ENERGY DISSIPATION CHARACTERISTICS OF CONNECTIONS FOR HYBRID AND FRP INFILL PLATES FOR SHEAR WALLS M. Dakhel 1*, T. Donchev 2*, H. Hadavinia 3 and M. Limbachiya 4 Department of Civil Engineering, Kingston University, UK. Email:
[email protected] 2* Department of Civil Engineering, Kingston University, UK. Email:
[email protected] 3 School of Mech & Auto Engineering, Kingston University, UK. 4 Department of Civil Engineering, Kingston University, UK. ABSTRACT: 1*
The latest developments in steel shear walls (SSW) are a clear indication about the effectiveness and practical applicability of systems consisting of steel frames and thin infill plates. One of the main factors defining the behaviour of such systems is the type of connection between the frame and the infill plate. The estimation of this factor is getting more complicated in case of recently developed hybrid and FRP infill pates. This paper assesses ways of improving energy dissipation properties of steel to hybrid (steel/GFRP) and steel to plain GFRP connections. Different combinations of connections between steel hybrid and FRP plates were developed and tested with the aim to achieve higher energy dissipation. The size of all the plates was 125mmX70mm, with the connection to the steel plates via two M8 bolts. Variations included the application of additional layers of GFRP at the zone of the bolted area, use of adhesive film and additional epoxy around the bolted area. Both ultimate load capacity and energy dissipation of the tested specimens were estimated and analysed. Conclusions leading to the use of the most effective types of connections are offered. KEYWORDS Shear walls, GFRP, connections, load capacity, energy dissipation
1. INTRODUCTION: This paper is part of a bigger project focused on assessing and improving the energy dissipation of hybrid steel/FRP shear walls. Connections between the infill panel and the outer frame are one of the main factors influencing energy dissipation. Due to the complexity and relatively new application of the hybrid infill plates, detailed assessment must be undertaken to find out the most efficient connecting system in aspect of increasing energy dissipation of the hybrid shear walls. There has been some research assessing the modes of failure and ultimate capacity of FRP connections. However, there is very limited research on improving energy dissipative properties of FRP and Hybrid steel/FRP connections. Such type of research is crucial in the long term development and application of the innovative FRP and hybrid steel shear wall (SSW) structural systems.
2. BACKGROUND: The energy dissipation in hybrid shear walls is developed in three different areas; the steel frame, the hybrid infill plate and the connection between them. Some of the main factors that contribute to the energy dissipation of steel and hybrid steel/FRP shear walls are; plate aspect ratio (span/height), thickness of infill plate, type of column to beam connection, the connection between the infill plate and the outer frame elements as well as the application of different types and orientation of FRP’s. Cut outs in infill shear panels is another factor that influences energy dissipation and ultimate load capacity, as assessed by Maleki (2010). He concluded that the stiffness, energy dissipation and load carrying capacity decrease for specimens with cut-outs. Maleki added that this issue could be resolved via the application of additional stiffeners. The connection of the infill plate to the exterior frame is one of the main contributors to energy dissipation. Choi and Park (2008), tested the effect of application of various details of SSW infill plate connection to the boundary frame elements. Two of them were M20 bolted connections at spacing of 100mm with a 6mm fish plate and two sided fillet welded connections to the same thickness fish plate. They determined that the effects of bolted
connections and welded connections with respect to energy dissipation were approximate the same, up to a drift of 3.6% and that bolted connections exhibited similar initial stiffness and slightly higher load carrying capacity. However, the ratio of cumulative energy dissipation capacity of the bolted connections to the welded connections from their research was 0.52. FRP structural connections have various failure modes in plane of loading some of the most common are; net section tension failure, shear out failure, bearing, cleavage, slip failure and possibly a combination of the mentioned modes (Zhou and Zhao, 2013). Turvey, G (2000) also confirmed in his overview that for pultruded FRP structural connections with single bolt fastening and an axial load at an angle of 0°, 45° and 90° to the pultrusion, there are four basic modes of failure and they are: bearing, tension, shear out , and cleavage. Turvey added that, when the ratios of the end distance to the diameter of the bolt and the end distance to width of specimens are high the failure mode is bearing. Rosner and Rizkalla (1995) developed a design procedure based on their research assessing the behaviour of bolted fibre reinforced polymer connections, taking into account the materials orthotropic properties and pseudo yielding behaviour. They obtained failure envelopes for fibre orientation of 0°, 45°&90°, depending on the ratios of the hole diameter to width (d/w) and edge distance to hole diameter (e/d). It was concluded that for the connections with a ratio of d/w & e/d falling on the part of the envelope that is curved the failure mode is net tension and for the straight part of the envelope the failure will be bearing or cleavage. Manalo et al. (2008) analysed the behaviour of hybrid GFRP/CFRP bolted connections both with adhesive and without. It was evaluated that the connections without adhesive failed via bearing of bolts and crushing of fibres, while for the connections with adhesive resulted initially in bearing and delamination between the CFRP and GFRP and eventually net tension of the connection at failure load. Manalo also assessed the effect of bolt torque, revealing that there is a slight increase in capacity with increase of torque.
