Letters: March 2005

NSC welcomes letters from readers on steel construction related issues. Please keep your letters brief — the Editor reserves the right to condense.

Eurocode safety margins

The report in NSC January 2005 about the SCI’s campaign to reduce safety factors in Eurocodes makes deeply disturbing reading. It appears that the SCI is arguing that because the new Eurocode is complicated and will increase design costs, this justifies reducing safety margins to compensate. This may make sense in crude marketing terms, but it makes no sense at all technically. If the new code is unfamiliar, over-complicated and difficult to understand, surely mistakes are more likely, so shouldn’t safety factors be increased, not reduced?

Engineers are entitled to ask how the proposed reduction in safety factors is justified technically. The SCI needs to produce evidence and open it up to public scrutiny by independent engineers.

A N Beale BSc CEng MICE FIStructE
Thomasons Consulting Civil & Structural Engineers
Leeds

The Editor replies: There is no strategy to reduce safety margins to compensate for any perceived complexities that may lead to increased design costs. In the UK, BSI is responsible for the development of the National Annexes (NAs) and as with most standards the NAs will be issued as drafts for public comment, giving engineers the opportunity to review the draft documents and comment on the proposed partial safety factors and the other Nationally Determined Parameters.

The responsibility for the NAs for Eurocode 3 is split between a number of BSI Committees. This approach adds an additional level of independence to the process by giving a large pool of engineers the opportunity to comment on the NAs and to scrutinise the technical evidence for the choices being proposed.

The process by which the numerical values for the Partial Safety Factors and other NDPs are determined is called ‘Calibration’. The calibration process compares the Eurocodes with the existing UK standards. The objective is to set the Partial Safety Factors and other NDPs such that there is a balance between safety and economy. The process encourages independent engineers to evaluate the evidence on which the Partial Safety Factors and other NDPs are based.

Safety margins will not be cut to encourage the use of the Eurocodes.  However, where we have better information to support changes that may result in more economical designs then engineers should be allowed to take advantage of this.

Eurocode safety margins

I read with interest the piece on the Corefast lift core installed in the cinema complex at the new Dundrum Town Centre near Dublin (NSC February).

The article stated that the whole Corefast core was ‘completed in five working days… six weeks quicker than a conventional concrete core’.  The

Bi-Steel product sounds very interesting and appears quick to build, but the important issue here is the overall cost of installation ie cost of the core and the cost of installing it.

I would be grateful if some information on this could be provided please?

Gavin Clifford
WSP Buildings, Manchester

The Editor replies: This is an important point and is one that often arises. Corus Bi-Steel typically finds that the Corefast core is marginally more expensive than a concrete core when one compares just the cost of the actual core materials, ie steel versus concrete/rebar on a direct substitution basis.  However, when one looks at incorporating appropriate design changes to utilise Bi-Steel’s exceptional strength, and taking into account the benefits that accrue from a faster build sequence, Corefast is more cost-effective .

To verify this Corus has recently commissioned one of the UK’s top construction cost consultants to carry out a study that involved comparing the costs of constructing a notional 20-storey office building, one with a Corefast structural core and one with a conventional insitu concrete core. The study is almost complete and its findings show that Corefast is a more cost-efficient methodology when one looks at the total cost to the client taking into account all factors including accelerated rental streams due to faster project completion, reduced finance costs and enhanced capital values due to more useable floor space. It also highlights the other benefits of Corefast including  a stiffer, higher integrity and more accurate structure, more off-site content, less site congestion and enhanced site safety.

The Report will be completed shortly and will be available free of charge from Corus Bi-Steel on 01344 751670 or from our website www.bi-steel.com