Composite Structures CIVIL3811/9811 PDF

Composite Structures CIVL3811/9811 Engineering Construction and Design School of Civil Engineering | Faculty of Engineering THE UNIVERSITY OF SYDNEY The University of Sydney Page 1  Composite Construction – Composite construction as we know it today was first used in both a building and...

Composite Structures CIVL3811/9811 Engineering Construction and Design School of Civil Engineering | Faculty of Engineering THE UNIVERSITY OF SYDNEY The University of Sydney Page 1  Composite Construction – Composite construction as we know it today was first used in both a building and a bridge in the U.S. over a century ago. The first forms of composite structures incorporated the use of steel and concrete for flexural members, and the issue of longitudinal slip between these elements was soon identified. Composite steel-concrete beams are the earliest form of the composite construction method. In the U.S. a patent by an American engineer was developed for the shear connectors at the top flange of a universal steel section to prevent longitudinal slip. This was the beginning of the development of fully composite systems in steel and concrete. The University of Sydney Page 2  Composite Construction The University of Sydney Page 3  Composite Construction Systems for Buildings – Composite floor systems typically involve structural steel beams, joists, girders, or trusses made composite via shear connectors, with a concrete floor slab to form an effective T-beam flexural member resisting primarily gravity loads. The versatility of the system results from the inherent strength of the concrete floor component in compression and the tensile strength of the steel member. The main advantages of combining the use of steel and concrete materials for building construction are: Steel and concrete may be arranged to produce an ideal combination of strength, with concrete efficient in compression and steel in tension. Composite systems are lighter in weight (about 20 to 40% lighter than concrete construction). Because of their lightweight, site erection and installation are easier, and thus labour costs can be minimized. Foundation costs can also be reduced. https://www.designingbuildings.co.uk/wiki/Concrete-steel_composite_structures The University of Sydney Page 4  Composite Construction Systems for Buildings – The construction time is reduced, since the casting of additional floors may proceed without having to wait for the previously cast floors to gain strength. The steel decking system provides positive moment reinforcement for the composite floor, requires only small amounts of reinforcement to control cracking, and provides fire resistance. – The construction of composite floors does not require highly skilled labour. The steel decking acts as permanent formwork. Composite beams and slabs can accommodate raceways for electrification, communication, and air distribution systems. The slab serves as a ceiling surface to provide easy attachment of a suspended ceiling. – The composite slab, when fixed in place, can act as an effective in- plane diaphragm, which may provide effective lateral bracing to beams. – Concrete provides corrosion and thermal protection to steel at elevated temperatures. Composite slabs of a 2 hours fire rating can be easily achieved for most building requirements. The University of Sydney Page 5  Composite Construction Systems for Buildings The University of Sydney Page 6  The floor slab may be constructed by the following methods: a flat-soffit reinforced concrete slab (Figure(a)) precast concrete planks with cast in situ concrete topping (Figure (b)) precast concrete slab with in situ grouting at the joints (Figure (c)) a metal steel deck with concrete, either composite or non-composite (Figure (d)) – The composite action of the metal deck results from side embossments incorporated into the steel sheet profile. The composite floor system produces a rigid horizontal diaphragm, providing stability to the overall building system, while distributing wind and seismic shears to the lateral load-resisting systems. The University of Sydney Page 7  Composite Construction Systems for Buildings The University of Sydney Page 8  Composite Construction Systems for Buildings – The most common arrangement found in composite floor systems is a rolled or built-up steel beam connected to a formed steel deck and concrete slab. The metal deck typically spans unsupported between steel members, while also providing a working platform for concreting work. The University of Sydney Page 9  Composite Construction Systems for Buildings The University of Sydney Page 10 Construction Sequence of the Composite Construction Erect steelwork Install deck, shear studs, and reinforcement Pour concrete At this stage, the steel is non-composite, deflecting and bending under the wet concrete and construction loads. The deck stabilizes the top flange. Concrete sets, beam develops composite strength Composite beam supports all additional and ultimate loads The University of Sydney Page 11  Composite floor plan Figure A1 shows a typical building floor plan using composite steel beams. Figure A1 a typical building floor plan using composite steel beams The University of Sydney Page 12  Stress distribution in a composite cross section. The stress distribution at working loads in a composite section