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Behaviour of grouted sleeve connections at elevated temperatures
ARTICLE IN PRESS
Thin-Walled Structures 44 (2006) 751–758 www.elsevier.com/locate/tws
Behaviour of grouted sleeve connections at elevated temperatures X.L. Zhaoa,, J. Ghojelb, P. Grundya, L.H. Hanc a
Department of Civil Engineering, Monash University, Clayton, VIC 3168, Australia Department of Mechanical Engineering, Monash University, Caulfield, VIC 3145, Australia c Department of Civil Engineering, Tsinghua University, Beijing, PR China
b
Received 8 October 2005; received in revised form 18 July 2006; accepted 21 July 2006 Available online 25 September 2006
Abstract Tubular steel jointing system that incorporates prestressed grout sleeve connections has superior strength under both static and dynamic loading. This paper reports an investigation into the effect of elevated temperatures on the load carrying capacity of such connections. Eleven specimens were heated in a furnace and the load was applied through an Instron machine. Three different grout lengths were chosen. The load deflection behaviour at different temperatures was compared. It was found that the ultimate load reduces almost linearly as temperature difference (TD) between outer and inner tubes increases. It is encouraging to observe the ductile behaviour of grouted connections at elevated temperature. Thermal analysis was also conducted to predict the temperature field in the connection. r 2006 Elsevier Ltd. All rights reserved. Keywords: Grouted connections; Steel tubes; Ductility; Elevated temperature; Thermal analysis
1. Introduction Tubular steel jointing system that incorporates grouted sleeve connections (see Fig. 1) rather than conventional bolted or welded connections has been an innovation in the construction of tubular structures [1–7]. The connection transmits axial force by shear on the steel and interface. The traditional method to enhance shear transfer was using shear keys in the form of welded ribs or weld beads. An innovative method was developed at Monash University to enhance the shear transfer by deliberately introducing radial prestress between the circular sleeve and the through member. The radial prestress is developed using a cement additive that expands the grout after setting. It offers enormous benefits in fabrication and construction through minimising detailed welding and low on-site technology required for erection. A series of research have been carried out at Monash University on tubular connections with prestressed grout [8]. Research performed so far includes the behaviour of Corresponding author. Tel.: +61 3 9905 4971/4953; fax: +61 3 9905 4944. E-mail address: [email protected] (X.L. Zhao).
0263-8231/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tws.2006.07.002
such connection under static, fatigue and high amplitude dynamic loading [9–15]. The outcome has been the development of construction methodologies and design procedures that significantly enhance load capacity and reduce cost. In all the tests the Calcium-Sulpho-Aluminate compound (CSA) was used as an expanding agent. There is an increasing application of this kind of innovative connection in both offshore and on-land structures, such as: (1) for construction of offshore pile foundations and jacket platforms; (2) for construction of multi-storey buildings, space frames, grain silos, airport hangers, mine processing facilities and stadia; (3) for retrofitting and repairing tubular members in trusses and frames, column splices and (4) for strengthening or repairing pipelines. However, very little is known about the behaviour of the jointing system (see Fig. 1) at elevated temperatures. The grout prestress may be affected by high temperature. This is because: (1) disintegration of grout at elevated temperature; (2) different expansion rate of the steel tubes and the cement grout and (3) under gradient temperature confinement provided by steel tubes can be reduced leading to loss of radial prestress due to thermal expansion and possible creep. The connection may fail suddenly with the
ARTICLE IN PRESS X.L. Zhao et al. / Thin-Walled Structures 44 (2006) 751–758
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thickness of the outer tube to Pmax ultimate load PmaxDT ultimate load obtained at elevated temperatures PmaxAmbient ultimate load obtained at ambient temperature T temperature DT temperature difference r density
Nomenclature cp Di Do k L tgrout ti
specific heat diameter of the inner tube diameter of the outer tube thermal conductivity grout length thickness of grout thickness of the inner tube
2.2. Test specimens
loss of prestresses especially when the connection is in tension. This issue must be addressed to ensure the safety of various structures using this innovative connection. This paper describes an investigation into the effect of elevated temperatures on the load carrying capacity of prestressed grout tubular connections. Only half sleeve connection (see Fig. 1(a)) is studied. Eleven specimens with three different grout lengths were tested. The specimens were heated in a furnace and the load was applied through an Instron machine. The load deflection behaviour at different temperatures is compared to study the effect of temperature on the ultimate capacity and ductility of grouted connections. The temperatures were measured using thermocouples. Thermal analysis was also conducted using thermal analysis system (TAS) program to predict the temperature field in the connections.
The basic dimensions of a specimen are shown in Fig. 2 where Do ¼ 165.1 mm, to ¼ 5.0 mm, Di ¼ 114.3 mm, ti ¼ 5.4 mm and tgrout ¼ 20.4 mm.The surface treatment for the outer sleeve is shot blasted whereas the surface treatment for the inner sleeve is mill scale. Thermocouples (OM, IM, P1, P2 and P3) were mounted on the specimens as shown in Fig. 2. Table 2 shows specimen label, grout length and temperature difference (DT in 1C) between locations OM and IM in Fig. 2. The lengths chosen are shorter than would be used in conventional design. Sufficient length in conventional design would be chosen to ensure that failure occurs first in the member by yielding rather than in the connection by slipping. Shorter lengths were chosen because the study focussed on reduction in connection strength.
