Bridge beams have evolved into the versatile and capable stalwart of the scaffolding sector that they are today. Chris Harrison, director at Apex Scaffold Design, looks at their development as an integral part of scaffolding.
Looking back 20 years, steel ladder beams and steel unit beams were all we had. Although they were strong, robust and versatile, their use was limited due to their weight – or, more accurately, their strength/weight ratio. Where a unit beam wasn’t sufficient, we had the giant leap to a Mabey Bridge, massively strong, un-versatile and heavy. Even though they still have their place today – ladder beams especially are particularly versatile – in my experience, scaffolders aren’t keen on them.
Developing better beams
Demand for improved beams was initially driven by the need to span temporary roofs further, moving from 450 deep aluminium beams to the 750 deep beams used in system temporary roofs from the likes of HAKI. Inevitably, these beams were adopted and used for other applications. As the scaffolding sector has developed, more and more beams have been introduced, with the use of aluminium saving weight, and a deeper beam achieving increased strength. Now we have a plethora of beams available from various manufacturers in various sizes, weights and strengths, meaning we can span further, build higher and support more loads than we ever could.
Figure 1 is a useful comparison for relative beam strengths. We can see that steel unit beams, even in pairs, are pretty useless at spans over 20m; at this length, the allowed load on a twin steel unit beam is 13 kN (1.3t) – this is only 0.65 kN/m along the full length of the beam, equivalent to a single lightweight platform. Whereas the modern aluminium beams will span well beyond 20m towards 30m and still offer a good, useable safe load capacity.
You can see in Figure 2 an example of when we were asked to design a 16m span bridge to support a tall and therefore heavy scaffold. The combined dead and imposed load the bridge needed to support was around 170 kN (17.0t), which did not include the bridge beam’s self-weight. The comparison graph (Figure 1) shows that the safe working load (SWL) for a twin 1200 beam is around 90 kN and therefore 180 kN for a double pair (4No. total – pair inside + pair outside), which is what we adopted – DESSA Asterix HD beams in this particular case.
Figure 1 also shows that for 750 aluminium beams/steel unit beams, each pair will support around 35 kN/20 kN respectively. This indicates that a total of 5No. double chords (10No. total) of 750 beams and 9No. double chords (18No. total) of steel unit beams would be necessary. Practically, this isn’t a workable solution – and this situation demonstrates how the development of stronger beams is allowing us to span further and build higher.
Challenges of working with stronger beams
The example in Figure 2 demonstrates one of the issues presented by these deeper, stronger beams. By spanning further and building higher, the loads supported at each end of the bridge are increased. In the steel ladder/unit beam days, a good rule of thumb was one standard per beam chord; the end reaction from each beam would, generally, not exceed the coupler or standard capacity, but this is no longer the case.
Large heavy duty bridges require large heavy duty support towers, as can be seen in the example where the end reactions were over 110 kN (11.0t) in total at each end. Heavy duty support towers with forkheads and steel spreader beams were required to support the loads, and in this particular case to spread the loads onto the supporting structure.
Another challenge is linked to the fact that beams have always been reliant on how they are laced and braced. And as stronger beams allow us to span further and build higher, this becomes even more important. Any beam’s bending capacity is directly connected and reliant on how the beam is laced and braced; the published figures for any beam will be based on the optimum restraint arrangement which will be stated by the manufacturer. If beams aren’t laced and braced correctly, this will significantly reduce the beam’s capacity.
Deflection can also become an issue, particularly as the span increases, and should be carefully considered in the design. In general, deflection limits adopted by engineers in the design of permanent structures need not be applied and can be relaxed when designing bridge beams in scaffold structures. Most of the new stronger beams are manufactured from aluminium which, by its nature, will deflect more than a comparable steel beam. As spans increase, we need to consider not only the aesthetics of excessive deflection but also the integrity of the scaffold/structure being supported.
An example of this are the ties in an access scaffold supported by a long span beam. In this case, the tie arrangement will need to be such that it can accommodate the vertical movement resulting from the deflection without detriment to the tie and fabric to which it is attached.
What’s the future?
We already have even stronger beams coming to market. Apollo has a 1500 deep X beam capable of supporting some very impressive loads. They are more than double the strength of 1200 deep lattice beams and boast a capability of a 30m span being able to support 10 tonnes shared by two beam chords. This beam was recently used by Tiger Scaffolding and Alwyn Richards on their impressive project for North Yorkshire County Council in Northallerton (the AccessPoint Spring 17 Site Report) to span 20m and support a large temporary roof.
How deeper and stronger do we think beams can go?
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Chris Harrison TISrtuctE EngTech