EXPERIMENTAL TESTING OF PRECAST CONCRETE PANEL CONNECTIONS

B.F. ALLAN, S.K. CHEN, R.S. HENRY, J.M. INGHAM

Department of Civil and Environmental Engineering, University of Auckland

SUMMARY Precast concrete panels are typically connected to one another, and to supporting frames, using either cast-in-place connections, welded embedded steel connections, embedded insert connections, or drilled-in insert connections. A number of failures were observed in connections between precast concrete panels during the Canterbury earthquake sequence, with failure most commonly occurring in bolted connections including panel-to-roof, panel-to-column and panel-to-panel (vertical joint) connections. The failure mechanisms were attributed to insufficient edge distance to the bolts, insufficient bolt anchorage, and insufficient tensile capacity to resist the large out-of-plane forces acting on the panels during the earthquakes. Following the Canterbury earthquakes a critique of these observed failure modes was undertaken and a literature review was initiated to establish past laboratory performance of the relevant connection types. Testing was carried out on bolted connections and the results are presented. INTRODUCTION Precast concrete panels are a widely used form of construction in low-rise industrial and commercial buildings, but there is a lack of reported information on how these panels have performed in previous large earthquakes. The current project arose out of concerns about the performance of precast concrete panels in the recent Canterbury earthquakes, where a number of buildings that incorporated precast concrete panels exhibited unexpected and undesirable failures (Henry and Ingham 2011). Consequently, the purpose of this project was to investigate the likely ultimate capacity of in-service connections when accounting for typically employed edge distances, and to correlate these capacities with anticipated demands arising during a design level earthquake. A review of the seismic performance of connections currently used in precast concrete panel construction was undertaken by classifying connections currently used in the New Zealand concrete construction industry and identifying failure modes that occurred in the Canterbury earthquakes. Experimental testing was then completed with the aim of replicating damage observed in Christchurch. It was hoped that replicating the observed damage would provide insight into the reasons for the damage and therefore highlight areas for potential improvement in the seismic performance of precast concrete panel connections.

REVIEW OF PERFORMANCE IN THE CANTERBURY EARTHQUAKES Earthquakes generate both in-plane and out-of-plane loading on precast concrete panels, and following the Canterbury earthquakes it was concluded that precast panels had generally performed well when resisting in-plane loads, with only a few panels showing some diagonal cracking (Henry and Ingham 2011). However, out-of-plane loading on panels had caused horizontal cracks across the middle of some panels and caused cracking around some connections (Henry and Ingham 2011; Baird et al. 2011). When considering damage to precast concrete panels in a structure having separate structural components to support vertical loads (such as a concrete or steel moment frame), out-of-plane loading is a more significant loading regime than is in-plane loading because out-of-plane failure can cause panels to collapse onto the ground surrounding the building, posing a significant life-safety threat to pedestrians. Fortunately, the out-of-plane collapse of panels was observed in only a few buildings in Christchurch, as reported by Henry and Ingham (2011) and Baird et al. (2011). Nevertheless, the Canterbury earthquakes have highlighted some potential deficiencies in the design and construction of structures utilising precast concrete panels (Henry and Ingham 2011; Baird et al. 2011), hence why the topic was investigated in the current study. Representative damage of panel-to-panel connections There were a number of examples in the 22 February 2011 Christchurch earthquake of failure in precast concrete panels at the panel-to-panel connections (Henry and Ingham 2011). While these failures did not result in the complete collapse of the precast panels, many panels required extensive repair. These failures highlighted an opportunity to undertake research addressing possible improvements in the design of panel-to-panel connections. The most commonly observed panel damage was cracking which occurred between the connection and the edge of a panel (Kam et al. 2010; Henry and Ingham 2011). Potentially, the bolt could tear out of the panel during an earthquake, which could lead to collapse of the panels (Henry and Ingham 2011). Figure 1 shows cracking around a bolted panel-to-panel connection. Possible causes of this damage were that no allowance had been made for differential movement between the two adjacent panels or that insufficient edge distance was provided between the bolt and the edge of the panel. Other forms of observed damage were excessive panel displacements and the crushing of concrete (Henry and Ingham 2011).