is shown schematically in Figure A2. The neutral axis is normally located very near to the top flange of the steel section. Therefore, the top flange is lightly stressed. From a construction point of view, a relatively wide and thick top flange must be provided for proper installation of shear studs and metal decking. However, the increased fabrication costs must be evaluated, which tend to offset the savings from material efficiency. Figure A2 The stress distribution at working loads in a composite section The University of Sydney Page 13  Composite Construction Systems for Buildings – A number of composite girder forms allow passage of mechanical ducts and related services through the depth of the girder (Figure A3). A3 Composite girder with opening for services The University of Sydney Page 14  Composite Construction Systems for Buildings – Successful composite beam design requires the consideration of various serviceability issues, such as long-term (creep) deflections and floor vibrations. Of particular concern is the occupant-induced floor vibrations. The relatively high flexural stiffness of most composite floor framing systems results in relatively low vibration amplitudes, and therefore is effective in reducing perceptibility. Studies have shown that short- to medium-span (6- to 12-m) composite floor beams perform quite well and have rarely been found to transmit annoying vibrations to the occupants. Particular care is required for long-span beams of more than 12 m. The University of Sydney Page 15  Long-Span Flooring Systems – Long spans impose a burden on the beam design in terms of a larger required flexural stiffness for serviceability design. Besides satisfying serviceability and ultimate strength limit states, the proposed system must also accommodate the incorporation of mechanical services within normal floor zones. – Several practical options for long-span construction are available, and they are discussed in the following subsections. The University of Sydney Page 16 Beams with Web Openings – Standard castellated beams can be fabricated from hot-rolled beams by cutting along a zigzag line through the web. The top and bottom half-beams are then displaced to form castellations (Figure A4). Castellated composite beams can be used effectively for lightly serviced buildings. Although composite action does not increase the strength significantly, it increases the stiffness, and hence reduces deflection and the problem associated with vibration. Castellated beams have limited shear capacity and are best used as long-span secondary beams where loads are low or where concentrated loads can be avoided. Their use may be limited due to the increased fabrication cost and the fact that the standard castellated openings are not big enough to accommodate the large mechanical ductwork common in modern high-rise buildings. The University of Sydney Page 17  Beams with Web Openings Figure A4 Composite castellated beams The University of Sydney Page 18  Beams with Web Openings – Horizontal stiffeners may be required to strengthen the web opening, and they are welded above and below the opening. The height of the opening should not be more than 70% of the beam depth, and the length should not be more than twice the beam depth. The best location for the opening is in the low shear zone of the beams. This is because the webs do not contribute much to the moment resistance of the beam. The University of Sydney Page 19  Fabricated Tapered Beams – The economic advantage of fabricated beams is that they can be designed to provide the required moment and shear resistance along the beam span in accordance with the loading pattern along the beam. Several forms of tapered beams are possible. A simply supported beam design with a maximum bending moment at the mid-span would require that they all effectively taper to a minimum at both ends (Figure A5), whereas a rigidly connected beam would have a minimum depth toward the mid-span. To make the best use of this system, services should be placed toward the smaller depth of the beam cross sections. The spaces created by the tapered web can be used for running services of modest size (Figure A5-1). – A hybrid girder can be formed with the top flange made of lower strength steel than the steel grade used for the bottom flange. The web plate can be welded to the flanges by double-sided fillet welds. Web stiffeners may be required at the change of section when the tapered slope exceeds approximately 6°. – Stiffeners are also required to enhance the shear resistance of the web, especially when the web slenderness ratio is too high. Tapered beams are found to be economical for spans up to 20 m. The University of Sydney Page 20  Fabricated Tapered Beams Figure A5 Tapered composite beam The University of Sydney Page 21  Fabricated Tapered Beams Figure A5-1 Tapered composite beam The University of Sydney Page 22  Haunched Beams – Haunched beams are designed by forming a rigid moment connection between the beams and columns. The haunch connections offer restraints to the beam and help reduce mid-span moment and deflection. The beams are designed in a manner similar to that of continuous beams. Considerable economy can be gained in sizing the beams using continuous design, which may lead to a reduction in beam depth up to 30% and deflection up to 50%. The haunch may be