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Building Envelope Design Guide - Atria Systems

by Todd Gritch, AIA, ACHA and Brian Eason, AIA, HKS Inc.

Last updated: 06-07-2010

Introduction

(a'treem), term for an interior court in Roman domestic architecture and also a type of entrance court in early Christian churches. Today atrium means an enclosed multi-storied space that is open vertically to multiple stories.

NFPA 92B the current standard for smoke control in large spaces defines atrium as a large volume space created by a floor opening or series of floor openings connecting two or more stories that is covered at the top of the series of openings and is used for purposes other than an enclosed stairway; or other mechanical and utility service to the building. The International Building Code (IBC) defines Atrium similarly as an opening connecting two or more stories other than enclosed stairways, elevators, hoist ways, escalators, plumbing, electrical, air-conditioning or other equipment, which is closed at the top and not defined as a mall.

Atriums have many advantages as a building form over conventional modern building configurations. Atrium buildings appeal to people not only logically, but also emotionally by providing a connection to the outside inside. By bringing natural light into the interior, atriums offer larger, more efficient floor areas than conventional buildings. Atriums provide more desirable work environments by providing more space with a connection to natural daylight and the outside environment. Many believe that access to natural full spectrum lighting creates a more healthful and productive environment. There have been several studies that support this view.

Atria view at The Plaza of the Americas-Dallas, TX

The view into an atrium can and in most cases is more entertaining and connective than an exterior view as illustrated below at The Plaza of the Americas in Dallas, Texas.

An atrium is a pleasant all weather gathering place providing shelter from the more extreme climate conditions outside. The atrium replicates a desirable outdoor environment by providing the benevolent aspects of the outdoor environment; natural light, moderate temperatures while sheltering us from the harsher elements of extreme temperatures, rain, and winds.

Because atriums are so complex, they create unique interrelationships between fundamental elements that must be understood and accounted for in the final design. Atriums will contain many compromises; the designer must understand the negatives as well as the positives of each component in the relationship to the complete atrium environment. Many atriums have been built where unintended consequences have compromised the design.

The complexity of atrium design does not lend itself to prescriptive standards, but sound life safety principles must be incorporated into every atrium design. Good atrium design will maximize the natural environment to minimize energy consumption.

Atriums can be configured in an infinite number of ways, but atrium configurations should be always a reasoned response to the climatic and life safety goals. Typical atrium configurations can be totally surrounded by building elements or partially enclosed. They maybe top lit, side lit or a combination of both. The configuration of the atrium will dictate many of the fundamentals of atrium the components. The first consideration of atrium design is an acknowledgement of the necessity of fire and smoke management. Building configuration is the most significant factor in smoke management and thus must be fundamental to the design.

The details associated with this section of the BEDG on the WBDG were developed by committee and are intended solely as a means to illustrate general design and construction concepts only. Appropriate use and application of the concepts illustrated in these details will vary based on performance considerations and environmental conditions unique to each project and, therefore, do not represent the final opinion or recommendation of the author of each section or the committee members responsible for the development of the WBDG.

Fundamentals

Geometry

The shape and geometry of an atrium is both the product of and the reason for the adjoining occupied portions of the building. Inhabited by office workers, residents, or other uses, these spaces are impacted greatly by the configuration of the atrium space. The configurations can refer to the shape in two or three dimensions, the scale or the layout of the surrounding spaces and how they are connected to the atrium.

There are several simple and several complex basic configurations of atrium space. They are:

Simple Types:

Geometric configuration of a single sided atriumGeometric configuration of a two sided atriumGeometric configuration of a three sided atrium

1. Single sided: Atrium abuts one side of the occupied portion of the structure.

2. Two sided: Atrium abuts two sides of the occupied portion of the structure.

3. Three sided: Atrium abuts three sides of the occupied portion of the structure.

Geometric configuration of a four sided atriumGeometric configuration of a linear atrium

4. Four sided: Atrium abuts four sides of the occupied portion of the structure.

5. Linear: Atrium sandwiched between two occupied portions of structure.

Complex Types:

Geometric configuration of a bridging-style atriumGeometric configuration of a podium-style atrium

1. Bridging: Atrium connects several occupied portions of structure.

2. Podium: Atrium sits at the bottom or below an occupied portion of structure.

Geometric configuration of a multiple lateral atrimGeometric configuration of a multiple vertical atrium

3. Multiple Lateral: Atrium spaces scattered throughout plan on single or multiple stories.

4. Multiple Vertical: Atrium spaces scattered throughout height of tower structure.

These different configurations can be developed to myriad architectural statements but the basic configurations remain recognizable. Which configuration is utilized by an individual designer is a function of (among other issues) personal taste, life safety issues, proposed uses of both the atrium and adjoining spaces, impact the atrium is wished to have climactically, geographic location, urban context, and scale of the atrium desired.

Natural Lighting

In the last couple of decades there has been dozens of scholarly papers and research studies on all aspects of indoor environment quality and its relationship to worker productivity and well being. In the United States, the oft-cited "West Bend" study by Walter Kroner and colleagues at Rensselaer Polytechnic Institute documented productivity gains from daylighting, access to windows, and a view of a pleasant outdoor landscape at the West Bend (Wis.) Mutual Insurance Company. According to the study, productivity gains in the new building increased by 16%, with the personal controls alone accounting for a 3% gain.(x)

Another frequently cited report is the Heschong Mahone Group study "Daylighting in schools," which was conducted on behalf of the California Board for Energy Efficiency. The researchers analyzed test scores for 21,000 students in 2,000 classrooms in Seattle; Orange County, Calif.; and Fort Collins, Colo. In Orange County, students with the most daylighting in their classrooms progressed 20% faster on math tests and 26% faster on reading tests in one year than those with the least daylighting.(x)

Natural light as it pertains to atriums is a basic element of the design. The light within the atrium as well as the light transmitted to the adjoining occupied space needs to be considered. The light coming into the atrium is impacted by several factors:

The average brightness of the local sky is a factor. This will affect the amount and type of glazing used for the exterior skin. Sufficient openings should be provided for the amount of light expected at the bottom of the atrium space. Additionally, the type of glazing, whether transparent or translucent will impact the amount and quality of natural light admitted to the atrium. Below is an example of natural light from a sky lit atrium at EDS Corporate Headquarters in Plano, Texas where significant daylight was desired at the floor level.

