Lifetime costs: Structural floors

There are dozens of ways to build structural floors, so choosing the one to suit your needs can be difficult. There are so many things to consider, such as whole-life performance cost and durability implications of common timber, steel, concrete and composite structural floors.

Structural floors

<B>Common floor constructions include:</b>

• Simple timber joists for spans up to 5 m or 6 m. Timber joists can be used in combination with secondary concrete or steel beams for wider spans. More timber-based composite I-beams are used in commercial developments and housing.
• Concrete-suspended floors are cast in situ. Pre-cast beam and block construction or pre-stressed planks with structural toppings or T-beams forms are used for 15 m spans or more.
• Steel floors may be of cold-rolled sections, usually for shorter spans. Hot-rolled sections for wider spans and greater loads. Specialist slim floors are used to reduce the depth of the structural floor for a given span and load, incorporating beams with cut-outs for flexible service provision.
• Composite floors, comprising steel structure, permanent steel shuttering with a concrete topping provide a fast-track solution in commercial schemes.

<B>Whole-life costs</b>
Structural elements should be designed to last the life of the building with minimal or no maintenance. As there is generally no maintenance, the whole-life costs associated with structural floors are generally defined by their initial cost.
Costs that may be incurred in the life of a floor may include checking and cleaning of air bricks to suspended-ground floors, repairs to ceiling or floor finishes due to deformation or creep of floor structure.
The cost of structural floors increases with the span and loads the floor is designed to sustain. But deciding which type of floor to use involves more than cost; the structural floor can't be viewed in isolation. In a large building with extensive services, the floor structure will affect installation of those services. It may be possible to design "holistically" so that the floor contributes to a reduction of services. For example, many buildings use concrete floors as a thermal store; allowing night-time cooling to reduce the need for air-conditioning.
Timber floors are generally the least expensive but single spans are limited to 5–6 m. Spans may be increased by design of double or triple floors using beams or girders, usually of steel, to carry timber joists. As costs increase, alternative floor constructions such as concrete or steel become viable.

<B>Whole-life performance</b>
The concept of whole-life performance provides a framework within which to manage decisions and appraise risks at each stage of a building's life starting from concept through design, construction, use and end of life.

<B>Concept and design phases</b>
At concept stage, the functional requirements of floors are determined, including anticipated loads, spans, sound insulation, fire proofing and integration with services. Once these are established, choices about the type, material and risks associated with alternative floor structures can be evaluated. The aim being to establish a solution that provides best overall value within the constraints of the design concept, the site and building market. However, there are typical trade-offs that need to be assessed, including:

• Speed of construction versus cost. In the case of office blocks, steel frames with composite floors of concrete on permanent metal shuttering allow for fast-track building to realise early income from office letting. In the domestic market, beam and block floors or pre-stressed concrete-floor planks are favoured as they provide a platform to work from that is not affected by inclement weather.
• Greater spans increase the depth of the floor, but reduce the number of columns and potentially the foundations, if pad or piled foundations are used.
• Where buildings require extensive services, floor-to-ceiling heights may be reduced by running services within the floor depth. Slim-deck floors, castellated or tapered beams can be specified in these cases.
• For suspended ground floors, it is advisable to use polystyrene infill blocks with pre-cast concrete beams to provide insulation as well as a surface for screed.

<B>Typical risks to consider</b>

• Moisture levels and risk of wetting in service showers are a particular hazard. This may lead to use of timber with preservative treatment, stainless-steel reinforcement to concrete, higher grades of concrete more resistant to moisture or enhanced protection to steel members.
• Deflection. Design standards define the acceptable deflection: for timber, 1/300 of the span. For steel floors, deflection is related to the finishes; if brittle finishes such as screed are used, acceptable deflection should be no more than 1/300 of the span for other finishes 1/200; concrete floors may deflect to 1/350 of the span or 20 mm whichever is the lesser. However, excessive loads may cause deflections in wide-span floors, which may induce cracking to floor finishes.
• Vibration of floors in use can be a problem where floors are thin and of a long span. Risk of vibration should be minimised at the design stage.
• A balance has to be struck between specifying the load-bearing capacity of the floor and potential future redesigns of the internal layout of buildings. Over-specification for loads usually results in a more expensive floor structure.

