Rethinking Street Trees, Part 2

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This is part 2 in an ongoing series of articles about Street Trees. In Part 1 (http://jfs-eng.com/rethinking-street-trees/), we discussed the notion of the tree pit and ways to integrate that structure into the fabric of street life. In the wake of Hurricane Sandy, let’s discuss Street Trees as a structural unit and why the root zone is so important. If one were to take a survey of the direction of street tree failure, the majority of street trees will fail towards the street or the building side and generally not parallel to the curbline. This phenomenon can be explained if we consider the tree as a structural unit and use the analogy of a retaining wall. Note that there are many other factors at play and this analogy is a simplified example. In this article, we’ll take a look at one failure mode for a street tree and examine how to reduce this failure mode.

Figure A: Retaining Wall

Figure A is a typical retaining wall section drawn in “engineer”. The pressure from the soil wedge generates a resultant force on the wall (a.) which in turn generates an overturning force on the toe of the wall (b.). This force generated is governed by the relationship of the toe length (L) and the wall height (B). Generally, an increase in the toe length (L) reduces the overturning moment generated at the wall toe (b.).

Figure B: A Street Tree Failure Mode

Figure B is a typical street tree translated into an analogous “engineer” parlance. We’ll make two assumptions here, first being that surface obstructions such as curbing and sidewalk stunt the street trees’ roots. The second assumption will be using an oblong NYC Tree pit standard of 5′ wide by 10′ long. In this case, we look at the resultant wind pressure street trees, here shown in blue. Based on these assumptions we can deduce that the resultant toe length (L) is longer parallel to the curb (Section B) than in Section A. Naturally, this reduction in the toe length L increases the resultant overturning moment in Section A which could lead to increased failure in that direction. We can also see how an increase in the beam (B) generates more resultant force due to increased leverage. A taller tree will naturally have a thicker trunk to account for this phenomenon.

Figure C: An Alternative Location

Knowing that this is the typical street tree failure mode, I offer an alternative plan view design for a street tree in Figure C. In every design we have to make a trade off, and here I am proposing that sacrificing parking space for the sake of street trees and sidewalk space will be a good thing. This proposal effectively increases the tree pit dimensions from 5’x10′ (50 SF) to 8’x20′ (160 SF). This increase in free root zone space will help promote tree health and will help with the overturning issue outlined in Figure B.

There are other benefits as well. By moving the tree from it’s typical location (a.), vertical obstructions are removed from the curbline making curbside loading easier for cars. By clustering street furniture near the tree, we can create a pocket park and streamline the sidewalk space for pedestrians. The expanded tree pit can also be combined with a bioswale system and will generally create more snow storage in the streetscape. In addition, this new configuration can be used to create a deeper driveway throat.

Although this proposed scenario may not work in every location, it’s worthwhile to consider this as an alternative planting pattern for street trees.