A good working relationship between specifier and structural engineer is vital to the success of a project. Here Barbour Index and Scott Brownrigg explain how to achieve it
For the specifier, careful liaison with the structural engineer is essential because the success of this working relationship can make or break a project. To help things along, a clear understanding of the terms and values used by structural engineers is vital.
The important areas for specifiers to watch are the visible surfaces of structural elements, including all forms of concrete, steel, cast metals, timber and civil works. Additionally the construction techniques, working space requirements, and waterproofing can have an important effect on the quality of the finished scheme.
The specifier should ensure that if structural elements are left exposed, the finishes are of a high quality. This must be agreed with the structural engineer early on in the design process. On many projects, the soffit, columns and external finishes can be formed in concrete so must be reviewed in detail.
Consider the quality of the finish wanted, including the extent of acceptable blemishes, exposed aggregate and – probably of most importance – the position of joints in the finished concrete. Slight variations in formwork placement can create unacceptably large discontinuities on the finished element, and these are difficult to remedy. Ideally, joints can be co-ordinated with the overall design.
There are several standards to use as a reference, for example BS 8110-1:1997, covering the structural use of concrete. The code of practice for design and construction defines the quality of surface finish as F1, F2 and so on for fine work and UF1, UF2 and so on for surfaces requiring further work.
For concrete floors, a power float finish may be suggested. Only use this for large areas that will not be covered by a raised floor. The power floating equipment is difficult to use in confined spaces and the floor pedestals used for supporting raised flooring systems are difficult to fix to the floor surface.
It is important to appreciate that structural engineers will specify structural element tolerances differently. Engineers are looking at materials joints and construction in an entirely practical and calculated way. Some of the time this will accord with the designer’s wishes, however ensure that you understand the engineer’s details as you may need to change joint details, add false details, consider weathering and drips or simply emphasise the quality of the finish required in controlling the aesthetic parameters. Normally, tolerances involving finishes and joints will need to be tighter if these elements are on view. Tolerances are commonly defined in accordance with the BS 5606 guide to accuracy in building. Further reference can also be made to BS 8110.
Tolerances are also crucial when it comes to structural flooring because this will affect raised flooring systems and subsequent floor screeds. The acceptable tolerance will vary depending on the final floor finish. Ensure the structural engineer is aware of what the follow-on trades need. This is particularly important when the structural floor will be exposed or will be covered by a thin sheet or coating. The BS 8204 code of practice recommends a surface regularity rating of SR1 for thin vinyl sheets less than 3 mm thick, and SR2 for thicker sheets used in combination with adhesives.
Be certain how dimensions are measured, particularly if calculated from the centreline of components. Ensure that you have checked the exact size of steel sections when detailing sizes as these will vary with section weight.
Be clear about the location of items such as holes and detailed features. Elements that are particularly critical should be discussed at a meeting; ensure that you also have a clear grasp of the structural principles involved and the areas that are important for the structure to work.
It may also be necessary to bear in mind the service engineers requirements. It is preferable for all three disciplines to be involved in discussions at the same time.
Consider how much movement there is likely in the structure as a result of thermal and wind loads, and allow for this in the detailing. This is essential for curtain walling installations.
Specifier’s and designers generally detail waterproofing and damp control measures, and engineers will usually specify tanking and gas control. However, on many occasions, particularly on large projects, the structural details can be complex and it may be more appropriate for the engineer to specify all the elements. The reason for this is that they should understand the performance characteristics of the materials that surround a waterproofing layer better than the designer. That said, many engineers feel this is not their responsibility and will try to avoid taking this area of work. Clarify who does what at the start of the project.
Ensure you have an understanding of the materials the engineer is specifying and its compatibility with other elements. Even if they are not on show some structural materials need careful detailing for the follow-on trades. This is particularly true for fire or thermal insulation sprayed onto steel. Most engineers will specify either a basic primer or none at all for steel sections. Be careful of the potential for steel to rust and cause the sprayed fire or thermal insulation to exfoliate. Most sprays will not stop moisture eventually reaching the steel, and even if primed not all primers will prevent the steel from corroding. The simplest decoration of structural finishes should be discussed before specifying the paint.