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Research by Petkune et al. (2014) investigated the effects of variation in the type of hybrid and FRP plates on the capacity of plate-to-steel element connections. Hybrid plates with CFRP prepreg unidirectional fabric attached to steel plate and CFRP only laminated plates were investigated as well as application of additional adhesive around the bolted area. It was determined that there is a significant increase in capacity when utilising adhesive around the bolted area. The presented research is focusing mainly on the improvement of energy dissipation in the connections between the frame and infill late. METHODOLOGY:
3.1. Specimens: Numerous specimens with a width of 70mm and a length of 125mm were tested. Three samples were tested for each set of specimens and the average results analysed. Varying factors were: the application of additional 2 layers of E722-02 UGE400-02 32%rw (GFRP) at a {-45/+45} orientation on either side of bolted area, application of DP110 Scotch-Weld adhesive and the use of EF72 adhesive film from TENCATE. The adhesive film was applied between the steel layer and the GFRP, as a solution to delamination issues for hybrid specimens. Figure 1 & Table 1 give information about of the geometry, type and number of samples. The orientation of the specimens with 8 layers of GFRP pre-preg plies were applied at a {-45/+45/-45/+45/+45/45/+45/-45} orientation. For the hybrid specimens incorporating steel plates the orientation is {-45/+45/S/+45/45}.The test specimens were tested using a displacement control load at an applied rate of 2mm/min. The applied displacement was continued after reaching maximum load capacity to fully assess the mode of destruction and energy dissipation of the specimens.
Figure 1: General dimensions of the specimens and cross sections
Table 1: Specimens Number of Bolts
2S 2G8 2G8D 2G8F 2G8FD 2SG4 2SG4F 2SG4FD 2SG4FE 2SG4FED
2 2 2 2 2 2 2 2 2 2
0.8m steel plate (S)
GFRP (G)
# of layers of GFRP
8 8 8 8 4 4 4 4 4
Additional 4 layers of GRP in bolted area (F)
EF72 adhesive film (E)
DP110 Epoxy (D)
Average thickness t1(mm)
Average thickness t2(mm)
0.8 2.13 2.13 3 3 2 3.05 3.05 3.15 3.15
0.8 2.13 2.13 2.2 2.2 2 2 2 2.1 2.1
3.2. Preparation: Application of adhesive film EF72 was used between the GFRP and the steel plate to increase the bond between the two layers. Before the film was applied the steel plate was abraded using sand paper and cleaned using acetone. Further to the application of the GFRP, the specimens were placed in a sealed vacuum bag between two plates attached to a 1 bar vacuum pump and placed in an oven for 1hour at 120 °C according to curing specifications of the manufacturer. Images of these steps are illustrated in Figure2.
Figure 2: Specimen Preparation, Starting from the left; (I) Abraded steel plates, (II) application of EF72 adhesive film, (III) application of pre-preg GFRP at +45, (IV) curing using oven and sealed vacuumed bag.
