Guide to Raised Access Flooring

What Factors Should You Consider When Choosing An Access Floor System?

When choosing a raised floor system, examine your specific application to determine the material, underfloor height, and weight load necessary for your facility. Much of the decision will depend on what's stored underneath, which could be a combination of data cables, electrical wiring, piping, and air handling systems. The elements to consider include:

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1. Application

Access floors are used in various commercial settings for many purposes. The particular application will affect many aspects of the raised floor. Different environments will require different heights and material construction. Common applications for raised flooring include:

• Data centers and server rooms: Raised floors are standard practice for data centers or the server room within an office building. Their usage is so commonplace that access floors are often referred to as access computer floors. Data centers will need to consider their configurations and whether they are using the platform for airflow or cabling. A low profile floor can work if it's accommodating cabling and not airflow. However, if using a raised floor for air circulation, a data center will need much more space under the floor. Separate solutions for cabling, such as a combination of overhead and underfloor cable management, may be required since thick cables could otherwise restrict airflow.
• Offices: While traditional raised floors are commonplace in data centers and server rooms, raised access floors are also a staple for office buildings and applications. They provide simple cable and electrical wiring management and may also be used for air distribution. Conference rooms and open office areas alike can benefit from a modular raised floor design with the ability to remove panels and insert new outlets easily. In an office setting, the primary consideration will be a low profile solution to maintain vertical space and ceiling heights. Applications without dense cabling needs should prioritize the lowest platform possible.
• Control rooms: Like data centers, control rooms contain equipment and often perform mission-critical operations. Found in IT departments, manufacturing facilities and factories, utility companies, and refineries, control rooms cannot afford downtime. Here, raised floors provide low-maintenance cable management and air distribution. When maintenance must be performed on the floor, cables, or air handlers, a raised floor's modular design will not disrupt control room operations.
• Educational facilities: Many older educational institutions have air quality issues linked to overhead and wall air distribution systems. As schools transition to smart classrooms with computer stations for every student or need enough outlets to support bring-your-own-device policies, a raised floor should accommodate both cabling and electrical wiring. When facilitating both airflow and wiring, the raised deck will need additional height.
• Casinos: The array of electronic gaming machines available at most casinos require access to electrical outlets throughout the gaming floor. Casinos also need to prioritize guest safety by keeping power cords and cables out of footpaths. To maintain a healthy gaming environment, casinos need to consider noise control and air quality, especially in facilities that allow indoor smoking.
• Emergency dispatch hubs and call centers: Telecommunications centers need and data cabling at every desk. In 911 call centers, each desk requires additional technology, since every dispatcher will have an array of monitors and communication devices to support critical operations. The dense cabling needs in these facilities require an efficient cable management system, accomplished through raised computer floors. Ease-of-maintenance is a vital concern since these facilities cannot afford downtime.

While many environments are well-suited for raised flooring systems, several others cannot support them. Access floors are not suitable for bathrooms, kitchens, and other areas that are exposed to moisture, especially when in-floor drainage is required. Any raised floor application will require subfloor assessment, and applications in high seismic activity zones will need additional considerations.

2. Height

While raised floors have customizable heights, they fall into two categories &#; low profile and standard, or full-height. Low profile floors are typically under 3 inches and can get as low as 2 inches. Low profile floors are usually the preferred options for retrofitting since they will not significantly impact ceiling height. They offer simple cable management and electrical wiring solutions.

While standard raised floors encompass any floor taller than 6 inches, they are usually at least 12 inches and can reach as high as 6 feet. Full-height floors are used whenever air distribution is necessary or when larger amenities, such as an 8-inch water chiller pipe, will go beneath the raised tiles.

If the floor requires underfloor air distribution, the height will typically be at least 24 to 48 inches. Access floors between 12 and 24 inches can support air distribution if that is the only thing they are used for. If wiring, cabling, and pipes are also necessary, a 24-inch raised floor may be too short. Your mechanical engineer can determine the exact amount of underfloor room you need to accommodate your ductless underfloor airflow, wires, cables, and pipes.

Full-height floors meant for air distribution are often too high for applications used just for wiring and cabling, as would be typical in a conventional office space. In general, it's best to use the shortest floor possible. A raised floor is measured by the finished floor height. So, when budgeting for underfloor space, subtract the tile's thickness and account for the floor hardware.

