Design Considerations for On-Structure Silva Cell Projects

Lincoln Center 2

These trees are planted in Silva Cells on top of an underground parking garage at New York’s Lincoln Center Theater (2012)

We see a lot of Silva Cell projects go in on top of existing structure such as parking garages. These sites are kind of like secret green roofs, although they don’t get as much fanfare! When planning for an on-structure project, here are a few things for designers to keep in mind. 

Vertical Dimension

The vertical dimension of the space is paramount: the height from the top of the underlying structure to the top of the finished pavement will ultimately determine the number of cell frames you can use. Our document, “Construction depths for Silva Cells” can help you get started by providing the vertical dimensions needed for one, two and three layer Silva Cell systems when used in conjunction with our recommended pavement sections.

In addition to the height of the Silva Cells and the pavement section over the top of them, there are two other considerations: the aggregate sub base that supports the Silva Cells and the “build up” between top of the underlying structure and the bottom of the Silva Cells. When Silva Cells are being installed over natural soil we recommend that they be installed on a minimum of  4” (100 mm) of compacted aggregate to ensure a solid foundation. When it comes to applications where the Silva Cells are being installed over concrete structure instead of natural soil, on the other hand, there can be some flexibility in the depth of the sub base.

It is also important to recognize that in almost all instances there will be a layering or “build up” of materials like vapor barrier, foam insulation, waterproofing membrane, and drainage products on the top of the underlying structure or between the underlying structure and the bottom of the Silva Cells. The overall dimension or thickness of this build up not only affects how much room there is for the Silva Cell system. The compressive strength of these materials needs to be taken into consideration when determining if or how much sub base aggregate is needed below the Silva Cells.

Load

Silva Cell systems when used in conjunction with one of our recommended pavement sections are designed to meet the AASHTO H-20 standard of 32,000 lbs per axle, or basically a large truck. Our recommended standard pavement sections range in thickness from 8” (4” of concrete over 4” of aggregate) to 15.5” (3.5” of pavers over 12” of aggregate).

H-20 Loading

Summary of top deck stresses under AASHTO H-20 loading

However, it is not always necessary to design to the H-20 standard.  Frequently with on structure applications limited access, weight restrictions and other similar factors limit the type of vehicular traffic there can be or even completely eliminates it. In these instances the design team has the opportunity to reduce the loading standard that needs to be met, which in turn can reduce the overall thickness of the pavement section over the Silva Cells and create more room for soil volume.

Weight of the Silva Cells

Weight is a major concern with any roof. Each Silva Cell frames weighs 10 lbs empty and at 24” x 48” x 16”, each frame holds 10 Cubic feet of soil.  The Silva Cell deck piece also weights 10 lbs, but decks are only installed on the top layer of frames.  The chart below provides the combined weight (empty) of the frames and decks for one, two and three layer systems.

Weight Single stack Double stack Triple Stack
Decks

10

10

10

10

Frames

10

10

20

30

Total (lbs)

20

30

40

 

We usually do not get involved with other weight related issues such as the weight of the soil and trees on a project, but you can read a more complete discussion of them in Jim Urban’s book, Up by Roots (see page 250 to 253). Jim rightly points out that soils can be heavy, and their weight will vary depending on porosity, bulk density, and compaction. A good rule of thumb he applies is 110 lbs to 120 lbs (50 kgs to 54 kgs) per cubic foot of saturated soil. There are light aggregates out there like perlite (expanded shale) for use on rooftops, but — regardless of soil type – make sure you use the saturated weight when doing any calculations.

Anchoring down the frames

In typical Silva Cell applications (not on structure) four 10” spikes per frame are used to keep the base layer of frames from moving around during construction. For on-structure applications it may not be possible to use spikes. We recommend using small wood spacers between the frames to help keep them stationary:

An example of how to anchor Silva Cell frames using wooden spacers in an on-structure application

An example of how to anchor Silva Cell frames using wooden spacers in an on-structure application

Many of our projects are actually on-structure green roofs. Here are a few examples.

Lincoln Center Barclay Capital Grove (New York, New York)

Lincoln Center 1

Lincoln Center Silva Cell installation (2009)

Lincoln Center 2

The trees planted in Silva Cells on top of an underground parking garage (2012)

In this project, a two-deep Silva Cell system rests on an underground parking garage. There is an aggregate cover over the roofing system, which is used as a sub-base for Silva Cell system. A system of plywood sheeting was used to secure cells in place during construction. The base course over the system includes two types of aggregate separated by geotechnical fabrics, and is designed for a custom load of 4,500 lbs (2,041 kg), in this case, a pickup truck. You can see a full case study here.

Mississauga Civic Square (Mississauga, Ontario)

Mississauga 1

Silva Cells were installed on-structure in Mississauga Civic Square in 2010

Mississauga 2

Silva Cells were installed on-structure in Mississauga Civic Square in 2010

Mississauga 3

The trees one year later (2011)

Silva Cells were installed on top of an underground parking garage as part of redevelopment of the public space outside of the Mississauga City Hall.

1808 1st Street Balcony (Vancouver, British Columbia)

1st Avenue 1

Silva Cells were installed on a community balcony in 2008.

1st Avenue 2

The 1st Avenue balcony tree in 2012.

This project was for a children’s play area on a balcony, so they only needed to meet pedestrian loading standards. Aggregate was placed on the roofing system, and foam was placed adjacent to demarcate the Silva Cell area. You can read the full case study here.

Cigna Headquarters (Bloomfield, Connecticut)

Cigna 1

Silva Cells were installed at Cigna Headquarters in 2010

Cigna 3

Silva Cells were installed at Cigna Headquarters in 2010

This interior courtyard used Silva Cells in a pedestrian-loading only area. The Silva Cells were placed directly on Mirafi 9800.

Cleveland Medical Mart and Convention Center (Cleveland, Ohio)

Medical Mart 1

Silva Cells were installed on top of a parking garage at the Cleveland Medical Mart in 2012.

In this on-structure project, a system of foam and aggregate was used as the sub base. This system will be topped with concrete and is adjacent to a roadway, so it was designed to meet H-20 loading. You can read more about this project here.

 

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