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858 Repeating the same mistakes

Report ID: 858

Published: Newsletter 56 - October 2019

Report Overview

A correspondent who investigates engineering failures has submitted a CROSS report to share common errors which engineers keep repeating.

Report Content

As part of the correspondents' role in investigating engineering failures, it has struck them how regularly certain shortcomings arise; that is, different engineers keep repeating the same mistakes. The correspondent has decided to submit a CROSS report so that they can share these common errors with a wider audience and thereby help to reduce the regularity with which such simple errors are made.

  • The design of padstones: These have been seen to be undersized or sized inappropriately (as a result of misunderstanding the extent to which the characteristic compressive strength of masonry can be increased locally below concentrated loads), or there is a lack of consideration concerning the implications of several padstones in close proximity and the interaction between them, or failing to take account of the effect of window and door openings.
  • Design of steel beams for torsion: Excessive torsional rotation of steel beams has been identified as an issue, for example where edge beams support masonry cladding, because the engineer has not considered the offset nature of the loading, the destabilising nature of the loading, or the lack of lateral restraint to the steel beam.
  • Incorrect use of wind loading pressure coefficients: Examples have been seen where general building pressure coefficients have been used for local elements (e.g. parapets, canopies and individual cladding elements). Such general pressure coefficients are likely to underestimate the forces. Care must be taken to use the correct pressure coefficients, which can be significantly higher for local elements such as parapets and canopies than the general building pressure coefficients.
  • Steel connections: A particular issue identified is the engineer designing the steel members on the assumption that no moment is transferred through the beam to column connection, but the fabricator then detailing the connection such that significant moment transfer occurs. This can result in the relevant column being overstressed.
  • Structural interfaces in domestic buildings: Not clearly detailing critical structural interfaces in situations where the implementation is left to a builder without further input from the engineer. The correspondent has seen examples of ad-hoc timber connections in roof structures and pockets cut into reinforced concrete beams, severely reducing their capacity. In such cases, the engineer had not produced a detail and the building contractor effectively produced their own design. Fortunately, these cases were found before construction was completed.
  • Generic design: Use of generic designs in unsuitable locations. Examples include street furniture not suitably designed for the wind loads on seafront location and parapet copings not suitably designed for resisting the wind loads on high-rise buildings.
  • Masonry cladding: Inadequate allowance for differential vertical movement in masonry-clad concrete framed buildings. This was an issue that came to the fore in the 1970s and was well-publicised. However, architects and engineers seem to have forgotten these lessons and the correspondent has recently seen many examples of inadequate movement joints. This is potentially serious, as the loads which are imposed on the masonry once the joints have closed can be sufficient to cause spalling and, in extreme cases, the masonry ties could be overstressed.

Comments

Like the reporter, the safety issues which the CROSS Panel see are often repeated. Many of the points raised in this report are obvious in hindsight to the experienced designer, most likely because they have encountered the issues themselves in the past, but they will not be at all obvious to less experienced designers.

Perhaps it is worth bearing in mind that if a career lasts 40 years, then around 2.5% of expertise leaves the industry each year, to be replaced by young entrants - therefore we all must be active in our learning and the promulgation of learning.

One way in which engineers may maintain and develop their knowledge of structural safety is to read CROSS reports. It is encouraging to see the IStructE taking the lead on this, whereby from 2020 members must declare that they have studied CROSS and SCOSS reports as part of their annual CPD commitment. Furthermore, during the Professional Review Interview, which is a key part of the qualifying route to Chartered and Associate-Membership, candidates will be expected to demonstrate that they are using CROSS and SCOSS reports to satisfy the Institution’s training objectives.

The reporter is thanked for sharing this collection of issues which they have encountered through their investigation work, and CROSS would welcome more reports from those who are directly involved in such engineering issues.

 

CROSS depends on you for reports - if you have experienced a safety issue that you can share with CROSS, please Submit a CROSS Report which is treated as confidential

If you have any comments regarding this CROSS report, please Submit Feedback

View other CROSS reports published in Newsletter 56


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