STRUCTURAL BEHAVIOR OF STEEL-CONCRETE-STEEL SANDWICH STRUCTURE WITH NEW TYPE OF SHEAR CONNECTORS

The aim of the current research is to investigate the structural behavior of steel-concrete-steel sandwich beams with a new suggested shear connectors. The shear connector was manufactured from deformed rebar in the form of stirrups. Four push out specimens were tested to obtain direct shear strength for the new type of shear connectors. Also four full size steel-concrete-steel sandwich beams were tested under three points loading. All beams were simply supported. The experimental results showed three failure modes as follow: (1) flexural failure; (2) shearing of shear connectors; and (3) buckling in compression plate. The use of proposed shear connectors from deformed steel bars produces a good connection between steel plates and concrete core, where the load capacity of steel-concrete-steel beam with long leg stirrups (L. connector) is larger than other beams. The ultimate load of beam B1 (steel-concrete-steel sandwich beam with L. connector) is 0.125 greater than the ultimate load of beam B2 (steel-concrete-steel sandwich beam with J-hook connectors).


INTRODUCTION
Steel-concrete-steel (SCS) sandwich structures are a new form of construction (Yan et al., 2015) [1], consisting of a layer of central plain concrete sandwiched between two steel plates, connected to the concrete core by shear connectors, Fig. 1. The performance of SCS sandwich structures depend mainly upon efficient interaction and transfer of stresses between steel plates and concrete core. This can be achieve by using shear connectors. The SCS sandwich structures can be classified into two main categories, depending on shear connector types: -SCS sandwich structures with mechanical shear connectors -SCS sandwich structures without mechanical shear connectors, in which the steel plate are glued to concrete core (Solomom et al., 1976).
The concept of steel-concrete-steel sandwich construction began when Solomon et al. (1976) submitted steel-concrete-steel sandwich beam as an alternative form of bridge decking for medium and long span deck. In steel-concrete sandwich structure, the mechanical connectors are usually used to transfer shear forces across the steel-plate and concrete core interface. The shear connectors are also used to prevent steel face plate separation and uplift (Liew and Wang, 2011). Double skin composite (DSC) elements are formed from two steel skins connected to an infill of concrete with welded stud connectors, see Fig. (1a). Wright et al., 1991, described design development and experimental studies on DSC system. A design model developed from previously reported scale model tests and similar to that used for reinforced concrete was proposed. A series of full scale DSC beams were tested and then used to verify the theoretical model. Comparisons of analytical and experimental results showed that the design model was suitable for most simple beams and gives a good prediction of behavior, (Wright et al., 1991). Engineering, Vol. 10, No. 3, July 2019 35 McKinlly andBoswell, 2002, was developed the original shear-stud concept to currently friction-weld round steel bars to both plates in a simultaneous operation. This process not only makes the units easy to handle, but also provides sufficient strength to resist the internal hydrostatic pressures due to fresh concrete. This product is called Bi-steel, see Fig. 1c. In this studied, investigated the elastic and plastic behavior of a series of sixteen full-scale, simply supported beams, under three-point loading. These tests compared existing, double skin method, and new construction methods, Bi-steel method, and were conducted until collapse due to local buckling of the compression steel plate. The test program was supported by a series of analytical solution covering the elastic and plastic performance of specimens. The analytically determined moment of resistance agrees well with experimental data, the standard deviation of the results being 4.25%, (McKinlly and Boswell, 2002).

Kufa Journal of
Schlesman and Russell 2004 studied applications of a SCS sandwich panels for nuclear reactors.
The idea was to provide high performance solutions against traditional constructions form such as reinforced concrete structure. The SCS member appeared to be stronger, more solid and more durable than the reinforced concrete member. When comparing two beams with the same load conditions, the SCS beam was shallower than the reinforced concrete beam. This was explained by the fact that the reinforcing steel face plates have an ideal locations. In addition, it was stated that the construction of SCS sandwich structures seems to be better than reinforced concrete structures. It can be estimated that the overall construction time can be reduced by 50% (Schlesman and Russell 2004).
The advantages of the SCS sandwich system are that the external steel face plates act as both main reinforcement and external mold. The steel face plate also acts as impermeable layer, blast, and impact resistant membranes (Farhan and Hussain, 2010). The steel-concretesteel sandwich applications has been extended to a verity of structures including submerged tunnels, storage vessels, shear walls in buildings and oil production structures, (Farhan and Hussain, 2010;Roberts, 1996).
In this study a new shear connector type is proposed. It was manufactured from deformed rebar in the form of stirrups, as seen from Fig. 2, and welded to the steel plates to serve as shear reinforcement to concrete core in addition to shear connectors. Four full scale simply supported beams were tested under three point-static loading. Stirrups type and the traditional J-hook shear connectors were investigated.

