CROSS Safety Report
Dangerous design of a retaining wall
This report is over 2 years old
Serious errors were made in the input to a computer program used for designing a 1.8m high retaining wall.
Key Learning Outcomes
For civil and structural design engineers:
A quality assurance system within your organisation, that includes the internal checking of calculations, can help prevent safety issues with computer programs from occurring
Competent supervision of design by experienced personnel can allow less experienced engineers to develop a feel for the right solution
It is good practice to carry out sense checks and validate all design outputs from proprietary design and analysis software
The importance of validating software is noted in the Institution of Civil Engineers Civil Engineering Journal August 2013 The importance of understanding computer analysis in civil engineering, and in the Standing Committee on Structural Safety (SCOSS) Topic Paper 2018 Reflective Thinking
For the construction team:
There have been numerous failures associated with freestanding masonry walls and a previous Alert was issued by SCOSS – Preventing the collapse of freestanding masonry walls
This report demonstrates the value and importance of technical acceptance checking in mitigating safety issues within a client organisation
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A design submitted for approval to the reporter, who was the technical acceptance authority for the client, was a 1.8m high brick retaining wall with a parapet on top, to support an access to a new building. From the reporter’s experience, the retaining wall looked slender and the design was queried.
The designer said that a computer design package had been used which showed a factor of safety of over 2 for overturning and sliding so they maintained that code checks were satisfactory. This did not seem credible to the reporter.
Also, the design showed inadequate fixity of the wall parapet so the lateral force from the parapet would not be added to the overturning moment and forces at the base of the wall.
Design input errors
The design was resubmitted and this time the calculations showed that the retaining wall had been designed as panels fixed vertically at 4.5m centres (no buttresses specified). Boundary conditions giving fixity at the movement joints between the panel had been wrongly entered into the design program. A dangerous assumption as the wall was a cantilever.
A further input error, and a lack of checking, had presumed a ground bearing pressure of 800kN/m2 rather than the bearing pressure of compacted fill on site of around 150kN/m2. This meant that the foundations were incorrectly passing the overturning and stability checks. When challenged on the above, the designer admitted fault with the bearing capacity values but still maintained that the assumed brick support conditions were adequate and anything else would be ‘overdesigning’.
When challenged on the above, the designer admitted fault with the bearing capacity values but still maintained that the assumed brick support conditions were adequate and anything else would be ‘overdesigning’
Are parapet fixings deemed as non-structural?
The designer also maintained that the fixings at the parapet into the wall were an architect designed item and hence non-structural. The wall was finally redesigned in reinforced concrete with a brick facing skin.
Several causes identified
The reporter feels the following causes can be taken from this incident:
Over-reliance on computer design packages
Inadequate experience of designer undertaking calculations
Inadequate checking of design (and sense checking)
Incorrect assumption that structural fixings were an architect designed item
Incorrect assumption that forces from fixings would not translate to the base of the wall
A designer not being open to having their design challenged
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This invaluable lesson emphasises the importance of experience, knowledge, and technical background. Software packages, especially those capable of providing design checks and conclusions, shall never be used as a black box or used alone. Software analysis tools might be used with practical experience, engineering judgment, and technical skills. Some non-geotechnical engineers when dealing with geo-structural elements (such as retaining walls) incorrectly use default or typical values for the soil and ground parameters (such as 800 kPa for the bearing capacity in this report).
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In this case there has been a catalogue of errors with design and a number of grossly incorrect assumptions. Had the authorities’ assurance engineer not called these into question, and the wall was built, it would have been a dangerous structure liable to failure during or soon after construction.
The importance of checking designs
The skills of design are only learned by much repetition of routine design tasks which develop a ‘feel’ for what is right and what is definitely wrong. Any experienced designer should have known, without calculation, that a 1.8m high retaining wall is not feasible in normal brickwork, and that a bearing pressure of 800 kN/m2 is not credible save on rock. If 800 kN/m2 were credible, base design would be governed by overturning. So, the safety concern here is not so much the individual errors, but that there are people producing designs who are either incompetent or badly supervised.
No firm should have allowed a design with such errors to be submitted to a third party. The safety lesson is that all designs should be checked by experienced personnel and that all inexperienced designers need to develop an eye for what is right without calculation.
The assurance engineer should never have had to pick up these fundamental errors. This should have been done within the checking and quality assurance regime within the design office of the originator. Indeed, had the client not had a final safety check, the original design could have been built.
Validating design and analysis software
It is essential that there is adequate supervision of design by experienced personnel to allow less experienced engineers to develop a feel for what is the right solution , rather than blindly using software and modelling in the hope that this will deliver satisfactory designs. It is also essential that both designs are properly checked and that all assumptions are verified before issue to a client or contractor.
It is essential that there is adequate supervision of design by experienced personnel to allow less experienced engineers to develop a feel for what is the right solution
This episode is a salutary lesson to the industry that we need to ‘up our game’ and ensure design, checking and supervision are undertaken by competent experienced personnel. It must always be remembered that cantilevers are safety critical structures and that design programs are only as good as the correctness of input.
The importance of validating software is noted in the Institution of Civil Engineers Civil Engineering Journal August 2013 - The importance of understanding computer analysis in civil engineering.
This report has been re-published to clarify the Key Learning Outcomes regarding the design of masonry walls.