Page 63 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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FABRICATION AND ERECTION*

FABRICATION AND ERECTION 2.25

Mill buildings, warehouses, shopping centers, and low-rise structures that cover large areas usu-
ally are erected with truck or crawler cranes. Selection of the equipment to be used is based on site
conditions, weight and reach for the heavy lifts, and availability of equipment. Preferably, erection
of such building frames starts at one end, and the crane backs away from the structure as erection
progresses. The underlying consideration at all times is that an erected member should be stable
before it is released from the crane. High-pitched roof trusses, for example, are often unstable under
their own weight without top-chord bracing. If roof trusses are long and shipped to the site in several
sections, they are often spliced on the ground and lifted into place with one or two cranes.
Multistory structures, or portions of multistory structures that lie within reach and capacity lim-
itations of crawler cranes, are usually erected with crawler cranes. For tall structures, a crawler crane
places steel it can reach and then erects the guy derrick (or derricks), which will continue erection.
Alternatively, tower crawler cranes (see Fig. 2.8) and climbing tower cranes (Fig. 2.9) are used
extensively for multistory structures. Depending on height, these cranes can erect a complete struc-
ture. They allow erection to proceed vertically, completing floors or levels for other trades to work
on before the structure is topped out.
Use of any erecting equipment that loads a structure requires the erector to determine that such
loads can be adequately withstood by the structure or to install additional bracing or temporary erec-
tion material that may be necessary. For example, guy derricks impart loads at guys, and at the base
of the boom a horizontal thrust that must be provided for. On occasion, floorbeams located between
the base of the derrick and guy anchorages must be temporarily laterally supported to resist imposed
compressive forces. Considerable temporary bracing is required in a multistory structure when a
climbing crane is used. This type of crane imposes horizontal and vertical loads on the structure or
its foundation. Loads are also imposed on the structure when the crane is jumped to the next level.
Usually, these cranes jump about six floors at a time.
The sequence of placing the members of a multistory structure is, in general, columns, girders,
bracing, and beams. The exact order depends on the erection equipment and type of framing.
Planning must ensure that all members can be erected and that placement of one member does not
prohibit erection of another.
Structural steel is erected by “ironworkers” who perform a multitude of tasks. The ground crew
selects the proper members to hook onto the crane and directs crane movements in delivering the
piece to the “connectors.” The connectors direct the piece into its final location, place sufficient tem-
porary bolts for stability, and unhitch the crane. Regulations generally require a minimum of two
bolts per connection or equivalent, but more should be used if required to support heavy pieces or
loads that may accumulate before the permanent connection is made.
A “plumbing-up” (fitting-up crew), following the connectors, aligns the beams, plumbs the columns,
and installs whatever temporary wire-rope bracing is necessary to maintain alignment. Following this
crew are the gangs who make the permanent connection. This work, which usually follows several
stories behind member erection, may include tightening high-strength bolts or welding connections.
An additional operation may involve placing and welding joists and metal deck to furnish a working
floor surface for subsequent operations. Safety codes require planking surfaces 25 to 30 ft (usually
two floors) below the erection work above. For this reason, deck is often spread on alternate floors,
stepping back to spread the skipped floor after the higher floor is spread, thus allowing the raising
gang to move up to the next tier. This is one reason why normal columns are two floors high.
In field-welded multistory buildings with continuous beam-to-column connections, the procedure
is slightly different from that for bolted work. The difference is that the welded structure is not in its
final alignment until beam-to-column connections are welded because of shrinkage caused by the
welds. To accommodate the shrinkage, the joints must be opened up or the beams must be detailed
long so that, after the welds are made, the columns are pulled into plumb. It is necessary, therefore,
to erect from the more restrained portion of the framing to the less restrained. If a structure has a
braced center core, that area will be erected first to serve as a reference point, and steel will be erected
toward the perimeter of the structure. If the structure is totally unbraced, an area in the center will be
plumbed and temporarily braced for reference. Welding of column splices and beams is done after
the structure is plumbed. The deck is attached for safety as it is installed, but final welding of deck
and installation of studs and closures is completed after the tier is plumbed.

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