Figure 1: Damage to panel-to-panel connection (photo: Jamie Lester)

Damage statistics for precast concrete panels Damage to precast panel connection was investigated for 26 case study buildings to identify the sources of failure. Of the 26 case study buildings, 54% failed due to connection failure (see Figure 2a). These connection failures were sub-divided into four main groups (see Figure 2b): failure due to pounding and compression from adjacent buildings; failure due to cast-in reinforcing connections (such as drossbach connections); failure due to bolted connections; and failure due to poor construction (such as nuts welded directly to the bolt which prevented movement). A significant 63% of the connection failures were bolted connections, being either cast-in anchors or mechanical anchors, with the majority of these bolted connection failures being due to pull-out of the anchor from the concrete (see Figure 2c). In a particular case study, pull-out of the anchor occurred as a result of stripping of the thread or necking of the bolts. Out-of-plane movement and insufficient edge distance were also identified as common failures in bolted connections (see Figure 2c).

(a) Principal reason for failure (b) Type of connection failure

(c) Failure mode for bolted connections

Figure 2: Quantitative review of panel failure modes

REVIEW OF PROCEDURES FOR TESTING CONCRETE ANCHORS Restrepo et al. (1996) undertook research on precast concrete panels and their connections. Commonly used connections were catalogued and classified, and cast-in-place and welded embedded steel connections were tested. Their research project is the most comprehensive testing to have been undertaken in New Zealand to date. Given the large number of failures observed in bolted connections in the Christchurch earthquake (see Figure 2b), it is of interest to the current project that Restrepo et al. (1996) did not test bolted connections. Eligehausen et al. (2011) undertook testing of single anchors to establish if high loading rates were required when analysing the behaviour of concrete anchors subjected to seismic loading conditions. The four anchor types investigated were adhesive anchors, torque-controlled adhesive anchors, sleeve-type expansion anchors, and bolt-type expansion anchors. Test specimens were reinforced concrete slabs constructed of normal-strength concrete, with the reinforcement symmetrically placed on the top and bottom of the slab to allow for uniform crack width throughout the member. Of interest to this project was the testing of the anchors in tension, with four general failure modes recognised as potentially

occurring when subjecting concrete anchors to tensile loading: pullout or pull-through; concrete breakout; splitting; and steel failure. Anchor spacing and a clear edge distance of at least 1.5hef were considered necessary to avoid the results being influenced due to adjacent anchors or edges of the test specimens. From the test results, it was concluded that rapid loading tests were not necessary to qualify anchors for seismic applications. One limitation of this experimental program was that it did not assess the behaviour of anchors as a bolt group. EXPERIMENTAL METHODOLOGY As part of the reported project, research was conducted to identify recorded damage to precast panels in previous earthquakes and the details of any prior testing of connections that had been performed. From this research, recurring forms of observed damage and a lack of prior experimental work were identified. As established in Figure 2, these recurring failure modes were:

Panel failure due to the poor performance of connections

Bolted connections (cast-in anchors and mechanical anchors) commonly failed due to pull-out failure of the anchor in tension, out-of-plane failure of the anchors due to relative movement of the panels, or insufficient edge distance.

Therefore, two types of anchor bolts commonly used in New Zealand for precast concrete connections were tested to replicate damage observed in the Canterbury earthquakes. Four different connection types were tested (see Table 1), with the anchors loaded in tension to determine their strength when subjected to out-of-plane actions. There were two stages to the experimental testing: Stage One testing consisted of anchors installed with unlimited edge distance, as reported in this paper, and Stage Two testing consisted of anchors installed with varying edge distances that were each equal to or less than 150 mm. Stage Two test results will be reported at the conference as the testing had not been completed at the date of paper submission.

Table 1: Tested connection types

Bolt Type Bolt Size

Number of Monotonic

Tests

Number of Cyclic Tests

Cast-in Type A Threaded Insert (Length = 95 mm)

M20 1 3

Cast-in Type B Threaded Insert (Length = 120 mm)

M20 1 3

Single mechanical anchor M16 1 3

Group of four mechanical anchors

M16 1 3

Specimen manufacture and test setup Test panels were designed to simulate realistic precast concrete panel elements, with the configuration of the panel reinforcement (see Figure 3) replicating details used in an earlier experimental study investigating precast concrete components joined to form structural walls (Ma 2000).

