How does structural bracing work

Appropriate allowances need to be incorporated in the structural analysis to cover the effects of imperfections, including geometrical imperfections such as lack of verticality, lack of straightness, lack of flatness, lack of fit and any minor eccentricities present in joints of the unloaded structure.

Global imperfections may be taken into account by modelling the frame out of plumb, or by a series of equivalent horizontal forces applied to a frame modelled vertically. The latter approach is recommended. In a braced frame with nominally pinned connections , no allowance is needed in the global analysis for local imperfections in members because they do not influence the global behaviour and are taken into account in when verifying member resistances in accordance with the design Standard.

Should moment-resisting connections be assumed in the frame design, local imperfections may need to be allowed for BS EN [1] , 5. The effect of frame imperfections is allowed for by means of an initial sway imperfection. This allowance is greater than the normally specified tolerances because it allows both for actual values exceeding specified limits and for residual effects such as lack of fit.

The design allowance in BS EN [1] , 5. For a detailed definition, see 5. This presumes that every row has bracing. BS EN [1] , 5. It is much easier to use equivalent horizontal forces than to introduce the geometric imperfection into the model. This is because:. According to 5. When designing the frame, and specifically the forces on the bracing system, it is much easier to consider the net equivalent force at each floor level.

The bracing system must carry the externally applied loads, together with the equivalent horizontal forces. In addition, the bracing must be checked for two further design situations which are local to the floor level:. In both these design situations, the bracing system is checked locally considering the storeys above and below for the combination of the force due to external loads together with the forces due to either of the above imperfections. The equivalent horizontal forces modelled to account for frame sway are not included in either of these combinations.

Only one imperfection needs to be considered at a time. The horizontal forces to be considered are the accumulation of all the forces at the level being considered, divided amongst the bracing systems. It is normal practice in the UK to check these forces without co-existent beam shears. The justification is that the probability of maximum beam shear plus maximum imperfections together with minimum connection resistance is beyond the design probability of the design code. In the analysis of bracing systems which are required to provide lateral stability within the length of beams or compression members, the effects of imperfections should be included by means of an equivalent geometric imperfection of the members to be restrained, in the form of an initial bow imperfection:.

For convenience, the effects of the initial bow imperfections of the members to be restrained by a bracing system may be replaced by the equivalent stabilizing force as shown in the figure right. The effects of the deformed geometry of the structure second order effects need to be considered if the deformations significantly increase the forces in the structure or if the deformations significantly modify structural behaviour.

The criterion should be applied separately for each storey, for each combination of actions considered. Typically, this will include vertical and horizontal loads and EHF, as shown in the diagram. In braced frames, lateral stability is provided only by the bracing; the nominally pinned joints make no contribution to the stability of the frame. Where second order effects are significant and need to be included, the most common method used is by amplification of an elastic first order analysis using the initial geometry of the structure.

In a braced frame, where the beam to column connections are nominally pinned and thus do not contribute to lateral stiffness, the only effects to be amplified are the axial forces in the bracing members and the forces in columns that are due to their function as part of the bracing system. The amplification factor is given in BS EN [1] , 5. Only the effects due to the horizontal forces including the equivalent horizontal forces need to be amplified.

If another metal piece is attached between two of the frame's bars, less wiggling will occur. Yet, for the sturdiest solution, a second piece of metal attached and crossed to form an X is needed.

This cross bracing effect reinforces any type of frame when the braces are added at sufficient places in the frame. Since crossed X patterns are diagonals in shape, these braces are known as a diagonal type of brace.

Anyone who has ever assembled furniture sold flat packed with diagonal braces included probably has a good idea of how strong cross bracing can make otherwise rickety structures.

Backs of bookcases and entertainment units are usually cross braced to give them strength to hold heavy loads. Braces of this style on bottoms of chairs or tables keep seats and tabletops from wobbling. In flooring installations, cross bracing is done in between floor joists to prevent them from moving around. Even large structures such as ships and buildings need the structural reinforcement achieved by this type of bracing. The lateral forces can be determined using equation 6.

