The aim of this article is to highlight the requirements that may exist for a given building project, and indicate how these requirements should drive the designer towards the most appropriate, sustainable, potentially reusable and cost-effective choice of floor system.
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The range of steel based floor systems is presented in general terms, with the advantages and disadvantages of each system identified so that these can be compared against the requirements of a given project. The article does not go into technical detail about the different types of composite, long span, and shallow floor solutions.
What drives the choice of floor system?
Different buildings have different requirements, so not surprisingly there is no 'one size fits all' most appropriate solution. Clearly the requirements vary depending on the type of use, but there are also some more subtle issues to consider and these are highlighted below.
It should not be forgotten that when considering intended use, it may be appropriate to pay attention to a different use in the future - many steel solutions offer flexibility that can result in high levels of sustainability over the lifetime of a building.
Simplicity and familiarity
As a rule of thumb designers should adopt the simplest solution that will meet the project requirements. Generally speaking the simplest solution will also be the most common, and familiarity will facilitate the design, fabrication and erection processes as no new learning is involved.
Within the context of steel floor systems, simple also means less labour and cost . For example, the simplest solution of a downstand solid web I-section beam as opposed to a truss means fewer structural elements, less fabrication , fewer surfaces to be fire protected and less time to design.
It is worth adding that this 'simple is best' philosophy also extends to frames as a whole - a simple braced frame will normally be a more economical solution than, say, a moment resisting frame.
For some projects the need to reduce to a minimum the construction time (on site) may play a determining role. Indeed, time is often one of the key drivers for choosing a steel solution. The need for speed may be driven by, for example, fitting in with vacation breaks for educational buildings , or bringing in income (e.g. retail buildings). It can lead to consideration of options that minimise wet trades on site (use of precast floor units), minimise the number of crane lifts and provide working platforms during construction (profiled steel decking), and that do not require propping between floors.
Services integrated within the structural floor depth
The volume of services needed in a building is clearly a function of the end use - hospitals being an obvious example of a highly serviced building - and design philosophy adopted by the services engineer, e.g. air-conditioned, naturally ventilated, etc.
When a lot of service ducts are to be accommodated it may be beneficial to adopt a floor solution that provides a flat soffit in order to maximise the flexibility in routing these ducts beneath the structural floor. It will also be easy to remove and/or replace these ducts to meet future needs.
Solutions that provide a flat soffit may not suit the longer spans needed to facilitate a column-free adaptable floor area. An alternative in a building that is both highly serviced and requires long-span floors is to integrate the services within the beam depth (as shown to the right), so that the total depth of structural floor plus service zone is minimised.
Need for adaptable space
Open floor area providing flexible, adaptable space
One of the long-recognised benefits of steel frame construction has been its ability to span significant distances. This is particularly true when composite solutions are adopted, given the efficiencies of that form of construction. This spanning ability allows the number of internal load bearing walls and columns to be minimised - open floor spaces can be created, or non load-bearing partitions (that are easily moved) used to form (temporary) individual areas. Adaptability may be more sustainable than deconstruction and reuse, for which steel is also suited. In recent years a number of steel framed office buildings have been reconfigured to provide residential units.
Daylighting requirements
'Deep' floor plans may mean that, for example, office workers are a long way from natural lighting. Long span solutions may not then be the most appropriate solution for certain situations, where a short span design (for example using shallow floors ) with an internal atrium may provide a more appropriate internal environment. The designer must seek the best compromise.
Aesthetics
If false ceilings are used then the aesthetics of the soffit of a given structural floor system are clearly irrelevant. However, clients may require exposed soffits, specified primarily so that the thermal mass of the floor is exposed. The soffit must also then be visually appealing. In some cases the presence of downstand beams interrupting the soffit may not be welcome, although it is also true that an expressed structure may be desired. A number of steel framed options may therefore be appropriate depending on specific requirements.
Acoustics
Deansgate, Manchester – office technology applied to an apartment building
The speed with which steel-framed buildings can be constructed, combined with excellent performance in service, was one of the reasons why steel frames with composite floors played such a central role in the boom in the multi-storey office market in the UK in the late 1980s and 1990s. When designers wished to transfer this technology to residential buildings some years later, it was recognised that possibly the biggest difference in requirements was issues associated with acoustics .
Good detailing is needed to avoid flanking issues, where sound travels around a barrier (such as a floor) by passing through an adjoining wall. An example, in accordance with the guidance provided in SCI P372, is shown below. The SCI has also developed an acoustic performance prediction tool for separating floors and walls to assist designers and architects.
Numerous apartment buildings have been constructed using steel frames, with a combination of good detailing and proprietary products used for raised floors, providing the necessary levels of performance. Deansgate in Manchester was an early example of this 'technology transfer' (see right).
Junction of a twin light steel frame separating wall with a shallow composite separating floor
Fire resistance
Fire resistance requirements depend on the use and height (number of storeys) of a building. Fire resistance periods between 60 minutes and 120 minutes are typical. The most common solution adopted to provide fire resistance is to protect the steel members so that they remain at a sufficiently low temperature (recognising that some loss of steel strength as temperature increases is acceptable as loads in fire are reduced compared to loading at ambient temperature). Intumescent coatings, (paint-like substances that char and expand with temperature to provide an insulation layer), are often used. If the steel elements are embedded in concrete this can provide the necessary insulation. Other options include board protection and the use of a cementitious spray.
Alternatively, when a 'fire engineering' approach is adopted the steel members are designed so that they are sufficiently strong, even when material strength has been lost due to exposure to fire, to resist the appropriate levels of loading. Extensive guidance, based on full scale fire testing of complete buildings, is available (SCI P375)
Exposed concrete floors supported on steel beams and used to provide thermal mass
Provision of sufficient thermal mass is an important part of a low energy building solution. The mass provides a heat sink that absorbs heat during the day, and then in combination with natural ventilation the heat is purged during the cooler night time. Composite floor slabs may even be constructed with integral water ducts to aid this purging. It is important that the thermal mass is exposed - so false ceilings may be a problem, as is plasterboard attached with dabs to otherwise massive walls. Horizontal elements (floors) are much more effective at providing mass than vertical elements.
When deciding how much mass is needed it is important to consider the occupation pattern of a building. Massive structures can absorb a lot of heat, but they also provide thermal inertia when wanting a building to heat up rapidly. There is a common misconception that a very massive building is necessary to exploit the benefits of thermal mass.
Floor stiffness
Stiffness is needed to ensure that a floor behaves correctly from a dynamic point of view, thereby assuring user comfort. This is a complex subject, as the real issue is how the floor responds (in terms of acceleration), which is a function of a number of variables including stiffness and the mass that is mobilised. The traditional approach, which is recognised as being crude, for designing a floor to respond acceptably is to check its natural frequency and compare that with a limiting value (which is a function of the floor mass). A more thorough approach is recommended, which often yields less conservative but satisfactory, results. See SCI P354.
A web-based Floor response calculator is available that allows designers to make an immediate assessment of the dynamic response of a floor solution. The software reports the results of approximately 19,000 arrangements of floor grid, loading and bay size, which have been investigated using finite element analysis. The results from this software provide an improved prediction of the dynamic response compared to the ‘manual method’ in SCI P354. The software may be used to examine complete floor plans or part floor plans, comparing alternative beam arrangements.
The required behaviour depends on the function for a given building/room. Some uses are less tolerant to floor movements (e.g. an operating theatre). Some uses (e.g. a gymnasium within an office building) are more likely to cause problems and warrant particular attention to avoid other building users being affected.
There has been considerable interest in the benefits of designing for deconstruction . The ability to dismantle a building and use the components again elsewhere is clearly attractive from a sustainability point of view, and steel lends itself to such a solution.
Research has been carried out on innovative shear studs, which allow composite slabs to be readily separated from the steel beams. Other initiatives involve the use of cross-laminated timber (CLT) used as a floor panel, used compositely with steel beams, but may be removed from the supporting steelwork as the building is deconstructed. Guidance on the recovery, testing and design of reused steelwork is available in SCI P427. Digital records of completed buildings, including information on the steel grade, impact properties and chemical composition should facilitate the efficient reuse of steel structures when a building reaches the end of its useful life.
As noted above, unless project specific drivers suggest the adoption of a more sophisticated alternative, then the simplest solution should be chosen and this will normally prove to be the most cost effective.
Cost is a fundamental consideration in the selection of the frame and floor system. Late in 2016, the BCSA and Steel for Life commissioned AECOM to provide a series of building type specific cost comparisons for office, education, residential/mixed-use, retail and industrial buildings, based on actual buildings. The buildings selected were originally part of the Target Zero study conducted by a consortium of organisations including Tata Steel, AECOM, SCI, Cyril Sweet (now Currie & Brown) and BCSA in 2010 to provide guidance on the design and construction of sustainable, low and zero carbon buildings in the UK.
The cost comparisons presented in the ‘Costing Steelwork’ series update the cost models developed for the Target Zero project, and provide up-to-date costs for the alternative framing solutions considered for each of the five building types.
The cost comparison studies illustrate that for a range of building types, on a like for like basis steel frame and floor solutions are highly competitive. The studies also highlighted the importance of considering total building cost not just structural frame cost, as the choice of the structural frame and floor configuration will have associated impacts on many other elements, including the substructure, roof and external cladding.
Benefits of different floor systems
Slab options
Composite slabs
Decking being laid out on a steel frame
Composite slabs, comprising lightly reinforced concrete cast on profiled steel decking, are an option whether the beams are downstand or integrated within the slab depth for a shallow floor form of construction. The slabs are normally reinforced using an upper layer of mesh and, occasionally, additional bars in the troughs (usually for longer periods of fire resistance and heavy loads). Fibre reinforcement may also be used. Spans of up to 4.5 m can be achieved using trapezoidal decking (80 mm deep). Some so-called deep decking profiles also exist (over 200 mm deep), that can span 6 m or so without propping during construction.