2. Material properties and test specimens 2.1. Steel tubes and grout
3. Test set up
Mild steel tubes were used as outer and inner sleeves. The size of the outer sleeve is 165.1 mm in diameter and 5 mm in thickness. The inner sleeve had a diameter of 114.3 mm and a thickness of 5.4 mm. One tensile coupon was taken from the outer and inner tubes. The tensile coupons were prepared and tested according to the Australian Standard AS1391 [16]. The measured yield stress is 290 MPa for the outer sleeve and 313 MPa for the inner sleeve. Based on these data the calculated yield capacity of the outer tube (OM) is 729 kN, and of the inner tube (IM) is 578 kN. A grout mix design by weight is shown in Table 1.
to
Grout
to
tgrout
Do
Di
L
Table 1 Grout mix design by weight
Grout
Do
ti (a)
Tests for prestressed grout sleeve connections at different temperatures were conducted to determine the load carrying capacity of the connections. The critical load case
(b)
Portland cement
CSA
Water
Superplasticiser
0.9
0.1
0.36
0.5 L at 100 kg mix
tgrout
to
Di
ti
Grout
tgrout
Do
Di
ti L
(c)
L
Fig. 1. Grouted sleeve connections: (a) half sleeve connection; (b) inner sleeve connection and (c) outer sleeve connection.
ARTICLE IN PRESS X.L. Zhao et al. / Thin-Walled Structures 44 (2006) 751–758
(i.e. tension) was applied. The part of the specimen containing the grout sleeve connection was put inside a furnace. The specimen was shielded from the heating panels by a stainless steel pipe (250 mm in diameter, 5 mm in thickness) to avoid any short circuit. The specimens were tested horizontally with the tension load applied through an Instron testing machine. A general view of the test set up is shown in Fig. 3. 4. Test results The temperature distribution along the grout length in a specimen was measured using thermocouples shown in Fig. 2. Details of temperature measurements were in accordance with AS1530.4 [17]. A soaking test was conducted for a specimen with a grout length of 350 mm. The soaking furnace temperature is given in Fig. 4(a). The temperature versus time curves are shown in Fig. 4(b) for each thermocouples. The upper three curves are for locations P3, OM and P2. The middle two curves are for locations IM and P1. The temperature difference (TD) between OM and IM is given in Fig. 4(b) as the lowest curve. The furnace temperature was raised to a certain degree within a certain time as shown in the third column of Table 2. It can be seen from the soaking test that the TD between OM and IM increases as time increases. When the TD to
P3
OM
Grout P2
753
reached a certain value (see column 4 of Table 2), load was applied to the specimen until the axial deformation reached around 20 mm. Load versus deflection curves are shown in Figs. 5(a)–(c), for each grout length. The maximum load obtained in the test (Pmax) is listed in Table 2. 5. Effect of temperature on load carrying capacity The non-dimensional ultimate load capacity (PmaxTD/ PmaxAmbient) is plotted in Fig. 6 against the TD between the outer and inner tubes. It can be seen that the ultimate load capacity reduces almost linearly as the TD increases. The average shear bond stress (defined as Pmax TD =p2 Di L) is plotted in Fig. 7 against the TD between the outer and inner tubes. The trend appears to have little sensitivity to grout length. It should be noted that the percentage of CSA used in the tests described was quite low, only 10%. It may be expected that a significant higher prestress and therefore shear bond strength could be obtained with higher percentages of CSA in the grout mix. The trends shown in Figs. 6 and 7 are based on very limited tests described in this paper. More tests are needed to include a wide range of parameters such as grout thickness and heating rate.
tgrout
P1 IM Do
Di
ti L Fig. 2. Grouted sleeve connection.
Fig. 3. Test set up.
Table 2 Specimen label, temperature difference and loading capacities Specimen label
Grout length (mm)
Furnace temperature
Temperature difference (DT) (1C)
Pmax (kN)
Average shear bond stress (MPa)
L1TD0 L1TD10 L1TD25 L2TD0 L2TD8 L2TD15 L2TD30 L2TD40 L3TD0 L3TD11 L3TD35
L1 ¼ 160 L1 ¼ 160 L1 ¼ 160 L2 ¼ 250 L2 ¼ 250 L2 ¼ 250 L2 ¼ 250 L2 ¼ 250 L3 ¼ 350 L3 ¼ 350 L3 ¼ 350
Ambient Reach 250 1C Reach 500 1C Ambient Reach 250 1C Reach 400 1C Reach 500 1C Reach 500 1C Ambient Reach 250 1C Reach 500 1C
0 10 25 0 8 15 30 40 0 11 35
234 162 158 286 238 276 157 126 462 368 259
4.07 2.82 2.75 3.18 2.64 3.06 1.74 1.40 3.68 2.93 2.06
in 60 min in 20 min in in in in
30 min 30 min 20 min 20 min
in 30 min in 20 min
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Thermal conductivity [18]:
Furnace Temperature (degrees)
600
k ¼ 54 3:33 102 T W=m C;
500
The material properties of grout are not readily available. The properties of dry Portland cement given in Bazant and Kaplan [19] are used in this paper as an approximation:
400 300
Density r ¼ 1500 kg=m3 . 200
Specific heat (reproduced as Fig. 8 of this paper).
100
L3=350 mm
Thermal conductivity of oven-dried hardened cement pastes vary within a relatively narrow range of 0.45–0.75 W/m 1C [19]. A constant average value of k ¼ 0.6 W/m 1C is used in the current model. The material properties of air are:
0 0
500
1000
1500
Time (seconds)
(a)
Density r ¼ 1:2 kg=m3 .
