Construction Industry Tolerances

Construction tolerances may be defined as the allowable deviation from specified or designed values. They primarily serve as a protection for both the "buyer" (building owner in construction) as well as the "seller" (the contractor in construction) of the product or service. The owner is protected by knowing that the workmanship of the project must fall within the accepted range of the prescribed tolerances. The contractor is protected by having an accepted variation from the specified project values or measurements.

Development of Tolerances

Since almost nothing in manufacturing or construction is exact, some trade groups have developed an extensive collection of tolerances that have become accepted standards in the construction industry. The following is an abbreviated list of some of the more commonly accepted standards.

• Architectural Woodwork Institute
• National Frame Building Association
• American Institute of Steel Construction
• American Concrete Institute
• Precast/Prestressed Concrete Institute
• Marble Institute of America

Very few, if any, of these tolerances or industry standards in use today are based on research or hard data. There is virtually no field research and analysis of most of the construction tolerances in use today. It should be noted that each industry develops its own tolerance standards that are derived from its own experience and collective judgment and its own economic considerations of what they have determined to be reasonable and cost-effective.

There has not been a concerted effort to assess the compatibility of these diverse tolerances when they interact with those of other groups and how they affect each other and, therefore, the resulting structure. In light of the fact that steel and concrete are used together, masonry is attached to or may support either steel or concrete building elements, every commercial and many residential buildings have framed glass, and sealants are expected to fill all sorts of the building joints, this has ultimately resulted in problems of fitting such elements together, invariably leading to the use of shims as an integral part of the construction practice.

Still, code committees often set standards based on such tolerances given the lack of potential compatibility and possible adverse interactions. These tolerances have become commonly accepted practice and appear in our project specifications. The ultimate question is how "good" are the tolerances set forth by one group when utilized in conjunction with those proposed by another in the construction process. And what are the potential implication of these over the life of the structure?

The project specifications spell out the expected quality standards for the project. These directly or indirectly include industry standard tolerances. These tolerances allow variations from a given dimension, quantity, practice, expected function, "look," or performance as well as the acceptable amount or range of the deviation, potentially affecting given values, dimensions, lines, grids, alignment, or location in the final product. These prevailing acceptable tolerances may end up affecting the production of materials, assemblies, fabrication, and/or installation activities related to the construction project. As a result, this generally affects the completed project in some shape, manner, or form.

Many of the components of buildings such as metals, stone, brick, and glass are produced in mills, plants, or shops. The steel sections that are produced in a mill are not exactly the size they are said to be. There is an industry acceptable variation in their dimensions. This variation is also found in brick. Brick is made by three basic methods, and there is some differential shrinkage depending on the different manufacturing processes. To a smaller degree, dimensional stability may also apply to the fabrication of stone slabs. Glass production is also subject to dimensional deviations as well. Precast concrete elements may also be subject to dimensional variation. In many cases, these deviations may not be critical to the performance of the final product, while in others they may.

To control cost, many building components are fabricated or preassembled off site. It is generally accepted that the quality of a product made in a plant is usually easier to control than in the field. Off-site fabrication is generally less costly as well, so, in some cases, it is preferred due to economic reasons. The components that go into the assembly may come from many different sources and possibly different industries. These prefabricated or assembled units create challenges to ensuring that these components "fit" into the site-built frame or into the partially built structure. Suddenly construction tolerances become important in ensuring that all these "puzzle" pieces fit together reasonably well without the need for costly remedial work.

One of the pillars of the project delivery process is to ensure that the completed structure meets the project’s quality requirements as well as all the applicable quality standards. Therefore, the accepted construction tolerances are expected to assist in ensuring that the proper technical functions like the structural connections, joints, air tightness, and moisture resistance performance as well as the appearance of the final installation are facilitated.

The pressures in the project delivery process for controlling cost has resulted in increasingly accelerated schedules and some forms of off-site prefabrication or preassembly. Materials made in mills or plants are shipped to fabrication shops where they are modified to reduce site work and speed up installation. This leads to steel components being fabricated in shops, concrete products are cast in plants, and aluminum frames are shop fabricated. Some building elements like plumbing, electrical, air conditioning, or fire protection are modularized, preassembled, or precut to reduce site work.

To meet these challenges, contractors have increasingly adopted practices that require greater attention to the fitting of these premade components into the site-built frame or partially completed structure. Since these premade components are not easily modified, contractors are faced with the need to ensure that these components accurately reflect the specified dimensions. The next thing the contractor has to ensure is that the site-built portion of the structure also accurately adheres to the specified dimensions. This would then ensure that all components will easily "fit" into their rightful place with minimal effort or adjustment.

This is when, as well as why, construction tolerances become an important factor in the "fitting" together of all the project components with reasonable accuracy to achieve the project delivery requirements. Construction tolerances play a role in numerous functions, such as structural reliability, safety, and anchorage integrity. Tolerances will come into play when related to joint moisture resistance and acceptable appearance.

