DC Velocity - January 2009

CONSTRUCTION SEQUENCE

The construction process itself also yields some benefits for a rack-supported structure.

A steel frame building will typically have the foundations poured first, followed by erection of the structural steel. The roof deck and siding are added next. Once enclosed, the underground work is completed and the slab is poured. When the slab has cured, the rack is erected one frame at a time. The difficulty arises in that there is usually a limited amount of access and head room. Final bolt-up of the frames and bracing must be done in the air, making the plumbing and alignment more difficult and ultimately more costly. This issue is further complicated as the height of the rack increases and/or the width of the aisles decreases.

Construction of a rack-supported building begins with the pouring of the slab, and the erection of the rack. Since there is ample space due to the lack of enclosures, the multiple frames are assembled in fixtures while lying on their side. This allows the support steel to be assembled, and for bolt-up to be accomplished at or near ground level. These modules are then “stood up” on the slab. Fixturing also assures extremely accurate alignment, thus minimizing the time required to complete the plumbing of the rack during erection.

SELECTION GUIDELINES

Structural Steel Frame Building is preferable when . . .

• Business requires changes in building configuration periodically

• There may be a need to remove the storage later in time and still use the building

• Storage is interspersed with other processes such as manufacturing

• Building height over 45' becomes disproportionately costly

Rack-Supported Building is preferable when . . .

• Storage density is to be optimized (storage/cu. ft.)

• Space utilization is optimized and footprint minimized (storage/sq. ft.)

• Accelerated depreciation is available and desirable

• Land is at a premium

• Site has high winds or seismic considerations (forces will be spread across multiple frames)

RACK-SUPPORTED CONSTRUCTION

Pour Slab

Module Assembly Fixture

Begin Rack Erection

Set Additional Modules

Install SRMs

Start Siding

Complete Rack

Complete Siding and Roofing

CONCLUSION There are pro’s and con’s to both steel frame and rack-supported structures. While the traditional steel frame building still has its place in environments where the space has multi-functional uses or may be apt to be reconfigured, rack-supported buildings have clear advantages when material storage is the primary consideration. These advantages should be thoroughly investigated in order to ensure the best course of action is chosen.

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