Field of the invention

The present invention relates to a steel-concrete composite structure, such as a steel- concrete composite bridge, and a shear connector assembly for use in a steel-concrete composite structure.

Background

It can be desirable to use prefabricated, pre-cast steel-concrete composite bridges to build a bridge easily and quickly, particularly in remote areas or other areas where ready-mix concrete is not readily available. Portable steel-concrete composite bridges can be particularly useful in the aftermath of natural disasters, such as earthquakes or flooding, to replace damaged or destroyed infrastructure. In a typical prefabrication method, concrete panels are usually connected to the steel girders using welded shear studs encapsulated in grouted pockets using epoxy mortar. However, waiting for grouting to harden can slow construction. Furthermore, once a bridge is constructed, it can difficult to repair or replace damaged or worn concrete bridge decks or welded studs.

For these reasons, there has been interest in providing replaceable shear connectors. In general, there are two approaches. The first approach is to use bolts which are embedded inside the concrete slab. Reference is made to L.N. Dallam: "High Strength Bolts Shear Connectors— Push out Tests", American Concrete Institute Journal Proceedings, number 9, pages 767 to 769 (1968), D.J. Dedic and W.F. Klaiber: "High- Strength Bolts as Shear Connectors in Rehabilitation Work", Concrete International, volume 6, issue 7, pages 41 to 46 (1984), G. Kwon et al.: "Behavior of post-installed shear connectors under static and fatigue loading", Journal of Constructional Steel Research, volume 66, issue 4, pages 532 to 541 (2010), D. Lam and E. Saveri: "Shear Capacity of Demountable Shear Connectors", Proceedings of the 10th International Conference on Advances in Steel Concrete Composite and Hybrid Structures (2012), M. Pavlovic et al.: "Bolted shear connectors vs. headed studs behaviour in push-out tests", Journal of Constructional Steel Research, volume 88, pages 134 to 149 (2013), and M.C. Moynihan and J.M. Allwood: "Viability and performance of demountable composite connectors", Journal of Constructional Steel Research, volume 99, pages 47 to 56 (2014). The second approach is to use bolts threaded through the hardened concrete slab. Reference is made to K. Sattlar: "Betrachtungen Uber Die Verwendung Hochzugfester Schrauben Bel Stahltrager-Verbundkonstruktionen", (Considerations on the Use of High-Tensile Bolts in Composite Concrete and Steel Girder Structures), Preliminary Publication, Sixth Congress, IABSE, i960, pages 333 to 349 (i960), W.T. Marshall et al.: "An Experimental Study of the Use of High Strength Friction Grip Bolts as Shear Connectors in Composite Beams", The Structural Engineer, volume 49, issue 4, pages 171 to 178 (1971), D.J. Dedic and W.F. Klaiber: "High-Strength Bolts as Shear

Connectors in Rehabilitation Work", Concrete International, volume 6, issue 7, pages 41 to 46 (1984), G. Kwon et al.: "Behavior of post-installed shear connectors under static and fatigue loading", Journal of Constructional Steel Research, volume 66, issue 4, pages 532 to 541 (2010), and S.S.M. Lee and M.A. Bradford: "Sustainable composite beams with deconstructable bolted shear connectors", Proceedings of the Fifth

International Conference on Structural Engineering, Mechanics and Computation, pages 1373 to 1378 (2013). However, none of these approaches have been widely adopted due to one or more reasons.

For example, there can be very little tolerance and great precision can be required in order to align pre-embedded bolts and/or pre-formed bolt holes during construction. Often such precision cannot be practically achieved in situations in which a prefabricated bridge might be erected, such as in remote areas or after a natural disaster.

Furthermore, in modern structural engineering, there is a drive towards achieving sustainable structures. One way to achieve sustainability is to use recyclable materials and to reduce waste. Steel beams can be reused or recycled. However, this requires the steel beams to be separated from the steel-concrete composite slab.

Summary

The present invention seeks to provide an improved steel-concrete composite structure which can be assembled easily and quickly by removing the need for fully grouted pockets and increasing the allowable tolerance for placement of steel and concrete components. The present invention also seeks to provide an improved steel-concrete composite structure which can improve sustainability of steel-concrete constructions by making it easier to separate and re-use steel components.

According to a first aspect of the invention there is provided a steel-concrete composite structure. The steel-concrete composite structure comprises a steel member having an upper surface and a plurality of shear connector elements (such as bolts) upstanding from the upper surface and a concrete slab having upper and lower surfaces. The slab is supported on its lower surface by the upper surface of the steel member. The slab comprises a plurality of through holes between the upper and lower surfaces, each through hole tapering towards the lower surface so as to form an inverted frustally- shaped seating surface. The concrete slab is configured and positioned with respect to the steel member such that at least one shear connector element projects into each through hole. The steel-concrete composite structure also comprises a plurality of removable inverted frustoconical plugs, each plug having top and bottom surfaces and an inverted frustoconically-shaped plugging surface, each plug having at least one through hole between the top and bottom surfaces. At least one plug is seated in a corresponding through hole of the concrete slab and each plug is configured such that at least one of the least one shear connector elements projecting into the corresponding through hole is received by a corresponding though hole of the plug. The steel-concrete composite structure comprises a plurality of fasteners (such as nuts), each fastener coupled to a corresponding shear connector element and arranged to discourage or prevent removal of a plug from a through hole of the concrete slab.

The steel-concrete composite structure can be assembled easily and quickly since the steel members, concrete slab and plugs can be prefabricated off site and the concrete slab can be easily aligned with the steel members. Furthermore, the plugs can be removed by simply undoing or breaking the fasteners thereby allowing the steel members and the slab to be cleanly separated. This can make it easier to recover and re-use steel components. The concrete slab may comprise a concrete grade between C8/ 10 and Cioo/ 115. The concrete slab may comprise C25/30 grade concrete. The concrete slab may be reinforced with any one of steel rebar, short steel fibres, fibre meshes and/or fibre reinforced polymer composites. The plugs may comprise a concrete grade between C8/ 10 and Cioo/ 115. The plugs may comprise C60/75 grade concrete. The plugs may be unreinforced. The plugs may be reinforced with any one of steel rebar, short steel fibres, fibre meshes and/or fibre reinforced polymer composites.