Composite slabs are an excellent choice when speed of construction is important. Bundles of decking are lifted into place on the steel structure, for distribution by hand. The number of crane lifts needed, when compared with the precast alternative, is greatly reduced. The ability to stack the pieces of decking into bundles also reduces transport time and costs.
During construction, once in place the decking provides other benefits in terms of acting as a working platform for storage of materials. When appropriately orientated and fixed to the steel beams it can restrain them against lateral torsional buckling. See SCI P300 .
Composite floor systems
In the final state the ribs in the decking serve as void formers in the slab, thereby reducing the weight of floor construction with the knock-on benefits this can have. It is also possible to suspend services from the soffit of a composite slab, using anchors that are designed to slot into the decking profile.
Steel frame construction is primarily used for large, simple structures. But nowadays, a wide range of buildings, such as offices, schools, public buildings, and even residential dwellings are made from steel.
This growing use is largely due to the many benefits that structural steel provides. But as with anything else in the world, using steel to build has its downsides. In this article, we dive into the advantages and disadvantages of structural steel structures.
Steel frame constructions offer the following benefits, which may put them above brick, concrete, and other construction materials:
Steel is a very homogeneous and uniform material. This means that steel products tend to be very reliable and predictable. As a result, there is better quality control and consistency in structural steel pieces. And, even when factoring in different specimens or grades of steel, variations for yield stress, strength, and elongation are much smaller than in other construction materials, like wood.
Steel products are available in many forms and sizes, and can even be fabricated as a custom order. This means that structural steel can be made to bear specific loads in buildings of all shapes and sizes.
Steel fabrication can be much easier and quicker than other kinds of construction materials. And, because work is fast, these structural steel pieces can be delivered and put to use much faster — making construction and erection more economical.
Structural steel has the ideal strength-to-weight ratio among all construction materials. Despite their incredible strength, they are much lighter than weight-bearing wood or concrete products by 30 to 50%. This makes steel a very advantageous material to use for building frames and platforms.
Steel has enhanced resistance — it’s especially resistant to fire and combustion. Using steel may reduce the fire risk of your building significantly. There are also special flame retardant coatings you can apply to steel to increase its resistance.
Aside from that, they are also pest and insect resistant. So, they are immune to the damaging effects of burrowing insects and animals, such as rats and termites.
Further, they are moderately weather resistant. Depending on the steel grade’s carbon content, coating, and powder treatment, it can reliably resist rust, high winds, and other effects of inclement weather.
Additions to existing steel buildings are quite easy to make and attach. Connections between new and existing structures can be made quickly and erected effectively — it’s not uncommon to see entire floors or wings added to existing buildings. Another example is steel bridges, which are often widened with new steel platforms and lanes.
In every construction project, there is the possibility of scrap components and faulty products. But, with structural steel, there is the possibility of recycling or recouping costs. Steel sections can be reused after a temporary or damaged structure has been disassembled. Other pieces will have scrap value, even with extensive damage.
If you’re planning to make a high-rise building or long-span structure, then structural steel is the material for you. Countless tall commercial buildings, long bridges, and more are made of structural steel. This is because structures are sturdy and reliable, and can be reinforced with steel plate girders or trusses.
Steel products are also non-porous and can be fabricated into seamless products. This is ideal for applications that need impervious material. This is why steel is preferred for structures like water reservoirs, oil pipes, gas pipes, and the like.
For temporary construction, steel is always the preferred material. This is because structures are easily assembled and disassembled through a few bolts. And, as mentioned, transport and erection are more economical when using steel pieces.
Despite all the advantages that steel offers for construction, it comes with some disadvantages.
Steel is susceptible to corrosion, especially without protective paint, galvanization, or coating. So, they typically require regular maintenance and painting to reduce the risk of corrosion. If not properly maintained, steel structures can lose as much as 1 mm of their thickness to corrosion every year.
In most countries, steel is not a highly available material. As a result, their initial costs are higher when compared to other structural materials, which may be in abundance. Sourcing new steel for repairs or additions in this case can also be quite costly.
Steel is known for its high thermal conductivity. In some applications, this may be an advantage. But in most cases, this can cause the structure to be inefficient in energy consumption and insulation. You may end up spending more on your cooling systems to keep the inside of your building cool and comfortable and counteract the thermal conductivity of steel.
Because steel fabrication is precise, they are delivered on-site in their final form. While this is a good thing, it does pose a problem if measurements were off or sudden adjustments need to be made.
Steel is hard to alter when on-site. You can possibly cut them down to size or weld components together, but this is a time-consuming process that can only be done by experienced and licensed workers.
Steel structures are rarely finished on their own. These structures typically need other components, such as insulation, drywall, sheathing, and other supplementary components to complete a building. In some cases, the construction time and overall costs might outweigh the benefits of using such a material.
Taking in all the advantages and disadvantages of steel frame construction, it’s easy to see why this material is an increasingly popular one in the world of construction.
If you’re in the planning stages of your construction project and are unsure of whether steel is the right material for the job, get in touch with Regan Industrial Sales, Inc. today. Our team can talk you through our different steel products and fabrication services to help you make a better-informed decision.
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