PERFORMANCE OF PRECAST CONCRETE CLADDING SYSTEMS UNDER IN-PLANE CYCLIC LOADING

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1 PERFORMANCE OF PRECAST CONCRETE CLADDING SYSTEMS UNDER IN-PLANE CYCLIC LOADING A BAIRD, A PALERMO, S PAMPANIN Department of Civil and Natural Resources Engineering, University of Canterbury SUMMARY In order to investigate the seismic performance of precast concrete cladding an experimental programme has been undertaken by performing in-plane, quasi-static cyclic tests of a moment resisting frame sub-assembly clad with a precast concrete panel. The contribution of the cladding to the strength and stiffness of the frame was monitored along with the performance of the cladding system through increasing drift levels. Various tie-back rod and slotted cladding connections were tested along with innovative dissipative connections to ascertain the effect they had upon the claddings performance and interaction with the frame. INTRODUCTION The performance of precast concrete claddings in earthquakes is invariably mixed. Many systems appeared to have survived the recent Canterbury earthquakes showing only minor cosmetic damage or no damage at all. Other systems show much greater damage with a small number failing completely (Baird et al., 2011). Even when no damage or failure was explicitly visible, later inspection often found that the cladding connections had been damaged to some degree. One inspection revealed that the cladding connections had actually failed and the panels were only still remaining on the structure due to being wedged between the beams of the structure. Shown in Figure 1 are some examples of damage to precast concrete cladding from the 22 nd February 2011 earthquake. Figure 1. Examples of damage to precast concrete cladding and its connections from 22 nd February 2011 earthquake (Baird et al., 2011) Explaining the reason for this difference in performance is critical for three reasons; firstly, to assess the residual capacity of damaged systems; secondly, to identify vulnerable systems that require increased strength, ductility or redundancy and finally, in the design of new cladding systems. In order to better understand the performance of precast concrete cladding systems, an experimental programme has been undertaken at the University of Canterbury using a fullscale test of a precast concrete panel attached to a concrete moment resisting frame. The

2 frame utilises Precast Seismic Structural System (PRESSS) beam-column connections which allows the frame to be tested repeatedly to high drift levels without sustaining damage. The frame is clad with a single precast concrete panel with a central opening. Several typical connection types and configurations are tested by performing in-plane, quasi-static cyclic tests of the system. As well as traditional connection types, an innovative connection type aimed at maximising dissipation of the frame-cladding system is tested. BACKGROUND Recent studies on the interaction of cladding panels with the primary structure have outlined how cladding panels can influence a structure s behaviour (Hunt & Stojadinovic, 2010, McMullin et al., 2004, Baird et al., 2011). The seismic performance of a cladding system is commonly determined using the inter-storey deflection (or drift) of the structure. The link between quantitative and qualitative seismic performance is achieved through the definition of the following performance levels: Operational, Immediate Occupancy, Life Safety and High Hazard (FEMA 356, 2000). Graphic illustrations of these performance levels can be seen in Fig Following the magnitude 6.3 earthquake that struck Christchurch on 22 February 2011, a damage assessment survey of facade systems was conducted (Baird et al., 2011). The survey included buildings within the Christchurch Central Business District greater than three stories in height, comprising a total of 371 facade systems. The survey was based on what is visible from outside the building, making it equivalent to a Level 1, or rapid safety assessment (ATC-20, 1989). The survey rated the performance of the facade systems using the performance levels shown in Figure 2. The facade performance composition from the survey is also shown in Figure 2. Operational Immediate Occupancy Life Safety High Hazard % 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Figure 2. Graphical representation of cladding performance levels (top) and facade performance composition following Christchurch 22/02/2011 earthquake (Baird et al., 2011) The survey classified the facade systems by eleven individual typologies based on those used in the Post-earthquake Building Performance Assessment Form (ATC-38, 2000). One of those categories used was heavy cladding (precast concrete or stone panels). Although 90% of heavy claddings were deemed to be either Operational or Immediate Occupancy (minor cracking and damage to panels), cases of complete disconnection of heavy claddings raised serious concern. The disconnection of several precast concrete spandrels resulted in the death of a woman sitting in her car on the street below (CERC, 2012). Because of the high risk that falling heavy claddings presents, further attention to these systems is required. CLADDING SEISMIC PERFORMANCE Capacity design (hierarchy of strength) principles can be used to assess the seismic performance of most cladding system. Assuming that the system is comprised of a structural frame member, a connector body and cladding panel, linked together with strong, stiff attachments, as shown in Figure 3, then the system can be simplified by focussing on the weakest in the system. For most cladding panel systems the weakest (and least stiff) element in the system is the connector body. The connector body is usually required to accommodate relative movement between the cladding panel and the frame as well as