An example of natural light from a sky lit atrium at EDS Corporate Headquarters-Plano, TX

The orientation of the glazing will have significant effect and needs to be a fundamental consideration. Glazing facing to the east or west should be avoided as it is difficult to control glare because direct sun at low angles will be admitted at some point during the day. Horizontal glazing at the roof should also be carefully considered because direct light is inevitable from this orientation. Diffuse natural light generally is the preferred form of natural lighting.

Reflectivity of wall surfaces facing the atrium should be a consideration. Bright surfaces will reflect light and maintain light levels deeper down into the atrium space and therefore transmit this light to the adjacent occupied spaces.

These issues must be balanced to provide an adequate amount of light at the occupied areas of the atrium as well as the quality of light desired for the proposed uses. Also a consideration is the amount and quality of light available to the spaces adjacent to the atrium. At the top of the atrium abundant light will be available and therefore it may be desirable to provide smaller openings to admit this light while providing more reflective surfaces in order to allow the light to penetrate deeper towards the floor of the atrium. Larger openings provided closer to the floor will admit a higher level of light, the deeper the atrium the more glazing will be necessary. To aid light in penetrating the adjacent spaces more evenly and deeper, light shelves can be utilized at each level facing the atrium. This strategy may require higher floor to floor dimension to operate sufficiently but will also reduce the need for general artificial light on each floor within a reasonable dimension of the atrium. In warmer climates, this reduction in artificial light may also improve the occupied space's thermal performance and reduce cooling loads.

The light allowed into the atrium can be varied or controlled by external or internal shading devices. These can be configured in a number of ways, vertically, horizontally or at angles to accomplish the desired shading as well as act as a design element. The need for shading devices depends on the strength of light reaching the atrium skin and this depends on the location geographically. External shading devices are used to prohibit light from directly entering the atrium and therefore controlling the heat gain associated with direct light. Internal shading lets the heat in but prohibits direct light from getting down to the usable space of the atrium.

Exterior Envelope

Resistance to the elements is the primary focus of the atrium enclosure. Several components can make up the skin of the atrium. They are the walls, roof and any sloping surfaces that act to keep water and wind out of the interior space and control the amount and quality of daylight penetrating the space. (Refer to Wall Systems)

In order to accomplish these goals, openings in the atrium skin should be limited to those required for ventilation and smoke evacuation at the top and bottom and pedestrian access or exit at the bottom. The Pedestrian access or exit should be accomplished through revolving doors or power sliders or swing doors in a vestibule configuration. This will help control drafts in the atrium induced by the stack effect in these large spaces. The enclosure elements of the atrium have to react to the structural building frame as well. This is accomplished through the use of movement joints to resolve a variety of differential movement between the skin and the frame, different skin elements as well as different building elements. These joints should be tracked horizontally, vertically and diagonally to termination and detailed to maintain weather tightness along their entire length and at termination or transition points. Below are some simple details showing the tracking of an expansion joint through different conditions and materials.

Plan Details at Expansion Joint tracking through different materials

Plan Details at Expansion Joint tracking through different materials.

Plan Details showing tracking of expansion joint through different skin materials

Plan Details showing tracking of expansion joint through different skin materials.

Exterior glazing systems used in atrium enclosures should be of a high performance curtain wall system designed and constructed expressly for the purpose of spanning large distances while controlling water and air infiltration without the aid of water shedding overhangs or other protection. The interface of these systems with adjacent material and systems should be detailed carefully taking into account different movement and movement characteristics. One type of movement that can occur is due to thermal expansion and contraction of building materials. Different materials can experience large differential movements over the same temperature change. Slip connections can typically be utilized within building systems to account for this differential movement. Deflection is another type of movement that should be considered. Deflection joints can occur at every floor level or every other floor level depending on vertical spans and loads being carried. Different structural support conditions can also telegraph through the exterior skin as expansion or construction joints. Attention must also be paid to geographic location, wind loading on components and cladding which will vary. Some coastal regions will require the consideration for large and small missile impact zones on the exterior skin and this will further increase the required performance characteristics for the system. Below is a view of the inside of such a curtain wall system and the structural back up required to support it.

View of the inside of a high performance curtain wall system designed and constructed expressly for the purpose of spanning large distances and the structural back up required to support itExample of an interior system on a slope-faced atrium at 311 South Wacker Drive-Chicago, IL

Roofing systems for atria can also pose severe challenges due to the desire to admit light through the roof using skylights or a translucent roof system. The roofing system should be designed to accommodate the expected volumes of water and/or snow by removing these from the roof in an adequately short period of time as well as supporting them structurally while this is accomplished. (Refer to Low and Steep Slope Roofing Assemblies)

Sloping skin elements provide a separate set of concerns. While neither wall nor roof, they take on characteristics of both. They must perform to maintain a weather tight seal, accommodate movement in multiple directions and tie into adjacent systems at difficult angles. Further, the prospect of providing exterior building maintenance (E.B.M.) systems on sloped surfaces is difficult, and additionally so if the slope is reversed to extend out while moving away from the floor. Support for staging rigs used to provide cleaning, maintenance, repair or glass replacement should be provided to keep the stage away from the glazing system as well as safely anchoring it to maintain lateral stability as the rig moves up or down the vertical of sloping surfaces. Vertical or sloping atrium faces have additional challenges as the interior surfaces requiring access may not be sufficiently close to a floor surface in order for maintenance to occur. Therefore an interior building maintenance system should be considered to operate similar to the E.B.M. system. These interior systems can be at least as complicated as the exterior systems and often times more so due to the design intent that they not be visibly heavy or obvious inside the atrium while offering the same level of access to the portions of the building skin as the exterior systems as witnessed below at 311 South Wacker Drive in Chicago, Illinois.

Glazing in a horizontal application requires protecting atrium occupants from falling glass either by utilizing wired glazing, laminated glazing or providing safety screening below the glazing.

Landscaping

There are three basic components to the landscaping or softscape of the atrium. The planting medium, the plants themselves and the space above the planting all contribute to the success of the planting.