450 P3
400
Specific heat cp ¼ 1005 J/kg 1C at 27 1C [20].
P2 Temperature (degrees)
Tp800 C:
350 300
Thermal conductivity k ¼ a þ bT þ cT 2 þ dT 3 þ eT 4 W/m 1C [20], where a ¼ 0.0265, b ¼ 6.19 105, c ¼ 8.55 109, d ¼ 3.93 1012 and e ¼ 1.40 1015.
OM
250
OM
P2
200
IM
P3
P1
Delta T
150
P1
IM
Delta T
100 50 0 0
10000
(b)
20000
30000
Time (seconds)
Fig. 4. Results of soaking test (L3 ¼ 350 mm): (a) soaking furnace temperature; (b) temperature versus time curves (soaking test).
6. Thermal analysis Thermal analysis is described in this section to predict the temperature field in the connections. This will form a basis for predicting the load carrying capacity of grouted connections at elevated temperatures. 6.1. Material properties The material properties used for steel are: Density r ¼ 7830 kg/m3. Specific heat in J/kg 1C [18]: cp ¼ 425 þ 7:73 101 T 1:69 103 T 2 þ 2:22 106 T 3 , 20 CpTp600 C:
6.2. Thermal modelling Modelling is conducted in TAS by Harvard Thermal (Harvard, Massachusetts, USA). TAS is a general-purpose software package which is based on the finite difference method for solving steady-state and transient complex thermal problems quickly and efficiently. The software can handle non-linear cases involving fluid flow, temperaturedependant properties and radiation. Fig. 9(a) shows the model of the entire system assembled. The furnace is assumed to be a quadrilateral volume made up from thin plate elements with the 250 mm diameter cylindrical shield protruding from both ends. The specimen consists of two concentric tubes with the gap between them filled with a special grout. It is centrally located inside the 250 mm diameter cylinder. Two heating elements are mounted on each side of the furnace and provide the energy to heat the specimen. Air, as a constituent component of the specimen, fills the tubes, cylindrical shield and the furnace. Heat transfer inside the furnace and the shield is by radiation and conduction whereas convection is assumed to be negligible. Convection is applied only to the parts that are exposed to the surrounding atmosphere. The ensuing radiation heat exchange is quite complex involving 1144 radiating surfaces in the model and 169,648 surface-to-surface radiation rays. As a result, more time is needed for the software to calculate the shape (view) factors for the radiation exchange than to solve the thermal problem. The experimental process was simulated by first heating the specimen for 1200 s at high heat input to bring the temperature of the furnace to 500 1C. After that a
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250 200
L1TD10
L1TD0
L1TD25
150 L1TD10
100 L1TD25
50
L2TD0
250
L2TD40
L2TD15 L2TD30
L2TD0 L2TD8
300 Axial Load (kN)
Axial Load (kN)
L1TD0
755
L2TD8 L2TD15
200 150 100 L2TD40
50
L2TD30
0
0 0
(a)
10 20 Axial Deflection (mm)
Axial Load (kN)
500
0
30
L3TD0 L3TD11 L3TD35
400
10
20
30
Axial Deflection (mm)
(b)
L3TD0
300 L3TD11
200 L3TD35
100 0 0
10
20
30
Axial Deflection (mm)
(c)
Fig. 5. Load versus deflection curves for each grout length: (a) grout length L1 of 160 mm; (b) grout length L2 of 250 mm and (c) grout length L3 of 350 mm.
5.0
1.2
1
L1=160mm
L3=350mm
L2=250mm
Trendline
Average Shear Bond Stress (MPa)
PmaxTD/PmaxAmbient
4.0
0.8
0.6 L1=160 mm 0.4
L2=250 mm L3=350 mm
0.2 trendline
3.0
2.0
1.0
0 0
10
20
30
40
50
Temperature Difference (degrees) 0.0 Fig. 6. Ultimate capacity versus temperature difference between outer and inner tubes.
0
10
20
30
40
50
Temperature Difference (degrees)
‘‘thermostat’’ was applied to the model to maintain a temperature range of 490–500 1C for the duration of the run (25,000 s). Figs. 9(a)–(c) show the temperature profile of the entire model, inner and outer sleeve assembly and the grout, respectively at 4500 s. These figures indicate that
Fig. 7. Average shear bond stress versus temperature difference between outer and inner tubes.
the central section of the specimen is the most heated area of the specimen and that the temperatures in the tubes and grout vary radially and axially.
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6.3. Comparison with measured temperatures
7. Conclusions
Figs. 10(a–e) present the comparison between the measured and predicted temperatures for locations OM, IM, P1, P2 and P3. The comparisons are only made within the time period of 5000 s because the TD between the OM and the IM reached the targeted value in less than 5000 s. There is a general agreement between measured and predicted temperatures.
The following observations and conclusions are made based on the limited test results and analysis.
6000
Cp, J/kg K
5000 4000
1. Temperature distribution in the grout connection was obtained. The temperature difference (TD) between the outer and inner tubes increases rapidly in the first 30 min. 2. The ultimate load carrying capacity reduces almost linearly as the TD between the outer and inner tubes increases. 3. It is encouraging to note the ductile behaviour of grout connections at elevated temperatures. 4. The predicted temperature agreed well with measured results.
