A 'Different' Approach to Below-Grade Garage Construction
Members of the parking industry are no strangers to the costs associated with both above- and below-grade parking structures. Historically, underground parking garages are anywhere from 1.5 to 2 times the costs of above-ground parking decks. The inherent benefits of underground parking are too often overshadowed by the costs of foundation construction. Below-grade methods have additional structural design issues, such as groundwater, deep-cut excavation and shoring, that above-grade does not.
An innovative design concept utilizing sheet piling as the permanent basement wall is changing the economics of parking garage construction. Although virtually new to the U.S., this technology has been in practice throughout Europe for more than 20 years. So much so that European designers consider sheet piling ahead of alternative concrete methods when evaluating design feasibility.
So why has this not caught on in the U.S.? It starts with "perception." Owner, consultant and contractor alike generally view sheet piling as a unattractive shoring material mostly used for marine applications. Furthermore, general site construction perceives sheet piling as a temporary works material. The concept of incorporating the structural attributes of sheet piling into the permanent building design is just not common practice.
Sheet Piling 101
For those of you not familiar with the product, hot-rolled steel sheet piling is manufactured by fewer than 10 producers throughout the world. There are numerous shapes and systems for a multitude of structural applications.
Because the U.S. is predominantly a Z-Pile market, we will focus on the Z-shaped sections. Z-Piles are produced from a continuous-casting steel process from some of the same mills that produce standard structural shapes for bridge and building erection. This "mini-mill" technology streamlines steel production so that all products can be hot-rolled from three continuous-cast near-net shapes.
Z-Piles are produced from the beam blank. Once cast, it moves through a series of stands in the rolling mill, making three or four passes at each. The intricate interlock design and differential thicknesses make this the most difficult structural shape to produce.
Sheet piles are widely known for the ability to resist lateral/bending forces, yet they are also very effective in an axial/vertical capacity. This allows the structural engineer to transfer load to the perimeter walls, which can reduce the number of interior support columns, thereby optimizing the usable area.
Cast-in-place concrete basement structures require temporary shoring systems, such as beam & lagging or diaphragm walls. These methods require bracing, dewatering and waterproofing -- all before the general construction can proceed. Such specialty contracting levels are significant cost factors in addition to the concrete foundation construction. The Arcelor Sheet Pile System for bottom-up construction also requires bracing and dewatering, but eliminates the waterproofing and reinforced concrete elements.
An impervious wall system is delivered to the site in pairs with a full-seal weld on the paired joint and a hydrophilic joint sealant in the female interlock. With wall thicknesses ranging from 3/8" to 3/4", the owner can maximize area with the building footprint right up to the property line.
Once a few feet of soils are excavated, temporary anchorage or bracing is installed to support the sheeting from lateral earth and water pressures. Excavation and dewatering continue until the final elevation is achieved. Bearing piles are driven at the base to support the structure. The base slab is cast-in-place on top of the piles, and work is ready to proceed upwards.
The lateral loads are transferred from the temporary anchorage or bracing to the floor slabs as each level is cast. The sheet piles are then pressure-cleaned, primed and painted to an aesthetic finish. An intumescent fire coating may also be applied, if needed. Incorporating the sheets into the permanent structural design results in time and material savings.
This cost analysis demonstrates the time scale of a traditional reinforced concrete basement wall constructed by bottom-up method. One crucial aspect of the bottom-up method is that erection of the superstructure cannot proceed until the basement work is complete. This is the critical path of the project and directly influences the project schedule.
Dean Abbondanza is with Skyline Steel. He can be reached at Dean.Abbondanza@arcelor.com.