(a) Test panel for individual anchors

(b) Test panel for anchor groups Figure 3 : Panel test specimen

Eight concrete cylinders were cast to identify the concrete compressive strength of each panel. Only seven concrete cylinders were used for Stage One tests as the last cylinder was used for Stage Two tests. The target 28-day concrete strength was 40 MPa, but the actual 28-day concrete strength of the cylinder was not measured. Instead, the concrete strength of the cylinders was measured on the day that the anchors were tested. The panels containing the cast-in anchors were constructed and cast one day before the panels having the mechanical anchors. Therefore, cast-in anchor tests were conducted over a period ranging from day 47 to day 51 and mechanical anchor tests were conducted over a period ranging from day 50 to day 53. The concrete strength of each cylinder is listed below in Table 2.

Table 2: Concrete cylinders strength

Cylinder

1 2 3 4 5 6 7 8 *

Panel for: Test Day Concrete Strength (MPa)

Cast-in anchors 47 34 31

51 35 36

Mechanical anchors

50 44

51 40

53 29 **

67 32

* Stage two test ** Concrete strength influenced by cracks formed at the base of the cylinder during casting

The panels were constructed in an Auckland precast concrete factory (see Figure 4), and transported to the laboratory once adequately cured. The panels were then secured to the strong floor using two 100x150 RHS, with a string-pot transducer used to measure the withdrawal displacement of either the mechanical anchor or the threaded rod of a cast-in anchor (see Figure 5). The string-pot was connected to the single anchors via a steel plate which was screwed into the top face of the threaded rod. On the extended end of the steel plate was a hook which the string-pot connected to. For the group anchors, two string-pot transducers were used to take an average displacement reading. These wires were connected to the plate via hooks which were welded directly onto the plate.

(a) Before concrete placement (b) After concrete placement Figure 4 : Panel manufacture at precast yard

Figure 5: Test setup- attachment of the jack and plate to the anchors being tested

Jack

Hold-down clamps

Load cell

String-pot displacement transducer

Loading Protocol Four tests were carried out for each type of connection. For the first test of each connection the load was applied monotonically, until failure, to determine the ultimate strength of the connection. For the last three tests the connection was loaded cyclically to simulate loading and unloading experienced during an earthquake. The loading sequence, shown in Figure 6, involved loading the bolt in sets of cycles. First the bolt was loaded to 50% of the measured ultimate strength from monotonic loading and then unloaded, for three cycles. Second, the bolt was loaded and unloaded three times to 75% of the ultimate capacity, and then loaded and unloaded three times to 100% of the ultimate capacity, before being loaded to failure if the bolt had not already failed.

Figure 6: Loading sequence for cyclic tests

RESULTS Cast-in Inserts Type A and Type B: Monotonic and Cyclic Loading Figure 7 shows the results of tests for cast-in type A anchors. The cracking that was observed for cast-in Type A inserts was similar to the cracking observed for cast-in Type B inserts.

(a) Cracking after monotonic loading (b) Elevation of anchor during cyclic test no. 1 loading

(c) Depth of panel cracking after cyclic test no. 1 loading

Figure 7: Photographs of cast-in Type A anchors during monotonic and cyclic load tests

The hold-down clamps and the face anchor located in the centre of the panel decreased the allowable crack length along the panel for testing of the cast-in Type B inserts. Despite this interaction, the crack pattern observed and the experimental cone of failure (shown in Figure 8) was similar to that observed in Figure 17.1 of NZS 3101:2006 for anchors not limited by edge distance. As shown in Figure 9 and Figure 10, similar cracking patterns were observed in all three of the cyclic tests on the cast-in Type B anchors. Cracks radiated out from the anchor toward the edges of the panel in a star pattern. A cone of failure similar to that described in NZS 3101:2006 was also observed during the experimental tests.

Figure 9: Cracking pattern of cyclic test no. 1 loading of cast-in Type B anchors

(a) Depth of panel cracking after cyclic test no. 2 loading

(b) Panel crack pattern after cyclic test no. 2 and cyclic test no. 3 loading

Figure 10: Photographs of slabs following Type B cyclic load tests

(a) Failure cone (b) Cracking due to the face anchor

Figure 8: Photographs from testing of cast-in Type B inserts

Single Mechanical Anchor: Monotonic and Cyclic Loading Figure 11 shows the concrete panel after testing of the mechanical anchors. A concrete failure cone was observed in this test, and the damage was much more localised when compared with the cracking of cast in Type A and Type B inserts. Figure 12 shows the mechanical anchors as they were being pulled out of the concrete, with all mechanical anchors behaving similarly to each other. A concrete failure cone was observed in all three of the tests and was similar to that described in NZS 3101:2006. It should be noted that prior to cone failure the anchors slipped relative to the hole, before engaging the concrete to form a cone failure.

