An architect designs a building with the expectation that all the floors will be level and that all the walls will be plumb. Tolerances of construction play an important role in producing a quality product that can help meet those expectations.
The process of building construction causes specific problems in the field of the achievement of accuracy and fit which require detailed examination in relation to techniques of construction, performance and aesthetic requirements, as well as the cost of erection of the building, its intended use and the possible replacement of components during its lifetime. The construction under site conditions of a large-scale object such as a building, involving the assembly of dimensionally variable components by means of a sequence of measuring and positioning operations, can result in deviations from the designed size and shape (induced deviations). Coupled with this are the inevitable dimensional changes resulting from movements and change of size of materials in response to changes in ambient, loading and other conditions (inherent deviations).
The establishment of required tolerances can become complex, and may be especially critical in the exterior walls of multi-story structures where the cladding must be attached to a structural frame. Significant deviations of the verticality of that frame, if not accounted for in proper positioning of the frame and adjustments of the connections, may be reflected in the building’s finished appearance.
If a floor is significantly out of level, this reflects badly on the quality of the constructed product. However, floor levelness is too often attributed to lack of construction tolerance control, while instead it is mainly influenced by the design methodology employed.
The structural engineer, when designing the framing system, must be cognizant of what is achievable, what can be expected, and how to get results consistent with the design intent. The ultimate questions that need to be answered by quantification are how straight is straight, how level is level, and how plumb is plumb? This is where tolerances come into play, but tolerance is not the whole story. Usually, it is only a part of the puzzle.
At the outset let’s understand what is tolerance?
An example of the relationship between the key basic terms of tolerance is given in figure below.
Tolerance is the difference between the upper limit of size and the lower limit of size. Tolerance is an absolute value without sign. In building construction, tolerance is commonly expressed by “± permitted deviation” so that the value of the tolerance is implicit.
Tolerances in civil, structural steel and other construction works are specified in respective codes of practices of that country. However, there are few reference codes which followed across the globe, e,g, ACI for concrete structure, AISC Code of Standard Practice for Steel Buildings and Bridges (COSP), ASTM for testing of materials etc.
In our country, BIS i.e. Bureau of Indian standards is followed.
The tolerances for steel construction can be further divided into three general areas, (i) Mill Tolerances, (ii) Fabrication Tolerances & (iii) Field Tolerances. Whereas, for civil construction Field Tolerances are main challenges.
Now, let’s have a look about tolerable limits of tolerances of some common civil works we execute in our day-to-day project works. These tolerances are broadly classified into two categories i.e. for Materials and for Workmanship.
Needless to mention, tolerance limit of these tolerances is depending upon project-to-project based on project’s various stake holders’ (e.g. client, consultant, contractors, engineering staffs etc.) intent, capacity, competency and cost of quality threshold. Hence, stakeholders should aware about the project specific tolerance limits viz-a-viz industry/local/country code of practices.
Now to standardise the good practice across border, ISO (the International Organization for Standardization) has come up with their ISO 1803, prepared by Technical Committee ISO/TC 59, Building construction, Subcommittee SC 4, Dimensional tolerances and measurement. ISO 1803 emphasises on Building construction — Tolerances — Expression of dimensional accuracy — Principles and terminology and itcancels and replaces ISO 1803-1:1985, ISO 1803-2:1986 and ISO 4464:1980.
In this edition the expression of dimensional accuracy is built up round the concept of target size. A list of basic terms, their definitions, and a list of terms relating to dimensional variability, together with their definitions, is included.
Compiled & Written by: Avijit Datta [9874697360]
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Abir Sengupta
From the standpoint of Architects , Structural Consultants & Construction Professionals, the allowable tolerance for various activities & Structural components is a matter of utmost importance and from, preparation of Technical documents to its execution and in determination of Quality compliance , the Engineer or the Architect must have the relevant knowledge. This Write-up acts as a practical guide for Building & other Construction Works , since it covers all the relevant construction works and activities. This Write-up is immensely helpful for me & I would like to express my sincere gratitude to Shri Avijit Datta for sharing such useful information. I genuinely believe that Shri Avijit Datta shall share such many more valuable blogs in the forthcoming days.