July 21st, 2016
Cellular and Porthole Beams™ have become a mainstay in construction projects across the world, providing added strength, reduced weight and convenient options for integrating building facilities. A number of production methods can be employed to create both straight and curved sections however, each has its own advantages depending on the demands of the application.
Greg North, current Regional Chairman of the BCSA (The British Constructional Steelwork Association) and Commercial Director for Barnshaws, one of the UK's leading producers of straight and curved cellular and Porthole Beams™ lays out the options for cellular beam construction and the associated merits of each approach.
In the construction of modern buildings, cellular beams are able to span wide areas and can be made part of the design language of the structure, enabling architects to implement new aesthetics into steel structures. These properties have made the perforated beam sections a regular feature on stadia, theatres and public buildings – allowing steelwork to be exposed to increase the grandeur of larger structures.
A straight cut decision
Cellular beams can be manufactured in straight or bent form, and usually form part of the roof or floor support structure. Each beam has a number of cells or apertures / holes cut into the section at intervals depending on the design. The holes both reduce weight and provide convenient access for cabling and pipework in addition to sometimes being used as a design feature.
Traditional castellated cellular beams that are characterised by the distinctive hexagonal shape of the openings have been superseded by the modern form of cellular beam. Using the latest design software and profiling techniques allows the overall beam depth, cell diameter and spacing to be specified to best suit the application, including varying all aspects across the span of the beam. These cells can be specified to enable the integration of electrical and data cabling, heating, AC and ventilation systems, without intruding into the space below the beam. Air conditioning ducts and wires can be passed directly through the holes in the beam, allowing space saving throughout the structure.
In larger, multi-floor structures, the ability to minimise floor-to-ceiling depth by running the various building services through the beams can save sufficient space for an additional floor to be included. Compared to a more traditional construction with universal beams, employing cellular beams maximises the potential space and improves the long-term profitability of the building.
CAD design software is used to calculate the load bearing requirements of a structural beam, the dimensions of which are determined by a number of factors. The decision to use a cellular beam will affect what is possible, while the choice of original steel section used to create the new beam, plus the manufacturing method will also have a large influence on the beam's size and characteristics.
Essentially there a three ways of achieving a cellular beam:
1. Cutting the required profiled holes out of a standard sized parent 'I' beam to create a Porthole Beam™.
2. Profiling staggered semi circles into a standard 'I' beam before cutting it into two halves along its length, aligning the cells and welding the beam together to create a deeper beam than the original.
3. Assemble a beam from three strips of plate steel, welding one to another to create an 'I' beam having cut holes in one of the strips.
Each system has its advantages, depending on the application, but the rules change slightly when creating a perforated beam which then needs to be curved to add strength and/or aesthetic appeal to the finished structure.
Cutting holes into a solid beam is the simplest process and does reduce weight, the holes are cut into the beam after it has been bent to the required radius. Once a beam is bent, it makes positioning the holes on the machine bed more complex, and with more acute angles of bend in the beam it makes the work piece more difficult to handle. It is usually the cheapest method, but typically produces a heavier ported beam.
Expanded cellular beams (cutting an 'I' beam in two and reassembling it) can result in a beam up to 60% deeper than the parent section, without increasing the overall weight, allowing greater spans to be created from smaller section solid 'I' beam stock. Welding two profiled tees together also allows for a lighter top tee to be combined with a heavier lower section to provide the required load capacity, which minimises the overall weight of the section.
Disadvantages of this process can include waste generated by the offset of a cell radius; the difficulties of integrating haunch and shear points and the time taken to weld the tees into a complete section. Due to the design of the sections, omitting cells in a beam is particularly difficult and requires forethought to integrate haunches and shears. In addition, the centre-line weld must be tested before the beam is installed, which increases the overall manufacturing time.
Introducing an aesthetic and functional curve
The process to curve a cellular beam starts with the creation of the two profiled tee-sections. The profiles must take account of the subsequent bending process; the radius of the holes in the lower tee-section will increase during bending while they will decrease in the upper section, so have to be compensated for when cutting the holes before being bent.
This slightly more complex cutting profile depends on the specified curvature of the beam and the radius of the holes. Usually two top tee-sections are produced from one original universal beam and each is partnered with a similar bottom tee after they have been bent to the required specification. The matched pairs are then aligned in a jig and welded together.
The process becomes slightly more complicated when the beam needs to be curved and tapered in order to accommodate the growing trend for more aesthetically pleasing and even lighter structures, particularly in roof sections. Essentially, the choice of production method then includes beam construction from plate sections which can be cut and assembled, then bent, or, pre-bent in the case of the upper and lower sections, the centre is cut to profile and perforated and then the three elements are welded together to create the finished fabricated beam.
This final option is the most expensive, but essential when a tapered beam is required, however it can be cost effective when a curved beam with uniform depth is required, this tends to be dictated by the relative market cost of plate versus 'I' beam section. When the depth requirement and overall weight target of the final beam fall within a popular 'I' beam section size, then the economics tend to favour Porthole Beam™ construction over fabrication.
Making your choice
Porthole Beam™ construction has simplicity and speed on its side, with the ability to tailor the profile of every hole and make allowances for tapers and haunches. The production time is reduced without the need for welding and there are no testing requirements or potential issues with misalignment.
However, time saved via less fabrication can be lost due to CNC machine capacity and calibration. As holes are cut into the section after it has been curved, the bent beam covers more of the machine tool bed, restricting capacity compared to cellular beams that are cut when straight. In addition, each differing section will require new CNC programming to achieve the geometrically perfect finish.
Fabrication is a case of design demands, or availability versus pricing conditions in the steel market.
Through promoting strength and maximising the height factor of steel structures, straight and curved cellular beams in all their forms typify the strides in modern construction efficiency. Designers can take full advantage of choice with regards to structural beams, specifying the correct section for their demands. Fundamentally, the simplification of all production processes of cellular beams signifies their relevancy to the continued progress of the industry.
Click here for more information on the data required for specifying Porthole Beams™