designed to develop the required moment, which is larger than the plastic moment resistance of the beam. In this case, the critical section is shifted to the tip of the haunch. The depth of the haunch is selected based on the required moment at the beam-to-column connections. The length of haunch is typically 5 to 7% of the span length for non-sway frames or 7 to 15% for sway frames. Service ducts can pass below the beams (Figure A6). Haunched composite beams are usually used in the case where the beams frame directly into the major axis of the columns. This means that the columns must be designed to resist the moment transferred from the beam to the column. Thus a heavier column and more complex connection would be required than would be with a structure designed based on the assumption that the connections are pinned. The University of Sydney Page 23  Haunched Beams A6 Haunched composite beam The rigid frame action derived from the haunched connections can resist lateral loads due to wind without the need for vertical bracing. Haunched beams offer higher strength and stiffness during the steel erection stage, thus making this type of system particularly attractive for long-span construction. However, haunched connections behave differently under positive and negative moments, as the connection configuration is asymmetrical about the axis of bending. The University of Sydney Page 24  Haunched Beams A6-1 Haunched composite beam The University of Sydney Page 25  Parallel Beam System – This system consists of two main beams, with secondary beams running over the top of the main beams (see Figure A7). The main beams are connected to either side of the column. They can be made continuous over two or more spans supported on stubs and attached to the columns. This will help in reducing the construction depth and thus avoid the usual beam-to-column connections. The secondary beams are designed to act compositely with the slab and may also be made to span continuously over the main beams. The need to cut the secondary beams at every junction is thus avoided. The parallel beam system is ideally suited for accommodating large service ducts in orthogonal directions (Figure A7). Small savings in steel weight are expected from the continuous construction because the primary beams are non-composite. However, the main beam can be made composite with the slab by welding beam stubs to the top flange of the main beam and connecting them to the concrete slab through the use of shear studs. The simplicity of connections and ease of fabrication make this long-span beam option particularly attractive. The University of Sydney Page 26  Parallel Beam System A7 Parallel composite beam system The University of Sydney Page 27  Composite Trusses – Composite truss systems can be used to accommodate large services. Although the cost of fabrication is higher in material cost, truss construction can be cost-effective for very long spans when compared with other structural schemes. One disadvantage of the truss configuration is that fire protection is labour-intensive, and sprayed protection systems cause a substantial mess to the services that pass through the web opening (see Figure A8). A8 Composite truss. The University of Sydney Page 28  Composite Trusses – The resistance of a composite truss is governed by: (1) yielding of the bottom chord, (2) crushing of the concrete slab, (3) failure of the shear connectors, (4) buckling of the top chord during construction, (5) buckling of web members, and (6) instability occurring during and after construction. To avoid brittle failures, ductile yielding of the bottom chord is the preferred failure mechanism. Thus the bottom chord should be designed to yield prior to crushing of the concrete slab. The shear connectors should have sufficient capacity to transfer the horizontal shear between the top chord and the slab. During construction, adequate plan bracing should be provided to prevent top chord buckling. When considering composite action, the top steel chord is assumed not to participate in the moment resistance of the truss, since it is located very near to the neutral axis of the composite truss and thus contributes very little to the flexural capacity. The University of Sydney Page 29  Composite Trusses The University of Sydney A8-1 Composite truss. Page 30  Stub Girder System – The stub girder system involves the use of short beam stubs, which are welded to the top flange of a continuous, heavier bottom girder member and connected to the concrete slab through the use of shear studs. Continuous transverse secondary beams and ducts can pass through the openings formed by the beam stub. The natural openings in the stub girder system allow the integration of structural and service zones in two directions (Figure A9 ), permitting story height reduction, compared with some other structural framing systems. A9 Stub girder system. The University of Sydney Page 31  Stub Girder System – Ideally, stub girders span about 12 to 15 m, in contrast to the conventional floor beams, which span about 6 to 9 m. The system is therefore very versatile, particularly with respect to secondary framing spans, with beam depths being adjusted to the required structural configuration and mechanical requirements. Overall girder depths vary only slightly, by varying the beam and stub depths. The major