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Institution Forums > General > Early Steels yield assessment View modes: 
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Site User - 02/11/2017 09:31:38
Early Steels yield assessment
Good morning, we have some roof trusses within a building that were installed in 1939 of which where calculated for loadings by two seperate structural engineers.
Some conflict arose with assessment under the yield stress as outlined in BS 449 and how it should be interpereted.
To clarify our position we had samples (7Nr) of the steels tested by a UKAS accredited lab for the area of the truss where the highest loads both bending moment and shear where calculated to be.
As would be expected we had a range of results, all higher than the permissible levels as outlined in BS499 for that era.
The question is; with upper and lower limits of the test yields is there defined guidance on which result to use or is it simply the lowest figure, we are trying to ascertain actual engineering principal as opposed to opnion of the consulting engineer.
Look forward to hearing on this.

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Reginald Beeton - 02/11/2017 10:23:34
RE:Early Steels yield assessment
As you probably know by now, BS449 is no longer current but is still used by some engineers. The approach used in that standard was fundamentally different from the current approach, the current standard for which is BS EN 1993.

If you want to do a historical check, to see what the result would have been in terms of the old standard, based on what is known as 'permissible stress' methodology, you would be looking for a permissible stress based on yield or ultimate stress (UTS) divided by a 'safety factor'. This gave permissible stresses (not accounting for buckling) in the region of 165MPa, if my memory serves. To get a comparable figure from your data, it might be prudent to use a 'characterisitc' strength, as mentioned below, and then apply an equivalent safety factor.

If you want to assess the structure for current usage and residual life etc., it would probably be better to use current standards. For that you would usually start from the basis of a 'characteristic' yield strength (or stress). This is defined as the strength (or stress) below which not more than 5% of the specimens fall. You can determine this from simple statistics if you have sufficient data points from the tests. You could also look at the standards for 'design assisted by testing' as these might capture overall behaviour in a more inclusive way.

It is also important to remember that back-analsyis of something is not the same as design. So, the various material and load factors have a different context in each case. In design, there are more uncertainties about such things as materials strengths and loads than in back-analysis where you might know the data with more certainty.

Another note of caution: If the steel is said to originate in the 1930's, you might, if you are unlucky, be dealing with what is known as 'early steels' which prevailed up until around that time (again you would need to check as I am going from memory). Early steels can sometimes be highly variable in nature, compared to modern steels and the strengh and toughness can vary dramatically from one location to another due to inclusions and variation in carbon content etc. It might be prudent to obtain the advice of a materials specialist.

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