Industrial Steel Buildings, Design Considerations

Lightweight steel portal frame building

Steel Frame

There are two major types of steel frame used for portal framed buildings today. First there are frames designed and manufactured with traditional hot-rolled steel. These tend to be robust structures, built from standard beams and sections. They are relatively heavy and may require substantial foundations. Hot-rolled steel frames are capable of covering large spans and may be more suitable for some monoslope designs. The frames are usually provided primer-painted and may require finish painting and occasional maintenance.lightweight galvanised steel portal frame structure

Many buildings these days are designed and manufactured using lightweight cold-rolled steel sections.

Although lighter in weight than hot rolled sections, the cold rolled steel is very strong (the rolling process actually enhances the strength of the steel) and allows a fully-engineered design of frame with lower foundation loadings. In addition, the sections, together with all cleats and brackets, are fully galvanised, which makes the frame virtually maintenance-free.
Each of these two types of steel frame has its own role to play. The cold-rolled steel frames are usually most cost-effective in small to medium-sized buildings (up to 20m span and 6m eaves) and in areas where low foundation loadings are important, while hot rolled frames are more suitable for the larger spans. A further option for larger buildings is a multi-span design, where two frames (either hot or cold-rolled) can be erected in parallel, sharing a central row of column.


 

Cladding and Insulation

Most steel buildings are clad with profiled steel sheets, which are available in a range of colours and finishes. There are also architectural profiles and special finishes which are available at substantial extra cost. In general, standard box-profile cladding fitted vertically, is by far the most cost-effective option.

Where an insulated building is required, composite panels, which consist of a layer of insulating foam sandwiched between two steel skins, are normally used these days – in the past, a built-up system using mineral fibre between outer and inner steel sheets, separately applied, was used; however this type of system is less thermally efficient and is less common nowadays.
When a building is to be heated to domestic or commercial levels (i.e. where people are permanently at work in it) then it needs to comply with Part L2 of the Building Regulations. This requires the calculation of the CO2 emissions, thermal performance of the building fabric and services such as heating and lighting, airtightness, and efficient energy use. These calculations must be carried out by a suitably qualified competent person, and the building must demonstrate compliance with the required criteria in all these areas.
Because much of the information required to carry out the calculations (such as details of the heating and lighting systems) is not available at the time of designing a steel building, we are unable to demonstrate full compliance with Part L2. However, our design and specification is based on materials with the U values that meet or exceed the requirements of Table 4 of the Design Standards Section 1 of Part L2A. We also follow the guidelines of the leading manufacturers of insulated panels and fixing systems in specifying, detailing and installing materials and components to ensure that the required levels of thermal efficiency and airtightness are achieved.

Where insulation is required to deter condensation, or when the building is heated only to a low level, then a thinner composite panel can be used. These buildings are exempt from Part L2 as stated in this extract from the regulations:

From Part L2A of the of the Building Regulations, Page 12.
Buildings that are exempt from the requirements in Part L
16 Examples of buildings which are industrial sites and workshops with low energy demand include buildings or parts of buildings designed to be used separately whose purpose is to accommodate industrial activities in spaces where the air is not conditioned. Activities that would be covered include foundries, forging and other hot processes, chemical processes, food and drinks packaging, heavy engineering and storage and warehouses where, in each case, the air in the space is not fully heated or cooled. Whilst not fully heated or cooled these cases may have some local conditioning appliances such as plaque or air heaters or air conditioners to serve people at work stations or refuges dispersed and not separated from the industrial activities.

Brick and Blockwork

Brickwork or blockwork walls can easily be incorporated into a steel frame structure. They will affect the frame design to a small degree. Erection of these walls normally lakes place after the frame has been erected and before it is clad

Doors

Roller shutter doors are the most popular and economic choice for goods doors. These will normally have galvanised laths and be hand operated by means of a chain. Options include electrical operation, coloured laths, Windlock guides and insulated laths, all of which incur additional cost. In general 0.6m (2ft) should be allowed above the door opening to allow space for the roller.
An alternative is the sectional overhead door, which uses hinged panels (with optional vision panels), these doors run on a track which protrudes into the building, usually horizontally, but which can be arranged to follow the internal roof line. Again, these can be both manually or electrically-operated.
Personnel doors are usually designed to be outward-opening and are usually made from steel for maximum durability and security. They are usually supplied primer-painted for customers to finish to their own requirements. Fire exit doors are of similar design and finish and are fitted with an internal panic latch or push pad but no external handle or lock. It is a requirement of Building Regulations that no part of the building should be more than 18m from a fire exit.
Combination doors are also available. These consist of a personnel door with external lock, together with an internal panic latch which overrides the locking mechanism, allowing people to escape from a fire, even if the door has been locked. These can be useful in smaller buildings, where access is restricted and it is not practical to provide separate personnel and fire exit doors.

Windows

Although buildings are normally provided with rooflights to around 10% of the roof area, there may be occasions where windows are required. Windows will normally be double glazed, UPVC or aluminium framed, and can be provided with opening casements and accessories such as trickle vents.

Boundaries and Firewalls

Where a steel-framed building is close to the boundary with another property, i.e. land that belongs to somebody else, then a ”boundary condition” may apply. If a wall of the building is between 1m and 5m from the boundary, then Building Control may ask for 1-hour firewall protection to that wall. This means that the composite panels will need to be upgraded to a fire-resistant version and that the frame will have to be specially designed so that when it collapses (after at least 1 hour) it will collapse inwards, away from the boundary: In fact, the building inspector is entitled to insist on firewalll construction if the building is within 25m of a boundary, but most inspectors would not go this far. Provision of firewalls imposes a modest increase in cost.
If the building is to be less then 1m from a boundary, then a 4-hour firewall I will be required. This cannot be achieved with composite panels and, under these circumstances, the only solution is to use brickwork or blockwork for the wall.
Whenever a building is close to a boundary it should be borne in mind that access will be needed in order to clad it. The minimum requirement is at least 1.5m of level hard standing; in the case of tall buildings, more clearance may be required.


Size and Layout

The size and shape of your building are determined by several factors. Foremost are the needs of your business. It may be that the height and width of the doorways are critical, it may be that there are minimum dimensions required by legislation (e.g. for MOT bays). In many cases the shape and size of the plot available will determine the building’s dimensions.
It is usually most cost-effective to minimise the span and eaves height of the building – increasing either of these has a disproportionate effect on the cost of the steel frame; equally buildings that are very tall and narrow will be expensive, as the wall area is large in relation lo the floor area. The most economical cost per square metre (floor area) is usually achieved by designing a rectangular building whose length is between 1.5 and 2 limes it’s width (span) and whose height is between 4m and 5.5m. Buildings which are oddly shaped (i.e. L-shaped or T-shaped) will incur significant extra cost. All buildings should be designed on a flat, level base.
The most cost-effective roof is symmetrical, with a central ridge. Offset ridges and monoslope designs again incur significant extra costs.


We hope you find this information useful in planning and choosing what type of building that best suits your needs, further articles are als

Please also see our Steel Portal Framed Buildings page for details on our products and services.