8.5.1
Traditional brace dissipaters
Traditional concentric braces dissipate energy by yielding in tension and buckling and yielding in compression. Because of the different strengths in tension and compression, and their susceptibility to low cycle fatigue fracture due to the formation and straightening out of large local curvatures generated by buckling in compression, they are generally not permitted to be major energy dissipating element in tall structures according to worldwide codes. They also need to be designed for significantly greater strength than other systems showing more ductility.
The bracing may be placed in different configurations, such as X, K, inverted V, or diagonal bracing as shown in Figure 8.10. Balanced diagonal bracing is the most common for moderate rise structures because it provides the same strength in both directions.
b) Non-linear lead extrusion damper Web Cap Plate Web Plate Beam Top Flange Plate
Column Continuity Plate
Column
Point of Rotation
(a) X (b) K (c) Inverted V (d) Diagonal (e) Balanced diagonal bracing Figure 8.10. Different bracing configurations for concentrically braced frames.
Frames with balanced diagonal bracing sustain buckling to the braces, but with appropriate bolted connections to the frame, the braces can be replaced after a major earthquake. The practical benefits of this concept were well seen in the 1987 Edgecumbe earthquake, with one major industrial complex storing spare braces for its braced frame main production process buildings and being able to replace damaged braces and restore full structural function within 24 hours of the earthquake.
8.5.2
Buckling restrained braces (BRB)
Buckling restrained braces (BRB) are restrained from buckling by means of a casing, as shown in Chapter 5. The steel is debonded from the casing material so that it can freely slide in the sheath. They can be used in concentrically braced frames as shown in Figure 8.11. The Psychology building at the University of Canterbury was retrofitted using this technology, as illustrated in Chapter 5.
The development path taken for BRB braces and systems in North America and Japan has been to specify an experimental testing regime and to require providers of braces to show compliance with this regime. This has led to the development of a range of proprietary, patented braces. However, the small size of the New Zealand market and distance from Northern Hemisphere brace suppliers means that a generic set of design and detailing requirements is a highly desirable outcome for New Zealand. A research project based around two such systems is currently underway at the University of Auckland with results expected by the end of 2011.
Figure 8.11. Diagonal and inverted V-braced BRB concentrically braced frame structures (AISC, 2007).
8.5.3
Friction braces– SFC– inconcentrically braced structures
Concentric bracing can also be used with friction devices for energy dissipation. These friction devices may be of two sorts as shown in Figure 8.12
(a) Asymmetric friction connection (AFC). (b) Symmetric friction connection (SFC). Figure 8.12. Friction connection types (from Chanchi et al., 2010).
The AFC has a slightly more pinched hysteresis loop than the SFC, when large sliding deformations are considered. This is because the bolts in the AFC must first move on an angle to activate the floating plate (i.e., the bottom plate in Figure 8.12(a). This will generally result in slightly lower permanent displacements.
8.5.4
Friction brace – AFC - in concentrically braced structures
Figures 8.13 shows concentrically braced systems which dissipate energy by means of the AFC brace systems. In Figure 8.13(a), the AFC is within the brace. Because elongated bolt holes can be long, large deformations may occur in the brace. Special care needs to be made with near the AFC area that an out-of-plane bending failure cannot occur. Also, the end connections of to the gusset plates must be detailed to ensure that in-plane bending of the brace does not cause any major problems. In Figure 8.13(b), horizontal sliding occurs in the gusset plate below the beam. In Figure 8.13(c), horizontal sliding occurs in and below the beam bottom flange. Figures 8.13(b) and (c) impose additional bending may be applied to the beam and this should be considered in design.
(a) AFC brace. (b) AFC attachment.
(c) AFC on beam bottom flange (MacRae, 2011).
Figure 8.13. Some AFC brace configurations (MacRae and Clifton, 2010). Plate attached to braces Gusset plate connected to beam with horizontally elongated bolt holes
Floating plate Is behind the gusset plate
Braces
Gusset plate stiffener
Gusset plate . Floating plate (AFC area ) Brace Plate attached to braces Sliding surfaces with shims Floating plate Channel Bolts
The AFC brace systems have the following desirable characteristics:
i) there is no significant damage to the frame (except perhaps to some bolts which may require tightening or replacement),
ii) the system has similar behaviour in both directions of loading,
iii) the hysteretic loop of the AFC together with the elastic response of the moment frame will have a significant post-elastic stiffness which encourages re-centring of the structure after an earthquake, and
iv) the technology developed does not require patents for use.
8.5.5
HF2V dissipaters in concentrically braced structures
HF2V dissipaters may be used in braced frames as shown in Figure 8.14. Here, brace buckling issues need to be addressed as well.
Figure 8.14. Braced frame with HF2V device.
8.5.6
Self-centring braces in concentrically braced structures
Innovative braces have been developed by Christopoulos et al. (2008). These have a flag- shaped hysteresis similar to that in Figure 8.3(c). This results in energy dissipated and no permanent displacement at the end of an earthquake. These desirable characteristics do come at a cost.