Details of specimens
The experimental program is divided into two parts. The first one is devoted for the behavior and strength of the suggested new type of shear connecters. In the second part, tests were conducted on SCS sandwich composite beams. Table 1 lists the SCS sandwich beams dimensions, plate thickness, and the properties of materials, and Table 2 lists the dimensions and plate thickness for push out specimens. Fig. 3 shows the typical push-out specimens, while   connectors can be calculated by the set of equations below (Abdul Razzaq, 2018): Where: n c and n t = number of shear connectors in compression and tension steel plate, respectively. P c,Rd and P t,Rd = shear resistance of the compression and tension shear connector, respectively.
f yst and f ysc = characteristic strength of compression and tension plates, respectively.
A st and A sc = area of tension and compression steel plates, respectively.
There is another parameter that limits the distance between the shear connectors, this is specified for preventing the buckling of compression steel plates (Abdul Razzaq, 2018).
where S c is the distance between shear connectors of compression steel plate and t c is the thickness of compression steel plate. Table 1 shows the distance between the shear connectors used in the different SCS sandwich beams.

Material properties
Ordinary Portland cement (type I) was used to cast the specimens in this study. The ordinary  Table 5. Table 6 shows the physical and chemical properties of the sand used for casting the push-out specimens and SCS sandwich composite beams.

Steel plates and reinforcement bar
Iranian steel plates were used in the manufacture of push-out specimens as well as the SCS sandwich beams. The steel plates were used with thicknesses of 4 and 8 mm. Three specimens from each thickness were tested to specify the properties of the steel plates. The dimensions of steel plate specimens are shown in Fig. 5. The test results conform to ASTM A36 and A572. Table 7 shows the properties of steel plates with thickness of 4 and 8 mm.
The deformed reinforcement steel bars were used in manufacturing the specimens. The bars are of diameter 12 mm. They were used as shear connectors. The tensile test was carried out according to ASTM standard (ASTM A615/A615 M-09) to determine the properties of reinforcement bars. Three samples with a length of 1 m were tested. Table 8 lists the results of the test.    Table 2 lists the dimensions and properties of push out specimens. The dimensions of the pushout specimens have been chosen such that be similar to those of the SCS sandwich composite beams. Therefore, the dimensions of the push-out specimens are: 600*500*300 mm, see. All push out specimens were loaded with a static load in several increments and the slip displacement associated with each load increment was measured by dial gauges. Fig. 6 shows the test set up for the push-out specimens.

Procedure for push out test
1. Before testing, the push-out specimens are prepared in order to record the required measurements.
2. For specimens that having plate of 4 mm thickness, the plates are supported at bottom of specimen to prevent the buckling through testing process.
3. Three dial gauges were installed, two for measuring the side displacement and the third for measuring the slip.
4. The load is then applied gradually on the specimen and each load increment does not exceed 10% of the expected ultimate load. The load is increased until up to failure.

5.
At each load increment all required measurements are recorded. Table 9 lists the experimental results for push out specimens. The mode of failure of each push out specimens was listed in Table 9. Fig. 10 shows the load slip relation of push-out specimens.

Failure mode
Examining the failure modes of push-out specimens, three main types of failure mode are observed and can be explained as follows: 1. Yield of shear connectors: where shear connectors are failed before the concrete core and steel plates, Fig. 11. This mode of failure was happened in test of specimens P.L8 and P.L4.
2. Buckling in steel plate: This type of failure usually occurs in the case of the thin steel plates (4 mm thickness). In this study, this type of failure occurred in P.L4 and P.J4,   1. The L. shear connector in specimen P.L.8 has the highest shear resistance, Table 9. This is due to two reasons: the first one, is the embedded length inside the concrete core. When the embedded length of the shear connectors increases, the resistance of the shear connectors to pull out increases too. The second reason, is the long length of the welding area of the shear connectors to the steel plate. For this type of connector the welding is along the horizontal part, which is in contact with the steel plate.
2. When comparing the behavior of models PL8, and PJ8, it is observed that the load slip relation are close to each other.
3. The failure mode for specimens PL8 due to yield in shear connectors, while the cause of the failure in the PJ8 specimen is the cut of the shear connector.
4. The shear strength of J-hook is the highest shear strength if the steel plate is 4 mm thick, as detailed in Table 9.
5. When comparing the behavior of PL4, and PJ4 specimens, it is noted that the load slip curves are close to each other.
6. The failure mode for PL4 is basically due to buckling of steel plates, while the cause of failure in the PJ4 is the cut of the shear connectors with the buckling of steel plates.   Fig. 17.

Modes of failure
There are four types of failure: a-flexural failure, Where the middle crack occurs when the load reaches 110 kN, for beam B2, and as the load increases, the cracks become widen, as shown Fig. 18.
b-Shearing of shear connectors: Where cutting occurs in the shear connectors close to the welding area with the steel plates, as shown in Fig. 18. This failure occurs for beams B2 and B3.
c-Buckling in compression plate, Fig. 19. This mode may be seen in beam B3 and B5.

Beams B1, B2:
Fig. 22a shows the comparison between the load deflection behavior of beams B1 and B2.

Beams B3 and B5
Fig. 22b shows the comparison between the load deflection behavior of beams B3 and B5.

Analysis of experimental results
From Table 10 and Fig. 14, and Fig. 22, the followings notes can be observed: