CROSS Safety Report
Effect of staff change on a design
This report is over 2 years old
A reporter illustrates how risk can be generated from staff turnover.
Key Learning Outcomes
For civil and structural design engineers:
It is good practice to start your calculations with a written statement of the principles that have been adopted and any assumptions that have been made
This report highlights the importance of a formal change control procedure and record keeping
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This report is from a reporter in Australasia who said that the ground floor of a warehouse was supported on concrete beams and columns. The perimeter beams also supported large site-cast two storey concrete panels. The panels, in turn, supported a concrete 1st floor and roof steelwork. The panels’ widths were the same as the column centres below. The reporter goes on to say that a contractor was awarded the contract 6 to 9 months after design was completed.
It became apparent to the contractor that it was not practical to cast such large panels. They rang the consulting engineers and asked if they could split the panels in two with a vertical joint at the beam mid-span. The engineer who received the query was not the original designer (who had left the firm). Apparently, the engineer confirmed that the panels could be split but when they were erected large cracks appeared in the beams.
Investigation revealed that the original designer had envisaged the full span panels would act as a deep beam and hence only load the beams near the columns. The beams had therefore been designed for a low bending moment at mid span. When answering the contractor’s question the second engineer had assumed the beams to have been designed for a uniformly distributed load from the panels so did not realise the inclusion of the mid span joint between panels would have this effect.
Clearly, says the reporter, the second engineer should have checked the design before answering the query. However, it was an easy mistake to make and illustrates how risk is generated from staff turnover. Perhaps the initial mistake was that the original designer did not investigate 'buildability.'
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It is good practice for designers to start their calculations with a written statement of the principles that have been adopted and the assumptions that have been made. UK practice is to use an ‘Approval in Principle’ to describe how the structure is to be designed and it must include statements on key assumptions e.g. deep beam action, and this information needs to be passed to the contractor on construction drawings.
The example here illustrates two basic issues: firstly the need for buildable designs and commensurate review procedures, and secondly the need for a formal change control procedure and record keeping. Staff turnover must not prejudice safety. More generally this is also a classic case of interaction between final design and construction methodology.
It also belongs to the generic group of failures whereby a practical problem had to be overcome leading to consequences not foreseen. A tragic example was the collapse the Hyatt Regency walkway in Kansas in 1981. The fabricator changed the hanger design from a one-rod to a two-rod system to simplify the assembly task, doubling the load on the connector, which ultimately resulted in the walkway’s collapse. One hundred and fourteen people were killed.