3 provide out of plane restraint. For systems that incorporate glazing, the connector body is typically strong and rigid and relative interstorey movement has to be accommodated within the cladding itself. This is usually achieved by use of gaps around the glass and within the glazing frame (called seismic frame). It is usually assumed that the attachment of the connector body is stronger than both the cladding and the connector body itself. For this investigation, precast concrete panels are tested and as such the performance and failure mechanism is expected to be governed primarily by the connector body. Three types of connector body types are to be tested, tieback, slotted and dissipative, as shown in Figure 3 (right). Structural Framing Member Reinforced concrete or structural steel spandrel beam or column Attachment Between frame and connector body. Remains elastic and very stiff Connector Body Designed to remain elastic OR allow movement OR become inelastic to dissipate energy Attachment Between cladding and connector body. Remains elastic and very stiff Cladding Panel Designed to remain elastic and typically very stiff Connector Body Type Tie-Back (traditional) Slotted/ Sliding/ Rotating (traditional) Dissipative (innovative) Characteristics Strength Stiffness Deform easily under lateral forces Disconnect the panel by allowing degree of freedom in one or more directions Dissipate energy in connector body under lateral forces Medium Low High Medium Low High Figure 3. Cladding system composition (left) (Pinelli et al., 1993), typical connector body types (right) EXPERIMENTAL SETUP In order to assess the seismic response of multi-storey buildings with claddings, a full-scale, single-bay, single storey frame subassembly has been constructed. The frame represents a portion of a reinforced concrete moment resisting frame. The beam and column members were individually cast with steel plates at the end of each member. The beam-column connections utilise Precast Seismic Structural System (PRESSS) technology which allow the frame to be tested repeatedly to high drift levels with different claddings without sustaining significant structural damage (Priestley et al., 1999). The frame is pinned at both column bases, with one pinned base being mounted on sliders to allow for frame elongation during loading. A hydraulic jack is attached with pin connections to the top of the west column and the steel reaction frame, as shown in Figure 4. Macalloy post-tensioning 1030 bars (f pu =1030 MPa) are used to tie the beam and columns together to provide the frame s moment capacity. The beams sit on steel corbels bolted into the column face s using cast in anchors. The frame members have a 28 day concrete strength of 50 MPa. The frame is subjected to increasing levels of drift using a quasi-static cyclic loading protocol in order to assess its seismic response (ACI 374.1R-05, 2005).

4 Hydraulic Jack Steel Reaction Frame Post Tensioning Bars R/C Beams : 400 x 450 R/C Columns: 400 x Figure 4. Experimental test setup (dimensions in mm) The precast concrete panel is 3.8 x 3.0 m in size, with a central opening of 2.0 x 1.6 m. The panel is 120 mm thick and reinforced with D12 s at 200 mm centres and D16 bars around external and internal edges. 1.0 m length D16 s also run diagonally at each corner opening. The panel has a 28 day concrete strength of 40 MPa. The precast concrete panel is attached to the beams using two different connections. One of these connection types is the bearing connection, as show in Figure 5. The bearing connections carry the gravity load of the panel back to the frame. The connections are metal angles securely bolted into place using anchors cast into the panel and frame and are located at the base of the panel. The bearing connection is not able to provide movement and acts as a fixed connection between the panel and the frame. The other connection type is a flexible, or movement connection, as shown in Figure 5. The movement connections resist out-of-plane forces due to wind and earthquake loading. They must also be able to accommodate in-plane relative movement between the frame and the cladding panel during earthquake induced movement. Since the panel is very stiff in the inplane direction and is fixed securely at its base, the top connections must be able to accommodate in-plane relative movement between the frame and the cladding panel during earthquake induced movement. A photograph of the test setup with the panel attached is shown in Figure 5. As mentioned previously, for this investigation, the performance and failure mechanism of the movement connection is expected to govern the cladding behaviour. Therefore, the testing involves varying the movement connection characteristics and the same panel is used for all tests. The testing sequence was such that the connections which risked damage to the panel were tested last so the panel remained undamaged for most of the testing. Two typical movement allowance connections types are tested; threaded rod tie-backs (commonly referred to as push-pull connections in the USA) and slotted connections. The number, size and length of tie-back rods are varied in order to assess the influence they have on the cladding behaviour. The slot length of the slotted connections is also varied. The threaded rod connection type (MP-TR1) is shown in Figure 6 (left) and the slotted connection type (MP-SL2) is shown in Figure 6 (centre).

