Tags: brickwork

Most designers think the builder should just cope when parts are built out of position, and builders are accustomed to doing so, often all too literally. Their methods are sometimes rough and ready.
When a part is built too far out of position to fit, it is more common to alter the details to ensure acceptable finished appearance, even at the expense of function, rather than demolish and rebuild.

It is not difficult to find:

• brickwork with too little bearing, because the structure was built too far back from the face of the wall;
• modified shelf angles, either burned off at the outer edge, or with structurally inadequate extensions;
• brick cut to make thinner veneer, and insulation omitted altogether, in order to keep a finished wall flat where the structure behind it was too close to the face of the wall;
• precast panels with joints varying in width from zero to more that twice the detailed joint size;
• fasteners for steel stud track that can be removed without tools, because they were placed too close to edge of slab, spalling the concrete.

When gaps are designed into assemblies to provide for structural live and dead load deflections, creep, and differential movement caused by thermal expansion and contraction or moisture, oversized or mispositioned parts may reduce the required gaps. Such problems are not confined to the brick veneer steel stud example. Tolerances need to be considered in the design of all building assemblies.
Builder and designer often share responsibility for these problems. Construction variations commonly exceed established norms. However, applicable codes and standards often explicitly allow variations that details do not accommodate. When there is no formal codification of allowed variation, the common practices of trades involved establish de facto standards. Designers who do not state an alternative implicitly adopt these standards, even if they are unaware of having done so.
The wall cavities measured during a study by CMHC a few years ago, were as much as 17 mm smaller, and as much as 20 mm larger than the dimension indicated in the details. The range from smallest to largest on the same building averaged 19 mm. The difference between the largest and smallest cavities on a building was never less than 10 mm, and in the worst case it was 37 mm. For most construction types, there is not much information available about what levels of accuracy are being achieved. Similar levels of variation do occur in other kinds of construction.
Designers should explicitly determine if the tolerances implied by the relationships between parts and provisions for adjustment in a detail are realistic. If a detail cannot accommodate variation that is acceptable under the applicable standards, by established trade practice, or by tolerances stated in the specifications, what will happen on site? The detail would have to be revised, perhaps at additional cost. Or, the builder may leave some insulation off to make room for the structure to avoid an ugly bulge in the wall. Some brickwork may be left insecure because of inadequate bearing. If the designer allows for adequate tolerances, and the builder meets them, there will be a great reduction of such problems. Reviewed by Martin Lapedus.