Planting while the primary focus can be extremely difficult to predict what types will live thrive and survive in an atrium space. A qualified landscape consultant should be retained during design to select planting and oversee the project through installation and beyond. There are some basic guidelines to plant selection. Sub tropicals should be used as they can handle the fairly consistent climate within a building.

The planting medium also has significant impact on the overall effect. Starting with planter sizes that are appropriate for the selected plants, proper soil mixes, drainage/ irrigation characteristics and nutrients that will be necessarily introduces to the planting areas following installation. These issues also should be left to the landscape consultant to determine and incorporated into the design by the building designer. Of these adequate drainage and water supply should be planned on by the designer from the start.

Once these issues are established, the design of the volume of space above and around the planting is where the designer can have the most influence. Plants derive their beauty from the energy they can absorb from their environment. Light, temperature, and humidity all contribute to the growth potential of the interior planting. The amount (both intensity and duration) and quality of light provided are factors to be considered. Light in the crown of the sky will be of higher intensity than at the horizon and therefore roof lit atria will provide higher light intensities than side lit ones. This light direction will also impact the growth patterns of the interior plants and therefore should be considered. If the light in the atrium is not of sufficient brightness to support the plants proposed, artificial light can be introduced. This light should be of the proper type, that is have acceptable color rendition and be of the right frequencies to promote plant growth. Typically it is not necessary to employ horticultural type lighting due to poor color rendition and little improvement in plant performance. The light range within the atrium should be maintained between 700 and 1,000 lux with a bare minimum of 500 lux for a duration of 12 hours per day(1). This light level can be achieved with daylight solely, and combination of daylight and artificial light or in extreme situations, solely artificial light. In any case, infra-red and ultraviolet light frequencies are detrimental to plant growth and should be filtered from the natural light if possible or not produced with artificial lighting. Artificial light should be turned off or reduced to minimum acceptable levels to provide safe pedestrian travel at night as the plants need to maintain good diurnal variation or distinguishable night/ day cycles.

Temperature also plays a large role in the landscape's success. Planting should be held back from sources of cold air such as unprotected entries (those without a vestibule or revolving door) cold spots due to uninsulated glazing, mechanical air distribution locations. Planting needs to experience some temperature variation during the day and night again to maintain the proper diurnal variation. A range of 70-75 degrees F is good during the day with a range of 60-65 degrees F at night. A minimum temperature of 50 degrees F should be maintained at all times unless plants have been chosen specifically to exist at lower temperatures.

The humidity in the atrium is also at issue. Plants will naturally increase the humidity in an interior space. Mechanical systems typically will counteract this phenomenon however and maintaining a high humidity inside may not be possible. This is not a large problem as most plants will be able to exist in this environment without much difficulty but its effect must be understood because it might be of consequence. The entire life cycle of the planting should be incorporated into the understanding of the plants role in the atrium. Plants that require extra maintenance to be appropriate to their atrium environment should be understood and incorporated into the Atrium planning. An example might be ficus types that drop an inordinate mount of leaves and thus require excessive maintenance.

Sound (Acoustics)

There are many impacts on the acoustical performance of an atrium space. The designer needs to decide early on the uses and types of activities to be supported both within the spaces and in adjacent occupied areas. These can range from gatherings for various events, musical performances and dances, lobby and reception function, or simply transitory from one part of the building to the other.

The basic functions of the atrium space and adjacent occupied areas will greatly impact any acoustical systems being considered. First, within the atrium space, what is the acceptable ambient noise level at the floor and in what range can the noise level be expected based on the design being considered. Medium to low thresholds of noise from the HVAC system at the floor will require consideration at the design stage. If the Atrium configuration has a dome and/or other round focusing surfaces, then the materials such as reflective and acoustical absorbing must be carefully considered. Secondly, the range of functions must be considered. Range of functions could be for instance musical concerts, parties and receptions, sit down dinners where individuals are sitting and having a conversation with those in close proximity. The musical performances want to have slightly higher reverberation to support the music, but the others want to have lower reverb for better speech privacy and ease of conversation. An example of a place that needs to be designed for both music and speech would be a church, the design has to strike a balance to serve both functions.

Acoustical parameters such as reverberation time and speech intelligibility must be considered for these functions. For example, if gatherings for corporate events or receptions are important functions then the reverberation time should be lower so the speech intelligibility will be reasonable. When the speech intelligibility is poor it is difficult to hear a conversation clearly. This is commonly known as the "Cocktail Party Effect." It may be necessary to have the acoustical criteria set for a variety of functions, but if there is a lobby and reception function then localized absorption is desirable for a clear conversation between the visitor and receptionist.

Thermal Control

Atria typically involve large open spaces connecting multiple floors. In some instances the space can be large enough that individual zones of greatly varying temperatures may exist within the atria. These zones may develop air currents within the atrium that may be stronger influences than the HVAC system. If the space is large enough, it is possible to create ‘rain’ indoors.

When only one wall of the atrium is an outside wall, it is possible in the warmer seasons for the air next to the wall that absorbs the walls transmitted heat to rise. Depending on the height of the space, air currents could develop and become a strong enough force overpowering the influence the diffuser placement has over the air movement. With multiple walls, the problem can still occur but tends to be less substantial due to more uniform temperature profiles.

Air currents due to load concentration may rise and displace the stratified air at the top, forcing the warmer air down. When cooling loads assume that stratification will occur, the design should not include heavy localized loads or unbalanced exposures. Architectural configurations of atria that include these requirements are rare. Therefore it is recommended that cooling loads do not assume stratification unless it can be reasonably shown that strong thermal currents will exist in the particular design. The design concept of spot cooling only the occupied areas is acceptable, but high diffuser throw velocity must be maintained to counteract any thermal induced air currents.

Under heating conditions, if the atrium is topped with a skylight or poorly insulated roof, warm moist air from the occupant level may rise and be cooled by the top exposure. This could create condensation.. This situation should be avoided, since it can potentially damage the structural components of the roof assembly. (Reference ASHRAE 1999 Applications, Chapter 4.8.)

Atria, unlike most designs in HVAC, should be viewed as a three dimensional volume from the start of the project. Normal engineering practices such as CFM per square foot, or square feet per ton usually do not apply to an atrium. The engineer should use sketches, sections, models, and plans to understand the space from three dimensions from the very start by working with the architectural team at the planning stages. It is key that the engineer understands all aspects of the atrium volume due to the impact on airflow movement and pressurization.