3000 2000
8. Future research
1000 0 0
200
400 600 Temperature, °C
800
1000
Fig. 8. Specific heat of idealized Portland cement paste with a water/ cement ratio of 0.5 [19].
It is encouraging to note that the grouted sleeve connection behaves in a ductile manner at elevated temperatures although the ultimate load carrying capacity reduces almost linearly as the TD between the outer and inner tubes increases. Future research may include:
Fig. 9. Temperature profiles at 4500 s: (a) furnace and tube assembly; (b) inner tube and outer tube assembly and (c) grout.
ARTICLE IN PRESS 250
250
200
200
Temperature (°C)
Temperature (°C)
X.L. Zhao et al. / Thin-Walled Structures 44 (2006) 751–758
150 Measured 100
Predicted
150 Predicted 100 Measured 50
50
0
0 0
1000
2000
3000
4000
5000
0
Time (seconds)
(a) 250
250
200
200
Predicted
150 100 50
1000
(b)
Temperature (°C)
Temperature (°C)
757
4000
5000
4000
5000
Measured
150 100 Predicted
50
Measured
2000 3000 Time (seconds)
0
0 0
1000
(c)
2000
3000
4000
5000
0
Time (seconds)
1000
2000
3000
Time (seconds)
(d)
250
Temperature (°C)
200
Measured
150 100
Predicted
50 0 0
(e)
1000
2000
3000
4000
5000
Time (seconds)
Fig. 10. Comparison of measured and predicted temperatures: (a) OM location; (b) IM location; (c) P1 location; (d) P2 location and (e) P3 location.
(a) Study of all three types of connections shown in Fig. 1 with more variables such as grout thickness, tube diameters. (b) Measurement of grout material properties. (c) Prediction of load carrying capacities at elevated temperatures. (d) Fire resistance of such connections subjected to standard ISO834 fire curve.
Acknowledgements The authors wish to thank the Australian Research Council for financial support. Steel tubes were supplied by OneSteel Market Mills, Australia. Tests were performed in
the Civil Engineering Laboratory at Monash University. Thanks are given to Mr. Graeme Rundle and Mr. Roger Doulis for their assistance. References [1] Tebbett IE. The reappraisal of steel jacket structures allowing for the composite action of grouted piles. In: Proceedings of offshore technology conference, Paper OTC 4149, Texas, USA, 1982. [2] Tebbett IE, Billington CJ. Recent development in the design of grouted connections. In: Proceedings of offshore technology conference, OCT Paper 4890, Houston, USA, 1985. [3] Department of Energy, UK. The strength of grouted pile/sleeve connections: Phase I, Final Report—Static Tests. Report no. ST22/ 80, April 1980. [4] Elnashia AS, Carroll BC, Dowling PJ, Billington CJ. Full scale testing and analysis of prestressed grouted pile/platform connections.
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[5]
[6]
[7] [8]
[9]
[10]
[11]
X.L. Zhao et al. / Thin-Walled Structures 44 (2006) 751–758 Proceedings of offshore technology conference, OTC Paper 5325, Houston, 1986. Wimpey Laboratory Ltd. Brown and Root UK Limited—BP Forties Field Phase I, Report on grout bond tests for large diameter piles. Report BM/112, September 1973. Yamasaki T, Hara M, Takahashi C. Static and dynamic tests on cement grouted pipe-to-pipe connections. Proceedings of offshore technology conference, OTC Paper 3790, Houston, 1980. Foo J. Prestressed grouted tubular connections. PhD thesis. Clayton, Australia: Monash University; 1991. Grundy P. Prestressed grouted sleeve tubular joints. In: Proceedings of Australasian structural engineering conference, Auckland, 1998. p. 815–20. Foo J, Grundy P. Performance of prestressed grouted tubular connections under dynamic loading. In: Proceedings of first pacific offshore mechanics symposium, PACOM’90, vol. III, Seoul, Korea, 1990. p. 33–42. Grundy P, Foo J. Prestress enhancement of grouted pile/sleeve connections. In: Proceedings of first international society of offshore and polar engineering conference, vol. IV, Edinburgh, UK, 1991. p. 130–6. Grundy P, Foo J. Performance of flattened tube connections. In: Proceedings of fourth international symposium on tubular structures, Delft, The Netherlands, 1991. p. 251–8.
[12] Grundy P, Ure A, Kim K. Shear transfer in a grouted cluster pile/ sleeve connection. In: Proceedings of second international society of offshore and polar engineering conference, San Francisco, USA, 1992. [13] Eimanis A, Grundy P. Load capacity of innovative tubular X-joints. In: Proceedings of fifth international symposium on tubular structures, Nottingham, UK; 1993. p. 485–94. [14] Grundy P. Grouted sleeve tubular splices. In: Proceedings of fifth international society of offshore and polar engineering conference, vol. IV, The Hague, 1995. p. 116–20. [15] Zhao XL, Grundy P, Lee YT. Grout sleeve connection under large deformation cyclic loading. In: Proceedings of 2002 ISOPE conference, vol. IV. Kitakyushu, Japan; May, CD-Rom, 2002. [16] Standards Australia. Methods for tensile testing of material. Australian Standard AS1391. Sydney: Standards Australia; 1991. [17] Standards Australia. Methods for fire tests on building materials, Components and Structures. Australian Standard AS1530.4. Sydney: Standards Australia; 1990. [18] Purkiss JA. Fire safety engineering—design of structures. Oxford: Elsevier; 1996. [19] Bazant ZP, Kaplan MF. Concrete at high temperatures: material properties and mathematical models. England: Longman; 1996. [20] Mills AF. Heat transfer. 2nd ed. Englewood Cliffs, NJ: Prentice-Hall; 1999.