Each through hole of the concrete slab may be configured to form an inverted frustoconically-shaped seating surface and each plug is seated in a corresponding one of the through holes of the concrete slab. Each plug may contain at least two through holes, and each plug may receive at least two shear connector elements into the respective through holes. Each through hole of the concrete slab may be configured to form an inverted rectangular-based frustopyramidally-shaped seating surface and at least one plug may be seated in a corresponding one of the through holes of the concrete slab. At least two plugs may be seated in a corresponding one of the through holes of the concrete slab. Each plugging surface may be coated with a layer of de-bonding material. The de- bonding material may be wax.

Thus, the replaceability of the plugs and/or of the shear connector elements may be improved by including layers of de-bonding material to facilitate removal of the plugs and/ or any regions of filling material.

A filling material may be disposed around the one or more plugs in the through hole of the concrete slab. The filling material may be cement based or epoxy based. Thus, the mechanical performance of the steel-concrete composite structure may be improved. The construction time may be improved relative to welded studs in a fully grouted pocket because the total volume of filling material is relatively reduced, thereby reducing the time taken for the filling material to set and/or cure. The shear connector elements may be removably attachable to the steel member. The shear connector elements may comprise bolts. Each fastener may comprise a nut which is received by a threaded portion of the corresponding shear connector element. The fasteners and the sheer connector element may be arranged to urge each plug against the upper surface of the steel member.

Thus, a frictional force is generated which opposes shear between the steel member and the concrete slab, which may improve the mechanical performance of the steel-concrete composite structure because the frictional force must be overcome before

commencement of sliding.

Where the shear connector elements are bolts, the bolts may received by through holes in the steel member. Each opening of the through holes in the upper surface of the steel member may comprise a countersunk seat. The steel-concrete composite structure may further comprise additional filling material disposed in the countersunk seat and around the bolt.

Thus, the mechanical performance of a steel-concrete composite structure may be improved by preventing or restricting lateral slip of the shear connector element with respect to the steel member. This can have the effect of constraining the shear connector element to deform with a double plastic hinge in response to shear loading.

Where the shear connector elements are bolts, the bolts may be received by through holes in the steel member. Each opening of the through holes in the upper surface of the steel member may comprise a seat configured to receive a bolt head or a nut. The portion of each bolt passing through the steel member is tensioned by a nut threaded over the bolt and tightened into the seat. The seat configured to receive a bolt head or a nut may be countersunk and the nut may be a cone nut configured to by received into the countersunk seat. The seat configured to receive a bolt head or a nut may be countersunk to an angle of 60 degrees.

Thus, the mechanical performance of a steel-concrete composite structure may be improved by preventing or restricting lateral slip of the shear connector element with respect to the steel member. This can have the effect of constraining the shear connector element to deform with a double plastic hinge in response to shear loading.

One or more intermediate layers maybe provided between the concrete slab and the steel member. The intermediate layer(s) may comprise a metal based material. The intermediate layer(s) may comprise a ceramic based material. The intermediate layer(s) may comprise a wood based material. The intermediate layer(s) may comprise a polymer based material such as a rubber or an elastomer. One or more fibre reinforced elastomeric elements may be disposed around the one or more plugs in the through hole of the concrete slab. The fibre reinforced elastomeric elements may comprise fibre reinforced rubber. The fibre reinforced elastomeric elements may be shaped to form a frustoconical or frustopyramidal shell. The fibre reinforced elastomeric elements may comprise a block of fibre reinforced rubber including one or more through holes to receive one or more shear connector elements and corresponding plugs. A fibre reinforced elastomeric element may be bonded to the inverted frustoconically-shaped plugging surface of a plug. The fibre reinforced elastomeric elements may be configured such that a hoop stress generated in the elastomeric element in response to a plug being urged against the upper surface of the steel member secures the steel member to the concrete slab.

Thus, the elastomeric elements may provide damping for dynamic loadings of the steel- concrete composite structure. The steel-concrete composite structure may be a bridge.

According to a second aspect of the invention there is provided a removable plug for use in a steel-concrete composite structure. The removable plug comprises an inverted frustoconical body having top and bottom surfaces and an inverted frustoconically- shaped plugging surface and at least one through hole between the top and bottom surfaces. The removable plug may comprise two, three, four, five or at least six through holes. The through holes of the removable plug may be disposed in a regular polygonal arrangement. According to a third aspect of the invention there is provided a method of building a steel-concrete composite structure comprising a steel member having an upper surface, a plurality of shear connector elements; a concrete slab having upper and lower surfaces, the slab comprising a plurality of through holes between the upper and lower surfaces, each through hole tapering towards the lower surface so as to form an inverted frustally-shaped seating surface, a plurality of removable inverted

frustoconical plugs, each plug having top and bottom surfaces and an inverted frustoconically-shaped plugging surface, each plug having at least one through hole between the top and bottom surfaces, and a plurality of fasteners. The method comprises coupling the plurality of shear connector elements to the steel member such that the shear connector elements are upstanding from the upper surface, positioning the slab so that it is supported on its lower surface by the upper surface of the steel member and arranged such that at least one shear connector element projects into each through hole, positioning the plurality of plugs such that each plug is seated in a corresponding through hole of the concrete slab and wherein each plug is positioned such that at least one of the least one shear connector elements projecting into the corresponding through hole is received by a corresponding though hole of the plug, and coupling each of the fasteners to a corresponding shear connector element so as to discourage removal of a plug from a through hole of the concrete slab.

