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Conclusion and Future Work7.1 IntroductionA complete study of the composite section under different loading combinations including shear, torsion and positive or negative bending moment was presented. The study based on examining thirteen push-out specimens in addition to two full-scale composite beams under the aforementioned loading types. The specimens were categorized into four different groups; each group was tested under different loading combination to overview the behaviour of composite section and the stud shear connector in all cases.Along with this experimental investigation, a non-linear analytical study was performed using the software package ANSYS. A three dimensional finite element model was presented to simulate the behaviour of composite beams with solid or composite slabs and push-out specimens. The different parts of the composite section were simulated using a variety of the elements present in ANSYS program. A 2 mm gap was left between the steel I-beam and the slab to make the connection between the slab and the I-beam depend only on the stud model. The stud in this gap was modelled using two different modelling techniques. The first technique was developed by 8-noded 3-D solid element; Solid model. The second technique was proposed as a combination of three elements; a rounded cross-section beam element in the direction of the stud shank, and two springs in the two directions parallel and perpendicular to the steel I-beam. This proposed model was called Beam-spring model. For the rest of the nodes at the interface between the steel I-beam flange and the slab, contact elements were used to insure physical separation and prevent penetration between these components. The accuracy of these models was tested by comparing the results with previous experimental and numerical data.The main important conclusions obtained from this study in addition to some recommendations and some outlines for the future work are summarized in the following sections;7.2 ConclusionFinite Element Simulation1. A newly proposed 3-D finite element model namely Beam-spring model for the stud shear connector was presented in this research.2. The superiority of this Beam-spring model to the other tested model; solid model; was due to the actual presentation of the Beam-spring model to the stud material and slippage in different directions, while the other model could not predict the actual slippage of the stud.3. The very good agreement of this proposed 3-D finite element model with the previous experimental and numerical data concluded its reliability and accuracy in predicting and studying the overall behaviour of the composite section. This model was used to assure the experimental results and complete the study of the torsional effect on composite beams. A good indicator for the mode of failure was obtained in the proposed model even the rib-punch failure occurred in the specimens with the studs in the unfavourable position.Torsional Effect on Composite BeamsThe current study revealed that the most critical loading combination on a composite section was torsion, shear and negative bending moment. The following lines will declare that.Composite Section under Shear and TorsionApplying torsion in addition to shear to a composite section causes the following effects:1. Reduced the ultimate carrying capacity of the composite section up to 34%.2. The most common mode of failure in these sections was the shear-bond failure. This type of failure prevents making a full benefit of the capacity of the studs. When the load applied to produce torsion reached 12% of the shear, stud failure was noticed.3. Using slab shear connectors as a suggested solution to prevent the expected shear-bond failure was not sufficient. The slab shear connectors punched the steel sheet and the shear bond failure occurred.4. Produces tensile axial force in the studs, which can reach 5 times that produced in an ordinary push-out test. This tension reduces the ultimate shear capacity of the stud up to 33%.5. If torsion was applied to the section first and then shear is applied, the failure load of the specimen is reduced up to 8% compared to a similar specimen with shear and torsion is applied simultaneously.6. If only one stud is used in each rib, the studs should be arranged at the outer flange of the steel I-beam. Stud failure is produced if the studs were arranged in the inner flange (near to the side of applying torsion) and reduction in the full capacity of the composite section is noticed. This position could be named as unfavourable position for studs in the sections subjected to torsion and shear.7. No danger was found from arranging the studs at the near side of the mid-span of the beam (the unfavourable position of the studs in section subject to shear), since torsion prevented the expected rib-punch failure in these sections.Composite Section under Shear, Torsion and negative bendingApplying torsion in addition to shear and negative bending to a composite section causes the following effects:1. Produces shear bond failure accompanied with rib-shear failure or stud failure which is considered very dangerous in composite structures.2. Produces tensile axial force is in the stud reaches about 6 times of that produced in the push-out test. This axial force reduced the shear capacity of the stud up to 48%.3. A reduction factor should be provided in the BS 5950 for the design strength of the stud shear connector when the stud exists in a hogging moment region accompanied with torsion and shear. The results of this research proved that the shear capacity of the stud under this