disadvantage of the stub girder system is that it requires temporary props at the construction stage, and these props have to remain until the concrete has gained adequate strength for composite action. However, it is possible to introduce an additional steel top chord, such as a T section, which acts in compression to develop the required bending strength during construction. For span lengths greater than 15 m, stub girders become impractical, because the slab design becomes critical. The University of Sydney Page 32  Composite Column Systems – Composite columns have been used for over 100 years, with steel-encased sections similar to that shown in Figure (A10-a) being incorporated in multistorey buildings in the United States during the late nineteenth century. The initial application of composite columns was for fire rating requirements of the steel section. Later developments saw the composite action fully utilized for strength and stability. Composite action in columns utilizes the favourable tensile and compressive characteristics of the steel and concrete, respectively. These types of columns are still in use today where steel sections are used as erection columns, with reinforced concrete cast around them as shown in Figure (A10-b). Figure A10 Encased composite sections The University of Sydney Page 33  Composite Column Systems – One major benefit of this system has been the ability to achieve higher steel percentages than conventional reinforced concrete structures, and the steel erection column allows rapid construction of steel floor systems in steel-framed buildings. Concrete-filled steel columns, as illustrated in Figure A11, were developed much later during the last century but are still based on the fundamental principle that steel and concrete are most effective in tension and compression, respectively. The major benefits also include constructability issues, whereby the steel section acts as permanent and integral formwork for the concrete. Figure A11 Concrete-filled steel columns. The University of Sydney Page 34  Composite Column Systems – Recently in Australia, Singapore, and other developed nations, concrete-filled steel columns have experienced a renaissance in their use. The major reasons for this renewed interest are the savings in construction time, which can be achieved with this method. The major benefits include: – The steel column acts as permanent and integral formwork – The steel column provides external reinforcement – The steel column supports several levels of construction prior to concrete being pumped. Figure A11-1 Concrete-filled steel columns. The University of Sydney Page 35  Local Buckling – If a composite section has a thin-walled steel section, which is able to buckle locally, then a reduction for the strength of the steel section must be made. The local buckling load and strength of a concrete filled steel section are significantly higher than those of a hollow steel section, as shown in Figure A12. Figure A12 Local buckling of box sections The University of Sydney Page 36  Composite connections – For composite frames resisting gravity load only, the beam-to- column connections behave as they do when pinned before the placement of concrete. During construction, the beam is designed to resist concrete dead load and the construction load (to be treated as a temporary live load). At the composite stage, the composite strength and stiffness of the beam should be utilized to resist the full design loads. For simple frames consisting of bare steel columns and composite beams, there is now sufficient knowledge available for the designer to use composite action in the structural element, as well as the semi rigid composite joints, to increase design choices, leading to more economical solutions. The University of Sydney Page 37  Composite connections – Figure A13 shows two typical beam-to-column connections: one using a flushed end plate bolted to the column flange, and the other using a bottom angle with double web cleats. Composite action in the joint is developed based on the tensile forces in the steel reinforcements that act with the balancing compression forces transmitted by the lower portion of the steel section that bears against the column flange to form a couple. Properly designed and detailed composite connections are capable of providing moment resistance up to the hogging resistance of the connecting members. In designing the connections, slab reinforcements placed within a horizontal distance of six times the slab depth are assumed to be effective in resisting the hogging moment. Reinforcement steels that fall outside this width should not be considered in calculating the resisting moment of the connection. Figure A13 Composite connections The University of Sydney Page 38  Composite connections Figure A13-1 Composite connections The University of Sydney Page 39  Steel/composite work marking plan drawing Figure A14 Steel/composite work marking plan drawing The University of Sydney Page 40  Vibration characteristics of steel–concrete composite floor systems – Trends towards long-span, lightweight floors in steel–concrete composite construction are resulting in structures possessing low natural frequencies, and potentially susceptible to vibration problems. The most common source of vibrations is caused by human activities on the floor, however, in some