Construction phase

In spite of the wealth of advice and good practice guidance available, floors are not problem-free. Common construction failings that impair the performance of floors and may need repair or replacement work include:

<B>Timber floors</b>

• Inadequate bearing to joists – normal requirement is 75 mm or 90 mm where lateral restraint is provided to walls to maintain structural integrity. If a bearing stress check is carried out, these may be reduced.
• Blocking or strutting not provided between joists to prevent twisting or rotation and minimise deflection.
• Joists not fitted tightly and accurately in metal joist hangers. Excessive cutting of joists or packing with compressible material may result in deflection or settlement or give the floor a "springy" feel.
• Excessive or wrongly positioned notches and holes cut through joists for services weaken the floor.

<B>Steel floors</b>

• Deflection of steel beams associated with wet concrete needs to be designed for, not just the dried mass of concrete floors.
• Structural steel located externally or in the perimeter of the building requires additional protection to maintain its longevity. Damage to this protection during erection may compromise its longevity.

<B>Concrete</b>

• Inaccurate setting out of walls and inadequate connections to walls may result in lack of bearing and failure of concrete pre-cast beams or pre-stressed planks.
• Where pre-cast concrete beams protrude into the cavity of the external wall they may provide a route for moisture penetration.
• Concrete beams are placed askew or set too far apart may result in reduced bearing for blocks, which could lead to displacement of blocks if point loads are applied.
• Insufficient or no grout between concrete beams and blocks will compromise the strength and sound insulation of separating floors.

<B>Suspended ground floors</b>

• Inadequate ventilation under suspended-timber ground floor increases the risk of rot from condensation.
• Gaps in insulation under ground floors result in higher heat loss and higher heating costs throughout the building's use.
• Lack of moisture protection from damp-proof courses or membranes to the underside of timber joists where they are supported on masonry may result in rot.

Use phase

Costs associated with floor structures, during the life of a building, are generally the result of poor design, workmanship or unforeseen changes. As these costs are unpredictable, a well thought-out and executed design is important. Timber floors in particular are springy, so some cracking of ceiling finishes should be expected and result in additional costs for floor maintenance. Protective coatings to steelwork may need recoating during the life of the structure.

End life

Reuse and recycling are options that may make a net contribution to the whole-life cost. Otherwise costs would result if materials were taken to a landfill site. Timber and steel joists and beams can be re-used – sometimes in shorter lengths. The grade of structural steel should be marked to give certainty of performance in future uses. Concrete, steel beams and reinforcement can be crushed and recycled. Steel and timber can be dismantled to reduce the negative environmental effects of demolition.

Factors affecting durability

Where structural floors are internal, the environment dry and the building heated, the risk of deterioration is minimal and a life beyond the nominal 60-year design life can be expected.
Moisture is the main agent of deterioration. Timber rots and steel reinforcement, beams and sheeting all corrode in the presence of moisture. Progressive rot or corrosion gradually reduces the bearing capacity of floors resulting ultimately in failure. Fortunately, for occupants, there is usually warning and failure is rarely catastrophic.

Mechanisms of failure

There are many causes for concrete failure. And some, by no means all, common failure mechanisms include:

<B>Carbonation of concrete</b>
One of the more common causes of concrete failure is carbonation. Failure results from breakdown of the alkaline properties of the concrete. It is the alkaline nature of concrete that protects the steel reinforcement. Without this protection, steel tends to corrode once exposed to moisture and oxygen.
The bad news for concrete internal floors is that acids in the atmosphere can and do carbonate the concrete, reducing the alkaline protection for the steel. In fact, carbonation rates are most rapid in warm conditions. In internal environments, however, there rarely is enough moisture present to cause steel corrosion. The maximum corrosion rate for reinforcement in concrete occurs at a relative humidity of 90-95%. To minimise the risk of failure of concrete floors specify:

• Low water-cement ratio.
• High cement content to increase density and alkalinity.
• Sufficient cover depth.
• Good compaction and efficient curing to minimise formation of cracks and voids.

<B>Creep and deformation</b>
Pre-cast floors may become permanently deformed under sustained loading over a long period of time, due to creep. The risks can be minimised by designing for sustained loads. Deformation is most significant in heavily loaded and wide-span structures over 12 m.

<B>Steel floors</b>
The most vulnerable components for steel-framed buildings are those most likely to be affected by water penetration; external members, steels within the external perimeter and ground floors. Risks are associated with condensation and wind-driven rain. Vertical steel members are usually affected, particularly where protective coatings are damaged. The edges of structural floors may be vulnerable to premature corrosion, although this is rare. Lightweight, cold-rolled steel structures affected by corrosion will have a much shorter life than hot-rolled sections.