FACTORS DETERMINING THE ACCURACY OF STEEL FABRICATION

The idea of constructing a prefabricated steel building could sound like an industrial version of buying at IKEA. Once the parts are on the job site, will you have problems putting it together? After all, this is a large complex product that is created off site, shipped to the site in pieces that must fit with each other and fit embedded items in the foundation or match up with an existing structure. You can’t eliminate variation in a product with so many makers and moving parts.

The manufactured building industry has spent years finding ways to make erecting a steel building faster and easier. With today’s prefabricated structures, technology and steel meet to create a precision product that is cut, rolled, punched, and ready to be put together by skilled workers in short order.

Steel associations and societies have identified the critical variations, determined the right limits for those variations, and providing guidance for managing variation.

STANDARDS
The American Institute of Steel Construction (AISC), the American Welding Society (AWS), and the American Society for Testing and Materials (ASTM), among others, develop and publish standards and best practices followed by a wide array of industries. The AISC Code of Standard Practices (COSP) and other documents provide guidance specific to steel manufacturing and construction from the mill to the job site.

Examples:

ASTM A6 deals with the general requirements that apply to rolled structural steel bars, plates, shapes, and sheet piling
AISC Code of Standard Practice for Steel Buildings and Bridges 2010
AWS D1.1M2015 Structural Welding Code - Steel
In particular, these standards set out tolerances for the milling, fabrication, and erection of steel for buildings.

TOLERANCES
Tolerances are one of the things that tell engineers and contractors where the structural steel will be within a structure once complete. The key to working with tolerances is learning how to use them to accurately create the building described in the contract and plans.

For steel, three tolerances come into consideration: mill, fabrication, and field.

Mill tolerances

Mill tolerances are found in ASTM A6, which covers hot-rolled steel shapes. They describe the range within which steel members must fit. This includes cross-section dimensions, weight per foot, and allowable variation from straightness for steel members produced at the steel mill.

Mill tolerances are developed for mills producing the steel and for the fabricators ordering from them.

NOTE: The fabricator is allowed, but not required, to rework members according to the COSP to correct out-of-tolerance variations. The mill may replace members that are severely out of tolerance.

Fabrication tolerances

Fabrication tolerances for rolled shapes control dimensions to aid erection fit-up and member positioning. Fabrication tolerances apply to two categories distinguished by the role of the member:

Both ends finished for contact – columns
Framed into other members – beams and braces
If a member will not be used as a compression member must only match ASTM A6 tolerances. Compression members are limited on camber and sweep to the member length divided by 1,000.

Sweep tolerance is the same for both straight members and those with specified camber.

NOTE: This is measured by the fabricator in an unstressed condition. Camber cannot be less than specification.

No members may contain twists, bends, open joints, or sharp kinks.

Field tolerances

Field tolerances also come in two flavors:

Site preparation tolerance
Erection tolerance
Site preparation tolerance is the responsibility of the general contractor or another owner-designated construction representative. Site preparation ensures the accuracy of:

Foundation, piers, and abutments
Building lines and benchmarks
Anchor rods, foundation bolts, and other embedded items
Many issues with tolerances in site preparation occur at the trade interfaces. The most common questions surround anchor-rod tolerances and improper anchor-rod placement. The AISC COSP 7.5 covers this particular area.

Erection tolerances, covered by COSP 7.12 and 7.13, includes tolerances as well as orientation/placement of individual members within a frame.

TRIAL ERECTION
Before shipping, a manufacturer can perform a trial erection of all or part of a steel building while it is still at the factory. Trial erection:

Ensures fit-up
Checks building geometry
Reduces risk of delay at the site
Protects steel surface treatments from damage
The use of CAD/CAM solutions and other technology has greatly reduced the issues inherent in prefabricating a metal building and then putting it together at the job site. Trial erection can add to the cost so you must balance it against the potential for problems in the field.

However, a trial erection, particularly for very complex buildings, is still a good way to find and correct problems while the product is still at the manufacturer such as the fit of interfacing structures. This practice can also bring to light unforeseen safety or construction problems. A design may require an uncommon geometry that forces workers to take potentially dangerous positions while securing a section.

STRENGTH IN FABRICATION
The strength of a steel building is predicated on the accuracy of milling, fabrication, and erection of a large, complex structure. Every column, beam, and brace must be manufactured to tolerances that are the result of years of construction practice. Variation is inherent in the system but you can mitigate the problems it introduces by following well-thought-out practices and standards.

Manufacturing a steel building to established tolerances and eliminating problems through trial erection reduces the time to completion of a building project, decreases the number of workers required onsite, and makes corrections before the materials even reach the site.

The end product is a strong, stable building that will be useful for decades.


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