Building Envelope Design Guide - Plazas, Tunnels, Vaults
Last updated: 03-14-2006
Introduction
Buildings frequently have plazas, vaults, tunnels or extensions below grade. The planning, development, detailing and construction of waterproofing for such features are significant. Although much more complex and far more maintenance intensive, these features are generally not treated with the same detailing attention that roof assemblies receive. In all such areas regardless of membrane detailing, protection, drainage and isolation along with thermal considerations must be incorporated into the design. Requirements for overhead waterproofing in tunnels or building obtrusions are significant and need to be treated accordingly. Design of the plaza surfaces, green-scape and tree or soil planter features, above the buried element also requires special design consideration.
Over the past 2 decades, plazas and special use features (fountains, planters and even athletic function) have become a more common and complex element in structure design. Tunnels and vaults, although a historic feature, are also experiencing increased design demand as they are upgraded to higher levels of service to accommodate; utilities, mechanical services or signal and communication conduits.
Plaza decks, tunnels and vaults are subject to some of the most accelerated and vigorous deterioration and distress of any structural system. Harsh exposure conditions from; exposure, moisture, thermal effects, weathering and traffic often reduce these systems serviceability at a rate even surpassing that of parking and bridge deck slabs. Accordingly, special analysis of plazas, tunnels and vaults as unique and novel; aesthetic, wearing, moisture protection and isolation and structural support systems, should be performed.
Plaza systems generally disintegrate and become unserviceable for 3 reasons, the first; ineffective design, the second poor construction and the third abnormal "unexpected" loadings. Other common causes of plaza deterioration or failure are; severe exposure including freeze thaw, chemical applications, overload and or improper materials selection and application.
Each of the above categories (listed features) is capable of working independently or in concert with other elements to cause premature plaza surface or waterproofing distress (service impairment) or subsequent damage over years of use exposure and traffic.
Description
This section provides specific description of materials and systems common in plazas, tunnels and vaults.
A plaza system is any supported slab that provides; green-scape, tree planters, vehicle and pedestrian movement over occupied space. Unlike the roof of a building, which is only exposed to weathering and the elements, plazas have numerous special features, supplemental to their common waterproofing characteristic. Depending on the type of structure the vehicular use may be limited to cleaning equipment or light vehicles. Under special circumstances plazas may be required to support truck and bus traffic or even construction equipment. A unique defining characteristic is the waterproofing requirement over occupied space, which at some level in the plaza system interfaces with the wearing surface and/or green-scape or tree-planters to produce high potential for distress, membrane leaking and or plaza surfacing degradation.
Figure 4 illustrates the basic components of a plaza system. The components of tunnels and plazas are similar.
Fig. 4. Plaza System Schematic
Description and guidelines are provided in the following sections:
- Wearing Surface
- Fill slab
- Isolation layer/Drainage Layer and flow path system
- Membrane and protection layer
- Structural Slab Support System
Wearing Surface
The wearing surface of a plaza is any surface subjected to pedestrian or vehicular traffic. Concrete, brick pavers, granite paving blocks, asphalt paving blocks and or pre-cast elements are all common to plaza wearing surface systems. In more recent years cement paving blocks consisting of high strength stamped or compressed cement units are also finding significant usages as plaza wearing surfaces.
Fill Slab
The fill slab is anything that occupies space on the plaza above the membrane and below the wearing surface. Historic fill slabs typically involve the use of; graded sand, asphalt material, pea gravel or a sand cement mortar setting bed. Pea stone is also commonly used which also provides the added function of improved drainage. More recently, Styrofoam voided areas or loose earth or lightweight concrete are used on horizontal applications of tunnels/vaults/plazas. These insulation systems have very high compressive strength properties (up to 10,000psf) making them resistant to crushing. In addition they are lightweight, cost effective, water resistant and serve as another layer of protection for the waterproofing.
Isolation Layer/Drainage Layers and flow path system (including drain basins)
Isolation/ drainage layers were provided historically beneath plazas where plantings were common surface features. The older drainage layers consisted of pea gravel and or a separator fabric over pea gravel to allow for drainage of surface moisture from the membrane to the inlet basins. Current isolation layers consist of a combination of geo-synthetic materials, used individually or in composite, or used in combination with the pea-stone drainage fill. The combination systems often reduce the demand for pea stone sill and speed construction. Two types of isolation must be considered when designing plazas, horizontal and vertical.
