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The reason why composite construction is often so good can be expressed in one simple way - concrete is good in compression and steel is good in tension. The scope of this article covers composite beams, composite slabs, composite columns and composite connections. Design of composite beams in the UK has traditionally been carried out to BS 5950-3-1[1].
More information on the relative status of the structural Eurocodes and the British Standards can be found by following the link here.
The plastic stress distribution in a typical downstand beam acting compositely with a composite slab is shown. Concrete is a material that works well in compression but has negligible resistance in tension.
For the concrete part (within the so-called effective width) of a cross section to carry compression, and the steel part to carry tension, the two materials must be structurally tied together. In exceptional circumstances through deck welding can be avoided by using single span lengths of decking (which butt up to rows of studs welded directly to the top flange in the fabrication shop), or cutting holes in the decking so that it can be dropped over the shop welded studs. Other forms of shear connection are available, including larger diameter studs and shot-fired connectors, but for buildings by far the most common option is 19 mm diameter headed studs. One of the advantages of welded studs is that they are considered to be ductile, which means that (in the absence of any fatigue considerations) the connection can be designed using plastic principles because it is assumed that force can be transferred between adjacent studs. When a beam is designed with full shear connection it means that sufficient connectors are present to either fully fail the concrete in compression, or fully fail the steel section in tension (whichever is the smaller force).
Until it was amended in 2010, BS 5950-3-1[1], which was written in the 1980s, took a fairly simplistic approach to the issue of minimum degree of shear connection. The benefit of joining the steel and concrete together structurally is to increase the resistance of the steel beam alone; typically this will be by around a factor of two. The components of a composite beam are as described above, but the same principles apply to composite slabs and composite columns. A composite column may be either a hollow section steel tube filled with concrete, or an open steel section encased in concrete. With all forms of composite construction it is important for the designer not to forget the construction stage.
The most common type of composite beam is one where a composite slab sits on top of a downstand beam, connected by the use of a through deck welded shear studs. Further guidance on practical aspects of decking placement may be found in the best practice guide SCI P300. Another common type of composite beam is one where, as with a traditional non-composite steel framed solution, a precast concrete slab sits on top of the top flange of the steel beam. A number of variations on the idea of downstand beams are available to meet long-span needs. Shallow floors offer a range of benefits , which must be considered in the context of a given project to identify when they are most appropriate.
A number of shallow floor solutions are available, including a range of rolled and fabricated options from Tata Steel. The slab may be either in-situ concrete on deep (225 mm) steel decking, or precast concrete.
Composite slabs comprise reinforced concrete cast on top of profiled steel decking, which acts as formwork during construction and external reinforcement at the final stage. The results of such tests have traditionally been translated into so-called m and k empirical constants that define the performance of a particular deck. The profiled decking is often designed to be continuous over two spans when acting as formwork.
It is possible to form openings in composite slabs, although this should be planned and the openings formed at the construction stage rather than having to cut out concrete. Further guidance on the design and detailing of composite slabs is given in SCI P359 and SCI P300 respectively.
Concrete filled hollow section compression members need no formwork and they use material more efficiently than an equivalent H section.
A programme, FireSoft, for the design of concrete filled hollow sections in ambient and fire conditions has been developed. Although design guidance exists for composite connections (SCI P213 ), they have been very little used in the UK (or indeed elsewhere in Europe).
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Whilst beams and slabs are very common in UK construction, indeed there exist a number of different basic types of composite beam, composite columns and composite connections are much less so.
The relative proportions of the steel section and slab mean that, as is commonly the case, the plastic neutral axis lies within the concrete. Hence for structural purposes it traditionally relies on steel reinforcement to carry any tensile forces (this is the role played by the steel part of a composite cross section, which is effectively external reinforcement), or must be pre-stressed so that even when subject to tension, an element is in net compression. For downstand beams this is achieved using headed shear studs, which are attached to the upper flange of the steel beam.
Reduced numbers of connectors may however be used, resulting in so called partial shear connection. A slab uses profiled steel decking in place of a steel section, and force is transferred via embossments and certain aspects of the deck geometry (rather than discrete shear studs).
Force is transferred between the two materials by friction and, where needed, discrete mechanical connectors, including shear studs that may be attached to an embedded steel section.
Assuming that there is no temporary propping, the steel part of a composite cross section must alone resist self weight and other construction loads as the concrete at that stage is ineffective.
The drivers that are relevant to a particular project will affect which flooring system is the most appropriate. This form of construction offers a number of advantages - the decking acts as external reinforcement at the composite stage, and during the construction stage as formwork and a working platform. The effective span range for this type of solution is around 6 to 12 m, which therefore makes it a competitor to a number of concrete flooring options.
They provide the opportunity to achieve longer spans (20 m or more) than are possible using a 'standard' solid web, rolled downstand beam. The unique ASB - Asymmetric Slimflor Beam - is rolled with a wider bottom flange than that found at the top.
Because the slab sits on the bottom flange, a key consideration is torsion applied to the beam.
The shallowness of the floors is achieved by placing the slabs and beams within the same zone. Some solutions achieve composite action between the steel beams and concrete by using discrete mechanical shear connectors - much like a downstand beam - whilst others achieve this primarily by wedging the concrete between the steel flanges. The steel is galvanized and may be of varying thickness, although about 1 mm is typical.
