BEHAVIOR OF STEEL-NORMAL AND HIGH STRENGTH CONCRETE COMPOSITE BEAMS WITH PARTIAL SHEAR INTERACTION

In this research, aimed to study the behavior of simply supported steel concrete composite beams with normal and high compressive strength of the concrete slab under the action of a span mid-point external load. The steel I-section beam is located at the bottom of reinforced concrete slab and connected with it by stud shear connectors. Eight composite beams were tested under the action of a monotonic load, half of them had a normal strength concrete slab while the others with a high strength concrete slab. Four degrees of shear connection interaction (100%, 80%, 60%, and 40%) were used for both groups of the tested beams. It was noticed that there are no essential differences between the modes of failure that occurred in the tested beams with normal strength concrete and those with high strength concrete. It was also found that there is an increase in the initial stiffness of the beams when the concrete changed from normal to high strength for different degrees of shear connections, but this increment reduced with increasing the degree of the shear connection. It was noted that the ultimate capacity of the tested beams was increased with enhancement of the strength of the adopted concrete from normal to high strength. The results showed that, when the concrete compressive strength was increased from 32.6 MPa to 72.8 MPa, the ultimate moment capacity of the specimens was increased from 28% for 100% shear connection, and it is increased to 38% for specimens with 40% shear connection.


INTRODUCTION
The expression "composite structure" is ordinarily comprehended inside the setting of structures and other building to infer the utilization of steel and concrete together into a part in a manner that the subsequent course of action capacities as a solitary thing. The point is to accomplish a more elevated amount of execution than would have been the situation had the two materials worked independently. Composite activity is accomplished by utilizing shear connectors mechanism. The shear connectors are typically joined to the top of a steel member by welding to transfer shear stresses between steel and concrete such that they will behave as one unit. Because of the composite activity, a huge increment in resistance and stiffness of the beams is obtained; this will reduce the depth of construction. In this way, composite beam is the main decision for long span structures with heavy applied loads giving the chance to accomplish an adaptable floor outline for the reception of evolving requests. With the approach of superior materials in composite beam these points of interest are used as advantage. Nie et al., 2004, presented an experimental study on the structural behavior of steel high strength concrete composite beams with full shear connection between the two parts of the composite beams, steel section beam and the high strength concrete slab. Eight specimens were tested in this study, seven of them with high strength concrete and one with normal strength concrete.
This study showed that the tested beams with high strength concrete have several favorable design aspects such as a larger margin beyond the steel yield and greater ultimate deformability. New design methodologies were planned so as to assessment the simply supported HS steel- The present work studies the behavior of eight simply supported steel concrete composite beams with normal and high compressive strength of concrete slab, in which the structural steel I-section beam located at the bottom of reinforced concrete slab and connected with it by stud shear connectors. Different degrees of shear connection between the two parts of the tested composite beams were investigated, ranged from 40% to 100% for both types of these tested beams.

Materials properties
The typical cross-section and the span length of all the tested beams are shown in Fig.1. Steel concrete composite beams were fabricated and then tested in this work. The properties of materials which used in the fabrication of the tested composite beams were including concrete, structural steel, reinforcement steel, and shear connectors.

Concrete
In this experimental study, two types of concrete mix design are adopted, after many trail mixes, to produce normal compressive strength (NS) and high compressive strength (HS) for concrete slab of composite beams with same concrete slump value. The materials that used included ordinary Portland cement (OPC), crushed gravel (G), sand (S), water (W), Silica fume (SF) and φ 16 Superplasticizer (SP). The quantities of materials that is entered in the formation of the concrete are listed in Table 1. Table 1 also shows the compressive strength of cubes at 7-days and 28days. All materials weights are for one cube meter of concrete mixture.

Structural steel I-section
The hot rolled steel I-section which is used in this work has dimensions of (140 mm outside height, 74 mm top and bottom flange width, 7 mm top and bottom flange thickness and 5 mm web thickness and with 13.1 kg/m weight), which is available in local markets, as shown in Fig.   1. The specimens for tensile test were cut from flange and web of I-section steel beam and the results of test are shown in Table 2.

Steel reinforcement bars
The steel reinforcement deformed bars with diameter 10 mm was used as shown in Figs.1 and 2. The test results of steel reinforcement specimen are shown in Table 3.  Elongation (%) 13 9 Min.

Stud shear connectors
The stud shear connectors with dimensions (75 mm height and 16 mm diameter), are joined to the top of a steel I-section by welding to transfer shear stress between two materials (concrete and I-section) making them as one units, as shown in Figs.2 and 3. Two specimens of stud connector were subjected to tensile test to get the yield stress and ultimate strength of stud material these values are given in Table 4.