- Horizontal Isolation—Special isolation systems are created with materials that provide a slip plane, allowing the upper plaza surface (when subjected to large thermal movements), effective shear relief from the lower plaza system. The buried system components may or may not accept or respond to heat from the structure below and often experience very limited thermal swings, relative to the exposed surface.
- Vertical Isolation—Rigid wearing surfaces, subject to direct or indirect exposure to the elements, expand and contract during summer and winter cycles respectively. By doing so, they are at risk of locking to and over stressing the membrane, causing failure and subsequent leakage into the occupied space below. To accommodate horizontal movement of the wearing surface, strategically and uniformly space isolation joints, commonly ¼ to ½ inches wide, are required on 20 to 40 foot centers. Such joints should contain a free draining compressible filler, with a suitable sealant. Vertical isolation joints also reduce the horizontal thrusting that potentially damages perimeter walls or penetration features.
Drain Basins are perhaps the most critical plaza performance feature and must have a 2 stage or "promenade" plaza drain assembly. The first stage of the drain basin, weep holes/ screens or perforations, remove surface moisture from the structural slab membrane interface. The second stage, the "exposed upper grate", removes moisture from the exposed wearing surface. Both must remain open and functional for the plaza to resist premature deterioration. Numerous performance evaluations confirm that preventing the "Bathtub effect", for plaza surfacing and subcomponents, including the membrane and isolation layer, are highly effective at reducing membrane failure and plaza wear surface disintegration. Maintenance of catch basins helps to promote long term plazas durability.
Membrane and Protection Layer
The structural slab supports the plaza surfacing system and includes a special waterproofing membrane or roofing material to prevent moisture leakage into the occupied space below. Structural slab membranes, often referred to in plaza applications as buried membranes are provided with a protective wearing system or protection course directly above the membrane, so that construction of the upper plaza can proceed with minimum risk of membrane damage. All of these components are essential to proper plaza performance.
In certain plaza systems, insulation is provided directly on top of the membrane and protection layer. This component then functions as a foundation layer for the upper plaza wearing surface, which should be provided with appropriate isolation systems between the insulating layer and the wearing course. These materials can be provided with compressive strengths up to 60 psi for use in horizontal applications such as tunnels/vaults/plazas to prevent crushing and settlement. A drainage layer at this interface, provided with horizontal isolation, is highly effective at promoting long-term system durability. The insulation also promotes long-term membrane performance by keeping it in a more agreeable service environment with reduced freeze thaw effects.
Structural Support Systems
The structural slab system used to support plazas can be any one of the basic slab designs. The 2-way flat plate was the most common historic slab system use for plazas. This was followed later by the one-way slab over beam systems, which were also common for plaza support. In the later years pre-cast structural systems have found use in plaza support as well as post tensioned slab systems.
It is recognized that the various types of structural systems have a greater or lesser degree of flexibility and deflection under plaza design live and dead loads. Accordingly, variable requirements on the plaza membrane system and the fill slab or interfacial isolation layers, are appropriate to proper plaza system design.
Fundamentals
Structural Support Functions—Tunnels/Vaults/Plaza areas typically have structural slabs and beam elements that are subjected to high dead and live loadings. Dead loads include overburden from soil, planters, pavers, fountains, mechanical equipment, etc. Live loads may include loadings from pedestrian walkways or from vehicular roadways. In many cases around governmental buildings, areas designed as pedestrian areas must also include consideration for emergency vehicular loadings.
Structural systems for tunnels/vaults and plazas are typically cast-in-place concrete systems, either conventionally reinforced or post-tensioned. The use of precast concrete elements for these areas should be avoided due the difficulties in obtaining effective joint and surface waterproofing.
Environmental Control Functions—The exterior environment that the tunnels/vaults/plazas are subjected to includes environmental control loadings such as thermal, soil, tree roots, moisture, insects, and noise. The interior environment that the tunnels/vaults/plazas are subjected to includes environmental control loadings such as thermal and moisture. The performance of the tunnel/vaults/plaza system depends on its ability to control, regulate and/or moderate these environmental control loadings on each side of the foundation wall to desired levels.