The smoke control system requirements for an atrium in many cases will dictate the HVAC design instead of the cooling or heating of the space. The smoke management system design should be well developed prior to designing the thermal comfort system. Once the smoke management intakes, fans, ducts, and diffusers are generally established based on the smoke management requirements, the potential to use these for thermal comfort can be considered. Again, it is key that the thermal comfort air system and smoke control system be implemented into the architectural design at the building planning stage. The smoke management system should not be compromised to take advantage of a designed thermal comfort system. The two systems must be designed in concert. The thermal air currents and any stratification during a fire event are completely different than in normal operation. This section only applies to the atrium thermal environment during normal operation. Refer to the Smoke Control section for the life safety design of the atrium. (Reference ASHRAE 1999 Applications, Chapter 51.)

The intended use of the atrium has an effect on the HVAC design and must be established early on in the design process. Is it a transient space only such as a hallway, or will seating be provided for people to lounge and interact? Will there be large gatherings of people during special events? What type of finishes and furnishings will be placed in the space? Will there be seasonal decorations such as a Christmas tree that will increase the fuel loading used for smoke control calculations? (Reference ASHRAE 1999 Applications, Chapter 51.13)

The use of the atrium may vary from the building it serves, atriums tend to be universal spaces used for many functions. Atriums are often used for large gatherings and functions. The anticipated occupant load the system is designed for should be documented and provided to building management. If the outside air ventilation system or the space thermal comfort system was not designed to accommodate dense occupant loads, the building management should understand the limitation on the use of the atrium.

If the space will be a transient space only, the design temperature range can be expanded. If seating, dining, or other uses where occupants will remain in the area for an extended time are intended then design temperatures should remain more stringent, similar to other spaces of similar use.

The following items should be considered:

  • Large volumes of air are involved in the smoke control system. This air may need to be heated before introduction or the sprinkler system maybe subject to freezing.
  • Air units must be designed to accommodate the 100% outside airflow to prevent component damage. This typically means involving a steam system in some way, which results in significant cost.
  • Can the applied heating system respond quickly enough to the smoke control mode.
  • How to heat the space if it has large expanses of glass.

Pressurization and Air Balance

Since an atrium by definition communicates or is adjacent to many different areas of the building, the pressure relationship between the atrium and other spaces is crucial to a successful design. In many cases, the atrium is also the main entrance to a building and the pressure relationship between the atrium and the outside is critical to the control of overall building pressurization.

The conditioning and ventilation of the atrium usually involves large quantities of air, so the infiltration or ex-filtration is an even smaller percentage of the total quantity of air being handled. This may require controls and instrumentation to be of a higher quality and accuracy than is typical in the remainder of the building. It also makes the initial, and ongoing, balancing of the atrium systems more critical to the overall success of the project. This must also work in concert with the smoke control pressure relationships. (Reference ASHRAE 1999 Applications, Chapter 51.12)

Stack effect and thermal currents may produce unanticipated influences on pressure relationships if not accounted for in the design. Therefore the atrium systems should be designed to allow some flexibility at start-up and in the future to adjust the balancing of the systems. This may include upsizing the atrium equipment to a condition greater than calculated for outside air or relief/exhaust quantities. This same thermal stratification has significant impact on the smoke control system operation in its ability to properly draw smoke and maintain pressure relationships. (Reference ASHRAE 1999 Applications, Chapter 51.13)

Since atriums are usually the focal point of the building and communicate with most all other spaces, atrium pressure should be considered as the datum that all other spaces are compared to. If the atrium is maintaining a slightly positive pressure relative to the outside, than most spaces should be designed to be neutral to the atrium and match the atrium pressure thus maintaining a positive building pressure. If too many spaces are designed positive to the atrium, the combined infiltration of air into the atrium may exceed its relief capabilities and over pressurize the building causing excessive air movement at building entrances. Due to seasonal thermal effects, the system should utilize automatic controls that will adjust the balance of the atrium based on outside temperatures or atrium pressures.

Fire Protection/Smoke Control

The most critical of all the technical issues to be solved in a successful atrium design is Life Safety because atrium buildings break with orthodox concepts of Safety. Life Safety design for any building is difficult. It involves more than a provision for emergency egress, it requires attention to who will be using the building and what they will be doing. Consideration must be given to communication, the protection of escape routes, and temporary areas of refuge allowing reasonable time for the building occupants to reach safety.

Because of its critical nature both NFPA 101 "The Life Safety Code" and The International Building Code have extensive code provisions for Atriums. Since the code provisions are extensive we will not recite them here but refer any design team to an exhaustive review of the requirements. Both NFPA and the IBC give significant explanatory material to atriums in their Life Safety Code Handbook and IBC commentary respectively. While similar they are not identical. A significant difference is that the IBC is prescriptive and arbitrarily limits the number of floors that maybe open to the atrium to three, where the Life Safety Code is more performance oriented and will allow the number of floors open to the atriums without enclosure be based upon the results of the required engineering analysis.

One of the basic premises of atrium requirements is that an engineered smoke control system combined with an automatic fire sprinkler system that is properly supervised provide an adequate alternative to the fire resistance rating of a shaft enclosure. It is also recognized that some form of boundary is required to assist the smoke control system in containing smoke to just the atrium area.

Both the Life Safety Code and IBC require that the atrium space be separated from adjacent areas by fire barriers having a fire rating of 1 hour or equivalent. Both codes accept adjacent spaces to be separated by properly constructed glass walls where automatic sprinklers have been installed to protect the glass. The sprinklers are to be located so as to wet the entire surface of the glass.

The development of most code provisions has largely been a response to specific fires and the desire to prevent recurrences. For example, in recent times many present code provisions were responses to the Coconut Grove Night Club fire, the Chicago school fire, and the MGM Grand fire. Conventional doctrine dictates that to achieve Fire and Life Safety that fires must be kept as small as possible and the effect of fire limited to as small an area as possible. This philosophy has resulted in conventional building configurations employing compartmentalized construction of fire rated floors and fire rated walls.