A filling material may be disposed around the one or more plugs in the through hole of the concrete slab. Each through hole may be filed with a quantity of filling material before the corresponding plug is emplaced into the through hole, so that the filling material flows around and inside the plug.

Brief Description of the Drawings

Certain embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which:

Figure 1 is a perspective view of a steel-concrete composite structure;

Figure 2 is a cut-away view of a portion of the shear connector assembly included in the steel-concrete composite structure shown in Figure l;

Figure 3 is a perspective view of a through hole included in a steel-concrete composite structure shown in Figure l;

Figure 4 is a cross-sectional view of a through hole shown in Figure 3 along a line labelled A-A;

Figure 5 is a cross-sectional view of a through hole shown in Figure 3 along a line labelled B-B;

Figure 6 is a perspective view of a removable frustoconical plug included in a steel- concrete composite structure shown in Figure 1;

Figure 7 is a plan view of a portion of a steel member and a fastener included in a steel- concrete composite structure shown in Figure 1;

Figure 8 is a cross sectional view of a first shear connecter assembly for use in a steel- concrete composite structure shown in Figure 1;

Figure 9 is a cross sectional view of a second shear connecter assembly for use in a steel-concrete composite structure shown in Figure 1;

Figure 10 is a cross sectional view of a third shear connecter assembly for use in a steel- concrete composite structure shown in Figure 1;

Figures 11A to 11E illustrate a method of constructing a steel-concrete composite structure shown in Figure 1;

Figure 12 is a cross sectional view of a test steel-concrete structure;

Figure 13 is a graph comparing the measured loading behaviour of a test structure shown in Figure 12 with a theoretical comparative example; and

Figure 14 is an exploded perspective view of a second steel-concrete composite structure.

Detailed Description of Certain Embodiments

In the following description, like parts are denoted by like reference numerals.

Referring to Figure 1, a steel-concrete composite structure 1 in the form of a beam bridge is shown. The bridge 1 includes rows of elongate steel members 2 supporting a deck 3 comprising concrete slabs 4. Under load, the steel members 2 are under tension, while the concrete slabs 4 are under compression.

Referring to Figure 2, each steel member 2 takes the form of an I-beam and has a respective upper surface 5. Shear connector elements 6 extend upwardly from the upper surface 5. In this case, the shear connector elements 6 comprise bolts.

Each concrete slab 4 has respective upper and lower surfaces 7, 8. Each slab 4 is supported on its lower surface 7 by the upper surfaces 5 of three steel members 2. In this case, the concrete slab 4 comprises C30/37 concrete, having a compressive strength of 37 MPa and reinforced with steel rebar reinforcements (not shown).

Referring also to Figures 3, 4 and 5, each slab 4 has through holes 9 between the upper and lower surfaces 7, 8. In this case, the through holes 9 are pre-formed into the concrete slab 4 at the time of casting. Each through hole 9 tapers towards the lower surface 7 so as to form an inverted frustally-shaped seating surface 10. In this case, the inverted frustally-shaped seating surface 10 has the shape of a frustum of a rectangular pyramid. The through hole 9 has a width Wi on the upper surface 8 of the slab 4 and a width W ₂ on the lower surface 7 of the slab 4. The through hole 9 has a length Li on the upper surface 8 of the slab 4 and a length L ₂ on the lower surface of the slab 4.

The through holes 9 are arranged in a line running along the slab 4, aligned with an underlying steel member 2. Thus, at least one shear connector element 6 projects into each through hole 9. In this case, two shear connector elements 6 project into each through hole 9. However, as will be explained in more detail, only one shear connector element 6 or at least three shear connector elements 6 may project into each through hole 9.

At least one inverted frustoconical plug 15 is seated in each through hole 9 of the slab 4. Each plug 15 is positioned such that a shear connector element 6 projecting into the corresponding through hole 9 is received by a corresponding though hole 16 (Figure 6) of the plug 15. In this case, the plugs 15 comprise C60/75 concrete, so that the concrete comprising the plugs 15 is stronger than the concrete comprising the slab 4. Each plug 15 is secured within the corresponding through hole 9 of the slab 4 by a corresponding fastener 17 which is arranged to discourage removal of that plug 15 from the corresponding through hole 9. In this case, the fastener 17 is a bolt threaded over an end of the shear connector element 6.

Securing the slab 4 to the steel members 2 using the plugs 15 helps to improve the replaceability and/ or reusability of a steel-concrete composite structure 1, because the plugs 15 may be removed and replaced by decoupling the fasteners 17. Employing an inverted frustally-shaped seating surface 10 in combination with the frustoconical plugs 15 can increase the tolerance for the placement of the shear connector elements 6 on the steel members 2 and the through holes 9 in the slab 4, thereby reducing the complexity and/ or cost of manufacturing and constructing pre-fabricated steel- concrete composite structures 1.

Referring also to Figure 6, each plug 15 has a top surface 18, a bottom surface 19 and an inverted frustoconically-shaped plugging surface 20. Each plug 15 has at least one through hole 16 between the top and bottom surfaces 18, 19. In this case, each plug 15 has a single, centrally positioned through hole 16. The top surface 18 has a top diameter D _1; and the bottom surface 19 has a bottom diameter D ₂ which is smaller than the top diameter Di. The plug has a height H, which in this case is less than a thickness of the slab 4. In this case, the slope angle of the plugging surface 20 is about 5 degrees.

In this case, the length L _t of each through hole 9 on the upper surface 8 of the slab 4 is at least twice the top diameter D _t of the plugs 15, such that two plugs 15 may be placed in each through hole 9 side by side, each plug 15 receiving one of the two shear connector elements 7 which project into the corresponding through hole 9.