3. Material and Finishes

Access floors come in a range of tile materials for different applications and finishes that can be used to customize their appearance. Lightweight construction will be a priority for applications where frequent access to the underfloor is required. Other materials may be necessary to withstand the weight load or environment where they are installed. The materials available include:

• Concrete: Cement raised floor systems have a layer of lightweight concrete sandwiched between a steel top surface and a formed steel well. These tiles are the industry standard for most IT applications and are often used in telecommunication rooms, mission-critical facilities, and assembly areas. Finishes include high-pressure laminate, vinyl, and bare steel designed for carpets or rubber flooring.
• Calcium sulfate: These raised floor system tiles have a heavy-duty core of recycled calcium sulfate with an ABS sealed trim. The upper surface is finished with high-pressure laminate or vinyl and the bottom is made from galvanized steel. These tiles provide the best resistance against humid environments and changing temperatures.
• Hollow steel: Hollow tiles are a lightweight option to facilitate frequent maintenance without sacrificing strength. The panels have a smooth steel surface and a hollow formed steel well on the bottom layer. They come with the same finishes as concrete panels.
• Wood core: Wood core tiles perform best for acoustic control while providing excellent durability with a commercial-grade composite wood core. These tiles come encased in galvanized steel on both the upper and lower surfaces. Their finish options include high-pressure laminate, vinyl, and bare steel painted finishes.
• Aluminum: Aluminum floor systems are best for high-tech environments or applications that require antimicrobial, easy-to-clean surfaces. Die-cast aluminum panels are the industry standard for everything from clean rooms, medical centers, and X-ray rooms to biomedical environments, microelectronic manufacturers, pharmaceutical companies, and laboratories. They're also a suitable option for data centers and are available in high-pressure laminate, vinyl, and bare steel finishes.

Choosing a finish involves matching the access floor to the rest of the building's architecture. Finishes allow for a customizable appearance, which can be valuable in an upscale office building or any environment that clients will see. They can also offer a practical application, with bare steel finishes designed for carpeting or non-slip rubber flooring.

4. Cooling System and Airflow

A raised floor system facilitating airflow distribution needs perforated tiles to allow the HVAC system to condition the facility. Adequate airflow prevents data centers and server rooms from overheating and maintains equipment by regulating hot and cold spots.

Standard airflow panels allow for 22% to 35% airflow, and universal high output air grates allow 55% or 66% airflow. Consult an HVAC engineer to determine how many perforated panels a particular layout will need. Be sure the specialist has experience with data centers and server rooms and understands the need for hot and cold aisles. 

As a general guideline, raised floors require one perforated tile per ton of air conditioning or one tile per 100 square feet of flooring. For the best results, use Computer Fluid Dynamics (CFD) modeling to understand the data center's airflow and identify hot spots. Too few perforated tiles will limit airflow and create recirculation, while too many will increase bypass air.

5. Cables

Data centers or other businesses with lots of equipment need efficient cable management techniques for their raised flooring systems. Server rooms may use overhead cable management while using the raised floor for airflow management. In this configuration, cable management won't be a consideration for access floor selection. 

If the floor is accommodating both cables and airflow, the raised floor will need to be high enough to ensure dense cabling does not disrupt airflow. Underfloor cabling in server rooms usually involves underfloor cable trays, which consolidate wires and cables and distribute them to the equipment housed in the data center.

Low profile raised floors are typically recommended when cabling won't be as dense, such as in standard office computer arrangements or computer bays in educational institutions. In a low profile access floor, cables can be threaded through cable raceways, which line the floor beneath the tiles and are built into a low-profile raised floor system. They can organize electrical and data wiring efficiently without the use of bulkier cable trays.

Whether using cable raceways or cable trays, grommets provide access points for cables under the floor to connect to the equipment requiring them. Ensure you select grommets that fit the size of the cords used, and consider whether you need air-guard grommets to prevent air leaks

6. Weight Load

Since access floor systems are raised above the subfloor, they need to support more weight than a traditional upper floor. If they support bulky equipment, like those found in a server room, they need to hold more weight. Many facilities install raised floor tiles with higher weight load ratings in high-traffic areas like walkways, and standard weight load panels in low-traffic areas. It's also important to consider the rolling load limit if heavy equipment such as scissor lifts or IT carts will be used.

