Cast-in Ferrule Connections Load/Displacement Characteristics in Shear

Transcription

1 Cast-in Ferrule Connections Load/Displacement Characteristics in Shear Ian Ferrier 1 and Andrew Barraclough 2 1 Product Manager - Connections, ITW Construction Systems ANZ. 2 Research and Development Manager, ITW Construction Systems ANZ. Abstract: Cast-in Ferrule connections are a very common, cost effective and versatile method of bolting steel members to concrete. The performance of ferrules cast into concrete and loaded in tension is well understood; however their performance in shear is more complex and not widely understood by engineers or published by suppliers. At first glance, ferrules appear to provide a rigid shear connection to a steel plate, comparable to a simple bolted connection between two steel plates. In fact the behaviour in shear is far from that, with the interaction between the steel ferrule and the concrete substrate as well as the bending applied to the bolt, giving these connections unique performance characteristics. Ferrule connections are actually capable of undergoing significant amounts of displacement when loaded in shear before ultimate failure is reached. This phenomenon can have advantages in many applications such as where load sharing is advantageous between individual anchorages in an array. The test program undertaken includes a total of 12 separate tests conducted in one batch of 40 MPa nominal compressive strength concrete, (3 samples of each of 4 configurations), encompassing ferrules, set at two different embedment depths, 2 different fixture plates and 2 bolt classes. This paper investigates and explains the effects that the 4 variables have on the load displacement performance of the connection loaded in shear and how this varies from a plane shear bolted connection. Keywords: Ferrule, ductility, bolted, connections, shear, 1. Introduction Cast-in ferrules are placed into position in the moulds of precast concrete panels prior to the placement of the concrete, screeding and finishing. They can be located either on the near or far face of the panel depending on the connection required. They can be held in place on the near side by chairs that sit on the casting bed and hold the ferrule at the right height to match the panel thickness, or simply puddled in after the concrete is poured. The ferrule thread is protected by the use of an antennae cap which pushes into the ferrule thread to prevent the ingress of slurry and also has small antennae that protrude from the surface and enable the ferrule to be located below the surface, generally 3-5 mm. On the far side they can be held with a variety of plates that either adhere to the bed or are held in place magnetically. Either of these options produces a recess at the surface of the concrete and the ferrule is located at the base of this recess, typically 10mm deep. 2. Experimental Program 2.1 Test Specimens A typical strength concrete used in the Australian precast industry was used throughout all series of the tests; being a maximum of 20 mm course aggregate, a typical 0.4 water/cement ratio, and nominal grade 40 MPa design strength, and 32 MPa at 3 days, supplied by a commercial ready-mix company. Nominal slump was 80 mm +/-5 mm. The tests were scheduled to be conducted 28 days from pouring and within a 24 hour period, and targeted mean compressive strength of 40 MPa. Concrete compressive strength, f c, was recorded by means of cylinder compression tests, prepared in accordance with AS (Methods of testing concrete - Determination of the compressive strength). 3 cylinder compressive

2 strengths were recorded for the batch of concrete at the end of the full test. The mean of these cylinder compressive strengths was calculated, f cm, and noted in Table Test # 1 Shear, Class 8.8 bolt, ferrule 10 mm below surface, 22 mm diameter hole in 10 mm fixture 20 mm x 95 mm footed ferrule (Reid part No. FE20095), set 10 mm below the surface of the concrete using a nailing plate. 10 mm fixture plate with 22 mm clearance hole. M20 x 60 Class 8.8 bolt with 30mm engagement (1.5 x nom dia.) in the ferrule. Flat washers under the head of the bolt to achieve the correct engagement. 240 Nm tightening torque. Fixture held away from direction of load to be applied during tightening. Figure 1. Class 8.8 bolt, ferrule 10mm below surface Test # 2 Shear, Class 4.6 bolt, ferrule 10 mm below surface, 22 mm diameter hole in 10 mm fixture As per Test #1, except; M20 x 60 Class 4.6 bolt with 30 mm engagement (1.5 x nom dia.) in the ferrule. 144 Nm tightening torque. Figure 2. Class 4.6 bolt, ferrule 10 mm below surface