As with other below grade systems, the most predominant environmental control loading for tunnel/vault/plaza systems is moisture. Moisture control is dealt with in a multiple screen/barrier type of design approach. For surface moisture loadings such as rain, snow and sprinklers the first line of control is the upper screen at the exposed surface. This upper screen may be comprised of relatively permeable landscape areas to impermeable pavers, concrete or asphalt surfaces that will shed the majority of surface moisture. The effectiveness of this initial screen in shedding moisture may influence the design of the other components of the system.
Moisture that penetrates through the upper screen needs to be efficiently directed off of the supported surface or to the drains provided in the tunnel/vault/plaza. This is accomplished through a drainage system of free draining granular material. Backfilling with native, poor draining soil is not recommended, as this will maintain an active water load on the tunnel/vault/plaza and limit its ability to control moisture ingress to the interior. As moisture moves from the upper screen and through the drainage system, moisture will inevitably make its way toward the surface of the tunnel/vault/plaza itself. A drainage system at the surface of the tunnel/vault/plaza is required to direct this water toward the exterior wall or to drains. In all cases a waterproofing membrane is required to effectively waterproof horizontal surfaces of tunnels/vaults/plazas. In no case would a damproofing or vapor retarder be sufficient to prevent leaking to the interior. The water is directed from the horizontal surface of the tunnel/vault/plaza to the adjacent walls. The side walls of tunnels and vaults are to be treated with similar consideration as foundation wall systems.
A critical design consideration is the continuation of the waterproofing on horizontal tunnel/vault/plaza elements to adjoining above or below grade waterproofing systems. This intersection, which may include expansion joints, flashings or differing materials, requires special consideration.
Thermal considerations may be a concern for shallow tunnels/vaults/plazas and insulation may be provided on top of or on the underside of the structural ceiling element. The use and location of the insulation is important on the control of moisture in terms of preventing condensation on interior faces of the tunnel/vault/plaza. Condensation is possible in below grade conditions in warmer more humid summer conditions as below grade spaces tend to be cooler in the summer because of the insulating effect of the backfill soil. This cooling effect combined with general poor air circulation in underground spaces can result in condensation on interior wall surfaces. The use of insulation above membranes also is beneficial in reducing the overall temperature range the waterproofing membrane material is subjected to which can reduce the potential of cohesive failure under elongations.
Finish Functions—Tunnels and vaults will typically only have finish functions on the interior face of the element while plaza systems have both internal and external considerations. The interior finish is dependent on the interior use whether it be a controlled office environment or a non-controlled parking environment. Typical finish systems may include paints, stucco, or framed walls with drywall. In many applications the interior finish is simply the interior surface of the material used for the foundation wall, i.e. concrete.
In plaza areas the exterior surface will likely be important from an aesthetic viewpoint with many plaza areas using a combination of landscaping and pavers.
Distribution Functions—The structural slab of the tunnel/vault/plaza may contain distribution systems such as electrical feeders, electronic conduit, mechanical piping or heating systems. Plaza soil areas also may contain mechanical and sprinkler lines.
Applications
There are two main types of systems used in plazas, tunnel and vault areas that are distinguished by the exterior use:
- Planter Areas
- Wearing System
Planter Areas
Major design points include providing a structural slab that slopes to the exterior of the element or to drain. A waterproofing membrane fully adhered and protected by a protection board. The insulation board may be needed in shallow applications to provide some thermal resistance but also when located above the membrane keeps the membrane above the dew point temperature preventing condensation on the underside. In some cases tapered insulation is provided below the waterproofing membrane to provide slope to the waterproofing membrane where the structural slab is flat. This practice is not recommended for the dew point considerations, as well as the effectiveness of the waterproofing membrane installation. A synthetic drainage layer located above the insulation material will direct any moisture penetrating upper soil screens. A well compacted granular drainage layer provides a solid, yet economical fill material in planter areas. The compacted granular drainage layer should be separated from the finer planter soil with an specifically designed filter or soil separator fabric.
Wearing System
Major design points are similar to that described for planter areas. In addition the wearing surface should be designed for aesthetic and functional purposes but also to provide the first screen to surface water. Crack free design and appropriately spaced and sealed construction/isolation joints are important. If at all possible in the design sloping this wearing surface to the drain or exterior will limit the amount of moisture entering the system. Proper design of the wearing surface, insulation board and synthetic drainage layer is a critical issue to handle vehicular loadings.