While atrium design breaks with conventional building configurations Fire / Life Safety in atrium buildings is comprised of the same three elements as in conventional buildings—means of escape, smoke control, and fire control. Means of escape, emergency egress is a fundamental plan issue and must be integral with the circulation concept of the building. Emergency egress must be incorporated from day one. Smoke control strategies are also fundamental and must be part of the initial ventilation concepts. Fire control and fire fighting provisions must also be integrated in to the original concepts.

The basic concept of means of egress planning is that occupants can move away from a fire and reach a protected means of egress by their own unaided efforts. This route must remain tenable throughout the evacuation process. A complicating element that must be addressed is that in an emergency people tend to use the route they are familiar with. Occupants of office buildings can be trained by fire drills but visitors will only know the way they came in. Means of egress protected exit stairs should be on familiar routes and in intuitive locations and signed very clearly. Mean of egress should not be unduly exposed to potential hazards. (Refer NFPA 101) The Life Safety Code being more performance based requires that an engineering analysis be performed to demonstrate that smoke will be managed for the time needed to evacuate the building. To accomplish this, the analysis must prove that the smoke layer interface will be maintained above the highest unprotected opening to adjacent spaces, or 72" above the highest floor level of exit access open to the atrium for a time equal to 1.5 times the calculated egress time or 20 minutes, whichever is greater. For a protect-in-place occupancy, such as Healthcare, the evacuation time is considered to be infinite, which means that the smoke control performance criteria must be maintained indefinitely.

The fire record has shown that smoke is the primary threat to life from fire in buildings. Smoke is the most rapidly developed threat. Proper smoke control in an atrium building is an absolute must. Smoke control systems that are integral to the buildings ventilation systems are preferred over stand alone systems. Integral systems are more reliable because their components are constantly being monitored and maintained. (Refer NFPA 92B Guide for Smoke Management Systems in Malls, Atria, and large areas.) NFPA 92B, quantifies the physics associated with atrium smoke control and presents methodologies for system design in an understandable and useful format. The guidelines of NFPA 92B allow the system designer to design a system and prepare associated documentation to access for adequacy in meeting the performance criteria.

The basic nature of fire and smoke must be well understood by the design team in order to incorporate smoke control in to the physical configuration of the atrium from the earliest schematic designs. Well designed smoke control cannot be added to a design, it must be integral to the design.

Effective smoke control depends upon rapid control of fire size to limit smoke quantities to manageable volumes. Fundamental to effective smoke / fire control is early detection and suppression. Smoke and or fire detection systems must be designed to identify and locate a fire early in its development. The fire prevention and smoke removal strategies for the atrium will vary dependent on the location of the fire and the configuration of the atrium.

Large volumes and high ceilings significantly complicate and possibly delay early smoke and heat detection. Systems that can detect the smoke near the occupied floor levels and close to the potential fire sources are best. Smoke detectors should be placed in ceilings in spaces surrounding the atrium but located within the atrium enclosure. These include balconies, seating alcoves, corridors, lobbies, and other spaces that have typical ceiling heights. To detect smoke in the high ceiling area, detectors should be located near the atrium floor to detect smoke prior to rising and potentially dissipating in the large volume. If the space is tall enough, smoke will cool and begin to descend back to the floor level, without reaching the upper ceiling level. Beam detectors are one potential solution to detecting smoke at lower levels of the atrium. If used, the location of beam detectors transmitters and receivers must be carefully chosen to allow the proper coverage, and to allow easy access for adjustment, testing, and periodic maintenance. Smoke and or heat detectors should also always be placed at the highest ceiling level as a precaution in case the other detection systems did not activate. The figure below in a highly simplified form represents many of the items that must be considered in the smoke control system in an atrium.

Example of an atria air flow

Example of an atria air flow.

Early suppression of a fire is essential to effectively limit the amount of smoke to manageable levels. Automatic sprinklers are the most effective means of fire suppression appropriate to atriums. The fundamental nature of atria presents challenges to the effective use of automatic sprinklers. In atriums with high ceiling heights, typical sprinkler designs provide marginal fire extinguishing capabilities and may actually be detrimental to the smoke removal system. Water from sprinkler heads located greater than 75 feet above the fire source may break up into a fine mist and evaporate before reaching the fire source. The evaporation of sprinkler water may cool the smoke and reduce the effectiveness of smoke removal systems, which were designed to pull the smoke from the top of the enclosure.

A qualified life safety consultant or fire protection engineer (FPE) involved early in the atrium design is the best source of potential fire detection and prevention system selection options. The FPE is knowledgeable in all of the atrium code issues. The codes and standards that address smoke control systems for atria are based on similar research and fundamental fire size and smoke generation models. ASHRAE 1999 Applications, Chapter 51 provides a broad design basis for smoke management that provides general directions to the designer. NFPA 92 is more specific and provides a set of calculations that can be performed by the FPE to determine the quantity of exhaust necessary to evacuate the smoke generated by the largest anticipated fire size. The calculations cannot anticipate all of the aspects that are unique to the atrium under design, and therefore should only be used for the most simple of atrium configurations. For all atria, and especially complex or tall atria, a qualified life safety consultant should be employed to assist in determining the smoke control system parameters. Currently the most comprehensive method of determining complex smoke management criteria is with computer fire modeling. (Reference ASHRAE 1999 Applications, Chapter 51.12) In many instances the results of the computer modeling will result in lower exhaust quantities being required, and therefore lowering initial project costs. The model allows multiple fire origins to be evaluated, and the resulting smoke removal system needs to be adequate for all anticipated fire origin location.

Computer modeling and visualization are important tools for understanding the processes of fire behavior. Fire models range in complexity from simple correlations for predicting quantities such as flame heights or flow velocities to moderately complex zone fire models for predicting time-dependent smoke layer temperatures and heights. Zone fire models' calculations can run on today's computers within minutes because they solve only four differential equations per room. Zone models approximate the entire upper layer with just one temperature. This approximation works remarkably well but breaks down for complicated flows or geometries. For such cases, computational fluid dynamics (CFD) techniques are required.