Figure 11: Localised cracking after monotonic (left) and cyclic loading (right)

(a) Cyclic test no.1 (b) Cyclic test no. 2 (c) Cyclic test no. 3 Figure 12: Similar pull-out behaviour of the single mechanical anchors during each cyclic

load

Group Mechanical Anchor: Monotonic and Cyclic Loading Figure 13 shows testing of a group of four mechanical anchors. As seen in Figure 13a, the monotonically tested group showed interaction between the cracking around adjacent anchors and a typical cone of failure. Individual anchors in the cyclic testing (Figure 13b and c) pulled out of the concrete without forming a typical concrete cone of failure.

(a) Monotonic test (b) Cyclic test no. 1 (c) Cyclic test no. 3

Figure 13: Photographs of the group mechanical anchors during and after loading

DISCUSSION All the connections for both monotonic and cyclic testing, except for Mechanical Group Cyclic 1 (MGC1), failed at loads which greatly exceeded their capacities as specified by their respective product catalogues. The premature failure of MGC1 (at approximately 90% of the manufacturer’s specified capacity) was attributed to the incorrect installation of one of the mechanical anchors. The hole that was drilled for the anchor did not reach the required depth because reinforcing steel was struck in the drilling process. As a result of the shorter hole the anchor had insufficient embedment depth. Due to the presence of reinforcement steel at the base of the hole and the shorter embedment length, the anchor was not able to have the correct torque applied because the anchor spun in the hole. Due to the incorrect installation, this anchor was not able to reach the manufacturer’s specified capacity. This result indicates that when installed correctly, the anchors perform to their design specifications, but that there is a significant decrease in strength when not installed correctly. A comparison of the load resisted by the insert or anchor at failure vs. the capacity of the bolts as specified by the manufacturers is shown in Figure 14.

(a) Failure loads and specified capacities

Anchor Legend:

CA: Cast-in anchor Type A CB: Cast-in anchor Type B MS: Mechanical single anchor MG: Mechanical group anchor /M: Monotonic /C1-C3: Cyclic 1 to Cyclic 3

(b) Normalised failure load/specified capacity

Figure 14: Ratio of load at failure vs. specified capacity

Edge Distances/Demand Strength As shown in the panel design (see Figure 3), two anchors were tested in each panel. For all but one of the panels the load at failure was higher for the first anchor tested than for the second. For the cast in anchors, this result was most likely due to the cracking of the concrete from the first test affecting the second test. The cracking in the first test travelled further than half way down the panel, effectively reducing the edge distance for the second anchor. As a result of this the second anchor in the panel had a lower load at failure than did

the first anchor. For the mechanical anchors the cracking for the first anchor did not reach half way, but a lower load at failure was still observed in the second test, which may have been due to the damage at the other end. The reduction in load at failure for the second test suggests that the edge distance has an effect on the capacity of anchor. This finding potentially explains many of the failures observed in Christchurch, where there was minimal edge distance provided for many of the bolted connections. Using Section 17 of NZS 3101:2006 the capacity of a cast-in anchor can be calculated. Edge distance is taken into account with the modification factor Ψ2.

Ψ2 = 1.0 𝑤ℎ𝑒𝑛 𝐶𝑚𝑖𝑛 ≥ 1.5ℎ𝑒𝑓

(1)

Ψ2 = 0.7 + 0.3𝐶𝑚𝑖𝑛

1.5ℎ𝑒𝑓 𝑤ℎ𝑒𝑛 𝐶𝑚𝑖𝑛 ≤ 1.5ℎ𝑒𝑓

(2)

