Overturning

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LDJ

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Good afternoon all,

I'm looking for some feedback on overturning calculations from my fellow scaffold designers.
Since I began designing scaffolds, I was taught, like many scaffold designers that the criterion for overall stability is a factor of safety against overturning or sliding of 1.50 and have therefore always factored my overturning moments by 1.50.
I have been reviewing a scaffold design this week where the designer has used the load combination 0.67 x (self weight + dead) + 1.0 maximum wind to calculate the maximum uplift/tension in the standards. This method in effect divides the permanent loads restoring moments by 1.50 to calculate the required anchorage/tension resistance.
This load combination is given in TG20:13 clause 4.3.3.2 (Load combinations for permissible stress / working load design) with further explanation and examples provided in clause 4.3.1.1. along with the statement "increasing the wind load by 50% produces a 250% increase in the tension/anchorage force and the required tension/anchorage resistance is underestimated by 57% if calculated at working load. In order to obtain a "correct" estimate of required tension resistance or overturning stability for a load combination involving opposing load effects using analysis at working load it is necessary to un-factor the favourable or restoring loads by 1.50."
Whilst this method resulted in a reduction in the amount kentledge required (which my clients will be thrilled about) the real issue for me is the reduced factor of safety against overturning (1.26 in this case) which directly contradicts TG20:13 sections 4.3.1, 4.3.1.1 & 3.9.1. "the ratio of righting moment to the overturning moment should not be less than 1.5"
Do any other designers use this method?
What are my fellow engineers thoughts on this approach?
 
Well I've got a few minutes so I'll wade in with this as a starter.

The basic requirement is to make sure that the structure does not overturn when the design overturning loading is applied to it. BS5975 allows you to multiply the overturning moment (wind plus notional loads) by 1.2. If the structure does not overturn all is well. (If the restraining force is provided by holding down bolts, you can take the working capacity and multiply it by 1.2 to achieve satisfactory restraint as well). The 1.5 factor quoted in TG20, which came from BS5973 also appears in Eurocodes but in them there is a complication in that the restoring loads should be factored by 0.9. I know that considered opinion is that the figure of 1.2 in BS5975 should not be used and that the Eurocode figure should be taken. If you combine the 0.9 & the 1.5, you get an overall factor of 1.67 which is used to compare characteristic loads.

(I have been designing falsework schemes for a number of years now and can't remember any that overturned when the factor of 1.2 as introduced by BS5975 was applied.)

Anyhow, we have 3 factors concerning overturning which are published in current standards, 1.2, 1.5 & 1.67. As far as I can see, it is justifiable to use any of them after due consideration.

Do not get confused by the worked example in TG20 - there is a fallacy in it. I suspect I know who got it put in and I think that I know what it was intended to show but it is wrong so I wouldn't take any notice of it.

Also, don't get mixed up between load factors of 1.5 and safety factors of 1.5. Because they have the same value, it does not mean they are the same thing.
 
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