Even when the actual exhaust quantities were determined by use of a computer fire model, smoke management systems shall be designed in adherence to all other requirements of NFPA 92. Requirements such as the use of direct drive equipment in lieu of belt drive, protection of control wiring, emergency power, and the design and installation of a fireman's control panel to allow manual operation of the smoke removal equipment is mandatory. (Reference NFPA 92)

Maintenance

The operation of the atrium can be diverse and requires close coordination between the designer and user to determine what aspects of the operation that are key. Some of the more critical elements are occupant comfort thru the air system, the mode of operation for the atrium and lighting considerations.

The key elements that the design engineer should consider in the atrium comfort system design are space temperature, energy efficiency and air system type. (Reference ASHRAE 1999 Applications, Chapter 4.8) These are described below.

Space Temperatures—The design temperature for the atrium can vary dramatically based on usage. If the space is to be a heavily occupied, constant use space, 75°F should be considered (summer design). However, if the space is a transient operation, a higher temperature of 78°F should be considered.

Energy Efficiency—Energy efficiency should be considered in the design such as these listed below.

  • Upper level stratification
  • Spot cooling where occupants are located
  • Night (or unoccupied) setback points
  • Triple pane glass
  • Motorized shading advices

None of these energy efficiency means however should compromise neither the airflow requirements nor the fire protection or smoke control design aspects discussed in other sections.

System Type—While variable air volume systems are recognized for their energy saving ability, they should be carefully scrutinized in the design process for their ability to maintain proper atrium pressure control. Typically, large vestibules are utilized at the atrium entry. Consider using a constant volume air supply in the vestibule to overcome wind pressure at the entry point.

It is important that the designer works closely with the building user in the planning stages to determine the many modes of operation that are anticipated .The building user will set the modes of operation. Thru a firm understanding of the modes of operation desired, the designer can provide as much flexibility in the building control system to achieve a detailed level of control for HVAC zone control, temperature reset, lighting functions, etc.

The lighting of an atrium can be a significant challenge and the use of a specialized lighting designer should be considered. Atrium lighting schemes must be evaluated on an individual basis according to the function of the space and nighttime operation. The following should be considered:

  • The lighting level should be maintained in the range of 15 footcandles either thru day-lighting or artificial means.
  • If indirect schemes are possible, they should be considered.
  • If the atrium is used as an egress, the lighting must be capable of immediate restart should normal power be lost.
  • Planting and vegetation needs should be addressed. Point source lighting may be necessary.
  • Develop a plan for lamp replacement in high ceilings.

Applications

The following are examples of atria that exhibit the above concepts in different combinations, locations and architectural styles. Hopefully they will help explain the concepts and give them physical representations.

Case 1: Plaza of the Americas, Dallas, Texas

An example of a bridging atrium at the Plaza of the Americas-Dallas, TX

The Plaza of the Americas in Dallas, Texas is an example of the bridging atrium, a complex form which utilizes the atrium to connect several buildings. In this case two twenty five story office towers, a twelve story hotel and a twelve story parking garage. There are retail shops at the two lowest levels, one below grade around an ice skating rink. Natural light is admitted to the atrium space through full height glazing at the side walls between the buildings being connected as well as through narrow skylights running across the roof in an angular fashion. Additionally, prisms are suspended inside the side wall glazing that produce interesting color patterns on the interior atrium surfaces. The light entering the atrium is transmitted to the adjacent buildings through floor to ceiling clear glass. The exterior atrium and building glass is heavily tinted and has a 30" high spandrel sill limiting vision glass area to limit the amount of heat gain. The Atrium is oriented with its long axis north-south. The atrium space is climatically tempered as opposed to controlled. This saves energy costs for the attached buildings while limiting the expenditure for both capital and operational for HVAC equipment for the atrium.

The smoke control is achieved through smoke vents at the bottom and top of the atrium exterior walls activated through smoke detectors. This passive smoke control design produced in the late 1970's is not the recommended method of smoke control based on today's codes or standard of care.

The exterior skin is a glazed curtain wall system supported by steel wide flange beams spanning between the building structures which the atrium connects. The roof system consists of deep long span steel trusses covered with exposed metal deck, insulation and roofing. While functional, this roofing system might seem a little light for the volume of space enclosed. Exterior building maintenance is achieved with traditional davits, tiebacks and stages at the exterior while botsains chairs rigged from the roof trusses accessed through a catwalk system. Stages could be rigged from the trusses also if required.

There is one bank of glass elevators exposed in the atrium which serve the parking garage which allow all tenants and visitors to enter the complex in a grand way seeing the whole atrium and building complex upon arrival. Each building also has an individual entrance accessible without entering the atrium. The lower portion of the atrium can be accessed from grade and has wide sidewalk like pathways adjacent to all buildings and overlooking the ice skating rink at the below grade level. Connection between the two levels is via two sets of escalators, the glass elevators or several sets of stairs.

Landscaping within the atrium is limited with small planting areas utilizing potted plants surrounded with mulch to give appearance of continuous planters. Additionally there are large palm trees in individual planters recessed into the lower level floor which are water proofed, drained and irrigated. This planting scheme minimizes the maintenance required for completely irrigated and drained planters while maximizing the effect of the landscaping provided.

This atrium application is also connected to adjoining properties via a sky bridge system which provides a link to a system of climate controlled accessways to other downtown buildings during the hot Texas summers. This atrium also provides a food court which is utilized by tenants as well as visitors from other area buildings. This provides economic help through leasing atrium space as well as producing income for tenants.

Case 2: Bayfront Medical Plaza, St. Petersburg, Flordia

Interior of the Bayfront Medical Plaza's two-sided atrium-St. Petersburg, FLExterior of the Bayfront Medical Plaza's two-sided atrium-St. Petersburg, FL

The Bayfront Medical Plaza is an example of a two sided atrium, a simple form which sits in the corner between two wings of a medical office building. The building is five stories with a two story space connecting the wings and the atrium on the face. The atrium is five stories high with full height glass on the exterior faces and punched opening between it and the building. It also connected to the adjacent parking garage via a sky bridge. The atrium is completely climate controlled with accessible floor space at the Ground Level and Level 2.

The Life Safety systems have a smoke evacuation system consisting of a coffered ceiling with vents and mechanical units for the venting of smoke. This system is activated by smoke detectors or sprinkler line flow switches. The building and atrium are fully sprinklered as well as the glass separating the building from the atrium in the punched openings.