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RESULTS & ANALYSIS: The main factors investigated were the load capacity and the energy dissipation. The energy dissipation was presented by the enclosed area under the load displacement graphs. For better comparison between the samples they were divided into three groups. The first group included all the specimens consisting of 8 layers of GFRP (2G8, 2G8D, 2G8F and 2G8FD), the second group comprised of the hybrid steel/GFRP samples with 4 layers of GFRP and a 0.8mm steel plate in between them (2SG4, 2SG4F and 2SG4FD) and the third group consisted of hybrid specimens that employed EF72 adhesive film between the steel plate and the first GFRP ply (2SG4FE and 2SG4FED). The behaviour of the samples from each of the three groups is displayed in graphs in Figures 3-5. Figure 3 illustrates all the specimens that incorporate eight layers of GFRP. The varying factors in the group consisted of the application of Dp110 epoxy around the bolted area (2G8D), the use of additional GFRP strips around the bolted area (2G8F) and the application of both epoxy and additional GFRP (2G8FD). The application of either the adhesive or the additional layer of GFRP shows that the ultimate load capacity for both 2G8D &
2G8F specimens was increased significantly by 103% and 90% respectively when compared to 2G8. However 2G8D exhibited complete failure at approximately 16mm displacement resulting in an increase of 106% in the energy dissipation, while 2G8F displayed gradual destruction allowing it to obtain slightly higher energy dissipation of 121% when compared to 2G8. When applying both additional layers of GFRP and epoxy, the specimen showed an increase in capacity from 8.73KN for 2G8 to 21.35KN. 25
Load (KN)
20 2G8
15
2G8D 10
2G8F 2G8FD
5
2S 0 0
5
10
15
20
25
30
35
Displacement (mm)
Figure 3: Load Vs Displacement of pure GFRP connections
The second group contained of hybrid steel/GFRP specimens with 0.8mm steel plate and 4 layers of GFRP presented in Figure 4. The variations were of the application of additional GFRP strips around the bolted area (2SG4F) and the application of both adhesive and additional GFRP strips around the same area (2SG4FD). For this group increase in load capacity and energy dissipation was not as drastic as for the pure GFRP specimens in the first group. For the reference hybrid specimen (2SG4) the maximum load and energy dissipation achieved was 12.98KN and 146.08KN.mm. When comparing 2SG4F to 2SG4 the ultimate load capacity was increased by 41% and energy dissipation increase by and 29% for 2SG4F. Applying both additional layers of GFRP and epoxy (2SG4FD) resulted in an increase in maximum load capacity to 24KN and increasing the energy dissipation to 276.5KN.mm. 30
Load (KN)
25 20 2SG4
15
2SG4F
10
2SG4FD
5
2S
0 0
5
10
15
20
25
30
35
Displacement (mm)
Figure 4: Load Vs Displacement of Hybrid connections without EF72 adhesive film
The graph in Figure 5 compares the hybrid steel/GFRP plates that have additional adhesive film between the steel plate and the GFRP layer as well as additional GFRP around the bolted area (2SG4FE) with the plate that has adhesive film between the steel plate and the initial GFRP layer and both additional adhesive and GFRP
around the bolted area (2SG4FED). 2SG4FED shows a clear increase of approximately 37% in the load capacity of the specimen and a 52% rise in energy dissipation when comparing it to 2SG4FE.
30
Load (KN)
25 20 15
2SG4FE
10
2SG4FED 2S
5 0 0
5
10
15
20
25
30
35
Displacement (mm)
Figure 5: Load Vs Displacement for specimens with adhesive film between the steel plate and the initial GFRP layer
30
300
25
250
20
200
15
150
10
100
5
50
0
0
Energy Dissipation (KN.mm)
Load Capacity (KN)
Figure 6 is allowing comparison of the maximum load capacity and the energy dissipation of all specimens. The application of epoxy around the bolted area significantly increases the load capacity due to the reduction in slippage between the specimen and the clamping steel plates. Applying additional FRP layers around the bolted area increases the bearing capacity of the specimen due to increased thickness of FRP around the bolts. The application of adhesive film in hybrid specimens has a minimal effect on both load capacity and energy dissipation, visible when comparing 2SG4F with 2SG4FE and 2SG4FD with 2SG4FED. This minimal effect of additional adhesive film is applicable only for the connections and the effect is not valid for other areas of the infill plate. It was indicated by Petkune et al. (2014) that the application of adhesive film between the steel and CFRP prepreg laminates over the whole infill plate can significantly improve the shear wall behaviour.
