Last edited 24 Oct 2016

Braced frame structures

Contents

[edit] Introduction

A braced frame is a structural system is commonly used in structures subject to lateral loads. The addition of a bracing frame increases a structure's stability against lateral loads such as wind loading and seismic pressure. The members in a braced frame are generally made of structural steel, which can work effectively both in tension and compression.

The beams and columns that form the frame carry vertical loads, and the bracing system carries the lateral loads. Braced frames reduce lateral displacement, as well as the bending moment in columns, they are economical, easily erected and have the design flexibility to create the strength and stiffness required.

The positioning of braces however, can be problematic as they can interfere with the design of the façade and the position of openings. Buildings adopting high-tech or post-modernist styles have responded to this by expressing bracing as an internal or external design feature.

[edit] Bracing systems

The resistance to horizontal forces is provided by two bracing systems:

[edit] Vertical bracing

Bracing between column lines (in vertical planes) provides load paths for the transference of horizontal forces to ground level. Framed buildings required at least three planes of vertical bracing to brace both directions in plan and to resist torsion about a vertical axis.

[edit] Horizontal bracing

The bracing at each floor level (in horizontal planes) provides load paths for the transference of horizontal forces to the planes of vertical bracing. Horizontal bracing is needed at each floor level, however, the floor system itself may provide sufficient resistance. Roofs may require bracing.

[edit] Types of bracing

[edit] Single diagonals

Singlediagonalbrace.jpg

Trussing, or triangulation, is formed by inserting diagonal structural members into rectangular areas of a structural frame, helping to stabilise the frame. If a single brace is used, it must be sufficiently resistant to tension and compression.

[edit] Cross-bracing

Xbrace.JPG

Cross-bracing (or X-bracing) uses two diagonal members crossing each other. These only need to be resistant to tension, one brace acting to resist sideways forces at a time depending on the direction of loading. As a result, steel cables can also be used for cross-bracing.

However, this provides the least available space within the façade for openings and results in the greatest bending in floor beams.

[edit] K-bracing

Kbrace.jpg

Braces connect to the columns at mid-height. This frame has more flexibility for the provision of openings and results in the least bending in floor beams. K-bracing is generally discouraged in seismic regions because of the potential for column failure if the compression brace buckles.

[edit] V-bracing

V-brace.jpg V-brace(inverted).jpg

This involves two diagonal members extending from the top two corners of a horizontal member and meeting at a centre point at the lower horizontal member, in the shape of a V. Inverted V-bracing (also known as chevron bracing) involves the two members meeting at a centre point on the upper horizontal member.

Both mean that the buckling capacity of the compression brace is likely to be significantly less than the tension yield capacity of the tension brace. This can mean that when the braces reach their resistance capacity, the load must instead be resisted in the bending of the horizontal member.

[edit] Eccentric bracing

Eccentric bracing.jpg

This is commonly used in seismic regions and allows for doorways and corridors in the braced bays. It is similar to V-bracing but instead of the bracing members meeting at a centre point there is space between them at the top connection. Bracing members connect to separate points on the beam or girder. This is so that the 'link' between the bracing members absorbs energy from seismic activity through plastic deformation. Eccentric single diagonals can also be used to brace a frame.

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