Thin-Walled Structures 44 (2006) 751–758 www.elsevier.com/locate/tws
Behaviour of grouted sleeve connections at elevated temperatures X.L. Zhaoa,, J. Ghojelb, P. Grundya, L.H. Hanc a
Department of Civil Engineering, Monash University, Clayton, VIC 3168, Australia Department of Mechanical Engineering, Monash University, Caulfield, VIC 3145, Australia c Department of Civil Engineering, Tsinghua University, Beijing, PR China
b
Received 8 October 2005; received in revised form 18 July 2006; accepted 21 July 2006 Available online 25 September 2006
Abstract Tubular steel jointing system that incorporates prestressed grout sleeve connections has superior strength under both static and dynamic loading. This paper reports an investigation into the effect of elevated temperatures on the load carrying capacity of such connections. Eleven specimens were heated in a furnace and the load was applied through an Instron machine. Three different grout lengths were chosen. The load deflection behaviour at different temperatures was compared. It was found that the ultimate load reduces almost linearly as temperature difference (TD) between outer and inner tubes increases. It is encouraging to observe the ductile behaviour of grouted connections at elevated temperature. Thermal analysis was also conducted to predict the temperature field in the connection. r 2006 Elsevier Ltd. All rights reserved. Keywords: Grouted connections; Steel tubes; Ductility; Elevated temperature; Thermal analysis
1. Introduction Tubular steel jointing system that incorporates grouted sleeve connections (see Fig. 1) rather than conventional bolted or welded connections has been an innovation in the construction of tubular structures [1–7]. The connection transmits axial force by shear on the steel and interface. The traditional method to enhance shear transfer was using shear keys in the form of welded ribs or weld beads. An innovative method was developed at Monash University to enhance the shear transfer by deliberately introducing radial prestress between the circular sleeve and the through member. The radial prestress is developed using a cement additive that expands the grout after setting. It offers enormous benefits in fabrication and construction through minimising detailed welding and low on-site technology required for erection. A series of research have been carried out at Monash University on tubular connections with prestressed grout [8]. Research performed so far includes the behaviour of Corresponding author. Tel.: +61 3 9905 4971/4953; fax: +61 3 9905 4944. E-mail address: [email protected] (X.L. Zhao).
0263-8231/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tws.2006.07.002
such connection under static, fatigue and high amplitude dynamic loading [9–15]. The outcome has been the development of construction methodologies and design procedures that significantly enhance load capacity and reduce cost. In all the tests the Calcium-Sulpho-Aluminate compound (CSA) was used as an expanding agent. There is an increasing application of this kind of innovative connection in both offshore and on-land structures, such as: (1) for construction of offshore pile foundations and jacket platforms; (2) for construction of multi-storey buildings, space frames, grain silos, airport hangers, mine processing facilities and stadia; (3) for retrofitting and repairing tubular members in trusses and frames, column splices and (4) for strengthening or repairing pipelines. However, very little is known about the behaviour of the jointing system (see Fig. 1) at elevated temperatures. The grout prestress may be affected by high temperature. This is because: (1) disintegration of grout at elevated temperature; (2) different expansion rate of the steel tubes and the cement grout and (3) under gradient temperature confinement provided by steel tubes can be reduced leading to loss of radial prestress due to thermal expansion and possible creep. The connection may fail suddenly with the
ARTICLE IN PRESS X.L. Zhao et al. / Thin-Walled Structures 44 (2006) 751–758
752
thickness of the outer tube to Pmax ultimate load PmaxDT ultimate load obtained at elevated temperatures PmaxAmbient ultimate load obtained at ambient temperature T temperature DT temperature difference r density
Nomenclature cp Di Do k L tgrout ti
specific heat diameter of the inner tube diameter of the outer tube thermal conductivity grout length thickness of grout thickness of the inner tube
2.2. Test specimens
loss of prestresses especially when the connection is in tension. This issue must be addressed to ensure the safety of various structures using this innovative connection. This paper describes an investigation into the effect of elevated temperatures on the load carrying capacity of prestressed grout tubular connections. Only half sleeve connection (see Fig. 1(a)) is studied. Eleven specimens with three different grout lengths were tested. The specimens were heated in a furnace and the load was applied through an Instron machine. The load deflection behaviour at different temperatures is compared to study the effect of temperature on the ultimate capacity and ductility of grouted connections. The temperatures were measured using thermocouples. Thermal analysis was also conducted using thermal analysis system (TAS) program to predict the temperature field in the connections.
The basic dimensions of a specimen are shown in Fig. 2 where Do ¼ 165.1 mm, to ¼ 5.0 mm, Di ¼ 114.3 mm, ti ¼ 5.4 mm and tgrout ¼ 20.4 mm.The surface treatment for the outer sleeve is shot blasted whereas the surface treatment for the inner sleeve is mill scale. Thermocouples (OM, IM, P1, P2 and P3) were mounted on the specimens as shown in Fig. 2. Table 2 shows specimen label, grout length and temperature difference (DT in 1C) between locations OM and IM in Fig. 2. The lengths chosen are shorter than would be used in conventional design. Sufficient length in conventional design would be chosen to ensure that failure occurs first in the member by yielding rather than in the connection by slipping. Shorter lengths were chosen because the study focussed on reduction in connection strength.