Membrane Selection
Fluid applied membranes or sheet membranes can be used in plaza systems. One of the most proven, durable waterproofing system is the hot applied modified bitumen systems. The combination of rubber derivatives and an asphalt base provide superior crack-bridging and chemical resistance performance. Urethane systems are typically 60 mils in thickness, where modified bitumen systems are 180 to 215 mils in thickness.
Unbonded applications are not recommended for these systems. Laps and detailing of penetrations are critical.
Building Façade Termination
Of critical importance in any building is the proper detailing and integration of the vertical building façade system and the below grade building system. The integration of the two systems requires careful consideration to insure that all moisture, air and thermal criteria for each system are satisfied at the transition interface. There is a combination of environmental design loadings at this interface such as surface water, runoff, and cavity wall drainage.
Below-Grade Penetrations
A general element that is common to all buildings yet frequently not fully detailed or addressed during design is penetrations. There are essentially three (3) types of penetrations that are common. Others also exist. These penetrations are any openings in the wall or structural system that once waterproofed provide an avenue of breech for moisture entry into the building. Sewer pipe penetrations, water line entry penetrations, drain basins in the floor slab or sleeves for electrical, gas or communication are all common penetrations, typically with their own design or detailed features. These features, however, leave much to be desired with respect to sealing and waterproofing. Penetrations can also become quite exotic such as steam penetrations or other features that require special treatment.
A special area requiring detail treatment and application of all previous features described for plantings, plazas, and waterproofing relates to planters that abut building faces. Of all systems that tend to be problematic and produce serious maintenance problems for structure owners, plantings at building faces tend to rank high on the list. Isolating the planter system from the building wall is considered a good first step to achieve effective treatment of aesthetics and building operations and envelope integrity. Where planter assemblies are constructed monolithic or integral walls and proper consideration is needed for insulation for heat flow and protection against moisture penetrations. Planter assemblies on raised balconies above buildings also represent special conditions and points of moisture entry that are aggravated by irrigation systems and electrical service facilities frequently provided in planters. In all instances proper isolation of the planting assembly or feature from the building façade components is essential. See common details for planters relative to façade termination detailing.
The previous discussion concerning penetrations recognizes that most services that enter buildings are provided underground. And many instances current design has such services entering the building at one or possibly two locations and vaults are designed under the structure to accommodate these services upon entry. Special vaults provide for effective treatment and special waterproofing, needed for each of the various systems. In all cases referring to the manufacturer's requirements for treatment of sleeves and/or special diaphragms and boots to accommodate sealing the penetration against moisture and/or other vagrant migration is necessary.
Overhead Floor Expansion Joints
Overhead floor expansion joints perform satisfactorily when the joint surfaces above are within the building line enclosure of the structure. Special problems and treatment is needed however, when the joints are exposed directly to the elements and plazas. For this case, treatment of the joints by raising the concrete joint openings above the surrounding surface is highly effective for preventing moisture collection at the joint and leakage when the joint seal system fails. Special design detailing that allows for construction of the expansion joint at the high point in a plaza structural slab profile is also effective for reducing exposure and thereby minimizing operational problems.
Although below grade areas are often used for electrical and mechanical space this practice must be treated by special waterproofing details to minimize the risk of leaking and in some cases excessive humidity levels affecting electrical components performance or durability.
Positive Drainage
The most basic requirement for proper plaza design, is proper slope to drain provided by the structural slab. Commonly known as gradient, this feature allows water that percolates down to the membrane, a means of flowing towards and discharging into the drain basin. Both historic and recent experience however, strongly suggests that without proper subsurface "structural slab" gradient both designed and constructed, undesirable ponding frequently occurs on the structural slab above the membrane. Typical flat slabs, regardless of the structural design system used for the supporting floors, can be a constant source of leakage headaches for the plaza owner.
The supported structural slabs in all plazas, tunnels and vaults should slope a minimum of 2% away from the building or toward the exterior of the element. It is not sufficient to just slope the wearing surface or even used tapered insulation to create slope while using a flat structural slab. A durable design will include positive drainage of structural elements to drain any water that penetrates to this point.