The Pro Access Floors Concrete floor system can withstand an ultimate load of up to 6,000 pounds, with a minimum ultimate load of 2,400 pounds. The panels individually can withstand anywhere from 1,000 to 2,000 pounds, depending on the size and material.

Raised Floors vs. Access Floors

Some people tasked with ordering floors for cabling and air distribution wonder if they need a raised floor or an access floor. These two terms mean the same thing, and there are many terms that all refer to the same type of flooring system. The name "raised access floor" is prevalent in the industry, and some call them "false floors." They're also referred to as "access computer floors" when used for computer equipment specifically.

Some customers call raised access flooring systems "Tate floors," after the popular brand. Tate floors have become industry shorthand for raised flooring in general, thanks to the brand's reputation. At Pro Access Floors, our customers can take advantage of genuine refurbished Tate floors and many other pre-owned access flooring systems from the top names in the industry at up to 60% off.

Additional Benefits of Access Floor Systems

Besides their primary use of storing infrastructure, access floor systems have additional features. They include:

1. Portable

Traditionally, raised floors were permanent. The pedestals had to be bonded to the subfloor using anchors or adhesives, which meant they couldn't be used for temporary installations. Low profile access floor pedestals do not require permanent bonding, so the entire flooring system is portable. If a building moves its server room to another area during future renovations, the access flooring can be packed up and redeployed in the new location. Likewise, if the company rents its building or needs to move to a larger facility, the flooring can come, too.

2. Durable

Commercial access floor panels are quite strong and can withstand impacts ranging from 150 to 400 pounds of force. Raised flooring panels can last 25 years or even longer, and their supporting pedestals are known to last up to 50+ years. So, when a facility's access floor panels near the end of their life, it's common to replace just the panels while utilizing your existing floor under-structure. When companies replace panels for aesthetic reasons, their durability and long-lasting design allow facilities to sell them to a third-party for refurbishment, re-lamination, and reuse.

3. Easy to Work On

If a panel breaks or becomes damaged, it can be swapped out for a new tile. With regular flooring, it's challenging to replace a single panel. When damage occurs, the entire floor will need to be replaced, disrupting the whole office while the renovation takes place. Instead of tearing out an entire floor, an organization can save money and reduce inconveniences by replacing raised floor panels one at a time, as needed.

Raised floors also provide easy access to what is underneath. When cables need replacement or wiring, pipes, and HVAC systems require maintenance, the tiles can be removed. Since the flooring system is modular, maintenance professionals can remove tiles in their work area while leaving the rest of the floor intact. In mission-critical facilities, this can limit downtime related to maintenance and repairs for systems housed under the floor.

4. Accessible for Electrical Outlets

Large open offices and manufacturing facilities need electrical outlet access throughout their environment. Most electrical work in traditional buildings exists within the walls, and integrating electrical wiring into a conventional, non-raised floor can be a significant undertaking. If the building instead has an access floor, it is much more affordable. Electricians can run electrical wiring under the floor. The modular access floor design allows outlets to be inserted anywhere they are needed, even after the platform is already built.

5. Low Profile

One reason many buildings don't have access floors is the perception that they take up feet of space. While full-height access floors have a finished floor height of 6 inches to 6 feet, low profile designs are sufficient for cable management alone. They take up significantly less vertical space, so a building can maintain relatively high ceilings. Access floors also save space by consolidating wiring. By keeping bundles of cables below the floor, facilities can reduce tripping hazards and leave more floor space open for pathways, furniture, or servers.

6. Easy to Rearrange Office or Floor Layouts

Server equipment usually needs replacement every two or three years, and data centers are often poised to expand quickly. As a result, both server rooms and data centers need to rearrange their server rooms to accommodate new servers and a larger footprint. A raised flooring system makes it simple to add new perforated airflow tiles rather than calling in an HVAC specialist for new ductwork. Since it is simple to add new cable grommets, electrical outlets, and airflow panels to access floors, these facilities can easily accommodate new and expanding equipment needs. 

