Below-Grade

Underground Parking Structures

Estimating costs for underground parking structures is considerably more complicated than costs for above-grade structures. Factors that contribute to the complexity include:

• The extent of shoring, if any, which will be required;
• Whether or not the parking is located below another use, which may dictate whether the structural system will be short-span or long-span;
• Location of water table in relation to the lowest level; and
• Whether or not the "lid" or plaza level over the parking is to be included in the cost of parking.

Shoring

The amount of wall to be shored depends primarily on the site. In some cases, the exterior walls may be far enough from property lines that slope cutting can be used on all sides instead of shoring. In other cases, all walls may be adjacent to property lines and require shoring. Any number of possibilities exists between these extremes. The unit cost of shoring depends on soil conditions and depth of excavation.

Other Uses Above Parking

If the underground parking is part of a freestanding parking structure with no other uses above, the same long-span structural system can be carried below grade and there is no penalty in parking layout efficiency.

If office, retail, residential, or hotel uses are above the below-grade parking, the structural system of the use above is carried down through the parking levels and parking layout efficiency may be severely impacted. For example, if the columns are spaced 32'-0" apart and code-required space width is 8'-6", only 3 spaces can be placed between columns, resulting in a 25% penalty, compared to a layout with a long-span structural system.

If parking is placed below a plaza or park with heavy landscape loads, a short-span structural system may be necessary, but in this case, the parking layout can dictate the location of columns rather than the building above, and the penalty in efficiency compared to that of a long-span structure is closer to 15%.

Other efficiency penalties that occur when another use is placed above parking include losses for elevators, mechanical rooms or electrical rooms that serve the use above and are located in the parking facility.

Water Table Location

If the water table is above the bottom of foundations, additional costs will be incurred for dewatering during construction. Cost of dewatering is a function of the depth below the water table, the area to be dewatered and the length of time dewatering will be required.

If the structure gets deep enough into water, the walls and grade slab must be designed for hydrostatic pressures, and, in some cases, additional structure weight must be provided to prevent "floating." These measures can add significantly to structure cost.

Allocation of Cost of Plaza Level

When another use is built above the parking structure, there is a grade-level or plaza-level structural cost that must be allocated either to the parking or to the building above. This is a cost that would not occur if the two uses were built side-by-side and the top parking level was open to the sky. The office, hotel or retail building would only have the cost of a grade slab included, which is much less than the plaza-level elevated slab supporting landscape loads or other elements.

The cost of the grade-level slab will also be higher than the cost of a typical parking level because it is normally supporting added dead loads such as paving or landscaping, plus higher live loads than parking requires. The costs of paving or landscaping would be in addition to the structural cost.

Calculation of Cost per Space

Normally, the cost per space can be approximated by multiplying area per space by the average cost per square foot. In the case of underground facilities, the cost of the grade-level deck, which provides no parking spaces, must also be taken into consideration. The cost per square foot of the lowest level is calculated by including the slab-on-grade, foundations, exterior walls, ventilation, fire sprinklers, excavation and shoring costs, in addition to the normal lighting, signing, painting, stairs, elevators, etc. that are appropriate for that level.

The cost of the typical parking level would be calculated in a similar manner, using the cost of the elevated beam-and-slab system in lieu of the slab-on-grade.

The cost of the grade-level, or plaza level, slab is calculated using only the structural cost of the beam-and-slab system and any improvements above it that are appropriate.

The area per space, as mentioned above, will depend on whether column spacing is dictated by another use above, or whether a long-span structural system can be employed. Area per space is also dependent on standards for space size and aisle width dictated by local planning codes.