Horizontal Isolation
Horizontal isolation, simply stated, involves providing a relief mechanism or system directly beneath the rigid wearing surface. The most widespread and aggressive forms of early membrane destruction are associated with wearing surfacing installation directly above the membrane and protection layer. Unless an effective shear relief plane is provided, normal thermal cycles cause the rigid wearing surface to expand and contract pulling on and often a rupturing the membrane. Water then migrates to the structural slab, becomes ponded and upon reaching a crack, leaks into the occupied space below.
Vertical Isolation
Because of normal expansion and contraction caused by thermal movements, isolation must periodically be provided, so that rigid elements don't lock up and bind against each other or their perimeter abutments. Significant forces can develop during normal expansion and contraction cycles. In extreme cases, rupturing, crushing or displacement can occur. Most common evidence of this characteristic is found in examining drain basin's, which are often sheared off at the structural slab/plaza wearing surface interface.
Sub Drainage
Not withstanding effective drain basin installation, (see next paragraph), the major problem with many aging plazas is failure to allow subsurface drainage. Historically, this function was provided by a loose layer of pea gravel, usually made up of one-quarter inch to 3/8 inch diameter round gravel. This layer, when properly connected to the drain basins, permits water to flow readily off the structural slab surface. (Ironically it also allowed horizontal shear relief or horizontal isolation layer when used beneath rigid surfaces) Regardless of the nature of subsurface drainage selected, it must be continuous along the flow path to the drain basin and covered with a proper separation and filter layer from planted areas and other potential contaminates, in order to maintain effectiveness. Without proper filtration, drainage grid cells will rapidly collect filtrate and plug even the best drainage layer.
Basin Drainage
Plaza drains must be designed to accommodate runoff not only from the wearing surface, but also from the membrane covered structural slab below. In order to be effective, drain basin's must have special weep ports at the structural slab level, so they can accommodate subsurface a runoff. Surface grates must have narrow lens openings, so that pedestrian traffic can safely traverse the exposed surface. Finally, drain basins should be isolated from the plaza wear surface to reduce the shear effects described earlier. Past experience has repeatedly shown that construction inefficiencies, relative to maintaining an open area at the weep ports, is the single most common cause of subsurface water entrapment.
Materials Properties
The importance of selecting the right materials for plaza system components can never be overstated. There is a wide selection of materials available depending on the final aesthetic and functional requirements desired. Because no consistent guidelines are available to the architect, each project is a novel design experience. Without proper guidance, architects and engineers and other design professionals normally do the best they can with locally available materials and recognized systems.
The best guidance that can be provided involves thoughtful consideration of the systems operating environment. Those systems designed and installed in harsh exposure environments, defined as areas where freezing and de-icing are common, should receive maximum design detailing for good drainage and durable surface and subsurface systems to withstand frost breakup.
For those areas subjected to normal thermal cycles and more moderate climate; isolation, drainage and proper separation layers are still all required. Damage to membranes, from improperly isolated wearing surfaces, can be just as severe and destructive in the southern states as in northern climates. There are few, if any plazas that can be effectively designed without proper horizontal and vertical isolation combined with the other factors described above.
Details
The following details can be downloaded in DWG format or viewed online in DWF™ (Design Web Format™) or Adobe Acrobat PDF by clicking on the appropriate format to the right of the drawing title. Download Autodesk® DWF Viewer. Download Adobe Reader.
Plaza System—General Concept (Detail 1.4.1) DWG | DWF | PDF
Plaza System—Wear Slab Base (Detail 1.4.2) DWG | DWF | PDF
Plaza System—Wear Slab Base @ Wall (Detail 1.4.3) DWG | DWF | PDF
Plaza System—Bonded Wear Slab (Detail 1.4.4) DWG | DWF | PDF
Plaza System—Planter Areas (Detail 1.4.5) DWG | DWF | PDF
Floor Drain—Plaza Area (Detail 1.4.6) DWG | DWF | PDF
Expansion Joint—Plaza Area (Detail 1.4.7) DWG | DWF | PDF
Emerging Issues
For emerging issues refer to General Overview section.
Relevant Codes and Standards
For codes/standards refer to General Overview section.
Additional Resources
WBDG
Products and Systems
Section 07 92 00: Joint Sealants, See appropriate sections under applicable guide specifications: Unified Facility Guide Specifications (UFGS), VA Guide Specifications (UFGS), DRAFT Federal Guide for Green Construction Specifications, MASTERSPEC®
For resources including texts, guides, and web pages refer to General Overview section.
