CROSS Safety Report
Repeating the same design mistakes
This report is over 2 years old
A correspondent who investigates engineering failures has shared common errors which engineers keep repeating.
Key Learning Outcomes
For civil and structural design engineers:
While designing padstones consider the effects that window and door openings can have, or the effects of several padstones in close proximity to each other
The effect of torsion on beams due to eccentric loads requires close attention and detailing
Be aware that different wind pressure coefficients are used for determining the wind load acting on local elements such as parapets, canopies and individual cladding elements
If a steel connection is designed not to transfer moment, then the connection needs to be detailed to prevent any transfer
Be aware of the importance of detailing critical structural interfaces, particularly where the implementation of the detail could be left to a builder without further input from the designer
Check to ensure generic designs can accommodate the wind and snow design loads for the locations that they are intended for
Provide allowance for differential vertical movement in masonry-clad concrete framed buildings
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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. They hope it will 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). The reporter also identifies the following issues:
A lack of consideration concerning the implications of several padstones in close proximity and the interaction between them
Not taken into account 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. A typical example is where edge beams support masonry cladding and the engineer has not considered the following:
The offset nature of the loading
The destabilising nature of the offset loading
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.
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
Moment transfer in 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 misuse
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.
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.
Expert Panel Comments
Find out more about the Expert Panels
Expert Panels comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-UK Expert Panels page.
Like the reporter, the safety issues which the CROSS Expert 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.
Increasing structural safety knowledge
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.
Reading CROSS reports to demonstrate CPD commitment
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 Institution of Structural Engineers (IStructE) taking the lead on this, whereby from 2020 members must declare that they have studied CROSS and the Standing Committee on Structural Safety (SCOSS) reports as part of their annual Continuing Professional Development (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.
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.
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