3 2.1.3 Test # 3 Shear, Class 8.8 bolt, ferrule flush with surface, 22 mm diameter hole in 10 mm fixture As per Test #1, except, ferrule flush with the surface. Figure 3. Class 8.8 bolt, ferrule flush with surface Test # 4 Shear, Class 8.8 bolt, Ferrule 10 mm below the surface, oversize hole (40mm dia.) in 10 mm fixture with welded 10 mm thick cover washer. As per Test # 1, except; 10mm fixture plate with 40 mm clearance hole with 10 mm thick washer with 22 mm clearance hole welded over top. Figure 4. Shear, Class 8.8 bolt, ferrule 10 mm below surface, oversize hole in fixture plate 2.2 Test Setup The test method employed to establish the ultimate shear capacities and associated displacement characteristic was to place them in direct shear, parallel with the face of the concrete element as would normally be experienced in application. The shear capacity will be affected by variable parameters, such as, concrete type, moment couple effects and stress distribution between the anchoring configurations. The ferrules were tested under shear loading using the below fixture, where the fixture was clamped to the surface of the concrete as the bolt was tightened into the ferrule cast into the concrete. During tightening, the fixture plate was held in the opposite direction, away from the direction of the load to be applied, to ensure all tests started from the same displacement datum. This fixture was designed to ensure that the load is applied parallel to and as close as possible to the surface of the concrete. Ferrule shear tests (12 off) were conducted at concrete compressive strengths

4 typical of what may be considered the upper limit of strengths to be expected in fully cured precast concrete panels. For a control test, 3 tests were conducted by a NATA laboratory to test the shear load displacement performance of a M20 Class 8.8 set-screw and nut used to connect two 20 mm thick plates and loaded putting the set screws in in shear. The results are summarised in Table 2. (a) Concrete shear test setup Figure 5. Test setups (b) Steel shear test setup Figure 6. Concrete shear test rig Figure 7. Concrete shear test setup detail

5 3. Results 3.1 Load/Displacement Data Test # Peak Load (kn) Peak (mm) Table 1. Ferrule Shear Results Mean Peak Load, (kn) Min Peak Load (kn) Mean Peak (mm) Comments Ferrule/concrete end of bolt Fixture anchorage failed prior to peak Fixture anchorage failed prior to peak Bolt shear failure Bolt shear failure Bolt shear failure Ferrule/concrete end of bolt Ferrule/concrete end of bolt Ferrule/concrete end of bolt Ferrule/concrete end of bolt Ferrule/concrete end of bolt insufficient bolt engagement (20mm) Table 2. Steel Shear Results Steel shear Steel shear Steel shear Table 3. Concrete Compressive strength results Concrete compressive strength f c Sample 1 f c Sample 2 f c Sample 3 f cm MPa

6 3.2 Failure images & load/displacement graphs (a) Gr8.8 bolt, ferrule 10 mm below surface, 22 mm fixture hole (b) load/displacement graph 1 Figure 8, Test #1 Class 8.8 bolt, ferrule 10mm below surface, 22 mm diameter hole in 10 mm fixture (b) Gr 4.6 bolt ferrule 10 mm below surface, 22 mm fixture hole (b) load/displacement graph 2 Figure 9, Test #2, Class 4.6 bolt, ferrule 10 mm below surface, 22 mm diameter hole in 10 mm fixture

7 (a): Gr8.8 bolt, ferrule flush with surface, 22 mm fixture hole (b) load/displacement graph 3 (note: plots 2&3 abandoned prior to ultimate). Figure 10, Test #3, Class 8.8 bolt, ferrule flush with surface, 22 mm diameter hole in 10 mm fixture. (a) Gr8.8 bolt, ferrule 10 mm below surface, 40 mm fixture hole (b) load/displacement graph 4 Figure 11. Test #4, Class 8.8 bolt, ferrule 10 mm below the surface, oversize hole (40 mm dia) in 10 mm fixture with welded 10 mm thick cover washer. Figure 12. Test # 4, ferrule failure due to insufficient bolt engagement

8 (a): M20 Gr8.8 bolt shear between 20 mm steel plates (b) load/displacement graph 5 Figure 13. Test 5, M20 Class 8.8 set screw shear tested between 20 mm thick Grade 250 plates with 22 mm holes. 4.0 Discussion The concrete compressive strength for this test was significantly higher than the 32 MPa that the test was anticipated to be conducted in. At the intended strength, the concrete provides less support to the ferrule as it is able to crumble in the compressive zone in front of the ferrule. This test therefore represents the shear performance at the upper limit of concrete compressive strengths that precast concrete panels may be when fully cured, not the normal or lower strengths where performance may be more critical. The failures seen in tests 1, 3 & 4 where a Class 8.8 bolt was used, the ferrule failed in bending, adjacent to the end of the bolt inside. In the test 2, the bolt failed in shear level with the top of the ferrule. Relative to test 1 (Class 8.8 bolt, ferrule 10 mm below surface and 22 mm clearance hole in fixture), the following was observed: 4.1 Ferrule position When the ferrule was cast flush with the surface and the 30 mm engagement between the Class 8.8 bolt and ferrule was maintained, a minimum 12% decrease in shear performance was observed compared to when the ferrules are cast 10 mm below the surface. 4.2 Bolt Class Referring to AS4100, Steel structures: 1998, section , the nominal shear capacity of a bolted steel connection can be determined as follows;

