Facade Tolerances
Lack of understanding of building tolerances is a major source of conflict on site.
Deviation: The difference between a specified nominal value and an actual measured value.
Induced deviations: The inevitable result of variations in the manufacturing process and the inability of human operatives and instrumentation to measure with absolute precision.
Inherent deviations: Inevitable inaccuracies due to the physical properties of materials.
Tolerance: Tolerances define the limits of induced deviation for which allowance should be made in design, and within which actual sizes are acceptable.
Tolerances for curtain walling are given in the CWCT Standard as follows:
Line +/- 2mm in any one storey height, or structural bay width, and +/- 5mm overall;
Level +/- 2mm in any one structural bay, and +/- 5mm overall;
Plumb +/- 2mm in any storey height, and +/- 5mm overall;
Plane +/- 2mm in any one storey height, or structural bay width, and +/- 5mm overall;
Widths of fixed joints should not vary by more than 10% of the minimum joint width;
Offsets at joints should not exceed 10% of the joint width in plane, or out of plane.
Design Allowances:
It should then be considered during design development that there will always be a direct interface to other trades, primarily the concrete and the primary steelwork structure.
Both these elements also come with their own set of tolerances which must at all times be considered. It is worth noting that you must always first refer to the contract specification for the allowable tolerances for each trades respective works, you must then refer to the contract building movement report, as any deviation and dead or live load movement will directly affect the facade design.
You would then be wise to consider the implications of the tolerances specified for the concrete and primary steelwork structure, and introduce a safety factor. Whilst this may not always seem cost relevant at the point of design, you should consider how damaging the time and cost impact can be if you are forced to make on site modifications due to out of tolerance primary works.
For example:
It is not unusual for concrete to be limited in plan to a tolerance of +/-20mm, and in level to +/-10mm, then you need to add into this the tolerance for the cast-in anchor channel where again you can consider +/-10mm, thus the cast in channel could indeed be out of position by upto +/-30mm. This can create other serious factors which if not accounted for will lead to the risk of the anchor fixation detail being changed to through bolts and washer plates, or indeed secondary steel supports or replacement of aluminium brackets with special fabricated steel brackets.
It is therefore always wise to anticipate not only what the specification calls for, but what experience tells us.
Deviation: The difference between a specified nominal value and an actual measured value.
Induced deviations: The inevitable result of variations in the manufacturing process and the inability of human operatives and instrumentation to measure with absolute precision.
Inherent deviations: Inevitable inaccuracies due to the physical properties of materials.
Tolerance: Tolerances define the limits of induced deviation for which allowance should be made in design, and within which actual sizes are acceptable.
Tolerances for curtain walling are given in the CWCT Standard as follows:
Line +/- 2mm in any one storey height, or structural bay width, and +/- 5mm overall;
Level +/- 2mm in any one structural bay, and +/- 5mm overall;
Plumb +/- 2mm in any storey height, and +/- 5mm overall;
Plane +/- 2mm in any one storey height, or structural bay width, and +/- 5mm overall;
Widths of fixed joints should not vary by more than 10% of the minimum joint width;
Offsets at joints should not exceed 10% of the joint width in plane, or out of plane.
Design Allowances:
It should then be considered during design development that there will always be a direct interface to other trades, primarily the concrete and the primary steelwork structure.
Both these elements also come with their own set of tolerances which must at all times be considered. It is worth noting that you must always first refer to the contract specification for the allowable tolerances for each trades respective works, you must then refer to the contract building movement report, as any deviation and dead or live load movement will directly affect the facade design.
You would then be wise to consider the implications of the tolerances specified for the concrete and primary steelwork structure, and introduce a safety factor. Whilst this may not always seem cost relevant at the point of design, you should consider how damaging the time and cost impact can be if you are forced to make on site modifications due to out of tolerance primary works.
For example:
It is not unusual for concrete to be limited in plan to a tolerance of +/-20mm, and in level to +/-10mm, then you need to add into this the tolerance for the cast-in anchor channel where again you can consider +/-10mm, thus the cast in channel could indeed be out of position by upto +/-30mm. This can create other serious factors which if not accounted for will lead to the risk of the anchor fixation detail being changed to through bolts and washer plates, or indeed secondary steel supports or replacement of aluminium brackets with special fabricated steel brackets.
It is therefore always wise to anticipate not only what the specification calls for, but what experience tells us.