Average Maximum Load Capacity KN
Average Energy Dissipation KN.mm (at the load of 1KN post faliure )
Figure 6: Energy Dissipation & Maximum Achieved Load
While assessing the failure modes it was reviled the most common modes of failure for connection with two bolts were shear-out, cleavage failure and a combination of both. Where adhesive was used to prevent slippage at initial stages of loading, in some specimens failure occurred in mid-section of the specimen resulting in higher load capacity. Some of the specimen’s failure modes can be seen in Figure 7.
Figure 7: Modes of failure starting from the left; (I) sample before testing, (II) shear out, (III) cleavage, (IV) combination of both shear out and cleavage and (V) failure at mid-section of plate.
CONCLUSION: From testing and analysing different variations of hybrid steel/GFRP connections the following conclusions ca be made: 1- For GFRP only plates application of either additional layer of GFRP or adhesive around the bolted area will increase both the ultimate capacity and the energy dissipation (Figure 3) 2- Simultaneous application of additional GFRP strips and adhesive around the bolted area will result in even higher ultimate load capacity and similar increase of the level of energy dissipation (Figure 3) 3- For hybrid plates the result of applying additional layer of GFRP and adhesive is similar to the results for plain FRP plates when both materials are applied simultaneously, with significant improvement of the ultimate load capacity and energy dissipation (Figure 4) 4- Additional epoxy layer between the steel plate and the adjacent GFRP layers at the zone of bolting is not so beneficial in increasing the capacity and energy dissipation as the application in internal zones of the infill plate. 5- Comparison of the obtained results with steel only connection indicates that in most of the cases the capacity and energy dissipation for the developed connections are significantly higher up to 2.9 and 2.8 times respectively. REFERENCES: Petkune, N., Donchev, T., Hadavinia, H. and Limbachiya, M., (2014) “Investigation in connections between steel, composite and hybrid structural elements”, MCM-2014: Mechanics of composite materials. June 2014, Riga, Latvia. Zhou, A., and Zhao, L,. (2013). Connection design for FRP structural members. In: ZOGHI, M. ed. The International Handbook of FRP Composites in Civil Engineering. Sep 2013 , 171 -190 Rosner, C. and Rizkalla, S. (1995). “Bolted Connections for Fiber-Reinforced Composite Structural Members: Analytical Model and Design Recommendations”, J. Mater. Civ. Eng., 7(4), pp.232-238. Turvey, G. (2000). Bolted connections in PFRP structures. Progress in Structural Engineering and Materials, 2(2), pp.146-156. Maleki, A., Donchev, T., Hadavinia, H. and Cheah, A. (2010) “Behaviour of steel shear wall systems with cutouts and stiffeners”, Proc. International conference in Structures and Architecture, Guimaraes, Portugal . Choi, I. and Park, H. (2008). “Cyclic test for framed steel plate walls with various infill plate details”. The 14 th World Conference on Earthquake Engineering October 12-17, 2008, Beijing, China Manalo, A. C., Mutsuyoshi, H., Asamoto, S., Aravinthan, T. and Matsui, T. (2008) “Mechanical behavior of hybrid FRP composites with bolted joints”. In: 20th Australasian Conference on the Mechanics of Structures and Materials (ACMSM 20): Futures in Mechanics of Structures and Materials , 2-5 Dec 2008, Toowoomba, Australia. Petkune, N., Donchev, T., Hadavinia, H., Limbachiya, M. and Wertheim, D. (2014). “Investigation of the behaviour of hybrid steel and FRP shear walls”. The 7th International Conference on FRP Composites in Civil Engineering., August 2014, Vancouver, Canada.
