Beams with a long span are designed differently.

 

Long-span Beam

Long span beams provide a number of advantages, including flexible, column-free interior areas, lower foundation costs, and faster steel erection. Many long span solutions can also be customized to allow for service integration without increasing the overall floor depth.

Long-span steel and (steel-concrete) composite beams are often designed according to BS 5950[1], BS EN 1993[2], or BS EN 1994[3]. Specific design guidelines, such as that on the design of beams with large web holes (see SCI P355), or manufacturer's information, are supplemented by this codified guidance for various types of beams. Such precise advice is frequently supplied in the form of design software and is usually based on extensive testing of a given product.

Options for long-span beams

The following solutions are listed in ascending order of spanning ability, with some overlap between them. The goal is to give a variety of options. Plate girders and beams with web holes are by far the most prevalent types of beams utilized today (be they cellular , fabricated, or rolled sections). Many solutions take advantage of the advantages of composite construction, which provides significant strength and stiffness gains over bare steel.

The use of parallel beams

The parallel beam method works well for spans up to 14 meters. Floor grids are made up of two levels of perfectly continuous beams that run in opposite directions. Services that run in either direction can be integrated into these two layers, allowing services to pass in any direction within the structural floor depth. Another advantage is that because the beams are totally continuous, the depth of the beams is reduced without the cost and complication of stiff, full strength connections.

A composite floor employing the parallel beam technique is seen on the left. SCI P074 provides specific design guidelines for this type of structure. Although this is based on BS 5950, the ideas can be applied to any Eurocode.

Web holes in composite beams

In order enable services to pass through a beam, web apertures are often made in the beam. This allows the structural and service zones to share space, resulting in a reduction in the overall depth of floor construction for a given spanning capability. Openings can also be made for aesthetic reasons, such as when supporting a roof with cambered beams. For spans of 10 to 16 meters, composite beams with web apertures have been shown to be a cost-effective solution.

The so-called cellular beam is a sort of composite beam with web holes that is created in a specific method and is thus explained individually below. Cutting the web apertures into the plate used to construct the web of a plate girder or the web of a rolled section is another option for forming the web openings. The best option to utilize is determined by the size, shape, and regularity of the apertures, as well as more commercial factors such as a desired supplier's procedure. Web-opening beams have no disadvantages in terms of erection or familiarity because they are very similar to a'standard' solid web beam.

Beams with web apertures must be designed with the knowledge that the openings bring a number of potential failure modes that are not present in solid web beams. The beam acts as a Vierendeel girder around the apertures, and web post buckling may dictate design (the web post is the section of web found between two adjacent openings, as shown in the figure below). Large holes may require strengthening to prevent the web posts from buckling.

Stiffened web holes in a composite beam

A composite plate girder with stiffened web holes is shown in the illustration to the right. Based on rigorous test programs that included fire testing, dedicated design guidelines (SCI P355) and software from specialist vendors are available.

Cellular composite beams

Cellular beams have several regular web apertures and are made by breaking two rolled sections longitudinally into two Tee pieces. The two Tees, which may or may not come from the same donor section (see below), are then welded together to form an I-section with distinctive web holes (normally, but not necessarily, circular). The cellular beam manufacturing process allows the bottom half of the finished beam to be made from a heavier donor section than the top half, allowing the bottom flange to be much larger than the top flange. This makes sense when the beams are to act compositely, as they typically are, and a concrete flange effectively substitutes the upper beam.

BS EN 1994 specifies design guidelines for beams with up to three degrees of asymmetry (the area of the bottom steel flange divided by the area of the higher steel flange). The more asymmetry there is, the more stringent the criteria for a minimum degree of shear connection, which must be reached to avoid excessive slip between the steel and concrete elements.

Despite the fact that cellular beams have regular gaps, some of them may be infilled and/or stiffened to handle local features such as incoming beams or heavy point loads. To allow for the passage of bigger service ducts, double (oval) apertures may be included. Specialist manufacturers offer dedicated design guidelines (SCI P355) and software based on rigorous test programs that include fire testing. The diagram below shows

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