Benefits Of Steel Construction
Dr. R Kuberan, Editor
Civil Engineering and Construction Review
In the multi-storey building sector, the benefits of steel construction
are largely related to the fast-track nature of the construction
process, which leads to a wide range of financial and process
benefits. Many innovations associated with the construction process
have further improved these inherent benefits and have increased
efficiency and productivity. This is very important in inner city projects
where lack of space for storage of materials and other facilities,
limitations on deliveries and logistics, and planning constraints, mean
that a higher proportion of work should be done in the factory and
less on site.
The benefits of steel in multi-storey construction arise mainly
from its prefabricated nature, its lightweight and the ability to phase
the various activities in series rather than in parallel.
Speed of Construction
Speed of construction is the most important benefit offered by steel construction, which leads to financial, management and other logistical benefits, many of which can be experienced in economic as well as sustainability terms. For an eight-storey office building, it is found that steel construction is up to 20% faster than reinforced concrete. But, importantly, the construction of the primary frame and floors is up to 40% faster and allows for early start in building services, installation, cladding and other activities. The fast construction process
is based on a synergistic use of steel frames, steel decking and in some cases, concrete or braced steel cores.
The financial Benefits of Speedy Construction Includes:
- Early completion, which leads to reduced interest on the borrowed capital and to early return in terms of revenue
- Lower cash flow
- Reduced management costs on-site, primarily due to the shorter construction period, but also due to the fewer personnel employed
- Reduced hire costs of site facilities
- Greater certainty and less risk in the construction process
Speed of construction is achieved by ‘just-in-time’ delivery of components and by rapid assembly of the steel framework. It is estimated that a single tower crane can install up to 20 steel elements per day, which corresponds to a floor area of approximately 300m2. Secondary benefits in steel construction arise from:
- Decking, in ‘bundles’ on the beams and installation of decking at a rate of up to 500m2 per day
- Avoidance of temporary propping by using steel decking spans of 3-4m for profiles of 50-80mm depth
- Fire protection by intumescent coating that is applied in the factory and, therefore, eliminates the time required for this process on-site
- Opportunities for reduction in the amount of fire protection by use of fire engineering analysis
- Use of mobile installation platforms to improve construction safety and speed up the installation process
- Prefabricated stairs that are installed as part of steel construction package
- Safety barriers can be attached to the perimeter steel beams
- Rapid concrete placement
- Light steel infill walls and partitions that are installed rapidly and can be prefabricated
- Modular service units that may be installed with the steelwork package
Steel construction of all types is lightweight, even when including concrete floors. The self-weight of a typical composite floor system is typically only 40% of that of a RC flat slab. When the total building weight is considered, a steel framed structure is up to 30% lighter than the equivalent concrete building, which leads to an equivalent saving in foundation costs. Further, steel construction is the preferred solution for building on:
Dr. R Kuberan,- Post-industrial or former built-on sites, often with pre-existing foundations
Civil Engineering and Construction Review
- Building over underground services and tunnels
- Building on railway lines and other ‘podium-type’ structures Steel construction virtually eliminates waste by the nature of its manufacturing process and all steel waste is recycled. Synergistic materials such as plasterboard can also be recycled.
Benefits of Adaptability
General expectations for all multi-storey buildings change substantially during their design lives. A building’s occupancy is also likely to change several times during its life. Increasingly, the nature of the occupancy may change. In the 1960s and 70s, many buildings we reconstructed to minimise cost without any allowance for future adaptation. These structures have not proved capable of responding to occupant’s changing needs, leading to their early demolition. Although difficult to quantify at the proposal development stage, there are clear qualitative benefits in specifying a structure that is inherently adaptable to changes in requirements during its design life. Key issues on adaptability are:
- Specifying longer spans, permitting greater flexibility of layout
- Providing space for additional services
- Specifying floor loadings that permit change of occupancy
Some examples of steel buildings are given below.