Referring also to Figure 7, in this case each shear connector 6 may be received by a through hole 24 formed through a portion of a steel member 2. The through hole 24 has an upper opening 24a in the upper surface 5 of the steel member 2 and a lower opening 24b in a lower surface 25 of the steel member 2. In this case, the lower surface 25 is provided by the underside of the steel I-beam flange which contacts the slab 4. The upper opening 24a is countersunk at a first countersinking angle 26a. As shall be described in detail hereinafter, the upper opening 24a may serve as a seat for a fastener 17. In this case, the first countersinking angle 26a is 60 degrees. In this case, the fastener 17 may be a conical nut 27 (Figure 9). Alternatively, instead of providing a seat for a fastener, the upper opening 24a may be filled with filling material 37 (Figure 8) which acts to enhance a dowel action which opposes slip between the steel member 2 and the slab 4.

The opening of the lower through hole 24b in a lower surface 25 of a portion of a steel member 2 may optionally comprise a seat for a fastener 17. When the lower opening 24b of the through hole 24 is to provide a seat, it may be countersunk at a second countersinking angle 26b. The lower opening 24b may receive a chamfered nut (not shown) having a chamfering angle which matches the second countersinking angle 26b. In this case, the second countersinking angle 26b is 30 degrees. The outer diameter of the countersinking of the lower opening 24b may be greater than or equal to the chamfered nut diameter. Where a countersunk upper opening 24a is used, the lower opening 24b need not also be countersunk. However, both of the upper and lower openings 24a, 24b may be countersunk in some steel-concrete composite structures 1.

Employing a seat for the shear connector element 6 can improve the mechanical performance of a steel-concrete composite structure 1 by preventing or restricting lateral slip of the shear connector element 6 with respect to the steel member 2. This can have the effect of constraining the shear connector element 6 to deform with a double plastic hinge from the upper surface in response to shear loading.

Referring to Figure 8, a first shear connector assembly 30 includes a shear connector element 6 received through a through hole 24 in the steel member 2 and protruding into the through hole 9 of the concrete slab 4. The through hole 24 includes an upper countersunk opening 24a. Optionally, the through hole 24 may also include a lower countersunk opening 24b. In this case, the lower opening 24b is not countersunk. The shear connecter element is received by the through hole 16 of the plug 15. The plug 15 is positioned within the through hole 9 of the concrete slab 4, with the bottom surface 19 in contact with the upper surface 5 of the steel member 2. In this case, the shear connector element 6 is an M16 steel bolt. The shear connector element 6 includes an upper threaded portion 31 which protrudes above the height H of the plug 15, and a lower threaded portion 32 which protrudes below the lower surface 25 of the steel member 2. A central portion of the shear connector element 6 maybe unthreaded in order to increase the cross sectional area of the shear connector element 6 in the plane in which the shear connector element 6 will be subjected to the greatest shear stress. A nut 29 is received over the lower threaded portion 32 and tightened against the steel member 2. Optionally, one or more washers 35 may separate the nut 29 and the steel member 2. In this case, the steel member 2 includes a lower opening 24b which is not countersunk. Alternatively, the lower opening 24b may be countersunk and receive a chamfered nut (not shown) having a chamfering angle which matches the second countersinking angle 26b.

A retaining washer 34 (also referred to as a "plate" or "repair" washer) having a relatively large difference between its inner and outer circumferences is received over the upper threaded portion 31, and secured against the top surface 18 of the plug 15 by a nut 29. One or more washers 35 may space the retaining washer 34 from the nut 29.

There may be a gap 36 between the inverted frustally-shaped seating surface 10 of the slab 4 and the frustoconically shaped plugging surface 20 of the plug 15. In this case, the gap 36 is approximately 5 mm at the point of closest approach between the plugging surface 20 and the seating surface 10. The gap 36 may be filled by a region of filling material 37. In this case, the filling material 37 is a concrete grout.

There may be a residual gap between the shear connector element 6 and the interior surface of the through hole 16 of the plug 15. In this case, the residual gap is also filled with the filling material 37. The residual gap need not be entirely filled with filling material 37, and is preferably at least half filled. The replaceability of the plug 15 is preferably maintained by making the length of the upper and lower threaded portions 31, 32 just long enough to receive the respective fasteners 17, but not long enough that the threaded portions 31, 32 contact the filling material 37.

Filling the gap 36 with filling material 37 helps to improve the mechanical performance of the steel-concrete composite structure 1. The construction time can be improved relative to welded studs in a fully grouted pocket, because the total volume of filling material 37 used is relatively reduced, thereby reducing the time taken to apply the filling material. When the filling material needs to dry, the time taken for the filling material 37 to set may also be reduced by reducing the volume.

The volume of filling material 37 in the residual gap around the shear connector 6 extends down into the upper countersunk opening 24a, producing a locking mechanism which can be effective in preventing or restricting lateral slip. This can have the effect of constraining the shear connector element 6 to deform with a double plastic hinge in response to shear loading, relatively increasing the energy absorbed for a given deflection. Additionally, in this case, the nuts 29 are tightened such that the bottom surface 19 of the plug 15 is urged against the upper surface 5 of the steel member 2, producing a frictional force which opposes shear between the steel member 2 and the slab 4. This helps improve the mechanical performance of the steel-concrete composite structure 1 because the frictional force must be overcome before commencement of sliding and transfer of loading to the dowel action of the filling material 37 and shear connectors 6.

Referring to Figure 9, a second shear connector assembly 39 is substantially the same as the first shear connector assembly 30, except that the shear connector 7 is additionally secured by a middle fastener 40. The middle fastener 40 is received over the portion of the lower threaded portion 32 of the shear connector 6 which extends above the upper surface 5 of the steel member 2. In this case, the middle fastener 40 is a cone nut 27 which is threaded over the shear connector 6 and tightened against the upper surface 5 of the steel member 2. Optionally, one or more cone washers (not shown) may space the middle fastener 40 from the steel member 2.

