Tags: tolerance

We have now

• Determined how much each component can be expected to vary in size and position.
• Looked at each step in the order of construction.
• Proposed a change to the standard, but aesthetically unacceptable, tolerances for brickwork.

It remains to revise the detail so that adjustments are available to compensate for extremes at each stage. This will make completion of the detail possible without reducing tolerances for subsequent stages. One possible revision is to adopt the slab edge detail shown in Figure 6. The slab position tolerance is the same as before. The brick position tolerance is plus or minus 5 mm. The shelf angle tolerance is 5 mm in any direction. The steel stud position tolerance is a more reasonable plus or minus 10 mm.
The shelf angle is shown supported on HSS brackets, with coarse adjustment of position provided by selecting an appropriate bracket size from a range of sizes, and positioning each bracket on the slab in relation to the datum floor level, not the local floor level. This brings the shelf angle close enough to make minor adjustments with shims and slotted bolt holes. While this appears more complicated than the typical detail in Figure 3, it is not necessarily more expensive. An angle cast in the slab would have to be heavier, because of the increased moment arm supporting the brick. It would also have to be cut when it projects beyond the brick, and extended when it does not provide enough bearing, and touched up afterward to prevent corrosion. Additional modifications would be needed in the vertical direction, concealed behind a larger flashing. A larger angle, with shims and grout between it and the floor slab is a possible solution. The resulting significant thermal bridge may be seen as an advantage, or disadvantage, depending on climate, location, and design objectives. The cavity would be larger than necessary so that at the bottom where the shelf obstructs it, there would be a minimum of 25 mm to ensure drainage.
When the shelf angle is too far inward by 5 mm, and the brick is too far out by the same amount, there will still be 65 mm of bearing. At the other extreme, the toe of the angle will be 5 mm back from face of brick (on average, it should be 15 mm back). From the preceding discussion, we know that the bed joint indicated should be 13 mm, and that the gap under the shelf angle, if 5 mm of movement is anticipated, should be indicated as at least 15 mm. A larger gap is needed if sealant will be used.
The problem of minimum fastener distance to slab edge is resolved by allowing fasteners to be off the centre of the stud track. The stud tracks should be positioned relative to the same datum used to position the shelf angle. Cumulative errors would result if they were measured from the edge of the slab, or the toe of the shelf angle.
If the studs are at their outward limit where the slab is at the inward limit, the track might hang out beyond the edge of the slab by up to 22 mm. If the fasteners have a 50 mm minimum edge distance they might have to be 75 mm from the outside face of stud, leaving 27 mm clear from the centerline of the fastener to the inside leg of the track. Before deciding that this is acceptable, confirmation is needed that performance will not be impaired by having fasteners consistently off the centre of the track, and that 27 mm is adequate tool clearance for installing the fastener. When they are installed, the fasteners have to be positioned from the slab edge, not the stud track.
The cavity, nominally about 52 mm allowing for bumps in the air barrier, could actually be anywhere from 37 mm (with the studs all the way out and the brick all the way in) to 67 mm. The distance from face of insulation to the middle of the wythe of brick, normally 97 mm, will vary from 82 to 112 mm, a range of 30 mm. Ties fitting this range may not be a standard size. More to the point, because the other halves of the ties won’t be fastened to the studs with perfect accuracy, two sizes of wire insert will have to be available to the mason at the jobsite, so that he can keep the ties bedded in the middle 30 mm of the brick wythe.
The vertical direction for the brick tolerances has already been considered. What about the stud length? If all the studs are precut, allowance is needed for errors in cutting. Consider also the expected structural deflection, plus 20 mm for variation in slab position. If the studs are cut 30 mm shorter than the average floor to ceiling dimension, then a connection is needed (such as clips) that still engages the studs and maintains the required stiffness when the gap from end of stud to underside of slab is 50 mm. The nested track connection shown will require studs to be cutto- fit or selected from an assortment of lengths. Reviewed by Moishe Alexander.