Eurocode 3 (EC3) vs BS5950

07 Jan

Firstly I would like to wish all readers a happy, healthy and prosperous New Year. To get the grey matter ticking over again after the Christmas break, I thought I would return to a more technical subject matter and discuss one of the Eurocode transition issues facing us, namely circular hollow section (CHS) steel section classification under EC3 when compared to the more familiar BS 5950.

Prior to the adoption of Eurocodes in the UK, all the load capacity verifications and charts for our standard CHS struts were determined from BS 5950; EC3 although on the face of it, being a limit state code, is a natural progression from BS5950 however numerous detail changes exist within the document. One specific change that we have felt particularly keenly in the last few months is the design and classification of circular hollow sections that we use in our long spanning struts.

Our 1220mm diameter x 16mm thickness grade S355 tubes which are used for our “super tube” range ; designed for long clear spans under heavy load conditions, fall under the category of class 4 “slender” sections. Delving further into the code we find ourselves directed to the murky depths of part 6 ‘Strength and Stability of Shell Structures’. After looking through and trying to understand this document (it’s not for the faint hearted!) it soon became apparent that this is a very complex way of doing things certainly when compared to BS 5950 and somewhat precludes the use of relatively simple and understandable “off the shelf” calculations to support the use of our equipment. As practising temporary works engineers looking for a better solution, we have investigated alternative methodology, which lies within the spirit of EC3, to provide a simpler means of classifying our equipment in terms of load capacity or resistance in Eurocode parlance.

The first approach (and the simplest) was to simply side step the problem by downgrading the strength of our steel from S355 to S275. This reduces the epsilon factor. From BS EN 1993-1:

Taking fy as 275 gives an ε value of 0.81 respectively. The 1220 x 16mm section now just sneaks into the class 3 (semi compact) designation defined by the following expression:

The design of class 3 sections is well defined within EC3 without resorting to section 6; so problem solved? Well yes in part but it does mean that we have to down rate the strength parameters of our equipment by approximately 20%. We do like to work our equipment hard and get value for money so this solution is not really ideal.

We decided to dig a little deeper to see what else we could come up with. This search led us to a “Non Contradictory Complementary Information” (NCCI) document (1) on the design recommendations for hot finished elliptical hollow sections. I appreciate that elliptical is not circular but the approach documented within the NCCI for dealing with slender class 4 sections seems much more logical, and certainly simpler to use. Basically the document suggests that for class 4 sections resistance properties for compression and bending can be calculated in a manner similar to that of a class 3 section but using effective section properties for area and modulus as defined by the equations below.

To err on the cautious side and also to follow the spirit of EC3 as much as possible by classifying the section as class 4 for all types of loading, we have replaced the “140” factor highlighted in yellow by “90” (i.e. the value used in EC 1993-1-1 table 5.2 – D/t > 90ε2 for class 4 CHS sections). This ensures that reduced values for are used when sections lie slightly outside of the boundary condition, otherwise the equation above could lead to being >1 in certain circumstances. By implementing this approach, the strength parameters of the super tube align much more closely with those calculated from BS5950; a code we have been using successfully for years and on many occasions. Perhaps more importantly this simplified approach gives practising engineers a feel for what is going on within the code.

Please feel to comment on this approach.

Once again best wishes for 2014.
Tony Gould