In this case, the cone nut 27 providing the middle fastener 40 is chamfered to an angle which matches the first countersinking angle 26a of the upper opening 24a in the steel member 2. This helps the middle fastener 40 to cooperate with the upper opening 24a to provide a locking mechanism. In this case, a portion of the internal threading of the cone nut 27 extending from the flat end of the cone nut 27 is removed. This allows the unthreaded portion of the shear connector element 6 to be partially received inside the cone nut 27. This can help to maintain the replaceability of the plug as described hereinbefore, and may also help to ensure that plastic failure of the shear connector element 6 does not initiate in the lower threaded portion 32. In this case, the middle fastener 40, 27 is received onto the shear connector element 6 before the shear connecter element 6 is received through the through hole 24. In this case, the opening of the through hole 16 in the bottom surface 19 of the plug 15 includes a widened end portion 41 forming a seat which receives and fits over the middle fastener 40. Using a middle fastener 40 can improve the mechanical performance of a steel-concrete composite structure 1. In particular, the cooperation of the cone nut 27 with the countersunk upper opening 24a produces a locking mechanism which acts to prevent or restrict lateral slip. This can have the effect of constraining the shear connector element 6 to deform with a double plastic hinge in response to shear loading, relatively increasing the energy absorbed for a given deflection.

Additionally, the middle fastener has an increased contact area compared to the shear connector element 6, which can reduce the peak stress in the filling material 37 and plug 15, thereby reducing the risk of local crushing. Concrete slabs 4 and shear connector assemblies 30, 39 according to the present invention can be easier to repair or replace than structures using conventional welded studs and grouted pockets. In particular, the concrete slab(s) 4 may be replaced if they have become worn or damaged. Where there is access beneath the slab(s) 4, fasteners 17 below the lower surface 25 of the steel member(s) 2 may be removed and the slab(s) 4 uplifted, including the shear connectors 6, plugs 15 and filling material 37.

Alternatively, or if there is not access beneath the slab(s) 4, the fasteners 17 above the plugs 15 may be remove and the slab(s) 4 may be uplifted including the plugs 15 and filling material 37, leaving the shear connectors 6 in place. Additional, the plugs 15 themselves may be replaced if they have become worn or damaged by removing the fasteners 17, extracting an old/worn plug 15 and replacing it with a new plug 15. The shear connectors 6 may additionally be replaced if they become worn, damaged or corroded. If regions of filling material 37 have been used to fill gaps 36, then the replaceability of the plugs 15 can be maintained by selecting a filling material 37 which is relatively easier to remove than the concrete comprising the plugs 15 and the slab 4. For example, a material such as cement mortar which may be readily chipped or broken away. Additionally, as described hereinbefore, the length of the upper and lower threaded portions 31, 32 may be controlled so that filling material 37 in not in contact with the threaded portions 31, 32.

Referring to Figure 10, a third shear connector assembly 42 is the same as either the first or the second shear connector assemblies 30, 39, except that one or both of the inverted frustally-shaped seating surface 10 of the concrete slab 4 and/or the frustoconically shaped plugging surface 20 of the plug 15 are separated from the filling - ι ₅ - material 37 by one or more layers of a de-bonding material 43. In this case, the de- bonding material 43 comprises wax. In this case, the thickness of the layers of de- bonding material 43 is relatively small compared to that of the filling material 37. For example, the layers of de-bonding material 43 may be less than 1 mm, less than 0.5 mm or less than 0.1 mm.

Including one or more layers of de-bonding material 43 helps to improve the replaceability of the plugs 15 and/or of the shear connector elements 6 by facilitating removal of the plugs 15 and/ or any regions of filling material 37.

A method of constructing the second steel-concrete composite structure 1 shall be described.

Referring to Figure 11A, a first stage of constructing a portion of a steel-concrete composite structure 1 is shown. Through holes 24 are provided in the steel member 2 at the locations which are to receive the shear connector elements 6. The upper openings 24a in the upper surface 5 of the steel member 2 are countersunk. Optionally, the lower openings 24b may also be countersunk. Referring also to Figure 11B, an intermediate stage of constructing a portion of a steel- concrete composite structure 1 is shown. Shear connector elements 6 are received into the steel member 2. Shear connector elements 6 may be temporarily secured in position using suitable means such as, for example, wire warped tightly around the bolt (not shown) or locking washers (not shown). In this case, shear connector elements 6 are provided upstanding from the upper surface 5 of the steel member 2 in groups of two shear connectors 6. Alternatively, shear connector elements 6 may be provided upstanding from the upper surface 5 of the steel member 2 in groups of one, two, three, four, five, six or more shear connector elements 7. Where the steel members 2 are I- beams, shear connector elements 6 are preferably provided in groups including an even number of shear connectors elements 6, with the same number disposed either side of the midline of the upper surface 5.

Referring also to Figure 11C, an intermediate stage of constructing a portion of a steel- concrete composite structure 1 is shown. The slab 4 is placed with the lower surface 7 of the slab 4 supported by the upper surface 5 of the steel members 2. The slab 4 is positioned such that each group of shear connector elements 6 is aligned with and received into a corresponding through hole 9 in the slab 4. In this case, each through hole 9 receives a pair of two shear connector elements 6. Alternatively, more or fewer than two shear connector elements 6 may be received into each through hole 9. Referring also to Figure 11D, an intermediate stage of constructing a portion of a steel- concrete composite structure 1 is shown. Plugs 15 are placed into each through hole 9 of the slab 4, such that each shear connector element 6 which protrudes into that through hole 9 is received into a corresponding through hole 16 of a plug 15. In this case, two plugs 15 are placed in each through hole 9, each plug 15 receiving one of the two shear connector elements 6 which protrude into the through hole 9. Alternatively, when more or fewer than two shear connector elements 6 protrude into each through hole, each shear connector elements 6 may be received by one of a corresponding number of plugs 15. If required, a gap 36 between the inverted frustally-shaped seating surface 10 formed by the through hole 9 in the slab 4 and the frustoconically shaped plugging surface or surfaces 20 of the plug or plugs 15 may be filled with a filling material 37. Where the gap 36 is filled with a filling material 37, filling material 37 may be poured into the through hole 9 to a certain depth first, with the plugs 15 inserted subsequently. This can help to reduce the incidence of trapped air pockets and ensure even filling of the gap 36 and the residual gaps around each shear connector 6.