The exterior skin of the atrium is a glass curtain wall system with tinted insulated glass in aluminum frames supported by a steel structural frame which is clad with GRG (glass fiber reinforced gypsum) panels and gypsum board furring. The roof is composed of steel wide flange beams covered with metal deck, insulation and roofing system. A suspended gypsum board soffit system creates the coffered ceiling which allows for mechanical systems and lighting to be placed aesthetically. Exterior building maintenance is achieved with davits and tie backs and stages while interior building maintenance is achieved through supports off of the steel structure.

Vertical transportation is independent of the atrium. The atrium acts as entry point for the building as well as collecting visitors from the parking garage across the street from the sky bridge and then down an escalator bank to the Ground Floor.

There is very little landscaping inside the atrium, several large potted plants, but extensive landscaping both hard and softscape outside of the atrium. The atrium also connects the main entry court with a more private courtyard behind the building.

This atrium provides a daylit entry to the building wings while offering a weather protected buffer for patients and visitors to get from their vehicles to the Doctor's offices. It is a simple yet welcome addition to the Hospital campus.

Case 3: Crawford Long Hospital, Atlanta, Georgia

Exterior view of the Crawford Long Hospital's one sided conservatory atrium-Atlanta, GA

Crawford Long Hospital is a one sided Conservatory atrium, which is the most simple form that utilizes the atrium as an entry point to the building it serves. The building is a five story hospital with a 14 story medical office building above for a total of 19 stories. The atrium is a three story volume with waiting rooms for various departments and patient rooms overlooking the space offering a comfortable connection to many patient and visitor services. Natural light is admitted to the space through large glazed punched openings in the front wall and large gable ended sky lights in the roof. The atrium faces south to take full advantage of natural light. There is a pedestrian connection via sky bridge to an adjacent cancer center and parking garage located across the entry drive.

The atrium volume uses an engineered mechanical smoke control system that draws fresh air in at the bottom and exhausts smoke laden air at the top.

The exterior skin of the atrium is an expression of punched openings glazed with an aluminum and glass curtain wall system incorporating glazed in granite panels and gypsum board and wood accents on the interior. The atrium structure is steel framing tied into the concrete frame of the building. The roof is steel framing with metal deck, insulation and roofing system clad on the interior with a highly articulated coffered suspended gypsum board soffit system that provides a place to provide additional lighting as well as concealing the extensive mechanical and smoke control systems. This atrium requires little in the way of exterior or interior building maintenance equipment due to its minimal height. Most glazed areas can be serviced from a man hoist which can be brought in as needed. Davits and tie backs are provided at the exterior perimeter where necessary.

Vertical transportation for the building (elevators) is adjacent but separate from the atrium. Escalators are provided within the atrium from Lobby Level to Level Two in order to connect with the sky bridge. There is also access to stairwells adjacent to the atrium. Horizontal circulation is provided from the main entry which has an attached canopy extending over the entry drive through the atrium to any number of destinations including the buildings vertical transportation hubs.

Landscaping within the atrium is lush and extensive located in planters depressed into the floor structure and above the space below, waterproofed and provided with irrigation and drainage. The planting is extensive and provides shade and reduces the scale within the atrium.

Case 4: EDS Corporate Headquarters, Plano, Texas

Example of a multiple lateral atrium design at the EDS Corporate Headquarters-Plano, TX

EDS Corporate Headquarters is a sprawling suburban low rise campus north of Dallas, Texas. The buildings are designed as clusters of office space connected with skylit multi-story atria. This is an example of a multiple lateral atrium design. The atria are completely climate controlled and some heavily planted to offer the illusion of outdoor space without the extreme temperature swings of the North Texas region.

The life safety systems include mechanical smoke evacuation and fully sprinklered interior space below the extensive sky lights in the ceiling plane. The spaces adjacent to the atria are not separated from the atria with partitions of any kind typically. Smoke control and containment were achieved through separate mechanical systems. This system was extensively tested and performed well.

The adjacent spaces to the atria form the majority of the walls of the atrium, therefore there was very little vertical exterior skin design involved in this case. There are however extensive horizontal vaulted sky lighting systems design including the incorporation of interior building maintenance systems.

There are many grand stair cases both adjacent to and within the atria as well as elevators and escalators providing vertical transportation in the buildings.

The largest of the multiple atria is located in the center of the campus and is flanked by two large catenary trusses providing support for the floors of the space adjacent to the atrium.

These atria provide daylight, accessible common spaces for all company departments to use and a large gathering space for full company gatherings.

Case 5: Methodist Willowbrook Hospital, Houston, Texas

Sketch of the linear atrium at Methodist Willowbrook Hospital-Houston, TX

Methodist Willowbrook Hospital is a four story Hospital and Medical Office Building in suburban Houston, Texas with a linear atrium dividing the building between the hospital function and the Medical Office Building function. The atrium is completely climate controlled and has limited planting in the space.

The atrium has a fully mechanical smoke evacuation system per the building and life safety code the building is fully sprinklered. The atrium is separated from the Hospital by a one hour fire resistive partition with glazed openings as allowed per the building code and from the Medical Office Building by a 2 hour fire resistant rated occupancy separation.

Since the Hospital and M.O.B. form the two long sides of the atrium, there is very little vertical exterior skin for the atrium. It is located only at the two short ends above entry vestibules or occupied space and consists of glazed aluminum curtain wall. The majority of the natural light is admitted by the sky light at the roof plane. This configuration limits the area available for mechanical access to fresh air necessary for the smoke management system. This configuration presented many design challenges to the mechanical designer.

There is vertical transportation located in the atrium in the form of elevators located in the center of the space on axis with the main entry to the building and two grand stair cases between the Ground Level and the Second Level wrapping the elevator core.

The atrium houses waiting areas for some Ground Level departments as well as a dining area for the Hospital cafeteria and a piano for entertaining patients, employees and guests during the day and at special functions in the atrium.

