame restraint) does not involve bracing. There are three bracing systems that provide the other forms of restraint:

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• discrete lateral restraints

• discrete torsional restraints

• discrete U-frame restraints.

Design aspects for bracing within the span are discussed inSections 6.3.2to 6.3.4 below. In all cases, the members forming the bracing system should be designed to the appropriate Sections of Part 3 (3/9, 3/10, 3/11, 3/14).

6.3.1 Forms of bracing

Lateral restraints

Discrete lateral restraint is provided by plan bracing to the compression flange, or by

transverse triangulated frames in conjunction with either plan bracing or a deck slab attached to the tension flange.

Plan bracing is usually only provided when there is no deck slab or other means to carry lateral loads back to the supports (lateral bending resistance of the main beams is sufficient in many cases, especially when the load can be shared between many main beams). Before the slab is cast, plan bracing to the top flange will provide the necessary restraint for single spans but, for continuous spans, transverse bracing will also be needed adjacent to the intermediate supports to restrain the bottom flange. Plan bracing to the bottom flange may also be used in longer spans where the designer chooses to provide lateral restraint to the compression flange (adjacent to the supports) at discrete points between effective transverse bracing (rather than increase the number of transverse bracings). For spans over about 60 m, plan bracing over the full length of the bottom flange may need to be provided as a means to increase torsional stiffness (effectively creating a box section) and thus improve dynamic performance (to resist wind-induced oscillations).

In most cases, direct connection of plan bracing to the flange plate should be avoided;

instead, it should be connected to horizontal cleats between web and transverse stiffener, just clear of the flange, or to cleats on top of the top flange but within the slab between the layers of reinforcement. Plan bracing below the top flange should be sufficiently clear to allow for the slab formwork. Plan bracing within the slab is preferable for durability reasons but it complicates the fixing of reinforcement and is therefore disliked by some contractors.

Typical triangulated transverse bracing systems are shown inFigure 6.2.

A triangulated ‘X’ system is suitable for deep main beams and a ‘k’ system is useful when an X would be very flat (the k system will be stiffer). In a triangulated system, the position of any horizontal members at the top flange should be considered carefully because they may conflict with supports to the formwork for the deck slab. These members are usually redundant once the slab has been cast but removal involves an additional operation (with potential hazards for the operatives), so they are usually left in place. Bracing members are usually lapped and bolted to web stiffeners; welding is rarely used.

As mentioned in Section 5.3.7, intermediate bracing that is continuous across more than two main beams will participate in the global action and will distribute load to several main beams. However, such continuity does not provide much benefit to the design of the main beams but introduces stress reversals in the bracing and

Figure 6.2 Triangulated transverse bracing systems

its connections; checks must therefore be made for fatigue. To avoid this fatigue situation, designers frequently use non-continuous bracing, where main beams are connected in pairs, with no bracing between one pair and the next. When continuity is required only during construction (for example to share wind loads between all the beams), temporary bracing is sometimes provided between adjacent pairs of beams (seeFigure 6.2).

Torsional restraints

During construction, when there is no deck slab, and where there is no plan bracing to provide lateral restraints, torsional restraint to the beams can be provided by triangulated frames (similar to those shown inFigure 6.2) between main beams or by stiff beams that are rigidly connected to stiffeners on the webs of the main beams.

Torsional bracing using stiff beams, in the form of a channel or similar deep member (thus often called ‘channel bracing’), is suitable for relatively shallow main beams (up to about 1200 mm deep). The beam is usually lapped and bolted to the web stiffeners; often the ends of the beams have deep gusset plates welded to them so that the bolt group has a higher moment capacity. See Figure 6.3.

Once the deck slab has been cast, torsional bracing effectively becomes part of a discrete lateral bracing system.

Figure 6.3 Torsional restraint by channel bracing

Discrete U-frame restraints

Discrete U-frames are created by stiff transverse beams, connected close to the tension flange, acting in conjunction with either plan bracing or deck slab at tension flange level, and with stiffeners on the webs of the beams.

When U-frames created by ladder-deck construction provide restraint to the bottom

compression flange, a moment connection is needed between cross-beam and stiffener. When the main beams are very deep, ‘knee bracing’ is sometimes provided to give a better and stiffer restraint to the compression flange.

6.3.2 Design of discrete lateral restraints

Elements providing discrete intermediate restraints to compression flanges should be checked at ULS in accordance with Clause 3/9.12.2.

The cases where discrete lateral bracing is provided include:

• where plan bracing is directly connected to the compression flange (midspan regions during construction)

• where plan bracing is connected to the tension flange and stiff (triangulated) transverse frames are provided at the positions where the plan bracing is connected (support regions during construction)

• where the deck slab is connected to the tension flange and stiff transverse frames are provided between compression and tension flanges (support regions of continuous bridges after the slab has been cast).

In these cases, the plan bracing members and any transverse bracing frames should be

designed to resist a lateral shear force F_Rgiven by Clause 3/9.12.2, in conjunction with wind or other forces. Note, though, that the force F_R is an internal self equilibrating force and should not, therefore, be included in the design loads on the bearings.

The magnitude of the force FRdepends on the stiffness of the restraint provided (subject to an upper limit) magnified by a factor to allow for the proportion of elastic critical buckling load (in lateral torsional buckling) that is being carried.

6.3.3 Design of discrete torsional restraints

Torsional bracing between a pair of beams, without any plan bracing or deck slab (i.e. during construction) does provide restraint against LTB (seeSection 5.4.2).

In such cases, each restraint should be sufficiently stiff and be capable of resisting a pair of equal and opposite forces FR(i.e. a couple), as required by Clause 3/9.12.2. Because these restraints do not provide any lateral restraint, the beams connected by such bracing must be designed to resist lateral forces (such as wind and construction sway loads) by bending in plan at the same time as they carry the vertical bending effects.

6.3.4 Design of discrete intermediate U-frame restraints

Discrete U-frames in conjunction with some form of plan bracing at the level of the cross members provide flexible restraint to a compression flange. [U-frames without plan bracing would have to be considered as torsional restraints without bracing, in accordance with Clause 3/9.12.2(b)].

Page 44 For discrete U-frames, the design forces FRfor the U-frames are given by Clause 3/9.12.3.2, in the same manner as Clause 3/9.12.2 for intermediate restraints, except that the stiffness used in the calculation is that of the U-frame, as defined in Clause 3/9.6.4.1.3.

Where the U-frame cross member is subject to vertical loading (for beam on slab construction this means U-frames adjacent to supports) an additional force F_C must be calculated (Clause 3/9.12.3.3).

During construction, before the slab is cast, transverse beams moment-connected to the main beams are only considered as discrete U-frames when there is plan bracing.

6.3.5 Design of continuous U-frame restraint

Where there is no discrete restraint to the bottom flange adjacent to supports, continuous restraint is effectively provided by the U-frame action of the deck and the girder web. In that case, the web must be checked for the effects of a continuous lateral force. The magnitudes of the force fRand a force fcdue to the bending of the deck slab are given by Clause 3/9.12.4.2.

These forces are determined in a similar way to those for the discrete U-frames.

6.3.6 Intermediate bracing in skew spans

Restraint to the main beams is needed orthogonally to the beam axis. Where this is provided by bracing between beams, this is best achieved by using planes that are square to the main beams, even when the bridge is skew. This will mean that the effective lengths of the beams adjacent to a support are not all the same but this does not raise any problems in principle.

Simple lateral ties to share wind load during construction (such as those shown in Figure 6.2) could be on the skew, linking two staggered planes of bracing, provided that the skew is small.