Referring also to Figure 11E, a portion of a completed steel-concrete composite structure 1 is shown. The upper ends of the shear connecting elements 6 protrude past the top surfaces 18 of the plugs 15. Fasteners 17 are applied to the upper ends of the shear connecting elements 6 to prevent or discourage the removal of the corresponding plugs 15 from the through holes 9 of the slab 4. In this case, the plugs 15 are secured using retaining washers 34, one or more washers 35 and nuts 29. In this case, the nuts 29 are tightened to secure the plugs 15 against the upper surface 5 of the steel member 2. Referring to Figure 12, a test steel-concrete structure 44 includes a pair of slabs 4 which are each coupled to a steel member 2 using a shear connector assembly 30. The test structure 44 shown in Figure 12 includes examples of the first shear connector assembly 30, however it will be apparent that either of the second or third shear connector assemblies 39, 42 may be used instead. The test structure 44 may be suitable for conducting full scale push out tests to Eurocode 4 standards. In this case, the steel member is an I-beam having first and second flanges 45, 46. In this case, the pair of slabs 4 includes a first test slab 47 coupled to the first flange 45 and a second test slab 48 coupled to the second flange 46. The steel member 2 and the first and second test slabs 47, 48 are orientated vertically with respect to a floor 49, upon which the slabs 47, 48 are supported. An upper end portion of the steel member 2 extends above the first and second slabs 47, 48, and a lower end portion of the steel member 2 is suspended above the floor 49. In this case, each of the first and second slabs 47, 48 includes one rectangular frustopyramidal through hole 9, with the long sides Li, L ₂ orientated horizontally. In this case, each through hole 9 receives two shear connector elements 6, and two plugs 15 arranged alongside each other.

A test load 50 is applied to the upper end portion of the steel member 2. The loading configuration of the test structure 44 applies a shear load between the steel member 2 and each of the first and second test slabs 4. The shear loading of the first and second test slabs 47, 48 can be substantially symmetric. Sensors (not shown) record the displacements of the first and second test slabs 47, 48 with respect to the steel member 2. Other sensors (not shown) may record the load and displacement of each individual shear connector element 6. Other sensors (not shown) may record additional information such as, for example, the size of any gaps which open up between the steel member 2 and the first and second test slabs 47, 48 during loading.

Referring to Figure 13, the measured mechanical shear behaviour 51 of an example of the test structure 44 is compared to the theoretical behaviour 52 of a comparative example comprising the same number of shear connecting elements which are welded studs of equivalent diameter, embedded into a concrete slab. The theoretical values 52 were calculated using a model proposed by Xue et al: "Static Behavior and Theoretical Model of Stud Shear Connectors", Journal of Bridge Engineering, ASCE, November/ December, page 623. The experimental data shown in Figure 13 is measured for a test structure 44 having the following dimensions. The top diameter D _t of the plugs 15 was 90 mm, the bottom diameter D ₂ was 70 mm and the height H was 115 mm. The width Wi of the through hole 9 on the upper surface 8 of the slab 4 was 116 mm, the width W ₂ on the lower surface 7 was 90 mm, the length L _t on the upper surface 8 was 216 mm, the length L ₂ on the lower surface 7 was 190 mm and the thickness of the slab 4 was 150 mm. The slab 4 comprised concrete with a compressive strength of 37.22 MPa and a tensile strength of 3.67 MPa, and was reinforced with steel according to EC4-1-1

recommendations. EC4-1-1 recommendations include reinforcement which are 10mm diameter ribbed bars, providing 180 mm spacing for vertical bars and 150 mm for transverse bars. The minimum concrete cover was 15 mm. Two middle bars were eliminated in the test due to presence of the slab through hole 9. The plugs 15 comprised concrete having a compressive strength of 73.87 MPa and a tensile strength of 3.708 MPa, and the plugs 15 did not include reinforcements.

The shear connecting elements 6 were grade 8.8 M16 bolts received by through holes 24 in the steel member 2. The upper opening 24a of the through hole 24 was countersunk to an first countersinking angle 26a of 60 degrees and the lower opening 24b was not countersunk. The M16 bolt steel had an elastic modulus of 204.43 GPa, a yield strength 824.76 MPa, an ultimate strength 946.47 MPa and a fracture strength of 703.32 MPa. The upper and lower threaded portions 31, 32 of the shear connector elements 6 extended for 25 mm from either end, leaving a 150 mm long portion unthreaded through the middle.

A third shear connector assembly 42 without a middle fastener 40 was produced, and a gap 36 between the inverted frustally-shaped seating surface 10 formed by the through hole 9 in the slab 4 and the plugging surfaces 20 of the plugs 15 was filled with a filling material 37 comprising a concrete grout with a compressive strength of 48.4 MPa. The concrete grout contained a 1:1 ratio of cement : fine sand, with a water/cement ratio of 0.5. The cement was Quickcem cement from Hanson company. %1.2 of cement weight, TamCem 60 superplasterizer, was also used. One layer of wax was used as the de- bonding material 43.

The test applied shear loading between the steel member 2 and the first and second test slab 47, 48 and lasted for approximately two and a half hours. The test was conducted under displacement control with a loading rate of approximately 0.1 to 0.2 mm per minute.

