Page 40 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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FABRICATION AND ERECTION*

2.2 CHAPTER TWO

At various stages in the development of a project, a fabricator can provide estimates based on differ-
ent levels of precision. During the early development stages some fabricators will be willing to give a
conceptual estimate using basic statistics about the project. In most cases, the final estimate will be based
on a precise take-off or listing of the material, a take-off of the work to be done, a calculation of the labor
costs to perform that work, and an evaluation of the conditions of the project. A structural steel estimate
will include the cost of materials, fasteners, purchased items such as deck and joists, preparation of
detail drawings, shop labor, inbound and outbound freight, and overheads.
Costs of material will depend on whether mill quantities can be purchased or the material must
be purchased from a service center at a higher price. Wide flange shapes are supplied from mills in
bundle quantities and usually in standard lengths between 40 and 60 ft. Sizes ordered in small quan-
tities or lengths that cannot be obtained economically from standard lengths may increase material
costs. The standard material specification for wide flange shapes in building construction, published
by the American Society for Testing and Materials as ASTM A992 steel, provides a 50-ksi specified
minimum yield stress. The standard material for other shapes and detail plate, ASTM A36 steel,
provides a 36-ksi specified minimum yield stress. Where special grades or supplementary require-
ments must be specified, material costs will be affected.
Time is usually not included in the estimating process to check design dimensions, evaluate each
connection against fabrication limitations, and to find and eliminate interferences. Time should be
included for unusual pieces and details that demand special attention.

2.1.2 Material Orders
Schedule is usually a primary consideration in steel fabrication. The steel frame is on the critical path
of most projects, and there is rarely extra time in the schedule. A steel fabricator starts a project with
two major items on the critical path: material acquisition and preparation of shop drawings.
In most cases, a fabricator will generate an advance bill of material starting almost immediately
after award of the contract. Advance bills of material are even more precise take-offs of the mate-
rial required for the project than was created for the estimate. Drafters generate the advance bills and
send them to the purchasing department. Purchasing sorts the advance bills, grouping like sections
and assembling piece sizes into economical sizes for purchase. Material orders are assembled and
placed with suppliers that can provide the material economically and on time. This is where small
quantities of a size will force the use of higher-price material from a service center. Also, deviations
from sizes in stock and unusual grades, or supplementary requirements, may result in the mill sup-
plying material on an extended schedule.

2.1.3 Shop Drawings
At the same time that some drafters are working on the advance bills, others begin the process of cre-
ating shop drawings. The more sophisticated designers and drafters of building structures generate
design information by creating a three-dimensional model using advanced design software. The
information is downloaded to detailers, who use these electronic files with detailing software to gen-
erate shop fabrication information.
Neutral file formats are available for data transmission that permit design software to generate
information in a format that can be used by detailing packages. The detailing software not only gen-
erates drawings, it is also capable of generating numerical control code to operate saws, drills,
punches, and thermal cutting and coping machines in the shop. The benefits of this method of design
and detailing are time saved, economic effectiveness, skill set requirements that are better suited to
the current workforce, and a reduction in errors associated with manual drafting.
Other fabricators and people working with other types of structures may generate shop drawings
by hand or use a combination of manual and automated calculation and drafting.
Detailers may be employees of the fabricator or independent contractors. Most fabricators employ
some detailers but use independent detailing firms to level the in-house workload. The detailer works
from structural design drawings and specifications to obtain member sizes and grades of material, con-
trolling dimensions and all information pertinent to fabrication and erection of the structural frame.

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