214 Erection Methods For Buildings

The determination of how to erect a building depends on many variables that must be studied by the erection engineer long before steel begins to arrive at the erection site. It is normal and prudent to have this erection planning developed on drawings and in written procedures. Such documents outline the equipment to be used, methods of supporting the equipment, conditions for use of the equipment, and sequence of erection. In many areas, such documents are required by law. The work plan that evolves from them is valuable because it can result in economies in the costly field work. Special types of structures may require extensive planning to ensure stability of the structure during erection.

Mill buildings, warehouses, shopping centers, and low-rise structures that cover large areas usually 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 limitations 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 structure. 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 erection 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.

FIGURE 2.8 Tower crane on crawler-crane base.

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 6 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 temporary 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 usually present is placing and welding 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

FIGURE 2.9 Variations of the tower crane: (a) kangaroo; (b) hammerhead; (c) climbing crane.
Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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