Composite beams details
The total span length of tested specimens was 2.4m. These tested beams were designed according to the plastic analysis and design method that adopted by Eurocode 4. The reinforced concrete slab had dimensions of (100 mm depth and 400 mm width) and reinforced with two layers of rebar in two directions (Ø10 mm @ 100mm in each direction) as shown in Fig. 1.
According to the plastic analysis, the required number of shear stud to get full interaction for the adopted composite beam cross section was equal to 20 studs (100% degree of connection) arranged with distance equal to 115mm along the beam length. The number of studs was reduced to be 16, 12, and 8 studs in order to get partial shear interaction with (80%, 60%, and 40%) degree of connection, respectively. In the design of the full shear connection composite beams sections for both normal or high strength concrete, the location of plastic neutral axis was kept in the concrete slab. Therefore, the distance between the stud shear connectors is the same for (NC) and (HC) as shown in Table 5. The 150mm cube compressive strength of concrete for each specimen is shown in Table 6.

Instrumentation and testing procedure
The deflections, applied loads, and slips between the steel I-section and the concrete slab were measured for all specimens. Monotonic loads were applied using Universal Testing Machine (TORSEE) 200 tons' capacity to all the tested specimens. The simply supported effective span for all tested beams was (2.3m) which loaded at mid-span as shown in Fig. 4. The applied load was increased successively up to failure. The measurements of the mid-span deflection, end slip, and crack development were recorded at the end of each load increment.

Modes of failure
All the specimens showed flexural failure modes started by yielding the steel beam and then crushing of the concrete flange in the mid span. There were no essential differences in the stages of behavior between the specimens with normal or high strength concrete except that the steel beams of the specimens with high strength concrete exhibited to more stresses until yielding occur compared with the specimens with normal strength concrete. The crack patterns may considered as flexural cracks at the mid span of the tested specimens and a shear flexural cracks beyond the mid span region, in spite of that the intensity of cracks in the HC specimens was slightly more than that appeared in the NC specimens as shown in Fig. 5. There was no separation (uplift) appeared between the concrete flange and steel beam for all the tested specimens.

Load deflection response
The experimental results for testing of all the specimens are summarized in Table 7. The load mid-span deflection curves for the tested NC and HC specimens are shown in Figs. 6 and 7, respectively.
For both NC and HC specimens, the load deflection curves can be divided into two stages. The first stage corresponded to the linear elastic response of the tested specimens. This stage was continued until the load reached about 45 -50% of the ultimate load for NC specimens and about 60 -65% of the ultimate load for HC specimens. Also, it was noted at this stage that there was an increase in the stiffness, which represented by the slope of the linear part of the curves, when the concrete was changed from normal to high strength for different degrees of shear connections. But this increment in the stiffness reduces with increasing of the degree of the shear connection, as shown in Figs.8 and 9. The second stage represents the response of the tested specimens beyond yielding of I-section, where the behavior became nonlinear and the stiffness gradually degraded until failure. The length of this part of the load deflection curve is reduced compared with the linear part as the degree of shear connection increased for both NC and HC specimens, in spite of that this nonlinear stage is appeared clearly in the responses of the HC specimens if compared with the response of the NC specimens.

Ultimate strength
The experimental results clearly show, and as expected, that the degree of shear connection increases, the ultimate strength of the steel concrete composite beams increases, approximately in the same manner for both NC and HC, as shown in Fig. 10. On the other hand, the ultimate strength of the tested specimens was increased with enhanced the increment of the strength of the concrete. When the compressive strength increased from 32.6 MPa to 72.8 MPa, the ultimate moment capacity of the specimens was increased by about 28% for specimens with 100% shear connection and 38% for specimens with 40% shear connection. Fig. 11 and Fig. 12 show the relative end slip (Johnson R.P.), between the steel beam and the concrete flange for the tested beams for different load levels for NC and HC specimens, respectively. Also, Table 7 shows the values of the end slip for all the tested specimens corresponding to the ultimate load. It is clearly appeared, and found as expected, that end slip value decreased with increasing the degree of shear connection. But it was noticed that the measured end slip for the beams with HC gives smaller values for different degrees of shear connection compared with those obtained from the tests NC, as shown in Figs. 13 and 14.

CONCLUSIONS
Experimental study was carried out of eight steel concrete composite beams in order to investigate their structural behavior with the effect of the degree of shear connection and the compressive strength of concrete slab. Four of the tested beams were fabricated with normal compressive strength concrete slab and the others with high compressive strength concrete slab.

It was concluded that;
 There were no essential differences between the failure mode that found in the tested NC and HC slabs.
 The initial stiffness of the beams was increased with changing the concrete slab from normal to high strength.
 As expected, with the increase of the degree of shear connection (increase the number of welded shear studs), the ultimate strength of beams increased approximately in the same manner.
 The measured end slip for beams with HC had small values for different degrees of shear connection compared with values obtained from the tests of beams with NC.

REFERENCES
Nie J., Xiao Y., and Wang H., "Experimental Studies on