Page 42 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch02.qxd 9/29/05 5:01 PM Page 2.4

FABRICATION AND ERECTION*

2.4 CHAPTER TWO

loads are transferred between beams and braces, and this is not always evident when only the maximum
loads from all of the load cases are shown. Therefore, it may be necessary to show the maximum loads
from each critical load case.
Welds. The American Institute of Steel Construction’s “Specification for Structural Steel Buildings,”
particularly Chap. J, and the American Welding Society’s “Structural Welding Code—Steel,” AWS
D1.1, both contain provisions governing welding in structural frames. AWS D1.1 contains many
provisions that require input from the engineer. Section 1 contains a list of engineer responsibilities
and Sec. 2 defines contract plan and specification requirements. Plans and specifications include
complete information regarding base-metal specification, location, type, size and extent of all welds.
If the engineer requires certain welds to be performed in the field, they must be designated. Special
requirements such as nondestructive testing and notch toughness must be shown. If the engineer sizes
any welds, the filler-metal strength classification must be shown. Alternatively, if reactions are given, the
detailer will be able to develop weld sizes and configurations.
Fasteners. The fastener specification must be shown on design drawings. When specifying high-
strength bolts, designers must also indicate the type of connection to be used: snug-tight bolts (the
typical and most economical choice), fully tightened bearing bolts, or slip-critical bolts. Any con-
nections specifically intended to slip, such as slotted or oversize holes, must also be indicated.
Tolerances Defined. Tolerances and provisions for adjustment should be considered together to
result in a structure that meets the user’s needs. Provisions for adjustment may entail extra costs, but
may still be less costly than special tight tolerances, and may be implemented more reliably.
Standard tolerances for building structures are defined in the AISC “Code of Standard Practice for
Buildings and Bridges.” For information on tolerances for highway bridge construction, refer to the
“LRFD Bridge Design Specifications” and “Standard Specifications for Highway Bridges,” both
published by the American Association of State Highway and Transportation Officials (AASHTO).
Structural steel tolerances provide for particular construction processes involved. Factors that con-
tribute to tolerances include mill rolling (defined in ASTM A6, “General Requirements for Delivery
of Rolled Steel Plates, Shapes, Sheet Piling, and Bars for Structural Use”), welding (defined in AWS
D1.1 or D1.5), and anchor placement, fabrication and, erection (defined in the AISC “Code of
Standard Practice”).

Control of Tolerances. Control demands planning, expertise, and adjustment of manufacturing
processes. Construction loads, residual stresses from rolling, welding shrinkage stresses, ambient
temperature effects, and construction loads may all have an effect on final fit and shape. Control of
tolerances may depend on the type of shape, size of the pieces, number of connections on a piece,
amount of welding, stiffness of the pieces, and whether there is adjustment in the erected structure. For
example, a wide-flange shape is easily connected to attachments on both ends, whereas a fabricated
box section having end connections and intermediate connections will present issues. The box sec-
tion may exhibit weld distortion and will be stiffer in torsion than a wide-flange shape by orders of
magnitude. Control of tolerances of a fabricated box will demand special fabrication practices such
as constraints in fabrication or milling of attachment points. Controlling welding sequences can
improve tolerances but is difficult to predict. Provision for adjustment is often a more practical
method to accommodate fabrication and erection tolerances. Other issues to consider with regard to
tolerances include the relationship between theoretical points on different floors (the AISC “Code of
Standard Practice” directs the contractor to establish reference points on each floor); movement of the
structure due to temperature and construction loads (it is not unusual for points in a large structure
1
to move 1 / 2 inches in a day); and possibly large numbers of pieces between points, which demand a
controlled relationship.
Special Material Requirements. There are occasions when special material requirements are
appropriate, and there are provisions in specifications for these requirements. ASTM material speci-
fications include supplementary requirements. When such special requirements are necessary, they

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.