Pedestrian Friendly: Planting Trees and Celebrating Stormwater in Urban Areas

When planning for vibrant urban spaces, tree grates and stormwater trench design can assure successful urban forestry and provide new tools for successful stormwater strategies.

Celeste Allen Novak AIA, LEED AP

Radiating, geometric, square, round and often emblazoned with a downtown logo, tree grates are just one of the many small elements which makes a downtown a great gathering place. Safe, appealing, tree - lined pedestrian street environments encourage people to walk and maintain their health through physical activity.  The DNA of a great city includes many elements that "allow us to work, live and have a relationship to nature."  These are the places which makes us happier, according to author Richard Florida.ⁱ The design of cities which incorporate nature, trees, and water along pedestrian friendly streets, requires planning and skills in urban planting practices. Providing street trees, planting beds and celebrating stormwater through design are elements that bring nature into and throughout the urban landscape.  

At a recent Creative Cities Summit in Michigan, architect Doug Farr, AIA, LEED AP, the chair of the U.S. Green Building Council's (USGBC) LEED for Neighborhood Development (LEED −ND ®) Core Committee, presented the components of pedestrian friendly design. 

Currently a pilot project, by 2009, LEED ND® will be the newest LEED® rating certification program.  This USGBC committee is working on the definition of what makes a superior pedestrian experience and currently are identifying those components as ½ human interactions and ½ environmental approaches.  The three major issues which should be addressed when designing urban spaces are connecting places, strengthening neighborhoods and developing a green infrastructure.

Trench grates can be placed along a fountain edge in a radial pattern. Photo courtesy of IRONSMITH

 

Designing the Small Elements That Make Public Spaces "Grate"

This article will provide designers with information on how to meet pedestrian friendly standards for streetscapes through best management practices for planting street trees and celebrating stormwater.  The article will discuss some of the methods for tree planting and the use and specification of  tree grates that will meet the Americans for Disabilities Act (ADA); expand for mature tree growth; and are constructed of materials appropriate to the  regional climate.  Architect Ludwig Mies van der Rohe said "God is in the Details."   On the street, these details may include bike racks, trench drains, and tree grates which can brand a downtown as an extraordinary public space.

Case Study: Planting an Urban Plaza: The Tempe Transportation Center

These trees are planted in a system which allows pavers to be placed over the grate. The tree is planted in structural soil, providing the elements for a healthy growth in this urban setting. Photo courtesy of IRONSMITH   Michele A. Shelor, ASLA likes to celebrate her talking trees at the Tempe Transportation Center.  Michele is the project manager for A DYE Designs for this LEED® Silver registered, mixed-use facility built for the City of Tempe.  In this project, there are a lot of talking trees growing from a unique paver grate system connected by structural soil tunnels.  In order to make it possible for trees to grow in a plaza, the planting area must be carefully designed.  The paver grate system includes an excavation area which is filled with a soil system of mortared rock which includes nutrients and a clay substrate.  In this project, three foot deep by three foot wide trenches were designed using structural soil so that the root system could grow under the plaza.  The trenches allow the trees to "talk" or grow toward each other through the nutrient channels promoting growth even in this paved environment.  The pavers are placed with sand joints so that water can filter into the root system.  The tree is planted below a paver grate which sits on concrete footer. There is a mesh panel welded to the frame of the paver grate that acts as the suspension system to supports the pavers above the soil.  The grate can be enlarged as the tree grows, but on the surface, the tree is protected by the minimal opening from bikes, pedestrians and litter.   The soil is prevented from being compacted at the base of the tree by the four by four invisible tree grate and throughout the plaza by the linked tunnels of structural soil. A tree planted in nutrient soil in the middle of a paved area needs a minimum of a 4 x 4 planting bed. Structural soil composed of larger rock infill and irrigation piping is part of the planting area. Photo courtesy of A DYE DESIGN This planting area is framed with a concrete curb below the level of the paver surface on which a metal grate is placed. Photo courtesy of A DYE DESIGN   Pavers are placed above grate in a sand fill; there is no mortar between the joints in order to allow for stormwater irrigation. Photo courtesy of A DYE DESIGN   The 40,000 square foot, Tempe Transportation Center includes this large pedestrian plaza with 15 bus bays and shelters that will serve the new METRO light rail and local and regional bus patrons. The building houses transportation offices for the city, a community/conference room, transit operations center, for-lease office space and ground-level retail space. The design teams LEED® strategies also included a "green" roof, gray water treatment, low-flow fixtures, photovoltaic solar collection, recycled acoustical ceiling tiles and wheat grass millwork.

