CROSS Safety Report
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A reporter raises concerns about carrying out alterations to old tenement buildings in Scotland.
Key Learning Outcomes
For asset owners and managers:
Regular inspections and maintenance can help keep a structure safe and help to identify any obvious safety issues that may need to be addressed
Be aware that safety critical defects may be hidden behind linings
Any alterations to tenements should be assessed and designed by a suitably qualified and experienced engineer
For civil and structural design engineers:
Any works involving major openings in tenement structures should include design details of temporary works and method statements for taking out walls
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Tenements are characteristically of traditional construction, with stone outer walls and brick inner walls and party walls, typically four storeys high, but this can extend up to eight storeys. Floor plans are repetitive, with variations at street level to accommodate main door flats in residential districts, or shops on commercial streets. Tenements appear massive and solid but (in the opinion of the reporter) many tenements are not particularly robust and are quite vulnerable to accidental damage.
Robustness and progressive collapse
The original construction would not comply with a number of present-day design requirements as there is a limited degree of tying of internal brick walls (including party walls) to external stone walls. If floor joists are removed, e.g. by fire damage, the system of walls may not have sufficient rigidity to remain standing. Again, if there is differential settlement of internal walls with respect to external walls, vertical cracking along their interface will weaken the wall system. There is limited tying of the floor joists running parallel to party walls or end gable walls. If the gable wall is weakened by, for example, a gas explosion, or if its foundation is undermined, progressive collapse of the tenement block may result.
If the gable wall is weakened by, for example, a gas explosion, or if its foundation is undermined, progressive collapse of the tenement block may result
If the tenements are built along an inclined street, the floor levels will be stepped, with the result that any lateral in-plane forces in the floors are transmitted into the next building by shear in the party wall. Beam/column ‘goal-post’ frames such as those used in alteration work are not usually designed to resist lateral loading or bending. Floor-to-ceiling heights in tenements are often over 3m high, leading to slenderness ratios of over 27 for 100mm thick load bearing brick walls.
Risk of settlement
Shallow foundations, with tenement walls built off large flagstones rather than concrete strips, are liable to differential settlement. In much of central Scotland, the bearing stratum is boulder clay, which has adequate compressive capacity for tenement loadings - however, some tenements are founded over old mine-workings or on the beds of rivers which have been diverted or routed into culverts. In such cases, large settlements or differential settlements may occur.
The low quality of bricks and lime mortar used for internal walls implies a low compressive strength for internal walls. In addition, a number of internal walls at ground level are bowed, or out of plumb and it may not be apparent whether this has developed recently or over many years. The clinker deadening installed between floor joists in the original construction increased floor mass and rigidity, and also damps any vibration of longer spans.
On the other hand, the clinker serves to retain water from leaking plumbing or other water ingress, so encouraging rot of the floor timber. Due to the cost of high level access necessary maintenance such as pointing and removing leaves from gutters may be neglected. Pointing is sometimes carried out in cement mortar rather than the original lime mortar.
A number of tenements have bay windows with slender stone mullions. In many cases, the bedding planes of the sandstone used for these mullions runs vertically. As the stone of the mullions delaminates, the cross-section in compression decreases and stresses increase, reducing safety. The stonework at the bay windows is more slender than elsewhere in the external walls. To prevent buckling, many Scottish tenements have had steel restraint straps installed around the bay windows.
In commercial streets, the owners of the shops and public houses often want to remove internal walls to increase sales area and improve security. Taking down loadbearing walls and installing beams is difficult to achieve without noise or movement transmitted to the adjoining properties.
Temporary works and method statements
The reporter believes that works involving major openings should include design details of temporary works and method statements for taking out walls. Generally, says the reporter, tenements should have an ‘MoT’ type appraisal of their construction at 5-yearly intervals or following any structural alterations.
Expert Panel Comments
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This report about stone-built tenement properties in Scotland raises issues affecting the existing stock throughout the UK and elsewhere. The direct and practical comments are welcome. Engineers with experience of surveying old buildings know of the degradation that occurs by alteration, abuse, lack of knowledge and poor quality of workmanship.
Engineers with experience of surveying old buildings know of the degradation that occurs by alteration, abuse, lack of knowledge and poor quality of workmanship
It is a real challenge to understand what the risks are and what balance to strike when deciding what work is desirable and what is essential. SCOTCROSS has had numerous reports of masonry and other materials and components falling from such buildings and detailed reports will be published.
Some of these issues will be addressed at the Institution of Civil Engineers (ICE) conference ‘Structural safety across the Lifespan of Buildings’ on 4th October 2006. The notion of having an MoT type report for buildings is interesting and the views of other engineers would be welcomed. In England and Wales, the Home Information Pack to be introduced in 2007 will contain information on the condition of dwellings that are for sale.
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