5 Figure 5. Photograph of test-setup (left) and cross section of cladding system (right) Also tested is an advanced connection aimed at providing dissipation. The connection consists of a U-shaped Flexural Plate (UFP) housed inside a square hollow section (SHS). As the frame moves relative to the panel, the UFP yields and dissipates energy. UFP dissipators have been used as a method of dissipation between coupling walls (Kelly et al. 1972). Their application as a cladding connection is new, however they are believed a relatively cheap and simple solution for adding extra dissipation to a structure. For this testing, each UFP connection was manufactured at a cost of $37.50; however economies of scale would certainly bring this price down. The UFP connections are able to be designed to provide a constant yielding force to large displacements. In this case, each UFP was designed to allow a maximum stroke of 300 mm (3.7% drift) and a yielding force of 15 kn. Shown in Figure 6 (right) is a photograph of a UFP attached to the panel and beam. The UFP connections are tested with and without the SHS housing, however it was found necessary to have the SHS to provide necessary out-of-plane restraint. Figure 6. Long threaded tie-back connection (left), slotted connection (centre) and u-shaped flexural plate connection (right) Shown in Table 1 is a summary of the tests conducted and the different movement connections used. Three tests were undertaken for each connection.

6 Table 1. Summary of different movement connections tested Test ID Movement Connection Type Size Length No. of Tests MP-TR1 Threaded Rod 20 mm x mm Rod 3 MP-TR2 Threaded Rod 12 mm x mm Rod 3 MP-TR3 Threaded Rod 20 mm x 2 85 mm Rod 3 MP-TR4 Threaded Rod 20 mm x 4 85 mm Rod 3 MP-SL1 Slotted 20 mm 150 mm Slot 3 MP-SL2 Slotted 20 mm 300 mm Slot 3 MP-UFP1 U-Shaped Flexural Plate 2 UFPs 300 mm Slot 3 MP-UFP2 U-Shaped Flexural Plate 4 UFPs 300 mm Slot 3 RESULTS The first tests were of the connections believed to pose the least risk of damaging the panel; the long threaded rod tie-backs and the slotted connections. Tests of the threaded rod tieback connections (275 mm long) exhibited a brittle failure mechanism due to low cycle fatigue during the 2.0% drift cycle. The slotted connections were able to go all the way to 3.5% drift, however for comparative purposes the test results have been shown in Figure 7 going to 1.5% drift. The cladding system that is tested is plotted in red and the bare-frame behaviour (without the cladding) is shown in blue. Figure 7. Force-drift behaviour of long threaded rod (left) and slotted connection (right) cladding systems (red) compared with the bare-frame behaviour (blue) It can be seen that the long threaded rod connections and the slotted connections have a small effect on both the strength and stiffness of the frame. In the long threaded rod, resistance is provided by bending of the rod and in the slotted connection resistance is provided by friction. The slotted connection is bolted with a spacer through the slot so the resistance of the connection is due to the friction from the weight of the panel wanting to fall out-of-plane. Even though the resistance provided by the connections is minimal (around 15 kn max) it can be shown that when a building is covered in cladding with connections of this type, the strength and stiffness provided can have a considerable effect upon the strength and stiffness of the building (Baird et al. 2012). Shown in Figure 8 (left) is the force behaviour of the frame-cladding system when the threaded rods are shorter (85 mm) so their ability to deform to higher displacements is limited. The force carried in the system is much greater and strength degradation is evident. Even though it was expected that the shorter threaded rods (85 mm) would fail earlier than

7 the longer rods (285 mm), this was not the case. The short threaded rod connections also fail during the 2.0% drift cycle due to low cycle fatigue. The failure always occurs at the interface of the nut, where the bending moment is largest. Shown in Figure 12 (right) are the failed threaded rod connections after being tested. The UFP connection behaviour, shown in Figure 8 (right) also transfer more force into the cladding than the long threaded rod or slotted connections, however it can be seen that there is no strength or stiffness degradation. Figure 8. Force-drift behaviour of short threaded rod (left) and UFP connection (right) cladding systems (red) compared with the bare-frame behaviour (blue) In order to compare the force being transferred through the cladding, the difference in force is found between the cladding-frame system and the bare frame and plotted for varying drift levels. The force carried in the cladding of the four tests above is shown in Figure 9. Figure 9. Force being transferred through the cladding for different connection types It can be seen that the force in the cladding for the long threaded rod and slotted connections is around kn and is reasonably constant. The force carried in the UFP connections is larger at around kn and also reasonably constant. It can be seen that in the short threaded rod connections, as greater drifts are reached the force being carried through the cladding becomes significant. It can also be seen that in the second and third cycles at each drift that the stiffness is lower than in the first cycle. This is due to damage occurring in the cladding panel. The short threaded rods were the last connection type to be tested and up until this point the cladding panel remained completely undamaged. Cracks begin developing from around 0.35% drift around the window corners, as shown in Figure 10 (right). These cracks continue to grow through greater displacements. Numerous other cracks form in the panel and the final crack pattern can be seen in Figure 10 (left).