 

Look Up and Down - Street Trees

Whether a southern palm tree or a northern maple, trees provide shade and a leafy canopy which filters the light, brings color and provides a home for birds in urban spaces.  Trees reduce the urban heat island effect and can be part of an urban stormwater detention system.  They can be planted in raised beds or along a confined setback area.  In downtowns, they are often planted below tree grates which are integrated into the sidewalk system.  The components of a successful sidewalk planting system include:

• Proper spacing
• Proper planting depth and protection from soil compaction
• Sufficient root growth area to support mature trees
• Flexibility to provide for increased tree growth
• Durability, materials and maintenance
• Safety and ADA compliance

Proper Spacing

Filtered light is created by spacing trees along the sidewalk.    The pilot program for neighborhood planning is LEED ND® Neighborhood Pattern and Development (NPD).  NPD Credit 7: Walkable Streets, recommends that designers space trees at intervals of no greater than 40 feet apart.ⁱⁱ This section also recommends that within five years, half the sidewalk should be shaded as determined by the width of the shade when the sun is directly above the tree.  A downtown sidewalk between 12 and 20 feet wide can provide space for walking, dining and street trees.   "Fifteen feet is usually the minimum width for an active retail street with outdoor cafes and street trees.  However, I have seen good streets as narrow as ten feet, usually with on-street parking to help buffer pedestrians from traffic, "said Cory Gallo, urban designer at JJR in Ann Arbor, Michigan.

Spaced to provide shading for pedestrians, trees also reduce the heat island effect, the elevated temperatures in urban environments that can impact the demand for energy, air-conditioning and also increase air pollution.ⁱⁱⁱ The health benefits of trees are well documented. Trees provide a filter for pollution breathing in carbon dioxide and returning it as breathable air. "When trees and vegetation reduce energy use, they also reduce CO2 emissions from power plants.  In addition, vegetation removes atmospheric CO2 by sequestration. Trees sequester − or store − between 35 and 800 pounds annually depending on their size and growth rate."^iv  There is up to a 60% reduction in street level particulates with street trees.^v  Other pollutants such as sulfur dioxide, ozone and nitrogen oxides are also absorbed by trees.^vi

Trees also reduce noise and the acoustics of a noisy street is modified through tree plantings.  Trees in urban areas may even reduce crime and domestic violence. Trees need sun, water and the nutrients from the soil to mature.  Paved, downtown streetscapes need to be well designed to nurture trees so that they will in turn, nurture urban pedestrians.

Growing Pains

Tree grates are one means to integrate trees into the streetscape. Properly installed, tree grates protect trees from pedestrians and pedestrians from trees. Tree grates eliminate compaction of the soil at the base of the tree due to constant foot traffic that can inhibit or damage root growth.  Tree grates also protect pedestrians from trip hazards due to the hole produced by an open tree well. Tree grates also allow for irrigation of trees if required while providing a pedestrian "bridge" over muddy or flooded planters.

Most cities require that street trees should be at least 2 inches caliper in trunk diameter.  Many cities provide a list of recommended trees to plant by in their particular climate zone in downtown areas.  For example, the city of Portland, Oregon provides numerous planting guidelines in their city code^vii and specific trees are listed as recommended for use in the Portland urban forest.  Many urban foresters recommend having a wide range of tree genus in any given location to provide biodiversity. This guards against a catastrophic loss of trees should one species fail due to drought, insect infestation or some other natural stress not present at the time of the initial planting. Most cities will also require that trees and other plant materials must be maintained and replaced as necessary and proper planting and irrigation techniques will reduce replacement costs.  

Tree planting areas should be sized for the tree at maturity, for example, the roots of a palm tree might require a different space than a Gingko tree. The designer should know the tree type and growth patterns for the regional climate when specifying trees.  Manufacturers provide grates for a variety of standard tree planter sizes or they can provide custom sizes as specified. 