2. Material properties and test specimens 2.1. Steel tubes and grout
3. Test set up
Mild steel tubes were used as outer and inner sleeves. The size of the outer sleeve is 165.1 mm in diameter and 5 mm in thickness. The inner sleeve had a diameter of 114.3 mm and a thickness of 5.4 mm. One tensile coupon was taken from the outer and inner tubes. The tensile coupons were prepared and tested according to the Australian Standard AS1391 [16]. The measured yield stress is 290 MPa for the outer sleeve and 313 MPa for the inner sleeve. Based on these data the calculated yield capacity of the outer tube (OM) is 729 kN, and of the inner tube (IM) is 578 kN. A grout mix design by weight is shown in Table 1.
to
Grout
to
tgrout
Do
Di
L
Table 1 Grout mix design by weight
Grout
Do
ti (a)
Tests for prestressed grout sleeve connections at different temperatures were conducted to determine the load carrying capacity of the connections. The critical load case
(b)
Portland cement
CSA
Water
Superplasticiser
0.9
0.1
0.36
0.5 L at 100 kg mix
tgrout
to
Di
ti
Grout
tgrout
Do
Di
ti L
(c)
L
Fig. 1. Grouted sleeve connections: (a) half sleeve connection; (b) inner sleeve connection and (c) outer sleeve connection.
ARTICLE IN PRESS X.L. Zhao et al. / Thin-Walled Structures 44 (2006) 751–758
(i.e. tension) was applied. The part of the specimen containing the grout sleeve connection was put inside a furnace. The specimen was shielded from the heating panels by a stainless steel pipe (250 mm in diameter, 5 mm in thickness) to avoid any short circuit. The specimens were tested horizontally with the tension load applied through an Instron testing machine. A general view of the test set up is shown in Fig. 3. 4. Test results The temperature distribution along the grout length in a specimen was measured using thermocouples shown in Fig. 2. Details of temperature measurements were in accordance with AS1530.4 [17]. A soaking test was conducted for a specimen with a grout length of 350 mm. The soaking furnace temperature is given in Fig. 4(a). The temperature versus time curves are shown in Fig. 4(b) for each thermocouples. The upper three curves are for locations P3, OM and P2. The middle two curves are for locations IM and P1. The temperature difference (TD) between OM and IM is given in Fig. 4(b) as the lowest curve. The furnace temperature was raised to a certain degree within a certain time as shown in the third column of Table 2. It can be seen from the soaking test that the TD between OM and IM increases as time increases. When the TD to
P3
OM
Grout P2
753
reached a certain value (see column 4 of Table 2), load was applied to the specimen until the axial deformation reached around 20 mm. Load versus deflection curves are shown in Figs. 5(a)–(c), for each grout length. The maximum load obtained in the test (Pmax) is listed in Table 2. 5. Effect of temperature on load carrying capacity The non-dimensional ultimate load capacity (PmaxTD/ PmaxAmbient) is plotted in Fig. 6 against the TD between the outer and inner tubes. It can be seen that the ultimate load capacity reduces almost linearly as the TD increases. The average shear bond stress (defined as Pmax TD =p2 Di L) is plotted in Fig. 7 against the TD between the outer and inner tubes. The trend appears to have little sensitivity to grout length. It should be noted that the percentage of CSA used in the tests described was quite low, only 10%. It may be expected that a significant higher prestress and therefore shear bond strength could be obtained with higher percentages of CSA in the grout mix. The trends shown in Figs. 6 and 7 are based on very limited tests described in this paper. More tests are needed to include a wide range of parameters such as grout thickness and heating rate.
tgrout
P1 IM Do
Di
ti L Fig. 2. Grouted sleeve connection.
Fig. 3. Test set up.
Table 2 Specimen label, temperature difference and loading capacities Specimen label
Grout length (mm)
Furnace temperature
Temperature difference (DT) (1C)
Pmax (kN)
Average shear bond stress (MPa)
L1TD0 L1TD10 L1TD25 L2TD0 L2TD8 L2TD15 L2TD30 L2TD40 L3TD0 L3TD11 L3TD35
L1 ¼ 160 L1 ¼ 160 L1 ¼ 160 L2 ¼ 250 L2 ¼ 250 L2 ¼ 250 L2 ¼ 250 L2 ¼ 250 L3 ¼ 350 L3 ¼ 350 L3 ¼ 350
Ambient Reach 250 1C Reach 500 1C Ambient Reach 250 1C Reach 400 1C Reach 500 1C Reach 500 1C Ambient Reach 250 1C Reach 500 1C
0 10 25 0 8 15 30 40 0 11 35
234 162 158 286 238 276 157 126 462 368 259
4.07 2.82 2.75 3.18 2.64 3.06 1.74 1.40 3.68 2.93 2.06
in 60 min in 20 min in in in in
30 min 30 min 20 min 20 min
in 30 min in 20 min
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Thermal conductivity [18]:
Furnace Temperature (degrees)
600
k ¼ 54 3:33 102 T W=m C;
500
The material properties of grout are not readily available. The properties of dry Portland cement given in Bazant and Kaplan [19] are used in this paper as an approximation:
400 300
Density r ¼ 1500 kg=m3 . 200
Specific heat (reproduced as Fig. 8 of this paper).
100
L3=350 mm
Thermal conductivity of oven-dried hardened cement pastes vary within a relatively narrow range of 0.45–0.75 W/m 1C [19]. A constant average value of k ¼ 0.6 W/m 1C is used in the current model. The material properties of air are:
0 0
500
1000
1500
Time (seconds)
(a)
Density r ¼ 1:2 kg=m3 .
