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Newsletter No 4 - November 2006

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Report Overview

CROSS, on behalf of SCOSS (Standing Committee on Structural Safety), have continued to receive reports on the concerns of engineers and the lessons to be learned. In this issue are several very practical reports; the protection of scaffolds from impact, liquid metal assisted cracking (LMAC), and issues about cold formed steelwork framing and fixings to steelwork. In addition there are two follow up reports on adapted masonry support props. Further material on previous reports is welcome and enables trends to be detected. Indeed a guiding principle of SCOSS is that evidence gathered from reports will be used to issue alerts or to influence change. The reports on scaffolding and on temporary props have been brought to the attention of HSE who are considering them in wider contexts, and the subject of LMAC has previously been the subject of a SCOSS report. Recommendations on these and other topics will be made early in 2007 in the next biennial SCOSS publication. If readers have a concern, either related to published reports, or on new topics, or wish to pass on a lesson that they have learned then make a report to CROSS.

Overview of Reports in this Newsletter

57 Adapted masonry support props

A reporter has come across adapted masonry support props in use and in his view they must only be used in the case of limited openings where the masonry can arch over leaving only a small area of masonry below the arching that can potentially break away. These shores, in his opinion, are usually adequate and indeed useful to support this small triangular area of masonry to limit the making good that would otherwise be necessary if all the masonry were removed.

Adapted masonry support props

Another reporter responding to the item has frequently seen builders using props with cantilevering head-plates which cause the supporting props to bend. Some years ago he photographed a set of such props which had very noticeable curvatures. The wall above had dropped in consequence causing damage to the building and resulting in a substantial claim against the builder. The reporter generally advises builders not to use such head-plates

53 Scaffolds and traffic protection

A reporter who is a Local Authority Structural Engineer is often consulted about, and sometimes has to arrange for, scaffolds to be erected on the Public Highway, or at least within close proximity of vehicular traffic. What amazes him is the lack of consideration shown both in British Standards and industry guidance regarding the risk of impact of vehicles with scaffolds and thereby putting at risk those working on a scaffold and those in the vicinity should a collapse or partial collapse occur.

49 Liquid metal assisted cracking

Another reporter had to advise on a large timber grillage system that had been connected at the nodes with steel connectors formed of a central bar (about 100mm diameter) fitted with radial gussets welded on to it. The whole assembly was then galvanised.

45 Liquid metal assisted cracking

According to the reporter possible problems with galvanising (if the recommendations in BCSA/Galvanisers Association publication 40/05 are not followed) may include; strain age embrittlement, hydrogen cracking, restraint cracking, distortion, and recently, Liquid Metal Assisted Cracking (LMAC).

48 Possible galvanising concern

The intention is that steel poles will be pressed to form the end connection serrations in the form of a ‘fir tree’. In this cold forming process, there will be some residual stress at the serrations and the flattened part of the pole. Before the hot dip galvanising, the pole material needs to be cleaned by pickling, which is an acid bath. Thus the two components for stress corrosion cracking are present - stress from cold forming and then the acid giving the corrosive environment.

39 Design of head track in cold formed steel framing

The reporter noticed that the in-house structural engineer for the steel frame company had not taken account of all the loads in the design of the head track.

44 Fixings to steelwork

The use of proprietary self tapping screws to fix to materials thicker than 10-12mm is questioned by a reporter.

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