Case 6: American Stores Corporate Headquarters, Salt Lake City, Utah

Exterior of American Stores Corporate Headquarters with its alternating series of 2 and 4 story vertical atriums-Salt Lake City, UT

American Stores Corp. HQ is a 24 story office tower in downtown Salt Lake City, Utah. It is composed of an alternating series of 2 and 4 story vertical atriums that work their way up the building from the ground floor up to the top. There were 4 two story and 4 four story atriums. The atriums were a parallelogram in plan configuration and enclosed on three sides by a combination of restrooms, file areas, corridors, and conference rooms. The fourth side of the atrium was an exterior wall of the building and had exposed steel framing. The service areas were separated from the atrium by a one-hour fire wall configuration. The Conference Rooms were floor-to-ceiling glass walls and were protected by a deluge sprinkler system on one side of the glass. A mechanical smoke evacuation system was used in all of the atriums.

 

Emerging Issues

One emerging issue affecting exterior building maintenance or more directly, window washing is Self Cleaning Glass. This was a major find a couple years ago but has not proven out commercially. The system works by applying a coating of microcrystalline titanium oxide to the exterior surface of the glass panel. The coating is on the order of 15 nanometers thick. This added coating is activated by U.V. radiation from the sun after installation and from that point on two processes take place to keep the glass surface relatively free of visible dirt. First a photocatalytic process takes place that breaks down organic portions of surface dirt and loosens it from the face of the glass. Second when water either sprayed or from rain strikes the glass, the coating has hydrophilic properties which is to say that water droplets tend to pool together more readily and sheet flow off the glass more easily carrying the loosened dirt with it. The glass may still require manual cleaning but technically shouldn't need it as often as uncoated glass. The cost implications of using this product which is available from several manufacturers depends on many factors but is not yet being widely used. However, this could change in the coming months or years.

A second emerging trend is sustainability. This issue has come to the fore recently due in large part to the LEED (Leadership in Energy & Environmental Design) Certification process instituted by the USGBC (United States Green Building Council). While the LEED system for certification encompasses all disciplines involved in building design, the atrium can be an asset in this arena and should be included in the evaluation when owners and designers wish to pursue green building certification.

Federal agencies, both civilian and military, were among the earliest advocates of green building nationally. Today, U.S. government buildings comprise about 10% of the projects registered in the USGBC's LEED program.

The Leadership in Energy and Environmental Design Green Building Rating Program is, in the words of the U.S. Green Building Council, "a national consensus-based, market-driven building rating system designed to accelerate the development and implementation of green building practices. In short, it is a leading-edge system for designing, constructing, and certifying the world's greenest and best buildings." This statement at once reveals both the brilliance and the shortcomings of LEED for new construction in its current form—and points the way toward improvements that need to be addressed in its next iteration.

LEED works so well, first of all, because it is simple to understand. LEED is divided into five categories related to siting, water conservation, energy, materials, and indoor environmental quality, plus an innovation and design category. Finally, while LEED is supposed to produce "the world's greenest and best buildings," the process does not in and of itself guarantee optimal results. Clearly, it takes more than following a checklist to create a well-designed, fully integrated sustainable building.

Zone Fire Models

In the 1970s, NIST and others developed zone fire models that describe how fires evolve in compartments. These models divide each compartment into two spatially uniform volumes or zones. The upper layer contains the hot smoke and combustion products from the fire and the lower layer contains air at near ambient temperatures. Zone fire models require solving a mass and energy conservation equation for both the upper and lower layers; that is, two control volumes. However, the models neglect the momentum equation within a zone, because they assume that flow within a layer is quiescent. A simple form of the momentum equation, Bernoulli's Law, is used though to compute vent flow between compartments, using pressure differences. Additional equations can describe other physical processes, such as fire plumes and radiative, convective, and conductive heat transfer. Zone fire models predict the interface height between the two layers and layer its gas temperatures remarkably well because of the tendency of hot gases to stratify or form layers due to buoyancy.

Interestingly, a zone fire model doesn't include the most critical parameter. The fire itself isn't modeled but inputted in the form of heat-release rate data.

Fire simulation with CFD

The current state of the art in computer fire modeling is exemplified by NIST's latest contribution to fire modeling, the Fire Dynamics Simulator (FDS). FDS predicts smoke and/or hot air flow movement caused by fire, wind, ventilation systems, and other factors by solving numerically the fundamental equations governing fluid flow, commonly known as the Navier-Stokes equations. The downside is that CFD calculations can easily take days to run since they solve for many variables in each of hundreds of thousands or even millions of grid cells. These calculations generate far more output than the simpler zone models. While simple line plots are adequate for visualizing zone-fire-modeling results, we need more sophisticated techniques for interpreting the massive amounts of data generated by the CFD models.

Summary

Atriums are more than a kit of parts that can be combined into various configurations. Each fundamental element must be fully understood. The design process begins with the site and the natural environment. All subsequent decisions are effected by these determining conditions.

The designer must successfully blend and combine elements of the fundamentals of geometry, lighting, envelope construction, landscaping, acoustics, thermal control, pressurization and air balance, fire protected, life safety smoke control, and maintenance into a cohesive whole.

The most fundamental concept of successful atrium design is a good understanding of the complexity of the atrium environment. Atriums are the most complex built environments that most designers will encounter. Atriums are composed of more component parts in more complicated relationships than any other building type. No fundamental component of an atrium should be accepted until its relationship with the whole is understood. For every component and every aspect of every component there will be beneficial aspects and also non-beneficial aspects. There will be "pros" and "cons" associated with every element. These attributes are not static, they will change with relationship to any or all of the other components or elements of the atrium.

In the final analysis successful atria are defined by three determinations; do they lift the human spirit, are they safe places to be and are they cost effective. Simple right.

Relevant Codes and Standards

  • NFPA 92B - Smoke Management Systems in Malls, Atria, and Large Areas, National Fire Protection Association,2000 Edition.
  • NFPA 101 - Code for Safety to Life from Fire in Buildings and Structures, National Fire Protection Association, 2000 Edition.
  • International Building Code Commentary, International Code Council, 2000 Edition.
  • Applications Chapters 4 & 51, American Society of Heating, Refrigeration and Air-Conditioning Engineers, 1999 Edition.

Additional Resources

WBDG

Space Types

Atrium, Lobby

Design Objectives

Functional / Operational—Ensure Appropriate Product/Systems Integration

Products and Systems

Section 07 92 00: Joint Sealants, See appropriate sections under applicable guide specifications: Unified Facility Guide Specifications (UFGS), VA Guide Specifications, DRAFT Federal Guide for Green Construction Specifications, MasterSpec®