In our recent blog post entitled ‘What is Effective Length’ we described the basic fundamentals of effective length and fixity and how they determine the capacity of the strut length. So how is this applied to IDH Scaffold Design and are Tie Patterns important?
Recap here on, ‘What is effective length?‘
Many designers take the effective length of a scaffold standard, arranged in a basic scaffold configuration, as 2m. I.E. 2m Bays and 2m lifts with conventional tie patterns, ledger and face bracing. However, this is incorrect and this is demonstrated in TG20:08 table 23 where, irrespective of tie pattern, the minimum effective length is 2.7m for a basic scaffold with a leg capacity of 18.2 kN and not the 29.1 kN a 2m length would otherwise give us.
This is confusing and TG20 does not satisfactorily explain why this is the case, albeit it goes some way using a Putlog scaffold as described in Fig 37 and Cl 18.104.22.168.
TG20 would do better to explain there are ‘multiple’ effective lengths in our basic scaffold – some standards have Le =2m and some Le = 2.7m. As the 2.7m Le standards have the lowest capacity it is these which would fail first and as such control the overall structural capacity of the scaffold.
The key wording is in Table 22 where it describes the ‘Vertical interval between lines of ties’ which is subtly different to vertical distance between ties. This can be demonstrated with Tie Patterns A and D in Fig 9 where pattern A shows a 4m vertical interval between lines and D, a 2m interval.
Hence, when we now look at Table 23, we can see for a 2m bay the effective length for a standard with a Type A Tie pattern is 3.2m and for a Type D Tie pattern it is 2.7m, giving leg capacities of 13.6 kN and 18.2 kN respectively. This is why the Tie Pattern is important!
But how can different standards have different Le’s? This is getting complicated now as we have to look at the scaffold as a unit, and no longer as individual components. As we described before the Le is the length which is ‘effectively restrained’. By understanding the importance of Ties and Tie Tubes (see ‘How do ties work’), we can analyse two cases – ledger braced standards and non-ledger braced standards.
When we consider the ledger braced standard, assuming the tie tubes, face brace and Type D tie pattern are installed correctly, we can say these standards have an Le of 2m and are therefore the strongest standards in the structure. But when we look at the adjacent un-braced standard, we have a condition where there are no ties on that standard vertically and the only restraint to stop this tube buckling outwards / inwards is the stiffness of the ledger tube (and possibly handrails). As there is no plan brace, this ledger can deflect outwards / inwards and as such only offers partial restraint to the standard to prevent it buckling. Therefore the Le in Table 23 is a function of how much the ledger can deflect – larger spans = larger deflection = less restraint = larger effective length in this standard = less capacity.
By Tim Burt