03.08 Building Tolerances
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Introduction
Accuracy of erection of the finished cladding may be of an order of magnitude greater than the accuracy required of the building frame. This is accepted practice and the difference between the two is accommodated by adjustable brackets and fixings. Provided the structural frame is erected to the anticipated accuracy then few problems arise and the specialist cladding contractor can fix the cladding to the required finished tolerances.
Problems occur when the structural frame is out of tolerance. At this stage it is unlikely that the frame will be dismantled and so brackets and fixings are modified to accommodate the new frame position. There is a cost penalty incurred as a result of the inaccuracy of the frame and thought may be given to applying cost penalties to the specialist frame contractor.
The problems associated with an inaccurate frame are greater if the frame encroaches into the cladding zone, in which case either the frame or the cladding have to be cut away. If the gap between the cladding and the frame is too large then it is possible to fabricate longer brackets. Of course they will not only be larger but will have to be stronger to carry the greater moments imposed on them. It is good practice to set frame tolerances as assymetric tolerances that ensure the frame should not be built within the cladding zone.
Expected tolerances
The tables below give the tolerances for components and assemblies that can be expected for different forms of primary supporting structure.
Most of the values are taken from standards or specifications to which contractors currently work.
BS5606 gives deviations actually measured on site during the 1970's. These are values that are typically achieved but tighter control of construction can produce better accuracy.
| Material/ standard | Length (L) | Width (b) | Depth (h) | Diagonal or squareness | Bow or straightness | Twist |
| Pre-cast concrete BS 8110: 1 (1997) | ± 9mm (L > 3 £ 4.5m) | ± 6mm (b £ 500mm) | as ‘width’ | 6mm difference (length of shorter side£1.2m) | 9mm (L > 3 £ 6m) | 6mm (b £ 0.6m & L £ 6m) |
| In-situ concrete1 BS 5606 (1990) | ± 13mm (h £ 600mm) | 10mm in 6m length2 | ||||
| Timber BS EN 336 (1995) | under-length not permitted3 | + 4/-2mm (b & h > 100mm4) | as ‘width’ | |||
| Hot rolled sections BS EN 10034 (1993) | ± 50mm; + 100mm where min. L requested | ± 4mm (b > 210 £325mm) | + 5/- 3mm (h > 400 £ 700mm) | 2% of b, max. of 6.5mm | 0.1%L (h > 360mm) | |
| NSSS (3rd Ed.) | ± 2mm after cutting | as BS EN 10034 | as BS EN 10034 | b or h/300 or b or h/10005 | L/1000 or 3mm (greater of) | |
| ENV 1090-1 (1996) | ± (2 + L/5000) cut length6 | as NSSS | as NSSS | |||
| Plate girder BS 5750: 2 (1992)&NSSS | ± 3mm | ± 6mm | ± 4mm | verticality of h/300 or 3mm (greater of) | L/1000 or 3mm (greater of) | |
| Permissible and measured/estimated manufacturing deviations of frame members | ||||||
Notes
1) Measured value;
2) Estimated value;
3) If overlength is likely to be a problem, a limit should be placed in the contract at time of preparation;
4) Class ‘1’ tolerances stated (i.e. for sawn surfaces) at a moisture content of 20 per cent;
5) Squareness of ends not prepared/prepared for bearing, respectively;
6) Permissible tolerance for the length of a component with both ends finished for full contact bearing, including end plates as applicable is ± 1mm.
| Columns | ||||||
| Position in plan | ||||||
| Distance between | ||||||
| Alignment | ||||||
| Plumb | ± 10mm (h up to 3m) | ± 12mm (h up to 3m) | ± 10mm (h up to 3m) | ± greater of: h/600 or 5mm | ± greater of: h/600 or 5mm; max = ± 25mm | ± h/300 |
| Beams | ||||||
| Level at opposite end | ||||||
| Level at adjacent floors | ||||||
| Floor beam level | ||||||
| Level of adjacent beams within 5m | ||||||
| Vertical alignment | ||||||
| Distance between | ||||||
| Overall | ||||||
| Height | (H £20m) | |||||
| Plan | (up to 40m) | (up to 40m) | (L £30m) | (L £30m) | ||
| Plumb | ||||||
| Opening | ||||||
| Permissible and measured/estimated erection deviations of frame members | ||||||
Notes
1) BS 8110: Part 1 and BS 8000: Part 2, Section 2.2 Sitework with in-situ and precast concrete both refer reader to BS 5606;
2) Estimated value (other values from BS 5606 are measured values).
Achievable tolerances
The tolerances given above for erected frames and walls are the minimum values allowed in specifications and standards. Greater accuracy can be achieved through;
- Better workmanship
- Different construction techniques
Examples of different approaches to construction are:
- The use of preformed galvanised steel frames that are used to form window openings in masonry walls. Brickwork is built around the form to achieve a perfect fit and the window is then installed into the steel former as a 'clip-fit' made to manufacturing rather than construction tolerances.
- Blockwork walls that abut a glazing screen can be built working towards the screen or away from it. In the later case the edge of the wall abuting the screen will be more accurately positioned. (note bricklayers are naturally left or right handed).
- Critical dimensions can be identified early on in a contract and the specialist frame contractor can probaly achieve greater accuracy of these dimensions albeit that accuracy elsewhere may not then be so good.