Horizontal bracing is employed to resist horizontal / lateral loads on the structure and distribute them to the outer columns and thereby into the vertical stabilizing system.
Horizontal bracing will also maintain the planar integrity of the structure and prevent it from deforming out of shape.
The bracing configuration works with the horizontal beams to form a horizontal triangulated truss, and like a vertical truss, the main beams, forming the chords of the truss, would effectively be in bending, while the secondary beams would act in tension, the bracing itself would be either in tension or compression depending on how it’s configured.
Whichever way the bracing system is configured it should be located so that it can effectively distribute and transmit the lateral forces to the vertical stabilizing system of the structure.
The bracing members should be set out at an angle of between 30° and 60°, with 45° being optimum.
The most common bracing section is the hot-rolled angle which may be configured as single, battened back-to-back, or battened and starred.
Ideally, angles should ideally be orientated so that the leg points downward to avoid the build-up of debris and moisture. But this is often ignored due to the difficulty of installing the bracing from the underside - it will not affect the integrity of the connection - so it's best to check with the steel erector.
Setting-Out Horizontal Bracing
When setting-out horizontal bracing, the same general recommendations that apply to Vertical Bracing, apply here, that is to say, the bracing members, together with the horizontal beams should ideally intersect at a common ‘node’ or Work Point.
Connecting to a Beam
There are two conditions when connecting horizontal bracing to a beam - the first is a straightforward gusset plate connection as shown in Fig. 1, while the second is when the connection is integrated with an incoming beam Fig. 2.
The main difference as far as the detailer is concerned is that in the first case, the gusset plate will be fixed to the web of the beam, while in the second instance, the gusset plates will be fixed to the incoming beam.
The distance, or 'drop' of the gusset plate (s1) when measured from the top flange will determine the elevation of the bracing relative to the floor steelwork level.
Setting the Vertical Elevation of Horizontal Bracing
Theoretically, the Gusset-Plate should be placed at the center (or ‘centroid’) of the beam because this is usually the default position on structural analysis models, but practically it will never work because of the varying beam depths that need to be accommodated. In practice, the moments generated by the Gusset-Plate offsets are inconsequential and are largely ignored, so it’s usually left up to the detailer to determine the elevation of the plate.
In determining the ‘drop’ (s1) of the Gusset-Plate it’s important to position it so that it will work for all beam sections, sizes, and depths within reason, and the main determining factor is the beam-end connection.
Common practice is to establish the Center of the Gusset-Plate between the first and second row of bolts when measured from the top flange of both the supporting and connecting beams. This will ensure the bracing will not interfere with the beam-end connection and provided the connections have been standardized, will be consistent for the entire structure.
This is a demonstration of the importance of standardizing flexible connections, particularly the vertical bolt-hole pitch, and reaffirms the wisdom of standardizing the vertical bolt-hole pitch across the full range of connection options.
There will naturally be exceptions that may require a purpose-designed solution, but this method will be found to work for the great majority of situations likely to be encountered.
It’s Conventional practice to detail the Gusset-Plate from the Top of the beam flange to the Top of the Gusset-Plate, even though the elevation was determined to the center of the plate.
Connecting to a Column
When connecting to a column things require a little more thought.
Ideally, the work point should be situated at the center of the column which gives, what appears to be a neat compact connection, but the gusset plate can become a bit complicated. The alternative is to set the work points at the centers of the flanges of the column - this will make things a lot easier for the detailer (and the workshop).
This method is generally OK for lightly loaded bracing systems where the eccentricity can be tolerated, but the column must be checked to ensure the offset will not affect the transfer of the horizontal loads to the vertical bracing system. The detailer should always confirm this with the responsible engineer,