How bracing plans ensure the structural fitness of a building is shown in the video.

Being economical to construct and simple to analyse, braced frames are a very common form of construction.The economy comes from inexpensive pinned connections between beams and columns.A concrete 'core' or diagonal steel members may be used for bracing.If the bracing system carries all the loads, beams and columns are designed under vertical load.

In a multi-storey building, the beams and columns are arranged in a pattern in both elevation and plan.The resistance to horizontal forces is provided by two bracing systems.

Three vertical planes of bracing are needed to provide resistance in both directions and to give resistance to the vertical axis.The locations shown in the figure below are examples of where more than three are usually provided.

If the floors act as diaphragms to provide horizontal bracing, the forces carried by each plane of vertical bracing depend on its relative rigidity and location, and on the centre of pressure of the horizontal forces.

The figure below shows a multi-storey building with vertical bracing in the form of diagonal steel members.

A reinforced concrete core can provide stability to a building.

It is better to place bracing at or near the extremities of the structure in order to resist the effects of torsional effects.There is a figure on the right.

It is necessary to assume that the horizontal forces are shared equally between the bracing systems in the same direction under consideration.

Equal sharing of forces should not be assumed where the bracing systems are located asymmetrically.If the floor is a stiff beam and the bracing systems are spring supports, the forces carried by each bracing system can be calculated.

The strength of each bracing system should be calculated using horizontal forces.The distribution of force to each bracing system can be calculated using the spring stiffness.

In a braced multi-storey building, the planes of vertical bracing are usually provided by diagonal bracing between two lines of columns, as shown in the figure below.Either single diagonals are provided, in which case they must be designed for either compression or tension, and either slender bracing members carrying only tension may be provided.

When crossed diagonals are used, it is assumed that the floor beams participate as part of the bracing system, since they are in compression.

A Frame stability design tool is also available, as well as guidance on the determination of equivalent horizontal forces and the consideration of second order effects.

The appropriate combinations of actions must be determined for the individual members of the bracing system.The combination of wind load and the leading action are likely to be the most difficult for bracing members.

bracing members that are inclined at 45 are recommended.This arrangement provides an efficient system with relatively modest member forces compared to other arrangements, and means that the connection details where the bracing meets the beam/column junctions are compact.The structure's sway will be increased by narrow bracing systems with steeply inclined internal members.More stable structures will result from wide bracing systems.

The table below shows how much of a difference a constant size of bracing cross section makes.

To transfer horizontal forces from the perimeter columns to the planes of vertical bracing, a horizontal bracing system is needed at each floor level.

Without additional steel bracing, the floor system will suffice to act as a diaphragm.If there is no diaphragm at the top of the columns, bracing may be required at roof level.There is a figure on the right.

All floor solutions involving permanent formwork such as metal decking fixed by through-deck stud welding to the beams have an excellent rigid diaphragm to carry horizontal forces.

If the floor systems are to act as a diaphragm, proper consideration needs to be given to the transfer of forces.If the steel is painted, the coefficient of friction may be as low as 0.1.This will allow the slabs to move relative to each other.For large shears, a more positive tying system will be required between the slabs and the steelwork in order to partially overcome this problem.

reinforcement in the topping may be used to connect slabs.Ties may be placed along both ends of a set of planks to make sure the whole floor is a single diaphragm.A 10mm bar at half depth will be satisfactory.

The capacity of the connection should be checked if plan diaphragm forces are transferred to the steelwork via direct bearing.Local crushing of the plank limits the capacity.The gap between the plank and steel should be made good with in-situ concrete.

Special measures are needed to provide an adequate diaphragm without the use of timber floors and floors constructed from concreted inverted tee beams.

A horizontal system of triangulated steel bracing is recommended where the floor can't be relied upon.Each direction may need a horizontal bracing system.

The horizontal bracing systems span between the supports, which are the locations of the vertical bracing.As shown in the figure on the left, this arrangement leads to a truss stretching the full width of the building, with a depth equal to the bay centers.

The floor bracing is usually arranged in a way that it acts only in tension.

Adequate allowances need to be included in the structural analysis to cover the effects of imperfections, such as lack of verticality and straightness, as well as any minor eccentricities present in joints of the unloaded structure.

Global imperfections can be taken into account by modelling the frame out of plumb or by a series of horizontal forces applied to a frame.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 don't influence global behavior and are taken into account whenverifying member resistances in accordance with the design Standard.Local imperfections may need to be allowed if moment-resisting connections are assumed in the frame design.

The initial sway imperfection allows for the effect of frame imperfections.There is a figure on the right.

There is an out-of-verticality 0 of 1/200 that is allowed for.For actual values exceeding specified limits and for residual effects such as lack of fit, this allowance is greater than normally specified tolerances.The design allowance is given in BS EN 1993-1-1.

According to the Eurocode, the number of columns in a row affects the reduction factor for the overall height.See 5.3.19 for a detailed definition.This assumes that every row has bracing.The number of columns in the bracing system should be used to calculate m.

Regardless of the height and number of columns, the value of may be taken as 1/200.

The sway imperfections may be neglected if the horizontal force exceeds 15% of the total vertical force.

Vertical sway imperfections may be replaced by systems of equivalent horizontal forces.It is easier to use equivalent horizontal forces than it is to introduce the geometric flaw into the model.This is true.

The design value of the horizontal forces is NEd at the top and bottom of each column, where the forces are in opposite directions.It is easier to consider the net equivalent force at each floor level when designing the frame and bracing system.The horizontal force should be equal to the total vertical design force applied at the floor level.

The bracing system has to carry the loads.The bracing needs to be checked for two design situations which are local to the floor level.

The bracing system is checked locally for the combination of force due to external loads together with the forces caused by either of the above flaws.The horizontal forces that are modeled to sway for the frame are not included in the combinations.At a time, only one flaw needs to be considered.

The horizontal forces are the forces that accumulate at the level being considered.

The UK checks these forces without beam shears.The design probability of the design code is beyond the justification.

In the analysis of bracing systems which are required to provide 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.

The stabilizing force shown in the figure right can be used to replace the initial bow imperfections of the members that are restrained by a bracing system.

The in-plane deflection of the bracing system is determined by q and any external loads calculated from first order analysis.

If the deformations significantly increase the forces in the structure, then the effects of the deformed geometry need to be considered.If cr is less than 10, the second order effects are significant.

For each combination of actions considered, the criterion should be applied separately.As shown in the diagram, this will include both horizontal and vertical loads.The bracing is the only thing that contributes to the stability of the frame.

amplification of an elastic first order analysis using the initial geometry of the structure is the most common method used for second order effects.The use of this method is subject to limitations.Second order analysis must be used if cr is less than 3.

In a braced frame, where the beam to column connections are not pinned, the only effects to be amplified are the forces in the bracing members and columns that are due to their function.