When rearranging an office space, electrical outlets and cable grommets can move, too. Offices can rearrange desks for the best space-saving and most productive layout without being restricted by outlet and cable locations. This feature can help high-growth companies looking to make space for more employees without moving to a new building.

How Should I Select a Raised Access Flooring Provider?

In the access flooring market, you'll find a range of suppliers across many different price points. With so many options, it's essential to work with a provider who can offer the highest quality of service at an affordable price. The traits to look for in an access flooring partner include:

1. Experience

A raised floor is a critical piece of a building's infrastructure and needs to meet strict safety requirements. Therefore, the raised floor provider needs to have a wealth of experience and product knowledge to help their customers find the right solutions for their buildings and install them correctly. A provider that also offers consulting and installation services is a sign of an experienced, trustworthy supplier. At Pro Access Floors, we've been in the business for over 25 years and have a team of experts ready to answer questions and install raised flooring systems nationwide.

2. Guaranteed Work

An access flooring provider should be able to guarantee a quality installation. Pro Access Floors guarantees both our low prices and our service. The new flooring systems come with a 1-year warranty, and our refurbished flooring systems come with a 90-day warranty. After offering the lowest price guaranteed, Pro Access Floors can also handle pre-installation, installation, and de-installation nationwide.

3. Fast Delivery

A professional raised flooring provider should offer fast turnaround times, so renovations can continue on schedule. At Pro Access Floors, we offer next day shipping on most orders, both new and pre-owned.

4. Inventory

A flooring provider must be able to deliver the style of tiles and understructure needed in the correct amount to cover a building's square footage. A large data center or multi-story commercial application will need to work with a provider that keeps a large stock of all the tiles, finishes, and understructure offered.

At Pro Access Floors, all our flooring systems are in stock and ready to ship. Our customers don't have to wait for their selections to be manufactured. Our refurbished flooring system selection includes many of the most trusted names in flooring, which are also ready to ship next-day.

5. Quality of Materials

While many access flooring providers offer similar materials, they don't always come with the same specifications. When assessing quality, ensure your provider offers high concentrated load ratings, superior rigidity, stability, and durability. Depending on your application, you may need tiles that are water-resistant and rot-proof. Look for flooring systems with a class "A" flame spread rating with strong acoustic and static performance. Consider your environment and ensure your provider can accommodate your humidity and temperature conditions to prevent de-lamination.

Call Pro Access Floors to Discuss Your Access Flooring Project

Pro Access Floors has an unmatched service record. We've served hundreds of Fortune clients with 100% satisfaction for more than 25 years. We guarantee our low prices and our installation and have a vast inventory of flooring systems ready to ship the next day. We work with high-quality materials rated to withstand high weight loads and offer options for waterproofing, temperature resistance, high-tech environments, and other applications. 

If you have questions about your access floor requirements or are ready to discuss your project, call us at 858-566- to get started. If you already know what you need, fill out our quick quote form to get pricing and freight costs fast.

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.

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.

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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.

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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.

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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.

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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.

For more steel flooring panelsinformation, please contact us. We will provide professional answers.

Additional reading:
Phenolic Foam Insulation for Superior Energy Efficiency

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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.

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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 s and s. 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

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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.

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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 , 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 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.

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Benefits of different floor systems

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Slab options

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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.

A number of methods can be used for controlling the concrete level during construction. The concrete depth may be kept constant, or the upper surface may be kept level. Depending which of these is chosen the weight of concrete will vary, so it is important that the designer communicates clearly with the site team. See SCI AD410. Further guidance on the installation of metal decking is also available.

When an exposed soffit is required - to expose thermal mass - a thermally transparent suspended ceiling may be used. The additional surface area of the soffit created by the decking (as opposed to a flat concrete face) can be beneficial.

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Precast units

Erecting precast floor slabs on a steel frame
(Image courtesy of Severfield (Design & Build) Ltd.)

Precast concrete units may be used in conjunction with steel beams. The units may be solid or hollow-core, and with tapered or bluff ends. They are normally prestressed. The beams may also be structurally connected to the slab units to make them 'composite', provided specific detailing rules are satisfied to ensure that the steel section and concrete (in-situ topping plus the precast units) act together. SCI P401 gives further information on this.

