In the past, virtually all construction activity was performed on site. Every
element of the structure was prepared and custom fitted into the building as it
was being constructed. Over time, with experience, certain construction
industry tolerances developed and evolved to become industry standards.
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.