The experimental loading curve 51 for the test structure 44 shows improved mechanical performance when compared to theoretical values 52 for welded studs of equivalent diameter embedded into a slab. The ultimate load and corresponding shear displacement on each shear connector element 6 for the test structure 44 are provided along with comparative examples in Table 1 below. All values correspond to shear connector elements in the form of steel M16 bolts.

Table 1

The loading performance of the example test structure 44 is believed to be due to one or more factors. Without wishing to be bound by theory, application of a pre-load to the plugs 15 produces a frictional force between the bottom surfaces 19 of the plugs 15 and the upper surface 5 of the steel member 2. Such a frictional force must be overcome before the slab 4 and the steel member 2 begin to slip relative to one another, which can improve the loading performance by delaying slip until a larger load is reached.

At higher loads, shearing is resisted by a combination of the frictional force and a dowel action of the shear connector elements 6 and the plugs 15. Using relatively high strength concrete for the plugs 15 helps to produce an effective dowel action by increasing the loading required to initiate local crushing of the plug 15 material by the shear connector elements 6.

Receiving the shear connector elements 6 through the steel member 2 using a countersunk upper opening 24a can help to constrain the shear connector elements 6 to deform with a double plastic hinge, which can increasing the energy absorbed.

Additionally, the peak stresses on the slab 4 can be reduced, because the plugs 15 have an increased bearing surface area compared to the shear connecter elements 6 alone. This can help to prevent cracking and/or crushing of the concrete slab 4. Second steel-concrete composite structure 5

Referring to Figure 14, a second steel-concrete composite structure 53 is shown. The second steel-concrete composite structure 53 is the same as the steel-concrete composite structure 1, except for the second through holes 54 in the slab 4 and the second frustoconical plugs 55. Each second through hole 54 tapers towards the lower surface 7 of the slab 4 so as to form a second inverted frustally-shaped seating surface 56. In this case, the second inverted frustally-shaped seating surface 56 has a frustoconical shape. In this case, four shear connector elements 6 are disposed in a square arrangement and received into each of the second through holes 54.

Each of the second plugs 55 has a top surface 18, a bottom surface 19 and a second inverted frustoconically-shaped plugging surface 57. In this case, each second plug 55 has four through holes 16 between the top and bottom surfaces 18, 19, disposed in a square arrangement. Each of the second plugs 55 is positioned in a corresponding second through hole 54 such that the four shear connector elements 6 are received by the corresponding through holes 16 of one of the second plugs 55.

Securing each of the second frustoconical plugs 55 with more than one shear connecting element 6 received into each second plug 55 can reduce the shearing load on each individual shear connecting element 6.

Although the second steel-concrete composite structure 53 has been described with four shear connector elements 6 received into each second through hole 54, different numbers of shear connector elements may be used. For example, one, two, three or more than four shear connector elements 6 may be used instead, corresponding to a second plug 55 including one, two, three or more than four through holes 16. Although the second steel-concrete composite structure 53 has been described with the shear connector elements 6 disposed in a square arrangement, the shear connector elements 6 maybe disposed differently and need not be arranged in a regular shape. The second steel concrete composite structure 53 is not restricted to frustoconical second through holes 54 receiving frustoconical second plugs 55. For example, the second through holes 54 and second plugs 55 may each have the shape of a frustum of a pyramid with any polygonal shaped base or a frustum of an oval based cone. Modifications

It will be appreciated that many modifications may be made to the embodiments hereinbefore described. For example, steel-concrete composite structures l, 53 have been described as bridges. However, a steel-concrete composite structure can take other form such as, for example, a building structure comprising a steel frame of a building and a concrete slab forming part of a floor. Steel concrete composite structures 1, 53 have been described with three steel members 2 supporting a concrete slab 4. However, fewer or more steel members 2 may be used to support a slab 4. Furthermore, a slab 4 need not be provided as a single piece, and a steel-concrete composite structure 1, 53 may include more than one slab 4 supported by the steel members 2.

Steel concrete composite structures 1, 53 have been described with steel members 2 directly supporting a concrete slab 4. However, the steel members 2 need not directly support the slab 4, and one or more intermediate layers (not shown) may be provided between the concrete slab 4 and the steel members 2. The intermediate layer(s) may comprise a metal based material. The intermediate layer(s) may comprise a ceramic based material. The intermediate layer(s) may comprise a wood based material. The intermediate layer(s) may comprise a polymer based material such as a rubber or an elastomer. The steel-concrete composite structures 1 need not be restricted to slabs 4 including rectangular frustopyramidal through holes 9 which receive two shear connecting elements 6 and two corresponding plugs 15. Alternatively, the steel composite structure 1 may use include through holes 9 which are frustoconical or a frustum of a pyramid with a base of any polygonal shape. Additionally or alternatively, each through hole 9 may receive only one, or at least three shear connecting elements 6, each of which may be received by a corresponding plug 15. Additionally or alternatively, each plug 15 may include more than one through hole 16 and receive more than one shear connector element 6. The inverted frustally-shaped seating surfaces 10, 56 and the frustoconically shaped plugging surfaces 20, 57 may be smooth. This can improve the replaceability of the plugs 15, 55 by making them easier to remove. The seating surfaces 10, 56 and plugging surfaces 20, 57 may be made smooth without substantially affecting the loading performance of a steel-concrete composite structure 1, 53 because the shape of the seating surfaces 10, 56 and plugging surfaces 20, 57 acts to urge the concrete slab 4 against the steel members 2.

The through holes 9, 54 have been described as being pre-formed into the slab 4 at the time of casting. However, the through holes 9, 54 may alternatively be formed in situ at the time of constructing/assembling the steel-concrete composites structure 1, 53.

