Roof Garden Talk with Tiffany

Photo by Shashi Chaturvedula on Unsplash

Dear Tiffany,

Thank you for letting me stay at your penthouse on this trip. The bedroom was very cozy and efficient. What I loved the most was the window seat, where I could watch the birds in the garden while enjoying the morning coffee, which was my favorite part of the day.

While I cannot thank you more for this beautiful roof garden, I discovered a few structural issues when picking up the newspaper downstairs. The staircase has several cracks, and I suspected they have something to do with this garden.

It is not unusual for concrete structures to have cracks, especially in Taiwan, which is in the seismic zone. Although your building is not newly constructed, most reinforced concrete structures can still have a life expectancy of about sixty years. Thus, I do not see these cement fractures as immediate structural failures.

On the other hand, I felt that giving you some heads-up analysis might be necessary. So you could begin considering how you would like to remodel your roof garden for more extended use. Here are a few structural principles that I use to analyze the overall landscape on the roof:

1. The column capital bears the most structure load.
2. The middle section of a beam is the weakest spot of the entire column span, which means it can carry the minimal structure load along the beam/girder as intended.
3. Extending the load-bearing area to one-quarter of the entire column’s span is still considered safe. That is because the compression and tension were still at balance without causing much deflection. (see graphic)

Morning journal: a nice coffee, a bird photo and structure nots- the column capital is considered to be the beam with a “fixed end.” The different behavior between cantilever beam and a fixed-end beam.

With these three principles, I started to review your roof plants’ locations.

Southwest side (between columns B-C): Two Japanese maples are on the street-facing side. You can keep the maple A but remove the maple B. Because planting location B is a cantilever situation, no supporting structure is underneath. With an added maple tree load on the top, it would be subject to a greater risk.