Office Building, Bishop’s Square, London
The Bishop’s Square Project near London’s Broadgate area comprises a composite steel structure of 18m span and only 650mm depth. There is an almost fully glazed façade and a ‘green’ roof space on three levels. The 12-storey building of close to 80,000 m2 floor
area comprised approximately 9,500 tonnes of steelwork and was erected in only 30 weeks out of an overall 20-month construction programme. Fire protection, in the form of intumescent coatings, was applied to off-site in a single operation by the steelwork contractor, which speeded up the following trades. The highly glazed façade was designed to satisfy onerous thermal requirements, which led to the use of triple glazing with integral louvres. Photovoltaic panels were installed on the roof to provide an energy source for lighting, thereby reducing running costs and CO2 emissions. The floor-to-floor height was only 3.9m, which necessitated a beam depth of only 650mm as part of a 1050mm overall floor zone. The 9m span heavily loaded primary beams had large rectangular openings and were tapered in depth close to the concrete cores to allow for distribution of large ducts. Secondary beams were designed as fabricated steel sections with a series of 425mm diameter circular openings of 425mm depth x 750mm length close to mid-span. An imposed load deflection limit of only 30mm was specified, which was achieved by beams of 138 kg/m weight with no stiffening.
Luxembourg Chamber of Commerce
The headquarters of the chamber of commerce of the Grand Duchy of Luxembourg was designed by Vasconi Archietcts and comprises an existing building and 20,000m2 of new office space. A conference centre of approximately 8,000m2 was provided together with 650 underground parking spaces on four levels. The total building area is 52,000m2 including car parking. The four and five storey composite structure consists of hot rolled steel sections and concrete floor slabs with integrated IFB sections
(a rolled asymmetric section with a wide bottom flange). The integrated steel beams are stiffened using a lightweight truss below the beams, leading to a 40% increase in span. Services are passed below the beams and through the truss to minimise the floor depth. The structure was assessed by a fire engineering analysis, which demonstrated that 60 minutes fire resistance could be achieved without additional fire protection. The IFB beams are particularly protected by the concrete slab and support the reduced load in fire despite the loss of the exposed truss.
Kings Place, Kings Cross, London
Kings Place in north London provides seven floors of office space, a 420-seat concert hall, are galleries and restaurants. The basement levels house the auditorium and other facilities. The flexible use structure is designed as a steel composite frame consisting of 12m span fabricated beams with multiple circular openings and supporting a 130m deep composite slab. In some areas, the composite floor is supported on a shelf angle. A novel part of the design was the fire engineering strategy, which demonstrated that the fire resistance of 90 minutes could be achieved by intumescent coatings only on the beam connecting directly to the columns; other beams were unprotected. The columns were protected by two layers of boards. The long span fabricated beams are typically 600mm deep and consist of multiple 375mm deep openings. The 130mm deep composite slab is reinforced according to fire engineering principles, which permit development of membrane effects in fire.
The primary and secondary beams connecting to the columns are protected by 1.6mm thick intumescent coating that was applied offsite to speed up the construction process. The coating was applied in a single layer, which was achieved by designing slightly heavier steel sections to reduce the load ratio in fire conditions. This holistic approach
was justified using a finite element model in which the properties of the steel and concrete were modified for the temperatures in both a standard fire and natural fire concept using the fire load and ventilation conditions established for the building use.
A.M. Steel Centre, Liege
The five-storey Steel Centre in Liege, Belgium is an innovative office building designed to achieve a high level of energy efficiency. It is 16m x 80m on plan and consists of an off-centre line of internal columns to create beam spans of 9m and 7m. The longer span secondary members are 500mm deep and are placed at 3m spacing, which support a composite floor. The secondary members use IPE330/ IPE300 sections to create cellular beams with regular 400mm diameter openings. The 9m span primary cellular beams are the same depth and use HEB320/HEA320 sections.
A fire engineering analysis was carried out to demonstrate that the composite beams could be unprotected except for those connected to the columns. The columns are concrete filled circular hollow sections, which are unprotected and achieve the required fire resistance, leading to a considerable reduction in fire protection costs. The building is supported on piles because of the poor ground conditions. The self-weight of the structure (<350 kg/m2) and of the curtain walling system was important in minimising the loads on the piles.
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