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Building Tomorrow's Parking Structures Today with Steel Frames
Advertising supplement provided by
The American Institute of Steel Construction, Inc.
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Functional Design Parameters

In establishing the feasibility, size, design concept and cost of a parking structure, the average floor area to be occupied by one parking space must be determined. This figure is obtained by dividing the total square footage of the structure (all levels) by its capacity, in total number of cars to be parked. The area-per-space ratio depends on many factors: size and shape of the buildable area; stall size, aisle width and ramp design; ratio of small to full-size cars; short or long span module; traffic flow system; mixed-use occupancy requirements (above or below); and local ordinances. The average area per space can vary from about 280 square feet to 400 square feet. For planning purposes 320 square feet to 350 square feet is typically used. When comparing average cost per space (i.e., cost per car) for design alternatives the design efficiency (area/space) must be considered. Steel columns are substantially smaller than corresponding concrete columns allowing for an increase in the number of spaces per floor.

In recent years, angle parking has become a popular stall design for self-parking structures, primarily because it is easier for drivers to maneuver and less driving aisle is needed than for 90° stalls. Angle parking, typically 50° to 70°, consumes a larger floor area per space than 90 degree parking with two-way aisles but the parking module, and perhaps even the over-all width of the structure, can be narrower. Angle parking uses one-way aisles, which may reduce congestion, but the narrower aisle may be perceived to be less secure for patrons walking to and from their cars. The 90° stall pattern allows a more uniform grid and appears to simplify ramp design.

Allegheny General Hospital, Pittsburgh, PA; Mulach Parking

For a “one-size-fits-all” design, assuming a 70/30 ratio of large to small cars and 90 parking, an 8 foot 6 inch. space width and a 60 foot wall-to-wall module may be used for planning purposes.

Bay sizes and interior spans in a parking structure are often dictated by factors other than parking efficiency. Framing for parking below steel-framed multi-story office or residential space having a 25 foot by 27 foot bay size will probably be more economical if the steel columns continue directly down through the parking levels to the foundation, avoiding structural transfers to longer bays at the transition floor. Long span modules, typically 55 foot to 65 foot, may be heavier, but cost 10% less due to fabrication and erection efficiencies. The number of stalls within the structure can be increased or the overall size of the structure can be reduced through the use of long span design. Advantages of long spans in parking structures include:

  • column-free areas that simplify maintenance and improve illumination
  • less opportunity for damage to automobiles
  • more openness and improved psychological effect on patrons (safety, comfort)
  • greater flexibility to re-stripe stalls in the future
  • fewer spaces lost to careless parkers in three-car bays

In parking structures with 55 foot to 65-foot spans, total floor depth will depend largely on the steel beam depth. Depending on the bay size, these spans are typically made with W24 to W30 composite design beams of A992 (standard 50 ksi) steel. It would appear that for the long span parking structure, a minimum total floor depth of around 30 inches must be expected. If shallower floors are required, shorter spans have to be considered. Thus, the designer should determine very early if there is a limitation on the depth of floor construction in the parking structure, because such a restriction can have a significant impact on the column grid and on selection, design and cost of the floor system to be used.

Depth of floor construction has emerged as an important factor governing the selection and design of floor systems for many types of buildings. Restriction of floor construction depth arises from local zoning (either total height or headroom), matching floors of an existing connected building, ramp grade requirements, and, for below-grade levels, shallow bedrock or a high water table. The minimum recommended clearance or headroom for parking structures is 7 foot 2 inches. (sometimes codified), but clearances as low as 7 foot 0 inches may be used. (The designer should consider detailing for a clearance that is 2 inches greater than the minimum required to allow for tolerances in beam camber and construction.) Where van access by disabled patrons is required, the minimum clearance to the lowest overhead obstruction is 8 foot 2 inches. To determine story height the designer should factor in the thickness of any mechanical and electrical services and fire protection that will run below the structural framing systems. Consideration should be given to running these services through web openings in steel framed structures.

Astute structural engineers have been successful in condensing steel beam floor construction to acceptable depths by the use of high-strength steel, LRFD (load and resistance factor design), cambering, composite beams, beam web penetrations, castellated beams and innovative steel beam/concrete floor systems.

In parking structures, as in all buildings, greater floor-to-floor height translates directly into increased cost of facade and other vertical building elements. With a simple and economical facade, the impact of this element on total cost is minimal. Decreasing depth of floor construction, below that of the most economical framing, may increase floor framing cost to some threshold that will exceed the savings accrued through lower total building height.

Depending on the length of horizontal run available, desired ramp grades may influence depth of floor construction. Generally, straight-run ramps with no parking stalls or pedestrian traffic on either side have 10% to 15% grades (slopes); a 12% maximum slope is suggested for long ramps. Any ramp with a slope greater than 14% will require a transition slope at the end of the ramp. Single lane ramp widths are usually 12 feet curb-to-curb, but 15 feet to 16 feet is advised if approaches or turns are particularly sharp. For the parking ramp itself, a 5% grade is preferred under ADA guidelines, although a 6% slope is generally acceptable.

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