loading considerations on scaffolds for design (2 Viewers)

outward bound

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Hi guys, i know this might sound a simple answer i just want your thoughts on the clarity - when calculating the loads on the scaffolds how do the designers generally calculate the live loads and dead loads - the love loads such as the materials is normally stacked on the outside 2 boards while the 2 or 3 boards on the inside will provide access for people - now because the load is concentrated on the 2 boards there is more weight placed on the outside standards so my question is this...do the designers split the weight over the inside and out for example 3kn per m2 = 1.5kN on the inside and out or do you calculate a higher percentage on the outside standards for this reason - in addtion the guardrails fixed on the outside of the scaffold means a higher dead load on the outside standards so is this taken into consideration as a concentrated loads over the outside standards accordingly

thanks guys but just clarity

OW
 
Self weight is self weight and occurs where it occurs, so for leg load you would calculate the material content on the outside leg including legs, ledgers, trans & braces along with guard rails boards and toe boards the same calc is performed for the inside leg which may or may not have guardrails etc.

The 3kN/mSq is exactly that its a uniformly distributed load (UDL) across the decked area between the standards and is considered shared between the two standards, in addition to that you may have inside boards from which the imposed loads are added to the inside leg.

There may be an argument that over the height of a scaffold the loads balance out via brace transfer etc but its not one I use
 
hi alan, its what i thought but wanted to double check, taking into account that materials are mainly concentrated on the 2 outside boards if access is left on the inside of the scaffold for 3 boards - but then its hard to say how many people might be working there and walking on the scaffold i guess it all depends on how they manage and use the meter square loading on the scaffold.
 
Hi OB, as Alan says it is far simpler to just apply the same UDL on all main boards between the standards (you couldn't control where the scaffold is loaded anyway or use 2 tags). For a design you can use fully loaded inside boards or just use 0.75 kN/m² and state this clearly on the design. I've never seen a design with 2 different live loads on the main boards as you may have say the live loads on the outside then the operatives on the inside using the load (bricks, tools or whatever) so the load would be roughly equal anyway when they are undertaking the task. I always add up the dead load of the inside and outside standards separately as the guardrails, toeboard, deadmen etc. make the outside standard heavier unless you have maybe 2 inside boards plus live on the inside (for an independent). This doesn't matter for birdcages as the load is generally the same over the whole area (although you can apply a heavier load over 6.0m² and have 0.75 kN/m² on the rest of the platform in accordance with BS EN 12811-1 if justifiable but this generally makes no difference to the worst case loading we usually calculate). TG20 also advises the main live load on 1 lift and 1/2 the live on another lift but this needs to be agreed with the site if you know they will be using more (or less) working lifts. Floors of permanent buildings are generally designed for a UDL across the whole area too to simplify for the designer/engineer and they usually also have a point load (concentrated load) requirement too (as do scaffolds) which vary depending on load class.
 
so although the udl has been worked out you also need to stipulate what the point loading is on transoms and ledgers based on the worse case i.e the middle of the bay..
 
You need to ensure that the boards, transoms and ledgers can support the concentrated load given in table 3 of BS EN 12811-1 although for standard scaffolds this is rarely a major factor. If you know there is a point load from a pallet truck, wheelbarrow, load trolley, vehicle or similar then you need to calculate that load separately and apply it to the scaffold. The concentrated load and UDL aren't generally applied at the same time.
 
would i be correct in saying that in the calcualtions that will be applied from tg20 that a safety factor of 1.3 or 1.5 should be included when calculating the dead loads in the scaffold
 
No, you are not correct. If you are using permissible capacities of components (eg tables 5.10) to check that the loads applied to the components don't overload them the input is purely the dead and live loads without any increase. In permissible stress design you would only introduce factors to the loads to deal with adverse situations like overturning where you need to make sure that the structure or system is stable.
 
thats what i thought - please tell me if i am wrong but the safety factor for the SWL of tubes and fittings have already been considered in EN39/BS1139 and EN74 which is reflected in the TG20 in the table for safe axial loads of steel tubes
 
thats what i thought - please tell me if i am wrong but the safety factor for the SWL of tubes and fittings have already been considered in EN39/BS1139 and EN74 which is reflected in the TG20 in the table for safe axial loads of steel tubes

That's correct. TG20 applies a factor of 1.65 to the calculated axial capacity and there's also a small factor included for manufacturing tolerances.
 
The data presented in TG20 tables 5.10 does not quite support what the text suggests and although they quote 1.65 as a global safety factor, we are probably better to just use the figures from the tables. That saves having to work out the critical euler loading for the effective length you are using and then applying partial factors different types of loads and the materials. The end answer is probably good enough to deal with real life whichever way you do it so it is better to go for the simpler method if you can.
I find that it helps to work out the total design imposed loading on the complete scaffold, convert it to tonnes and then consider whether that is likely to be exceeded or even approached so that you get a feeling for how close the design is to real life.
 
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