Add to this the direct forces on the bracing caused by lateral loads, for example by wind loading. If plane frame or grillage models have been used to determine the bracing stiffnesses they could also be loaded with wind load on the windward face to determine the distribution of forces from wind loading. Bracing which remains in the structure permanently will also be affected by traffic loads and other variable actions, even if it is only required for temporary loads.

To determine the effect from live loads two options are available. The easiest option is to extract the worst case distortions from the global grillage analysis and impose these results onto a plane frame local model of the intermediate bracing.

However this is very conservative and it may be difficult to achieve a satisfactory design using this method. Alternatively, the actual bracing can be input into a comprehensive 3-D model of the structure. The latter method has the advantage that the loading on the bracing will be less than for the first method, the disadvantage is that a 3-D model takes longer to set up.

When designing the bracing members, do not forget that bracing members are generally slender and members that are subject to compression should be checked for buckling resistance. Support bracing has a different set of loads to resist. These are forces due to non-verticality of webs at the support, due to distortion introduced at skew supports, due to eccentricity of bearing reactions, and due to imperfection in alignment of compression flanges of the main girders.

The method of PD [4] clause 10 can be used to determine all four of the above effects. The equations are used to determine a force F S. The most complicated part of the force F S4 is only applicable to skew supports. These forces are applied at the level of beam flanges to produce a torque. The forces are applied to a maximum of two beams, so for the case of a multi-girder bridge a variety of load cases may need to be considered.

The above figure indicates that the support bracing should be designed for the wet concrete condition. This is often critical, but the design should also consider the finished condition.

In the finished structure the loads will be greater but the loads will be shared between the concrete deck and the steel bracing, so this condition may be less critical. There are many types of bracing arrangement possible. As noted previously the general preference is for torsional bracing rather than lateral bracing. For torsional bracing in multi-girder bridges , the K type bracing is usually preferred, rather than X bracing for tall main beams, but if main girders are shallow, relative to spacing, channel bracing would be better.

For ladder deck bridges , the bracing will be formed by transverse beams. A constant depth transverse beam is preferred, and if possible knee braces should be avoided. In skew bridges it is best to keep intermediate bracing normal to the main beams.

However if skew exceeds this value it is best to keep support bracing normal to the main beams and double up the support bracing as shown below. More detailed guidance on arrangement of bracing is given in a separate article on skew bridges.

Most bracing is required only for the "wet concrete" construction condition. Once the concrete has hardened, the bracing is redundant. Bracing may even be a nuisance in the finished condition because it can attract large effects due to traffic loads and it can be difficult to make the bracing work.

The question is therefore why not remove the bracing? In general it is considered best to leave bracing in place. Although the weight of bracing is not much as a proportion of the overall tonnage, it is likely to be too heavy for manual handling and it can be quite difficult to manoeuvre the bracing out from under a completed bridge deck.

The bracing may have taken up load and it may not be easy to remove bolts. Also leaving the bracing in place means that should the bridge need to be demolished in the future, the bracing could be used to stabilise the steelwork while the deck is broken out.

Bracing is almost always connected with bolts rather than welds. This allows the bracing to be easily assembled on site although in many cases beams are delivered to site already braced in pairs ready for lifting. Slip resistant connections are normally used. Guidance on slip resistant connections generally is given in a separate article on connections for bridge steelwork , and Guidance Note 2.

Navigation menu Home. Share Tweet. Tools Printable version. From SteelConstruction. Typical bracing of a multi-girder bridge. Typical bracing over one span of a multi-girder bridge slab not shown. Plan bracing systems. Structural action of U frame bracing. Beam on springs analogy.

Loading a plane frame to determine plan bracing stiffness. Loading a grillage to determine torsional bracing stiffness.

U-frame stiffness. Application of F S forces to support bracing. Bracing arrangements in skew bridges. Actions on structures.