Page 57 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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FABRICATION AND ERECTION*

FABRICATION AND ERECTION 2.19

The AISC “Code of Standard Practice for Steel Buildings and Bridges” defines the clause
“Architecturally Exposed Structural Steel” (AESS) with more restrictive tolerances than on steel not
designated as AESS. The AESS section states that “permissible tolerances for out-of-square or out-
of-parallel, depth, width and symmetry of rolled shapes are as specified in ASTM Specification A6.
No attempt to match abutting cross-sectional configurations is made unless specifically required by
the contract documents. The as-fabricated straightness tolerances of members are one-half of the
standard camber and sweep tolerances in ASTM A6.” It must be recognized that the requirements of
the AESS section of the Code of Standard Practice entail special shop processes and costs and they
are not required on all steel exposed to public view. Therefore, members that are subject to the pro-
visions of AESS must be designated on design drawings.
Designers should be familiar with the tolerances allowed by the specifications covering each job.
If they require more restrictive tolerances, they must so specify on the drawings and must be pre-
pared for possible higher costs of fabrication.
While restrictive tolerances may be one way to make parts of a structure fit, they often are not a
simple matter of care and are not practical to achieve. A steel beam can be fabricated at 65°F and
1
installed at 20°F. If it is 50 ft in fabrication, it will be about / 8 in short during installation. While
1 / 8 in may not be significant, a line of three or four of these beams in a row may produce unacceptable
results. The alternative to restrictive tolerances may be adjustment in the structural steel or the parts
attaching to it. Some conditions deserving consideration include parts that span vertically one or
more stories, adjustment to properly set expansion joints, camber in cantilever pieces, and members
that are supported some distance from primary columns.

2.10 STEEL FRAME ERECTION

Fabricated pieces are usually received at an erection site from fabrication plants on over-the-road,
railroad, or floating carriers. The pieces are unloaded into a receiving (shake-out) area, where they
are temporarily stored and sorted to comply with a planned installation sequence.
In some cases, pieces are preassembled into larger lift units or modules, to improve installation
efficiency or to provide stability. There are also occasions when transport of each piece has to be
scheduled to arrive from a fabrication plant at a predetermined time to be lifted directly from the car-
rier for placement in the structure. Such a situation occurs when erection is done in a congested loca-
tion or where there is no viable shake-out area.
Forces on frame members during construction vary significantly from those in the completed
structure. In most cases these forces are not critical to the design of the members, but in some cases
they are. The erector analyzes member strengths and connections in load conditions related to the
sequence of erection. If the construction loads are critical, the erector advises the engineer and/or
fabricator and arranges appropriate design modification.

2.11 ERECTION EQUIPMENT

Lifting equipment is the single most critical item in the erector’s inventory. Hook speed, reach, and
capacity control the progress of erecting. Rigging, primarily wire rope slings and hardware, has to
be designed for efficient attachment and removal. Steel buildings and bridges are generally erected
with cranes, derricks, or specialized units. Mobile cranes include crawler cranes, rubber-tired rough
terrain cranes, and truck cranes; stationary cranes include tower cranes and climbing cranes. Stiffleg
derricks and guy derricks are generally considered stationary hoisting machines, but they may be
mounted on mobile platforms. Guy derricks can be used where they are jumped from floor to floor.
A catenary high line is an example of a specialized unit. Rubber-tired gantry-type cranes are often
used in shipping and shake-out yard operations. These various types of erection equipment used for
steel construction are also used for precast and cast-in-place concrete construction.

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