A four foot by four foot tree grate may fit into the sidewalk spacing, but a four foot by four foot planting hole may not be large enough to sustain proper tree growth.  Roots are the collectors of water and nutrients for a tree as well as its anchor, so a healthy root structure is critical to a healthy tree. Typically a tree needs as many cubic feet of soil for roots as the area in square feet under its crown or leaf spread. Roots are usually limited to 3 feet below grade so a tree expected to have a 24 foot wide canopy would need 148 square feet of planting area at grade for optimal root growth. The compacted soils beneath most sidewalks and roadways are poor areas for roots as they lack oxygen and moisture necessary for root growth and tree metabolism. Therefore, it is best to provide as much planting area with nutrient rich and properly draining soil as possible. Using large tree grates and / or incorporating structural soils are options. A four by twelve foot tree grate or a longer grate which can incorporate several trees as shown in this Chicago streetscape are two options to provide for the mature growth of the street trees. In this way tree grates can be used to "buy" some pedestrian space while optimizing tree growing space. Trees can also be planted in raised planter beds where space is available.

Windy, wet, dark and cold downtown Chicago is a harsh place for a tree.?The designers of the Hyatt Project solved many growing pains by creating a long planting bed using this structural steel and cast iron tree grate system. Photo courtesy of IRONSMITH

 

In addition to planting space, tree grates should also allow for proper trunk growth.  Grates should be selected with openings adequate for the tree trunk at maturity.  Many grates will have "knock out" panels that can be removed through the use of a portable grinder with an abrasive wheel. After scoring, a sharp tap with a hammer will remove the excess material to enlarge the grate for future growth.  An expandable grate will provide a series of concentric rings designed for strength and to maintain the aesthetics of the tree grate even as it is expanded.   Grates that are expandable will list the expansion size to which the grate can be adjusted.

Case Study: Planting Trees in Downtown Chicago

When the Hyatt Center was constructed, the designers had to work within a complex streetscape.  Coordinating architectural firm A. Epstein was also the architect of record, working in conjunction with design architect Pei Cobb Freed and Partners, LLP, New York. The Hyatt Center is a 1,800,000 square foot, 48-story office building on a one-half block site bounded by Wacker Drive, Monroe Street, Franklin Street and Arcade Place in Chicago, Illinois.  The project incorporates approximately 300,000 square feet of office space for the Pritzker Companies, including Hyatt Hotel's corporate headquarters; 475,000 square feet for the law firm of Mayer, Brown, Rowe and Maw; 270,000 square feet for commercial banking firm Goldman Sachs; and 430,000 square feet of speculative office space. Two below-grade levels provide parking and one mezzanine level contains food service and a health club facility for the building tenants. The civil engineering and site work for this project was more involved than most typical downtown developments due to the project location, adjacent to Wacker Drive, a bi-level roadway system. Normal details such as the sidewalk, curbing, driveways, utility service connections and landscape coordination issues normally associated with a downtown development at one level, needed to be addressed at two different street levels. The sidewalks along Monroe and Franklin, the west boundary of the site, are vaulted sidewalks. The designers needed to accommodate several large electrical vaults in place below the walks. This condition complicated the issues involved in reconstructing the sidewalks and curbs.  The City's landscaping requirements for street trees along Monroe and Franklin, including sub-soil drainage for the trees, required a considerable coordination effort with the various agencies involved. In order to provide proper spacing for the trees, and add to the pedestrian space, large tree grates spanned the length of the sidewalk.

 

Tree guards can be added to a tree grate to protect trees from pedestrians. Photo courtesy of IRONSMITH

A new type of grate system supports pavers above the tree well.  This system was used in the Tempe Transportation Center case study and allows for more pedestrian walk areas with a smaller open grate surface.   This system:

• Greatly reduces the area for weeds and trash accumulation
• Cuts down on tree area maintenance
• Allows easy fertilization and irrigation
• Keeps roots from lifting pavers
• Seamlessly incorporates any size tree pit, utility corridor, etc.