8 trut easurements Figure 10. Crack pattern following testing of short threaded rod connections (left) and closeup of development of corner crack (right) Rotary potentiometers were used to measure the diagonal strut displacement of the panel. The strut displacement gives a good measurement of the amount of deformation and consequently the likely amount of damage occurring in the panel. Shown in Figure 11 are the strut displacements for increasing levels of drift of the four tests introduced previously. It can be seen that the strut displacement is virtually nil in both the long threaded rod and slotted connection tests. The strut displacement in the UFP connection test is no greater than 1.5 mm. No damage was observed to the panel so this is presumed to be within the elastic range of the panel. The level of strut displacement in the short threaded rod test is an order of magnitude greater than in the UFP test and explains the evidence of cracking throughout the panel shown in Figure 10. Figure 11. Strut displacement for different connection types The development and expansion of cracks in the panel is evidently able to be linked to the strut displacement. By plotting the force in the panel (found from the difference between the test case and the bare frame) against the strut displacement we can see how the panel deforms in shear due to increasing drift, as shown in Figure 12 (left). Positive strut displacements correspond to a lengthening of the strut and negative displacements correspond to a shortening. The plot shows that larger tensile displacements develop in the panel. The hysteretic behaviour in this direction is also evident.

9 Figure 12. Relationship between the force being transferred through the cladding and strut displacement (left) and short threaded rod failure (right) CONCLUSIONS The aim to better understand the seismic performance of precast cladding systems was explored using an experimental programme testing various cladding connections. As shown by the tests which utilise threaded rod and slotted connections, even typical cladding connections provide some strength and stiffness to the structure. The effect of this is ignored in the design and analysis of the structure as it is believed that the cladding does not interact with the structure. Another approach may be to accept that the cladding is going to have an influence on the structure s behaviour during an earthquake and employ connections which are designed to not only accommodate the necessary design forces and movement allowances, but also dissipate some of the energy of the earthquake, reducing demands on the structure overall. U-shaped flexural plates show the potential to be a suitable device to fulfil this purpose. The detailing of connections for precast cladding is critical in assuring the cladding remains undamaged and attached to the structure following an earthquake event. Connections that provide only limited movement allowance, e.g. short threaded rod connections, induce large strut actions within the cladding which results in damage to the cladding panel. Care also needs to be taken in assuring connections have adequate ductility as shown by the brittle failure mechanism of threaded rod connections. The loading protocol likely has a biased effect upon the estimation of damage to the threaded rods due to low cycle fatigue; however the brittle failure mechanism of these connections is of particular concern. The authors intend to investigate further numerically the potential losses due to cladding connections damage when various connections are used as well as using finite element software to better understand residual capacity of damaged connections. REFERENCES ACI 374.1R-05 (2005). Acceptance Criteria for Moment Frames Based on Structural Testing and Commentary. American Concrete Institute ATC-20. (1989). Procedures for Postearthquake Safety Evaluation of Buildings & Addendum. Applied Technology Council.

10 ATC-38. (2000). Database on the Performance of Structures near Strong-Motion Recordings: 1994 Northridge, California, Earthquake. 2000, Applied Technology Council. Baird, A., Palermo, A. and Pampanin, S. (2012) Understanding Cladding Damage: An Experimental and Numerical Investigation into a Christchurch Earthquake Case Study. NZSEE Conference, April, Christchurch, New Zealand Baird, A., Palermo, A. and Pampanin, S. (2011) Facade Damage Assessment of Multi-storey Buildings in the 2011 Christchurch Earthquake. Bulletin of the New Zealand Society for Earthquake Engineering 44(4): CERC (2012). Canterbury Earthquakes Royal Commission 43 Lichfield Street. (accessed 12 Apr 2012) FEMA 356. (2000). Prestandard and Commentary for the Seismic Rehabilitation of Buildings. Federal Emergency Management Agency Hunt, J.P. & Stojadinovic, B. (2010). Seismic performance assessment and probabilistic repair cost analysis of precast concrete cladding systems for multi-storey buildings, PEER Report, University of California Kelly, J.M., Skinner, R. I. & Heine, A. J. (1972). Mechanisms for energy absorption in special devices for use in earthquake resistant structures. Bulletin of NZSEE, 5(3) McMullin, K., Wong, Y., Choi, C. & Chan K. (2004). Seismic performance thresholds of precast concrete cladding connections, 13 th World Conference on Earthquake Engineering, Vancouver, Canada, Pinelli, J.P., C., Craig, J.I., Goodno, B.J., and Hsu, C.C. (1993), Passive Control of Building Response Using Energy Dissipating Cladding Connections, Earthquake Spectra, 9(3), pp Priestley, N., S. Sritharan, J. Conley and S. Pampanin (1999). Preliminary Results and Conclusions From the PRESSS Five-Story Precast Concrete Test Building, PCI Journal, Precast/Prestressed Concrete Institute, 44(6)