450 P3
400
Specific heat cp ¼ 1005 J/kg 1C at 27 1C [20].
P2 Temperature (degrees)
Tp800 C:
350 300
Thermal conductivity k ¼ a þ bT þ cT 2 þ dT 3 þ eT 4 W/m 1C [20], where a ¼ 0.0265, b ¼ 6.19 105, c ¼ 8.55 109, d ¼ 3.93 1012 and e ¼ 1.40 1015.
OM
250
OM
P2
200
IM
P3
P1
Delta T
150
P1
IM
Delta T
100 50 0 0
10000
(b)
20000
30000
Time (seconds)
Fig. 4. Results of soaking test (L3 ¼ 350 mm): (a) soaking furnace temperature; (b) temperature versus time curves (soaking test).
6. Thermal analysis Thermal analysis is described in this section to predict the temperature field in the connections. This will form a basis for predicting the load carrying capacity of grouted connections at elevated temperatures. 6.1. Material properties The material properties used for steel are: Density r ¼ 7830 kg/m3. Specific heat in J/kg 1C [18]: cp ¼ 425 þ 7:73 101 T 1:69 103 T 2 þ 2:22 106 T 3 , 20 CpTp600 C:
6.2. Thermal modelling Modelling is conducted in TAS by Harvard Thermal (Harvard, Massachusetts, USA). TAS is a general-purpose software package which is based on the finite difference method for solving steady-state and transient complex thermal problems quickly and efficiently. The software can handle non-linear cases involving fluid flow, temperaturedependant properties and radiation. Fig. 9(a) shows the model of the entire system assembled. The furnace is assumed to be a quadrilateral volume made up from thin plate elements with the 250 mm diameter cylindrical shield protruding from both ends. The specimen consists of two concentric tubes with the gap between them filled with a special grout. It is centrally located inside the 250 mm diameter cylinder. Two heating elements are mounted on each side of the furnace and provide the energy to heat the specimen. Air, as a constituent component of the specimen, fills the tubes, cylindrical shield and the furnace. Heat transfer inside the furnace and the shield is by radiation and conduction whereas convection is assumed to be negligible. Convection is applied only to the parts that are exposed to the surrounding atmosphere. The ensuing radiation heat exchange is quite complex involving 1144 radiating surfaces in the model and 169,648 surface-to-surface radiation rays. As a result, more time is needed for the software to calculate the shape (view) factors for the radiation exchange than to solve the thermal problem. The experimental process was simulated by first heating the specimen for 1200 s at high heat input to bring the temperature of the furnace to 500 1C. After that a
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250 200
L1TD10
L1TD0
L1TD25
150 L1TD10
100 L1TD25
50
L2TD0
250
L2TD40
L2TD15 L2TD30
L2TD0 L2TD8
300 Axial Load (kN)
Axial Load (kN)
L1TD0
755
L2TD8 L2TD15
200 150 100 L2TD40
50
L2TD30
0
0 0
(a)
10 20 Axial Deflection (mm)
Axial Load (kN)
500
0
30
L3TD0 L3TD11 L3TD35
400
10
20
30
Axial Deflection (mm)
(b)
L3TD0
300 L3TD11
200 L3TD35
100 0 0
10
20
30
Axial Deflection (mm)
(c)
Fig. 5. Load versus deflection curves for each grout length: (a) grout length L1 of 160 mm; (b) grout length L2 of 250 mm and (c) grout length L3 of 350 mm.
5.0
1.2
1
L1=160mm
L3=350mm
L2=250mm
Trendline
Average Shear Bond Stress (MPa)
PmaxTD/PmaxAmbient
4.0
0.8
0.6 L1=160 mm 0.4
L2=250 mm L3=350 mm
0.2 trendline
3.0
2.0
1.0
0 0
10
20
30
40
50
Temperature Difference (degrees) 0.0 Fig. 6. Ultimate capacity versus temperature difference between outer and inner tubes.
0
10
20
30
40
50
Temperature Difference (degrees)
‘‘thermostat’’ was applied to the model to maintain a temperature range of 490–500 1C for the duration of the run (25,000 s). Figs. 9(a)–(c) show the temperature profile of the entire model, inner and outer sleeve assembly and the grout, respectively at 4500 s. These figures indicate that
Fig. 7. Average shear bond stress versus temperature difference between outer and inner tubes.
the central section of the specimen is the most heated area of the specimen and that the temperatures in the tubes and grout vary radially and axially.
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6.3. Comparison with measured temperatures
7. Conclusions
Figs. 10(a–e) present the comparison between the measured and predicted temperatures for locations OM, IM, P1, P2 and P3. The comparisons are only made within the time period of 5000 s because the TD between the OM and the IM reached the targeted value in less than 5000 s. There is a general agreement between measured and predicted temperatures.
The following observations and conclusions are made based on the limited test results and analysis.
6000
Cp, J/kg K
5000 4000
1. Temperature distribution in the grout connection was obtained. The temperature difference (TD) between the outer and inner tubes increases rapidly in the first 30 min. 2. The ultimate load carrying capacity reduces almost linearly as the TD between the outer and inner tubes increases. 3. It is encouraging to note the ductile behaviour of grout connections at elevated temperatures. 4. The predicted temperature agreed well with measured results.