instances, mechanically induced vibrations from air conditioning plant, etc. may also be problematic. In this paper the serviceability assessment of floor vibration occasioned by walking activities is considered. Over 30 years ago concerns were raised regarding vibrations induced by walking on steel–concrete composite floors that satisfied traditional deflection criteria. In response to these concerns, design criteria based on a simple impulsive loading function from a person rising onto the balls of the feet, and suddenly dropping onto the heels has been used as a measure of floor acceptability. More recently, design procedures have been developed that more realistically consider the excitation of the floor from walking activities. The University of Sydney Page 41  NATURAL FREQUENCY – For free elastic vibration of a beam, or uniform section, the natural frequency is given by: Equation 1 where EI is dynamic flexural rigidity of the member (Nm2), m is the effective mass (kg/m), L is the span of the member (m), and kn is a constant representing the beam support and/or loading conditions. Some standard values of kn for the first mode of vibration of elements with different boundary conditions are as follows. The University of Sydney Page 42  NATURAL FREQUENCY – A convenient method of determining the natural frequency of a beam f, is presented within the SCI (Steel Construction Institute) guide ,by first finding the maximum deflection (in millimetres) caused by the weight of a mass m. For a simply supported element subjected to a uniformly distributed load Equation 2 For beams with different loading types and/or boundary conditions, similar results can be found with the numerator in equation. (2) varying between 16 and 20. However, for practical design, a value of 18 will normally produce results of sufficient accuracy. According to the SCI guide, the maximum deflection given in equation. (2) should be based on the gross second moment of area of the composite beam, with a load corresponding to the self-weight, and other permanent loads, plus a proportion of the imposed load that may be considered as permanent (taken as 10% for offices). The University of Sydney Page 43  NATURAL FREQUENCY Equation 3 The indicated concept can be extended to enable an estimate of the fundamental (first mode) frequency f0 of a complete composite floor system. This is achieved by using engineering judgement on the likely mode shape and the support conditions this will impose on the individual structural components. For example, the SCI guide suggests that on a simple floor comprising a slab continuous over a number of secondary ( joist) beams, which in turn, are supported by stiff primary (girder) beams (Figure A15 and A16), there are two possible mode shapes that may be sensibly considered: The University of Sydney Page 44  Secondary ( joist) beam mode – Secondary ( joist) beam mode: the primary beams form nodal lines (i.e. they have zero deflection) about which, the secondary beams vibrate as simply supported members (Figure A15). In this case, the slab flexibility is affected by the approximately equal deflections of the supports and, as a result of this, the slab frequency is assessed on the basis that fixed-ended boundary conditions exist. Figure A15 Secondary ( joist) beam mode The University of Sydney Page 45  Primary (girder) beam mode – Primary (girder) beam mode: the primary beams vibrate about the columns as simply supported members (Figure A16). By similar reasoning, because the equal deflections at their supports, the secondary beams (as well as the slab) are assessed on the basis that fixed-ended boundary conditions exist. Figure A16 Primary (girder) beam mode The University of Sydney Page 46  The frequency of the whole floor system – The frequency of the whole floor system can be calculated for each mode shape, by summing the deflection calculated from each of the mentioned components, and placing this value within equation 2. The lowest frequency value determined by consideration of these two cases defines the fundamental frequency of the floor f0 (and its corresponding mode shape). Alternatively, it can sometimes be convenient to use the component frequencies directly, to evaluate the fundamental frequency of the floor by Dunkerly’s approximation Equation 4 The University of Sydney Page 47  The frequency of the whole floor system – where fc1, fc2 and fc3 are the component frequencies (Hz) of the composite slab, secondary beams and primary beams respectively, with their appropriate boundary conditions, as defined before. As described earlier, two-mode shapes exist which may be sensibly considered in design a secondary ( joist) beam mode; and a primary (girder) beam mode (Figures A15 and A16). The lowest frequency value determined by consideration of these two mode shapes is the first mode, or fundamental frequency, of the floor structure. In assessing the level of response, it is presently assumed by the SCI design guidance that the floor only has this mode of vibration. The End The University of Sydney Page 48  References – 1. Hicks, S., Vibration characteristics of steel–concrete composite floor systems. Progress in Structural Engineering and Materials, 2004. 6(1): p. 21-38. – 2. Uy, B. and J. Liew, Composite steel-concrete structures. Civil Engineering Handbook, 2003. The University of Sydney Page 49