loading combination is decreased down to 0.5, while the BS 5950 recommended a design strength of the stud in hogging moment region equals 0.6 of the characteristic resistance of the stud, and no consideration for torsion was taken. On the other hand, no consideration in the Egyptian Code of Practice for the reduction in the characteristic strength of the stud in this case.4. Can cause rib-shear failure in the composite slab. So, it is suggested that the BS 5950 increases the specified minimum width of the composite slab in a composite section subject to this loading combination, and the Egyptian Code of Practice should specify a minimum width of the slab of push-out specimen, since no dimensions were provided.5. Causes additional shear and slippage at the interface between the composite slab and the steel I-beam in the direction perpendicular to the steel I-beam in addition to the known longitudinal shear.6. Applying a value of load to produce torsion to the composite section is more critical than applying the same percentage of load to produce negative moment since torsion reduced the load carrying capacity of the specimen by 8% and increased the axial force in the stud by 4% than negative moment.Composite Section under Shear, Torsion and positive bendingApplying torsion in addition to shear and positive bending to a composite section causes the following effects:1. No danger was observed in applying torsion to a composite beam under positive bending and shear, since no stud failure was noticed within the loading value and a degree of shear interaction of 60 % and up.2. Torsion transforms the axial force in the studs form compression to tension, and increasing the value of torsion increases this tension. So, care should be taken for this change and the produced tension should be considered while designing composite beam with this loading scheme, since the shear capacity of the stud is reduced with the existence of tension force.3. Adding torsion to shear and positive bending reduces the ultimate capacity of the composite beam up to 40% than the similar beam without torsion.4. Increasing the percentage of load to produce torsion by 32% reduces the shear carried by the stud down to 18% and increases the axial force by 10%.5. Serviceability limits should be checked carefully when designing the composite section under torsion, shear and positive bending. The recorded slab deflections at serviceability load and failure load exceeded the calculated value according to the BS 5950 Part4 by 2 and 5 times, respectively.6. At a certain load level of applying this loading combination, zero axial force is produced in the stud in spite of applying torsion to the specimen. For the studied dimensions of composite beams and this loading sequence, applying load to produce torsion of about 10% of the total load produces zero axial force in the edge stud.Proposed Equations and Relations1. A relation was proposed to calculate the produced axial force in the stud under the different loading combinations.2. Some modifications were performed to the exponential empirical equation which describes the load-slip curve of the push-out specimen;Q = Qu (1 – e -?s)?These modifications were performed to make the equation match the behaviour of push-out specimen under different loading combinations. For the pure push-out specimens, the values of ? and ? were modified to 0.5 and 0.6 respectively. In case of loading the section with shear and torsion, the values were modified to 0.7 and 0.41, while in case of applying shear, torsion and negative bending they were modified to 0.9 and 1.2, respectively.7.3 Recommendations:From the obtained results of this research, it is recommended to;- Use a reduced characteristic strength of the stud shear connector when it exists in a composite section subject to shear, torsion and negative bending moment. This reduction can be about 0.5 to avoid stud failure which is a sudden and a severe failure type. Neither the BS 5059 nor the Egyptian Code of Practice considered this recommended reduction.- When only one stud is used in each rib in a composite section subject to torsion, it is recommended not to arrange the stud at the side of the load which produce torsion.- Increase the bond between the steel sheet and the concrete to reduce the shear bond failure which occurs in the composite section subject to torsion.- A careful check should be done for the serviceability limits when designing a composite section under torsion, shear and positive bending.7.4 Future WorkThe proposed extent of the continuation of this study can be summarized as follows:- Increasing the bond between the concrete and the steel sheet using some type of adhesive or other shape of the steel sheet with different embossments to reduce the shear-bond failure which happened in the sections under torsion.- Perform some other experimental tests on full-scale composite beams with different degrees of interaction to specify the minimum allowed degree of interaction in the two cases of composite beams under torsion, shear and positive or negative bending moment.- Using other dimensions and other types of shear connectors in composite beams subject to shear, torsion and positive or negative bending moment to specify the best shape and dimensions of the shear connector suitable can be used in a composite section under these loading combinations.- With the aid of the proposed finite element model with its proved accuracy, a further study of composite beams under shear, torsion and positive or negative bending with the effect of cyclic load to generalize the study on beams in bridges.
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