Some grates include protective tree guards attached to the tree grate.  A tree guard is a means to support the young tree while it is being established on the sidewalk.  It can prevent pedestrians, bikes, skateboards, or chairs, bumping into trees, trees becoming bicycle racks and sometimes even prevent dogs from watering the trees.  Tree guards are fabricated in halves and drilled and then bolted to the tree grate.  They can also be mounted on a cross bar as an option with the grate frames.  The standard height is 5' and the standard tree opening is 16", however, any height and opening may be specified.

Custom Design: Site-Specific Solutions, Aesthetics and Branding

Besides providing solutions for healthy tree plantings in the built environment, tree grates are also a design element.  By the nature of the production process cast metals permit and even invite design innovation that can enhance a sense of place.  Corporate or city logos can be incorporated into the castings to provide a branding element for the space. Special sizes and shapes can be cast to reflect or coordinate with surrounding flat work or building elements. For example, in addition to the standard square, round, and rectangular shapes, triangles, arcs and diamonds have also been used as part of a sidewalk design element.

Case Study: Theme Park Design Branding
One theme park with "street trees" needed both tree grates with a distinct design theme and also grates that had no openings. Historically the park had installed standard tree grates however, over time, due to nightly washing of the surrounding hardscape, the facilities crew discovered that the trees were literally drowning.  In response, new grates were designed with a pattern that suggested the slots of a standard tree grate but had no real openings to minimize wash water in the tree planter. In addition, the grates incorporated a raised collar around the tree opening to keep water from washing into it. The success of this solution in one area of the park led to using the same collar and no slot solution in other locations. This "slotless" design also allowed more design opportunities for the motif.  The park designers found it helpful to have the design concept produced virtually with 3D design software to see how well the slotless design "read" before manufacturing.	Slotless custom tree grate at a theme park. If an area needs to be washed down frequently, this tree grate is designed to capture the water before it reaches the tree root sending it to infiltration systems and/or storm drains. Photo courtesy of IRONSMITH
The grate shown at top is a computer generated rendering from 3D design software. The grate shown at bottom is an actual photo of a completed grate panel. The lower image shows the gray color of cast iron before it starts to develop a patina. Photo courtesy of IRONSMITH

 

Sustainable Design:  Materials and Durability

Main street sidewalks, indoor malls, downtown business districts are horizontal surfaces with a lot of pedestrian traffic.  The wear and tear on the walking surface can include damage from foot traffic, dining furniture, heat lamps, bicycles, baby carriages as well as from cleaning, road salt, snow clearing and maintenance crews.   Metal products for all street applications should be durable.  Typical metal products used for outdoor wear include recycled cast gray iron, aluminum or bronze.  Most steel construction shapes are made from 75 percent recycled product and are 100 percent recyclable.   Cast products can be specified for LEED® credits to be made from 100 percent recycled product and are 100 percent recyclable.  There is very little difference in durability, between cast iron, bronze or aluminum and the designer usually chooses a product, based on the design aesthetic and/or budget.  Special alloys of aluminum and bronze can also be requested of a manufacturer. Most grate frames and tree guards are fabricated from steel.

Natural or unfinished gray iron develops an attractive rust patina that does not penetrate or weaken the casting and is maintenance free.  Natural aluminum will age to a pewter gray and when provided with a brushed finish, can approximate the look of stainless steel at significantly less cost. .   Designers can also specify an array of factory finishes; primed  and painted in an exterior enamel, primed and painted an exterior polyurethane, powder coatings, and/or  with a zinc undercoating.  Paint finishes can be matched to any color choice.  Industrial paints are recommended for enamel or polyurethane finishes.   Polyurethane finishes will hold up to foot traffic much better than enamel and are far less likely to fade due to solar UV exposure.  

One of the disadvantages of "wet finishes" such as enamel and Polyurethane paints is their volatile organic compound content (VOC). Powder coating is the application of an electrostatic charged dry powder baked onto a surface.  Select a powder coat color that is available in a polyester base for good weather-ability and do not use epoxy coatings. For heavily trafficked surfaces and outdoor surfaces, particularly in harsh winter climates, powder coating is more durable than either enamel or polyurethane finishes and will require less maintenance.  Cast iron is extremely susceptible to rust and even when well painted or coated, some rust should be expected.  For this reason the following guidelines should be used when specifying finishes for cast products.  