Composite slabs are normally designed to be simple spanning at room temperature, but continuous under fire conditions. As with all composite elements they are attractive because they play to the relative strengths of both steel and concrete. This is the first time that guidance has been given in a code for use in the UK, which may explain why composite columns have been rarely used to date. Concrete infill adds significantly to the compression resistance of the bare steel section by sharing the load and preventing the steel from buckling locally. Rectangular sections have the advantage of flat faces for end plate beam-to-column connections (using Flowdrill or Hollo-bolt connections).
In theory they appear to be attractive, as slab reinforcement can be used to avoid the need to add to the steelwork connection, for example with extra rows of bolts in an extended end plate.
Both ends of the board slide into the anchor joints or stacking joints and screw into place. If you are in any way dissatisfied with a product you ordered, we'll exchange it, replace it or refund your money within 30 days of purchase. When cutting these boards, I recommend using a fine-tooth saw, especially if you are going to use a hand saw. There's lots of margin for error, since the tabs on the anchor joints will cover any ragged edge if your cut is not smooth.
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Its success is due to the strength and stiffness that can be achieved, with minimum use of materials. The reduced self weight of composite elements has a knock-on effect by reducing the forces in those elements supporting them, including the foundations. This usually happens if the applied loading is at a low enough level, for example, in the common cases where a beam design is governed by construction stage or serviceability considerations.
The relative benefits decrease with span, as the size of the steel beam increases relative to the size of the slab. Particular detailing is required for the shear connection when precast units are used, so that the body of the precast units can be mobilised as part of the concrete compression flange.


This geometrical form is common to all shallow floor solutions, as it enables the slab to sit on the upper surface of the bottom flange - rather than the upper surface of the top flange as found with downstand beams. For internal beams this will mainly be a concern during construction, requiring appropriate sequencing to minimise torsion.
An added benefit is that a flat soffit is achieved - there are none of the interruptions found with downstand beams. For ASBs the composite action is enhanced by the presence of a profiled pattern rolled into the upper surface of the top flange. Because it is so thin, there is a need for stiffeners to avoid local buckling when it is acting as a bare steel section to support the wet weight of concrete and other construction loads. Designers obtain the relevant information (implicitly) from software or brochures provided by the decking manufacturers. This continuity is achieved thanks to nominal reinforcement, which also fulfils other roles such as crack control, that continues over intermediate supports (its influence - assumed to be beneficial - is ignored for room temperature design). Two hours fire resistance can be achieved without the need to fire protect the steel decking. This can result in a high resistance for a relatively small cross sectional area, thereby maximising useable floor space.
Rules are provided for composite H sections, either fully or 'partially encased' (web infill only), and for concrete filled hollow sections. The gain in fire resistance may be at least as valuable, especially if it permits the column to be left unprotected or only lightly protected. However, it is difficult to achieve the correct detailing for composite connections, because the needs for strength, stiffness and ductility can border on the mutually exclusive - too little reinforcement will reduce connection ductility (rotation capacity) because of potential rebar failure, too much will reduce ductility because of concrete crushing failure.
You can stack them to whatever height you want, but we feel the height of 4 timbers is about as high as you should go. This is because the boards, which are hollow with an internal stringer, are not strong enough to support a longer length of garden soil without bowing. Design of composite beams and composite slabs(for buildings) is now covered by BS EN 1994-1-1[4].
The profiled metal decking that forms the basis of the composite slabs is sandwiched between the base of the stud and the top flange, and the welding process joins all three together.
However, codes also specify a certain minimum degree of connection that is needed to prevent excessive slip between the steel and concrete, which would result in failure of the connectors. When acting compositely the top flange of a steel beam is restrained laterally by the slab, but during construction lateral torsional buckling (LTB) may reduce the effective resistance - only when the decking runs transversely and is properly fixed does it prevent LTB - further guidance is available covering both design, in SCI P359 , and detailing, in SCI P300. The decking is lifted into place in bundles, which are then distributed across the floor area by hand. Tata Steel also produces fabricated beams based on open and hollow sections (SFB and RHSFB) with a plate welded to the lower flange to achieve this geometry. Edge beams must be designed to carry torsion in the final state, making the RHSFB a good option given the torsional resistance and stiffness of hollow sections. Encasing the steel sections within the slab also has benefits in terms of fire performance, with (often) no need to use added fire protection. Friction then prevents interface slip as the beam is loaded and deflects, providing a reliable form of shear connection to assure the strength and stiffness of the composite element. Additional reinforcing bars may be placed in the decking troughs, particularly for deep decking. The re-entrant stiffener shown at the top of the decking not only stiffens the upper flange but can also be used to support hangers for relatively lightweight items suspended from the soffit.
Infill concrete retains free water which in other situations would be lost; its latent heat of evaporation significantly delays temperature rise. After that the weight of the soil, especially after a rain, can make the bed bow out in the middle of the span. The floor depth reductions that can be achieved using composite construction can also provide significant benefits in terms of the costs of services and the building envelope. They are sometimes required in shallow decking when heavy loads are combined with high periods of fire resistance. Interlock is achieved through embossments (dimples) that are rolled into the decking profile, and by trapping the concrete around the re-entrant parts of the profile. Composite columns requiring formwork during execution tend not to be viewed as cost-effective in the UK. It also fails to explicitly recognize the benefits to be had when the beam has regularly spaced large web openings, or is only part utilized in bending (because SLS considerations govern design). There are no standard decking profiles, so the interaction achieved by the embossments, etc of each propriety deck is different. SCI P405, published by SCI in 2015 as a replacement for NCCI produced by SCI (Pn002a ) allows the minimum degree of connection to be relaxed when certain criteria are satisfied.



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