Potential Impact of Tolerances

In high-rise construction, a review of the accepted tolerances by various organizations may highlight the potential for creating challenges when utilized without a focus on their impact on one another. This is especially concerning in buildings that are over a few hundred feet high. This becomes evident when the "out of plumb" tolerances (variations) associated with the building frame are reviewed with those of the building cladding. The steel frame of a high-rise structure may migrate outward by over 2 inches and migrate inward by over 1 inch from the plumb axis of the building. In concrete, the frame may lean in or out by 1 inch in either direction.

If the leaning out tolerance is progressively cumulative in the outward direction, the building skin will visibly slope and potentially create a visual problem. If the variation randomly moves in and out of plumb as the structure goes up, it can create a number of problems. Visually the façade will weave in and out causing an aesthetic problem. The distance from the curtain wall, which needs to be flat, and the structure that is varying will affect the length of the connector. This will affect the ability of the typical designed connector to safely transfer the dead lead of the curtain wall as well as the expected live loading (wind) to the structure. The connecter may deflect or rotate, potentially causing problems to the integrity of the curtain wall or its ability to seal out the weather.

Although concrete frame tolerances are more stringent than those of steel, concrete frames have an added potential concern as the column is allowed to rotate 1–8 inches on its axis. This could create issues for the following finish trades if the column is freestanding or integrated into the skin of the building. Concrete columns are also allowed to move 1–8 inches in translation. That means they can migrate away from their true location by that amount. If the column is integrated into the skin of the building, it may create a real problem with the skin modules or possibly the fenestration. Careful attention to controlling these tolerances is critical to avoiding potential resulting problems.

Potential problems or conflicts are not just limited to the frame and cladding. There are potential areas of concern between the curtain wall and the fenestration. The resulting opening may be larger or smaller. This may create a problem in the joint being too small or too large, creating a potential sealant problem. If it is too small and the window does not fit, that could create a real headache. The opening of the window unit may be out of square, causing its own challenges. These problems would come from controlling fabrication tolerance or field measurement tolerances.

The window unit itself may have tolerance issues where the frame and the glass tolerances come into play. The accepted tolerance for glass is a variation of +/- 3/32 of an inch in length or width. The glass panel may be +/- 1/4 of an inch out of square. While the frame may have an accepted tolerance of +/- 1/16 of an inch out of plumb or out of flatness. The frame may also be out of square by as much as 1/8 of an inch.

The window frame may be built somewhat smaller or larger than designed. It may be somewhat out of square. The glass may be manufactured slightly smaller or larger, or it too may be somewhat out of square. If the frame is out of square in one direction, and the glass is out of square in the opposite direction, then there is the potential that the rabbet depth for a good portion of the window may be deficient, leading to potential for the glass to "pop out" of the frame under severe wind conditions, or the space between the frame and the glass will be so close that any thermal movement of the frame may fracture the glass.

Joints and Tolerances

As construction progresses, these tolerances will accumulate. In some cases, one may negate the adverse effect of the other, but there may be cases where they may be additive and potentially become problematic. Material is manufactured, and these operations have tolerances. These may be plus or minus a small fraction of an inch. Then the material is fabricated, and the fabrication process has built in tolerances. These tolerances may be built into the equipment used, or the measurement (+/- a fraction of an inch) made by the operator (+/- a fraction of an inch). The craftsperson may measure something and round the measurement to the nearest part of an inch. Usually, many of these tolerances negate each other as some are positive while others may be negative. But there always is the possibility that most, if not all, of these tolerances (fraction of inches) may be positive or all negative, and the cumulative effect may be problematic.

Virtually all buildings have some form of joints. These may be nonmovement joints as well as movement joints (see Figure 1). The width of these joints may vary from the design as they may end up too narrow or too wide as a result of cumulative tolerances. This deviation may impact the aesthetics of the façade, or this may impact caulking and sealant application. When it comes to expansion joints, this variation may affect air and water tightness. 

Usually, joints are filled with a sealant to keep moisture and air from infiltrating into the structure. When joints are expected to accommodate movement due to expansion or contraction, the width of the gap, generally, is a function of the forces that need to be accommodated. The appropriate sealant is selected based on the anticipated movement of the joint. This outcome may be affected by the cumulative effect of manufacturing, fabrication, installation, measurement, and workmanship tolerances.

Conclusion

Knowing the effect of nonuniform, accepted construction tolerances, there are some things that can be done during design and construction to address the potential conflicts and possible resulting problems. The designers may choose to list tighter tolerances, and the contractor may ensure that the work is performed with greater attention to deviations from the defined or specified criteria. This will benefit all parties as it will minimize rework as well as result in fewer future maintenance issues.