• Use only dark colors; black, dark brown, dark green, dark gray, etc.
• If light colors are required, specify a zinc undercoat to help reduce rust bleed through.
• When no rust can be tolerated, specify cast aluminum.

In urban areas with heavy pollution, in cold climates where there is snow removal using salt and in humid climates which are near the ocean, specify unfinished or painted aluminum or unfinished cast iron with the rust patina not painted cast iron.  New rust is bright orange which will naturally rust to a dark brown; however, there are liquid products which can be applied to accelerate the darkening of the patina.  Darker patina's can be achieved by applying a low grade asphalt product that is absorbed into the steel.
Too much watering, salt snow melt applications and power washing are man-made elements which in urban setting can harm a tree's growth.  Without enough water, or with too much water, a tree will have a hard time surviving in the urban environment.   Besides assuring that the tree is planted with enough root space, the designer can choose tree grates which provide irrigation below the grate and protect the tree from too much water.

Universal Design: Safety, High Heels and Trip Hazards

The Americans for Disability Act (ADA) was initiated in the mid-1990s and its impact had far reaching consequences for designers who care about access both inside and outdoors.    However, some of the first tree grates which were designed with smaller openings were a response to Las Vegas nightclub owners as women were catching their high heels on the tree grate openings.   Older grates have two to three inch "slots" and did not meet the new ADA legislation for trip hazards.   In addition, the larger slots collected more street litter.    The patterns in tree grates have evolved over the years, and many of the traditional patterns were modified to provide a more even walking surface.

The elaborate tree grate designs are based on historic ornamental patterns used for well over a century on downtown streets.
Old style tree grate	New style tree grate
Photo courtesy of IRONSMITH

 

The ADA does not specifically address tree grates; however gratings within the "accessible pathway"^viii should have openings not greater than ½" wide perpendicular to the general direction of travel.  Since the minimum requirement for the accessible pathway is 36" tree grates usually do not intrude into it.  However, in a large plaza or with large paving grate systems along a sidewalk, there is a danger of high heels, or canes becoming caught in a larger grate opening.  Selecting an ADA compliant tree grate will assure a higher level of pedestrian safety.

The Occupational Safety and Health Administration recommends that walking surfaces have a static coefficient of friction of 0.5. A research project sponsored by the Architectural and Transportation Barriers Compliance Board (Access Board) conducted tests with persons with disabilities and concluded that a higher coefficient of friction was needed by such persons. A static coefficient of friction of 0.6 is recommended for accessible routes and 0.8 for ramps.^ix Designers should avoid specifying a high gloss powder coating for walking surfaces as they may be slippery when wet.  Low gloss textured finishes and unfinished grates will likely meet ADA requirements. In addition, the designer may also specify a slip resistant additive to the enamel or polyurethane paint used on some grates to improve slip resistance.

Low Impact Development − Stormwater Trenches

For many years, stormwater has been managed and hidden through engineering, not celebrated and exposed through art.  In the city, beautiful fountains were sculptures using water as a resource, but not a means to integrate the management of water quality with elements of art and play on the street.   A new practice of stormwater management often referred to as LID or Low Impact Development provides a new set of tools for urban designers.  New fountains and new stormwater plans include a variety of ways to celebrate water and provide opportunities for humans to interact with waterscapes, a word coined by designer and urban planner, Herbert Dreiseitl. Dreiseitl quotes Leonardo Da Vinci to explain his own fascination with water in urban settings. "Take thought, when you are speaking of water, that you first recount your experiences, and only afterwards, your reflections."^x In Dreisetl's plans, communities are invited to think about water as a sustainable resource adding one more natural element to the urban setting.

Low impact development practices are based on the principles that rainfall should be managed at the source through the insertion of smaller water quality control systems.    The goal of LID is to "mimic a site's predevelopment hydrology by using design techniques that infiltrate, filter, store, evaporate, and detain runoff close to its source. Techniques are based on the premise that stormwater management should not be seen as stormwater disposal. Instead of conveying and managing / treating stormwater in large, costly end-of-pipe facilities located at the bottom of drainage areas, LID addresses stormwater through small, cost-effective landscape features located at the lot level. These landscape features, known as Integrated Management Practices (IMPs), are the building blocks of LID. "^xiLID principles have been embraced by numerous cities, many of whom have adapted special land development codes to encourage these initiatives.   Some of these practices require the use of new, more visible trenches and drains, and include new sidewalk drain systems, new catchment basins at the end of roof top down spouts, tree box filters and fountain edges.

