STESSA 2012 Santiago, Chile Design Considerations for Braced Frames With Asymmetrical Friction Connections - AFC By J. Chanchi, G.A. MacRae, J.G. Chase, G.W. Rodgers, A. Mora, G.C. Clifton

Objectives  Description of asymmetric friction connections (AFC) and their potential advantages in braced frames.  Description of some AFC configurations.  Description of AFC design parameters.  Explanation of basic considerations to design and model AFC.  Conclusions

Previous Research  Symmetrical friction connections in braced frames by Tremblay (1993) and Grigorian&Popov (1994)  Asymmetrical friction connections in moment resistant steel frames by C.Clifton (2005)  Asymmetrical friction connections in braced frames by Butherworth (1999) and Macrae (2008)

Description of AFC  Basic Geometry: Connection with standard holes in bottom and cap plate and slotted holes in central plate. Shims are placed between plates.  Sliding force depends on size and number of fully- tensioned bolts

Description of AFC  Typical Hysteresis loop for AFC with brass shims and 2 M16 bolts grade 8.8 obtained at University of Canterbury (2011)  No degradation was obtained in tests when brass, aluminium and bisalloy shims were used. On the other hand, steel shims had a poor performance

Advantages of AFC  Long slotted holes can provide a lot of deformation capacity  Sliding force can be controlled easily by changing bolt size and number of bolts.  The seismic energy is dissipated by friction in the AFC.  No damage is expected in the frame and low damage is expected in the connection itself. Only bolts require replacement after a severe earthquake  The cost is similar to that of conventional steel construction.

AFC Configurations  Applications on braced frames: B B AFC cross section B - B Slotted plate AFC detail

AFC Configurations  Applications on braced frames: C - C C CC Slotted gusset AFC detail AFC cross section D - D AFC cross section Slotted bottom flange D D AFC detail

AFC Design Parameters  Slot length must be obtained from maximum expected frame deformation multiply by oversize factor Фs > 1  Sliding force Fs=μn*n*Ntf n: number of shear planes (n=2) Ntf: fully tensioned bolt load, which depends on bolt size and steel grade. It is given in design tables as Proof load. μn: normalized friction coefficient. μ= for steel and brass shims (MacRae 2010) and up to μ=0.26 for brass shims according to tests at UC (2011).

AFC Design Parameters  Recent real scale tests at UC have shown that AFC hysteresis loop for braced frame applications is almost rectangular in contrast with bilinear loop for beam-to- column moment connection (Clifton 2005) and MacRae (2010).

AFC Design and Modelling Special Considerations:  To design the brace and its gusset plates, an overstrength factor must be incorporated when calculating sliding force. This prevent brace failure prior to sliding. The minimum value 0.26/0.21≈1.25 is recommended.  To minimize moment transmission from column-to-beam connection it’s recommended: (i) Axes of brace, beam and column intercept at panel zone centre. (ii) Pinned end brace connections.

AFC Design and Modelling  The AFC must be modelled with a perfectly plastic hysteresis loop.  A non-linear time history analysis must be performed to obtain frame forces and non-desirable residual displacements,as well as maximum lateral displacement and required slot length.

AFC Design and Modelling  Example: Concentrically single braced frame. Push over analysis with a target lateral displacement of 100mm 3.0 m 2.5 m AFC detail 4 M16 Gr.8.8, Brass shims 200UC PFC 200UC59.5 P = variable

Conclusions  AFC can dissipate the energy and structure remains almost undamaged. Only AFC bolts may require replacement.  The AFC cost is similar to that of a traditional fixed connection.  Deformation and force requirements can be achieved by changing the number of bolts, slot length or bolt size.

Conclusions  There are different AFC configurations which can be used on braced frames. Four of them were presented.  The design considerations for AFC and conventional braced frames are similar but an adequate brace detail is required to minimize moment transmission. Also, an overstrength factor must be included when designing.

Questions Questions ?