The plugs 15, 55 have been described as being urged against the steel members 2 by pressure exerted by the fasteners 17. However, the fasteners 17 need not be tightened and may simply prevent the plugs 15, 55 from being removed. In such a case, the fasteners 17 may be lock nuts or a pair of nuts 29 may be tightened against each other.

The shear connector elements 6 have been described as removable bolts. However, the shear connector elements 6 need not be bolts, and may alternatively be studs, pins or dowels. The shear connector elements 6 have been described as being removeably coupled to the steel members 2. However, the shear connector elements 6 may be irremovably coupled to the steel members 2, for example, by welding or brazing. The fasteners 17 need not be nuts 27, 29, and other types of fastener may be used. For example, the fastener 17 may be a pin or wire inserted through an end of the shear connector element 6 which protrudes above the top surface 18 of a plug 15, 55, such that the shear connector element 6 is prevented or discouraged from being withdrawn from the through hole 16 of the plug 15, 55.

The slab 4 has been described as comprising C25/30 concrete having a compressive strength of 30 MPa. However, other grades of concrete may be used for the slab 4, such as, for example, any concrete grade between C8/ 10 and Cioo/115, dependent upon the expected loading of a steel-concrete composite structure 1, 53. The concrete slab 4 has been described as being reinforced with steel rebar. However, other types of concrete reinforcement may be used such as, for example, short steel fibres, fibre meshes, fibre reinforced polymer composites or any other type of concrete reinforcement which would be suitable for the expected loading of a steel-concrete composite structure 1, 53. The plugs 15, 55 have been described as comprising C60/75 concrete having a compressive strength of 75 MPa. However, other grades of concrete may be used for the plugs 15, 55 such as, for example, any concrete grade between C8/10 and Cioo/115 dependent upon the expected preloading of shear connector elements 6 and the required dowel action. The plugs 15, 55 may be reinforced using steel rebar or other types of concrete reinforcement such as, for example, short steel fibres, fibre meshes, fibre reinforced polymer composites or any other type of concrete reinforcement which would be suitable for the intended loading. The height H of the plugs 15, 55 has been described as being less than the thickness of the slab 4. However, in particular where a further top surface (not shown) is to be laid overlying the upper surface 8 of the slab 4, the height H of the plugs 15, 55 may be equal to or exceed the thickness of the concrete slab 4. The slope angle of the inverted frustoconically shaped plugging surfaces 20, 57 has been described as being about 5 degrees. However, the slope angle of the

frustoconically shaped plugging surfaces 20, 57 may be less than 5 degrees, between 5 and 10 degrees, up to 15 degrees or more than 15 degrees. Larger slope angles may provide a larger force urging the concrete slab 4 against the steel member 2 in response to shear loading. Larger slope angles may increase the axial loading of the shear connector elements 6, which can reduce shear strength.

An upper openings 24a of a through hole 24 in a steel member 2 which receives a shear connector element 6 have been described as being countersunk to form a seat, and a lower opening 24b has been described as optionally countersunk to form a seat.

However, other types of seat may be used such as, for example, a square seat to receive a carriage bolt or a cylindrical seat into which a nut 29 may be tightened using a socket wrench. The upper opening 24a of a through hole 24 has been described as being countersunk to an first countersinking angle 26a of 60 degrees and a lower opening 24b of a through hole 24 has been described as being optionally countersunk to an second countersinking angle 26b of 30 degrees. However, the first and second countersinking angles 26a, 26b need not be restricted to these values and may instead be, for example, less than 15 degrees, less than 30 degrees, less than 45 degrees, less than 60 degrees or more than 60 degrees. The gap 36 has been described as being approximately 5 mm across. However, the gap 36 may be larger, for example between 5 to 10 mm or more than 10 mm. Alternatively, the gap 36 may be smaller, for example, the gap may be substantially eliminated if sufficiently precise placement and alignment of the shear connector elements 6 and the concrete slab 4 is possible. The filling material 37 has been described as a concrete grout and may be, for example, cement or epoxy based. Grouts used are preferable durable and display minimal shrinkage on setting and over time.

The gap 36 may be filled by pre-filling each through hole 9, 54 with a quantity of filling material 37 before emplacing the corresponding plug 15, 55 into the through hole 9, 54, so that the filling material 37 flows around and inside the plug 15, 55. Alternatively, the gap 36 may be filled after the plug 15, 55 has been secured by a fastener or fasteners 17.

If the filling material 37 is low viscosity before it sets, sealant (not shown) such as, for example, silicone rubber sealant, may be applied around the perimeter of the opening of the through hole 9, 54 on the lower surface 7 of the slab 4 to prevent the filling material 37 from penetrating between the lower surface 7 of the concrete slab 4 and the upper surface 5 of the steel member 2.

The de-bonding material 43 has been described as wax. However, other types of de- bonding material 43 may be used such as, for example, chemical release agents used to treat the surfaces 10, 20, 56, 57 such that they do not bond to the filling material 37. Alternatively, the plugs 15, 55 may be placed within a frustoconical shell (not shown) comprising, for example, polymeric material, so that the frustoconically shaped plugging surfaces 20, 57 do not directly contact the filling material 37.

The gap 36 has been described as being filled with filling material 37 which is concrete grout. However, the filling material may be replaced by high strength fibre reinforced elastomeric elements (not shown). For example, a sheet of fibre reinforced rubber shaped into a frustoconical or frustopyramidal shell, or a block of fibre reinforced rubber including through holes to received the plugs 15, 55. Such elastomeric elements may be inserted into the through holes 9 of the slab such that, when plugs 15, 55 are inserted and fasteners tightened, the resulting hoop stress secures the steel member 2 to the slab 4. When elastomeric elements are used, they may be bonded to either of the plugging surfaces 20, 57 or the seating surfaces 10. When elastomeric elements are used, either of the plugging surfaces 20, 57 or the seating surfaces 10 may be oiled to assist during insertion. Using elastomeric elements may provide damping for dynamic loadings.