3000 2000
8. Future research
1000 0 0
200
400 600 Temperature, °C
800
1000
Fig. 8. Specific heat of idealized Portland cement paste with a water/ cement ratio of 0.5 [19].
It is encouraging to note that the grouted sleeve connection behaves in a ductile manner at elevated temperatures although the ultimate load carrying capacity reduces almost linearly as the TD between the outer and inner tubes increases. Future research may include:
Fig. 9. Temperature profiles at 4500 s: (a) furnace and tube assembly; (b) inner tube and outer tube assembly and (c) grout.
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150 Measured 100
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150 Predicted 100 Measured 50
50
0
0 0
1000
2000
3000
4000
5000
0
Time (seconds)
(a) 250
250
200
200
Predicted
150 100 50
1000
(b)
Temperature (°C)
Temperature (°C)
757
4000
5000
4000
5000
Measured
150 100 Predicted
50
Measured
2000 3000 Time (seconds)
0
0 0
1000
(c)
2000
3000
4000
5000
0
Time (seconds)
1000
2000
3000
Time (seconds)
(d)
250
Temperature (°C)
200
Measured
150 100
Predicted
50 0 0
(e)
1000
2000
3000
4000
5000
Time (seconds)
Fig. 10. Comparison of measured and predicted temperatures: (a) OM location; (b) IM location; (c) P1 location; (d) P2 location and (e) P3 location.
(a) Study of all three types of connections shown in Fig. 1 with more variables such as grout thickness, tube diameters. (b) Measurement of grout material properties. (c) Prediction of load carrying capacities at elevated temperatures. (d) Fire resistance of such connections subjected to standard ISO834 fire curve.
Acknowledgements The authors wish to thank the Australian Research Council for financial support. Steel tubes were supplied by OneSteel Market Mills, Australia. Tests were performed in
the Civil Engineering Laboratory at Monash University. Thanks are given to Mr. Graeme Rundle and Mr. Roger Doulis for their assistance. References [1] Tebbett IE. The reappraisal of steel jacket structures allowing for the composite action of grouted piles. In: Proceedings of offshore technology conference, Paper OTC 4149, Texas, USA, 1982. [2] Tebbett IE, Billington CJ. Recent development in the design of grouted connections. In: Proceedings of offshore technology conference, OCT Paper 4890, Houston, USA, 1985. [3] Department of Energy, UK. The strength of grouted pile/sleeve connections: Phase I, Final Report—Static Tests. Report no. ST22/ 80, April 1980. [4] Elnashia AS, Carroll BC, Dowling PJ, Billington CJ. Full scale testing and analysis of prestressed grouted pile/platform connections.
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X.L. Zhao et al. / Thin-Walled Structures 44 (2006) 751–758 Proceedings of offshore technology conference, OTC Paper 5325, Houston, 1986. Wimpey Laboratory Ltd. Brown and Root UK Limited—BP Forties Field Phase I, Report on grout bond tests for large diameter piles. Report BM/112, September 1973. Yamasaki T, Hara M, Takahashi C. Static and dynamic tests on cement grouted pipe-to-pipe connections. Proceedings of offshore technology conference, OTC Paper 3790, Houston, 1980. Foo J. Prestressed grouted tubular connections. PhD thesis. Clayton, Australia: Monash University; 1991. Grundy P. Prestressed grouted sleeve tubular joints. In: Proceedings of Australasian structural engineering conference, Auckland, 1998. p. 815–20. Foo J, Grundy P. Performance of prestressed grouted tubular connections under dynamic loading. In: Proceedings of first pacific offshore mechanics symposium, PACOM’90, vol. III, Seoul, Korea, 1990. p. 33–42. Grundy P, Foo J. Prestress enhancement of grouted pile/sleeve connections. In: Proceedings of first international society of offshore and polar engineering conference, vol. IV, Edinburgh, UK, 1991. p. 130–6. Grundy P, Foo J. Performance of flattened tube connections. In: Proceedings of fourth international symposium on tubular structures, Delft, The Netherlands, 1991. p. 251–8.
[12] Grundy P, Ure A, Kim K. Shear transfer in a grouted cluster pile/ sleeve connection. In: Proceedings of second international society of offshore and polar engineering conference, San Francisco, USA, 1992. [13] Eimanis A, Grundy P. Load capacity of innovative tubular X-joints. In: Proceedings of fifth international symposium on tubular structures, Nottingham, UK; 1993. p. 485–94. [14] Grundy P. Grouted sleeve tubular splices. In: Proceedings of fifth international society of offshore and polar engineering conference, vol. IV, The Hague, 1995. p. 116–20. [15] Zhao XL, Grundy P, Lee YT. Grout sleeve connection under large deformation cyclic loading. In: Proceedings of 2002 ISOPE conference, vol. IV. Kitakyushu, Japan; May, CD-Rom, 2002. [16] Standards Australia. Methods for tensile testing of material. Australian Standard AS1391. Sydney: Standards Australia; 1991. [17] Standards Australia. Methods for fire tests on building materials, Components and Structures. Australian Standard AS1530.4. Sydney: Standards Australia; 1990. [18] Purkiss JA. Fire safety engineering—design of structures. Oxford: Elsevier; 1996. [19] Bazant ZP, Kaplan MF. Concrete at high temperatures: material properties and mathematical models. England: Longman; 1996. [20] Mills AF. Heat transfer. 2nd ed. Englewood Cliffs, NJ: Prentice-Hall; 1999.