Construction Tolerance: Paying Attention To The Next Trade

Over the past several decades, I have seen more and more contractors trying to make construction fit and comply with the Contract Document dimensions that typically do not take construction tolerances into account. Making the construction ‘work’ under these circumstances typically leads to dimensional imperfections and problems in the field for the next contractor, such as not getting the proper system rating of a wall or column because the correct thickness of material per the tested assembly cannot be achieved within the confinements of the installation as designed. Conditions might create a situation where to achieve the required ADA required distance from a wall, the finishing contractors need to make unnecessary adjustments to materials or work in a place that ends up being an extra cost to the subcontractor, general contractor, or owner.

We could do better, and it starts with the designer, taking fabrication and construction tolerance into account and holding contractors to the allowable tolerances set forth by the industry.

There are many acceptable tolerances in construction, including those conveyed through the following reference samples from building codes or other guides typically referenced in the Contract:

● Handbook of Construction Tolerances 2 ed  by David Kent Ballast, AIA, CSI

● ACI 117, Specifications for Tolerance for Concrete & Materials

● MNL 116 & MNL 117, Quality Control for Structural Concrete / Architectural Concrete

● MNL-135, Tolerance Manual for Precast & Prestressed Concrete

● ASTM A6, Specification for Rolled Structural Steel, Plates, Shapes, & Sheet Pilings

● AISC 303, Code of Standard Practice for Steel Buildings & Bridges

● ASTM C55, ASTM C90, ASTM C129, ASTM 744 – CMU

● ASTM C62, ASTM C216, ASTM 652, ASTM C1088 - Masonry

● ACI 530.1/ASCE 6/TMS 602 – Specifications for Masonry Structures

● Dimension Stone Design Manual 2022

● Indiana Limestone Institute 22^nd ed.

● AAMA CWM-19, Metal Curtainwall Manual

● ANSI H35.2-2017, Dimensional Tolerances for Aluminum Mill Products

● GANA Glazing Manual

● SFM-1-87, Aluminum Storefront and Entrance Manual

These guides give a designer the tools needed to properly allow for the installation of materials while ensuring the construction process accounts for the next item being installed.

Example 1: Steel Tubes in Cold Formed Metal Framing (CFMF)

Typically, we see steel tubes within a CFMF wall across relatively straight and long spans. When structural steel tubes are installed within the CFMF wall, steel tube installation tolerances, and if the tube is to be fireproofed (which has its own thickness tolerance), must be accounted for when choosing the tube size. Steel tube installation has the following tolerances to consider:

1. Column Anchor bolt placement tolerance: ¼" center to center

2. A 4" x 10' length tube has the following tolerances:

4" tube - Fabrication Twist ±5/64"           

6" tube - Fabrication Twist <±3/32"

3. Fabrication Bow ¼."

4. Installation plumb 1:500, ¼" for a 10' column.

(Fabrication tolerance generally cannot infringe on the installation tolerance, which means the tolerances could be additive).

5. CFMF wall has the following tolerances:

1/8" in 10' placement tolerance

1/8" in 12' plumb tolerance

4” tube tolerance =  ¼” anchor bolt + 5/64” fabrication + ¼” bow + ¼” placement = 53/64” for steel + 1/8” CFMF = 61/64 from center + any fireproofing thickness.

6” tube: ¼” anchor bolt + 3/32” fabrication + ¼” bow + ¼” placement = 27/32” for steel + 1/8” CFMF = 31/32” from center + any fireproofing thickness.

As these dimensions demonstrate, placing a 6" tube in 6" CFMF will likely not work. Placing a 4" tube centered in the 6" CFMF wall will likely work (by 5/23") unless installing fireproofing, which will probably need an 8" CFMF wall.

Example 2: Concrete Shear walls and Metal Panels

Another example is the installation of metal panels on a concrete shear wall. It is essential to understand the concrete installation process for the project because the in-and-out tolerance will differ based on how the wall is installed–either by slip forms or form-and-pour. With both methods, accepting installation tolerance in the design and specifying a clip system capable of moving in and out will elevate unnecessary costs due to construction tolerance before the metal panel installation.

Example 3: Precast Planks and Roof Installation

The final example occurs when installing a roof directly on precast panels. Precast panels will typically have a natural chamber, which becomes more pronounced as a panel extends in length. The precast panels also have an up-and-down tolerance. Both issues must be addressed when installing a roof, which typically involves flat insulation panels. Installing the insulation panels on a slight curve will create voids under or above the insulation, and the up-and-down tolerance will also likely create voids. One potential fix is to install a concrete topping, which also has its own tolerance requirements. The concrete topping cannot be too thin for the mix design and may require an engineered concrete topping bag mix or self-leveling concrete material. Based on the tolerance of the concrete mix design being considered, the precast plank concrete overlay might be two inches or more at the high point of the panels. These tolerances must be considered to design the parapet's height correctly.

"Having a general guide on hand helps when designing and considering the material and installation tolerances"

This can be used for slab edge installation and general hold dimensions when columns are part of an exit passage hallway, stairway, or elevator. Having a general guide on hand helps when designing and considering the material and installation tolerances.

Technology could help with this important task by having these tolerances built into the system as an option, giving the designer the proper information when designing the building. Just a notification identifying the tolerances of an installed system might be enough to make the appropriate adjustments early in the design.

We can always do better as an industry by working together to enhance the design and benefit the owner. Next time you design critical dimensional elements with construction tolerance challenges, ask yourself what you can do to improve the process and specifically set up the contractor and the next trade for success.