Where: Cmin = Smallest edge distance hef = effective anchor embedment depth, mm

By using this modification factor the concrete breakout strength of the anchor is reduced when the edge distance is less than 1.5hef. However, the minimum value of Ψ2 is 0.7 which suggests that even when the anchor is at the edge of the concrete the anchor is able to carry load. There are also no limits specified in section 17 of NZS 3101:2006 regarding minimum edge distances. Seismic Load on Panel Using section 8 of NZS 1170.5 the horizontal force on a precast concrete panel can be calculated. Calculations for a representative precast concrete panel indicate that the loads expected on the connections are well below the capacity of the anchors tested. For a representative precast panel the load on a connection was calculated as 8 kN per connection. For a representative anchor in 40 MPa concrete with an edge distance of 100 mm and an embedment depth of 100 mm the concrete breakout capacity is 46 kN, and therefore the capacity of the anchor greatly exceeds the demand on the connection. This result indicates that a correctly installed anchor should be able to withstand the forces experienced in a design level earthquake. The basis for this conclusion can be identified in Figure 14, in which of the 16 tests that were performed, 15 anchors failed at loads which were (on average) 2.1 times their specified design capacities. These findings suggest that the magnitude of the Canterbury earthquake greatly exceeded the design level earthquake of NZS 1170.5:2004, hence pull-out failure was commonly observed and/or that reduced anchor capacity due to edge distance effects is an important factor. This latter topic is investigated in Stage Two of this project. CONCLUSIONS AND RECOMMENDATIONS

1. During the Canterbury earthquakes there were failures of precast concrete panels and their connections.

2. Failures observed in the bolted connections of precast concrete panel were due to pull-out of the anchors in tension, high out-of-plane loads, and insufficient edge distances.

3. The loads at which cast-in anchors and mechanical anchors fail exceed their respective design capacities when installed correctly.

4. The failure of bolted connections during the Canterbury earthquakes were associated with the anchors being subjected to a force with a magnitude significantly greater

than their intended design (resulting in pull-out failure) and also potentially due to insufficient edge distance. This latter topic is considered in Stage Two of this test project.

5. It is recommended that further research into the effects of edge distances be conducted and that if necessary, NZS 3101:2006 section 17 be revised to accommodate the experimental findings.

ACKNOWLEDGEMENTS The authors would like to thank Adam Leach, Jamie Lester, Didier Pettinga, Rod Fulford, John Marshall, Barry Davidson, Nic Brooke, Alessandro Palermo, Andrew Baird, Steve Thomas, Glyn Sykes, Mark Ash, Ronald Lumantarna, Kam Weng Yuen, and Pia Abercromby for their generous assistance in this project. Reid Construction Systems and Ramset are thanked for supplying the anchors used in this study. Stresscrete are thanked for providing construction facilities and the concrete used in this project. The comments contained in this report are those of the authors only and do not necessarily represent the views of those named above. REFERENCES Baird, A., Palermo, A., and Pampanin, S. (2011). Façade damage assessment of multi-storey buildings in the 2011 Christchurch Earthquake. Bulletin of the New Zealand Society for Earthquake Engineering 44(4): 368-376. Eligehausen, R., Hoehler, S. M., and Mahrenholtz, P. (2011). Behaviour of Anchors in Concrete at Seismic-Relevant Loading Rates. ACI Structural Journal 108(2): 238-247. Ellen, P. E. (1961). Tilt-Up Design and Construction. New Zealand Concrete Construction 5(6): 94-101. Henry, R. and Ingham, J. (2011). Behaviour of tilt-up precast concrete buildings during the 2010/2011 Christchurch earthquakes. Structural Concrete 12(4): 234-240. Ma, M. (2000). Methods of joining precast concrete components to form structural walls. ME Thesis, University of Auckland, Auckland, New Zealand NZS 3101:2006 Concrete Structures Standard. Section 17. Standards New Zealand, Wellington, New Zealand NZS 31170.5:2004 Structural Design Actions Part 5 : Earthquake actions New Zealand. Section 8. Standards New Zealand, Wellington, New Zealand. Kam, W. Y., Pampanin, S., Dhakal, R. P., Gavin, H. P., and Roeder, C. (2010). Seismic performance of reinforced concrete buildings in the September 2010 Darfield (Canterbury) earthquake. Bulletin of the New Zealand Society for Earthquake Engineering 43(4): 340-350. Restrepo, J. I., Crisafulli, F. J., & Park, R. (1996). Earthquake resistance of structures : the design and construction of tilt-up reinforced concrete buildings. Christchurch, N.Z. : Dept. of Civil Engineering, University of Canterbury, 1996.