CROSS Safety Report
Near miss when modifying brickwork
This report is over 2 years old
A reporter’s experience of creating a new door opening in an existing brick wall on the first floor of an old building is not always as simple as it would appear.
Changes to existing load paths while structures are being modified can occur suddenly and have the potential to cause brittle failure in older structures.
Key Learning Outcomes
For structural and civil engineers:
When modifying an existing building, check carefully that load paths are correctly identified before work on-site commences
For construction personnel:
Check the continuity of support between structural members in case some movement has occurred, with resulting loss of support (e.g., base of a brick wall and an underlying concrete beam
Make allowance with temporary works (e.g., propping of concrete beams) in case of unexpected sudden change in load paths
For clients and building owners:
Recognise the need for inspection by a competent structural engineer before work is allowed to start on any changes to load-bearing walls or other structural elements
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A reporter’s client had wanted to create a new door opening in an existing brick wall. The brick wall was on the first floor in an old building (>80 years old) and was supported on a suspended concrete beam (refer Fig.1). The proposed new opening was located over an internal concrete column. An angle lintel was installed at the head of the proposed opening by partial grout removal and the opening was formed.
Sudden movement and cracking of old brick wall while being modified
As the bricks were being removed, sudden cracking occurred in the beam and adjacent slab. The reporter’s opinion is that over time, the concrete beam had experienced creep and was no longer supporting the brickwork; the brick wall was 4m high and was spanning between the columns as a deep beam. Cutting a hole in the brick wall above the concrete column removed the support for the wall at this point, resulting in a sudden load re-distribution back into the concrete beam. According to the reporter, the beam was originally designed to support the weight of the wall and the cracking in the beam appeared to be related to the sudden shear load transfer at the end supports.
Near miss - potential for sudden brittle failure of old concrete beam
The reporter considers this to be a near miss, as the shear capacity of the concrete beam was unknown, and a brittle failure could have resulted. The reporter’s view is that the damage would have been avoided if the concrete beam had been propped prior to forming the opening as this would have allowed for a gradual transfer of load back on the beam. However, this had not been anticipated as the reporter thought it was a straightforward operation to install an angle lintel in a simple wall.
Similar event - concrete beam suddenly cracked
The reporter had a similar experience on another project when a suspended concrete beam cracked after there was a sudden re-distribution of load during modifications to brick walls above. They say it was harder to predict in this case due to the more complex nature of the structure above, but they have now learned always to prop the concrete structure when it is supporting masonry that is being modified.
Expert Panel Comments
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This is an interesting and not uncommon problem when modifications are made to old buildings that may result in some unwanted surprises if a conservative approach is not adopted. In this example, the reporter has learned always to prop the structure first before carrying out modifications that might affect its behaviour. Although it is unlikely to change the final load on the member, the point of the propping is that it can be released slowly in a controlled manner so that any load redistribution can take place slowly and not suddenly, avoiding potential stress amplification due to impact.
Need to understand alternative load paths
However, propping alone may not alleviate the problem if the structure will be adversely affected by the modifications such as changing the applied load pattern. Thus, it is important to have an understanding of the structure in question before any work is carried out. With cases such as this, where a masonry wall is tending to arch over the supporting structure, the initial investigation could include the removal of the skirting (if present) at the base of the wall to check if there was a horizontal gap or crack at this location.
If the age of the structure is known and drawings are available giving reinforcement details, then it should be a relatively straightforward exercise to check the adequacy of the structure for any resulting adverse load following the modifications. If reinforcement details are unknown, and if the modifications are going to increase significantly the load on the member, then further investigation should be carried out to identify the reinforcement at critical sections.
Old structures designed with limited understanding of structural principles
In the case reported above, the reporter states that they were satisfied that the beam had been originally designed to carry the wall, but notes that the shear capacity was unknown. In an 80-year old building probably built before World War 2, shear design was not well understood and often bent up bars over the columns were used for shear reinforcement. When investigating and evaluating the performance of older structures,, reference should be made where possible to relevant textbooks of the day such as: Salmon E.H., Materials and structures, Volume II, The theory and design of structures, Longmans, Green and Co, 1938.
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