Trench grates can be specified in standard widths and the manufacturer can assist with installation details.   Trench drains are cast iron, aluminum, or bronze with steel frames. Most cast iron tree grates are so heavy that they act as their own deterrent to unauthorized removal.  Aluminum grates are lighter and aluminum has a greater value at recycling centers, so to reduce vandalism, grates can also be ordered with pilfer-proof bolting systems.

Often, in order to create a more natural path for water in the street, designers are challenged to design off the grid, in radial patterns, or along a change in the grade surface.  Stormwater trench grates can be designed to meet these challenges as long as the designer understands that these flat planes should be integrated into the slope or the curve in flat, level, segments.  The radius must be on "plane" following the change in grade without any leveling changes midslope.   Most concrete installers will assist the designer with the proper slope to the drain along the curved surface.

A radius trench drain serves to drain storm water as well as capture the splash from the fountain in this retail plaza. Photo courtesy of IRONSMITH

 

Conclusion:  Nature does Nurture

Urban design enhances the pedestrian experience in downtowns, malls, and streetscapes.  According to a Michigan Cool Cities initiative, what makes a downtown "cool" is a variety of features, from a leafy green street, to a sidewalk café.   Governor Granholm kicked-off the "Cool Cities" initiative in June, 2003 throughout the state of Michigan, in part, as an urban strategy to revitalize communities, build community spirit, and most importantly, retain the "knowledge workers" who are leaving Michigan in alarming numbers."  Since 2003, numerous cities in Michigan have submitted for grants to upgrade their streetscapes add tree grates and stormwater trenches and create more pedestrian friendly downtowns.  Mayors of cities from all over the United States, encourage designers to create urban spaces where people can live, work, shop and play. 

In 2006, the U.S. Conference of Mayors unanimously adopted the 2030 challenge which proposed to curb greenhouse gas emissions and provide for a 50 percent reduction in the use of carbon fossil fuel.  By requiring that street trees become part of all site plan approvals, cities are able to make some progress toward this goal. Trees provide health benefits, encourage walking and in addition, studies have shown that the observation of nature, such as seeing trees from an office window  restores people's mental, social, and creative functioning. A list of benefits provided by trees in an urban forest includes enhanced air quality, real estate values, reduced air pollution and water protection.

Trench grates integrate with tree grates to provide drainage to this Columbus, Ohio street. Photo courtesy of IRONSMITH

 

Vibrant downtowns build the economy and provide a focus for cultural growth.   Pedestrian friendly downtowns are those which improve the environment and revitalize community spirit.   Planting a healthy urban forest and providing an opportunity for water to become part of the streetscape are some of the tools used to create successful urban spaces. 

 

Celeste Allen Novak AIA, LEED AP, principal of RizzoloBrown + Novak Architects, is an architect and writer specializing in green design and community planning.

ENDNOTES: i - Florida, Richard.  Creative Cities Summit 2.0, Keynote Luncheon, October 14, 2008. ii -http://www.usgbc.org/ShowFile.aspx?DocumentID=2845 iii -http://www.epa.gov/hiri/ iv-http://www.epa.gov/hiri/strategies/vegetation.html v - Coder, Dr. Kim D.,"Identified Benefits of Community Trees and Forests", University of Georgia, October, 1996. vi - Nowak, David J., "Urban Trees and Air Quality," November, 1995 vii -http://www.portlandonline.com viii -http://www.access-board.gov/adaag/html/adaag.htm#4.3 ix -http://www.access-board.gov/adaag/html/adaag.htm#A4.5.1 x - Dreiseitl, Herbert. Ed.  Waterscapes, Planning, Building and Designing with Water.  Birkhauser. Boston. 2001 xi -http://www.lid-stormwater.net/background.htm#What_is_LID

IRONSMITH provides solutions to designers for trees in pedestrian hardscapes along with Stormwater and security elements. Supplying the broadest line of ADA compliant tree and trench grates, IRONSMITH works with designers on all their custom built elements for standard and radius installations in architectural surfaces.www.ironsmith.biz