9 Figure 14. Extract from AS4100, Steel Structures:1998 For a M20 Class 4.6 bolt with one shear plane through the threaded area, V f = 60.7 kn. As mentioned above, the failure mode with a Class 4.6 bolt was close to this type of failure as the bolt failed in shear at the top of the ferrule. 4.3 Oversize holes in fixture plate and large washer welded over top The arrangement of test # 4 (Class 8.8 bolt, Ferrule 10 mm below the surface, oversize hole (40 mm dia) in 10 mm fixture with welded 10 mm thick cover washer ) exposes the bolt into significantly more bending moment, due to the increased distance from the failure zone of the ferrule. Examining the photo of the failed fixing, the bolt exhibits some bending; the only bending exhibited in the Class 8.8 bolts. In comparison with test 1, we observed a 28-29% reduction in shear capacity. 4.4 Bolt engagement in ferrule An error made in setting up one of the test # 4 (Class 8.8 bolt, Ferrule 10 mm below the surface, oversize hole (40 mm dia.) in 10 mm fixture with welded 10 mm thick cover washer) trials, resulted in only 20 mm of engagement between bolt and ferrule. This test failed at approximately 7% lower load than the other two tested with 30 mm of engagement. 4.5 Displacement at ultimate between 4 variants Referring to the mean displacements at peak load recorded in Table 1 & 2, there is little difference between the results for the 4 tests with the ferrules cast in concrete. Comparing these results from test 5 (M20 Class 8.8 set screw shear tested between 20 mm thick grade 250 plates with 22 mm holes), a smaller displacement at ultimate was recorded for the ferrule tests in general. 4.6 Comparison with bolt put into shear between two grade 250, 20 mm thick plates, Referring to the results for test 5, (M20 Class 8.8 set screw shear tested between 20 mm thick grade 250 plates with 22 mm holes), the results show a marginal (4%) increase over the Test 1.1 ultimate load (140 kn for test 1.1 verses 145kN mean ). The nominal shear capacity for a M20 Gr8.8 bolted connection with one shear plane through the threaded area, V f = 126 kn.

10 5. Conclusion It was unfortunate that the test setup failed twice in test 1, leaving us with only one result extending to an ultimate capacity of 140 kn at a displacement of 6.2 mm. The two results effected (130 & 134 kn) had displacements of only 3.8 & 4.3 mm respectively when the fixture anchorage failed, much lower than the displacement at ultimate for the 140 kn result. We could expect then that these two results would have gone on to similar ultimate loads had the test setup not failed. Relative to the most common of the ferrule connections, test 1 (Class 8.8 bolt, ferrule 10 mm below surface and a 22 mm clearance hole in 10mm fixture), in the relatively high strength concrete used for this test: When the ferrule is located flush with the surface of the concrete, the shear capacity is reduced by a minimum of 12% compared to when ferrule is located 10 mm below the surface provided the same 1½ diameters of engagement between the bolt and the ferrule is maintained. A 46% decrease in shear performance was recorded when Class 4.6 bolts were used as an alternative to Class 8.8. It is pertinent to note that no increase in displacement at ultimate load was recorded for the Class 4.6 bolts, suggesting that the use of Class 4.6 bolts does not offer increased ductility in shear over Class 8.8 bolts. A 34 % decrease in shear performance was recorded when the fixture has oversize holes and a large washer welded in place after installation. This is a common practice used to allow easier on site assembly. A significant loss of shear capacity occurs when insufficient engagement between bolt and ferrule is used. No discernable difference was observed between the four ferrule tests presented in this paper in regard to the displacement recorded at ultimate shear load. However all the ferule connections exhibited less displacement at ultimate shear load compared to the bolted steel to steel shear connection. 6. Acknowledgement We thank ITW Construction Systems for their financial, and Research and Development & testing support, and their on-going assistance to further engineering knowledge. 7. References 1. Standards Australia AS4100, Steel structures, Sydney Australia, Standards Australia AS , Methods of testing concrete - Determination of the compressive strength, Sydney Australia, Standards Australia, 1999.