Floors using precast units offer a number of benefits. The spanning ability of the units is such that the spacing of secondary beams can be increased (compared to when traditional decking profiles are used). The construction system is most efficient for column grids of approximately 9 m by 9 m. The units provide a flat soffit.

For semi-exposed applications, such as car parks, precast units may be a more durable alternative than steel decking (although with the correct detailing and coatings it is certainly possible to use decking in such applications).

Precast floors

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Downstand beam systems

Trapezoidal decking installed on downstand beams

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 through deck welded shear studs. This form of construction offers a number of advantages - the decking acts as external reinforcement at the composite stage, and during the construction stage acts as formwork and a working platform. It may also provide lateral restraint to the beams during construction. The decking is lifted into place in bundles, which are then distributed across the floor area by hand. This dramatically reduces the crane lifts when compared with a precast based alternative.

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. 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. 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. See SCI P401 for more information.

A number of variations on the idea of downstand beams is available to meet long-span needs. The use of long span beams results in a range of benefits, including flexible, column free internal spaces, reduced foundation costs, and reduced erection times. Many long span solutions are also well adapted to facilitate the integration of services without increasing the overall floor depth.

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Shallow floors

(Image courtesy of Kloeckner Metals UK Westok)

The USFB system

Shallow floors offer a range of benefits such as minimising the overall height of a building for a given number of floors, or maximising the number of floors for a given height of building. Additionally, a flat soffit is achieved - there are none of the interruptions found with downstand beams - which gives complete freedom for the distribution of services below the floor. These benefits should be considered in the context of a given project to identify when they are most appropriate.

The shallowness of the floors is achieved by placing the slabs and beams within the same zone. This is achieved by using asymmetric steel beams with a wider bottom than top flange, which enables the slab to sit on the upper surface of the bottom flange with adequate bearing, rather than the upper surface of the top flange as found with downstand beams. The floor slab may be in the form of a precast concrete slab or a composite slab with metal decking (either shallow or deep decking may be used). An added benefit is that some forms of shallow floor construction inherently achieve composite interaction between the beams and slab, thereby enhancing structural efficiency.

A number of shallow floor solutions are available, including Ultra Shallow Floor Beams (USFB) from Kloeckner Westok, and ArcelorMittal's Slim Floor solutions.

• USFB with precast hollocore slabs
  (Image courtesy of Kloeckner Metals UK Westok)

• USFB with deep decking
  (Image courtesy of Kloeckner Metals UK Westok)

Kloeckner Metals UK Westok&#;s USFB system comprises a shallow and asymmetric Westok cellular beam with reinforcement placed through the cells to anchor the slab to the beam. This simple detail provides a straightforward and cost-effective arrangement to prevent disproportionate collapse and is also used to resist torsion in the final condition. For composite slabs with metal decking the reinforcement is placed in the troughs of the metal decking. With hollowcore slabs, the reinforcement is placed in alternative cores of the precast unit. To restrain the top flange of the USFB in the Normal Stage, the insitu concrete should be cast flush with, or with at least 30 mm cover over, the top flange.

USFB cross-section
(Image courtesy of Kloeckner Metals UK Westok)

The USFB is manufactured from standard rolled sections, and is available in increments of 1 mm depth. They are typically 150-300 mm deep and are sized and designed using Westok&#;s freely available Cellbeam software package based on each individual project requirements and floor grids etc. The software carries out all of the necessary structural checks, including torsion checks in the Construction Stage. USFBs can economically span up to 10 m with structural depths that compare very favourably with R.C. flat slabs. As such, they are popular in many sectors, particularly Education, Commercial and Residential.

&#;Plug Composite Action&#; can be mobilised for USFBs, which has been demonstrated using full-scale laboratory testing, to further enhance the capacity of the section. To mobilise &#;Plug Composite Action&#;, the following detailing should be adopted:

• Composite slabs with metal decking: Concrete cast level with, or above, the top flange
• Precast units generally: Minimum 50 mm topping level with, or above the top flange
• Hollowcore units: Every 2nd core broken out and filled with concrete and reinforced through the cell
• Solid in-situ slabs: Concrete cast level with (or above) the top flange

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