Step 1 - Generative Design Framework
Here are 3 decision areas I went through in the brainstorming process:
- Structural
- Design Variables
- Floor areas (must be contained within the lot)
- Number of floors, above and below grade
- Evaluators
- Gross floor area (weight and seismic base shear)
- Floor area x story height (material weight and overturning moment)
- Most Important Tradeoffs to Consider
- Construction time increases greatly with building height (stories are assembled in series).
- Occupancy
- Design Variables
- Floor areas (occupiable)
- Number of floors
- Inter-story heights (z-offset)
- Elevator/stair shafts (unoccupiable area)
- Evaluators
- Gross floor area (capacity)
- Entry/evacuation time (commuting/safety)
- Insolation (natural light)
- Most Important Tradeoffs to Consider
- Taller buildings require more shaft area to maintain the same evacuation time
- 3-stories above ground will take longer to evacuate than 2 stories above and 1 basement story? That’s my intuition…
- Basement may have limited light exposure (design dependent), less market value
- Affordability/Maintenance
- Design Variables
- Floor areas (must be contained within the lot)
- Number of floors, above and below grade
- Inter-story heights
- Evaluators
- Cumulative insolation on roof only (solar power)
- Thermal insulation demand (cumulative insolation from all exposed surfaces)
- Building volume (HVAC capacity)
- Exposed surface area (for facade maintenance/cleaning)
- Most Important Tradeoffs to Consider
- Solar panel area on the roof increases at the expense of growing insulation, HVAC, and facade maintenance demand.
Step 2 - Generative Design Study
Elaborating on Occupancy [for 2 units], we’ll focus on maximizing occupiable floor area and minimizing floor transit time.
- Occupancy
- Design Variables
Floor areas (occupiable)Number of floorsInter-story heights (z-offset)- Elevator/stair shafts (unoccupiable area)
- Number of shafts
- Number of basement floors and above-ground floors
- Evaluators
- [Maximize] Gross floor area (capacity) + Revenue
- [Minimize] Entry/evacuation time (commuting/safety)
- Most Important Tradeoffs to Consider
- Taller buildings require more shaft area to maintain the same evacuation time
- 3-stories above ground will take longer to evacuate than 2 stories above and 1 basement story? That’s my intuition…
- Basement may have limited light exposure (design dependent), less market value
First, we will define mass floors to start with a maximum occupiable floor area.
Then, we will subtract out the shaft area in the center of the floorplan (assume we place a shear wall core in the center).
[Output 1] Calculate the remaining gross floor area (sum of the floors), and the revenue assuming certain fixed price per SF and an additional price for floors above ground.
[Output 2] Assume a fixed floor area per person and calculate the elevator round trip time (RTT)
- Assume elevator average load per elevator is L = 8 people
- Assume people on each floor Pi = (occupiable floor area) / (constant SF per person)
- n = number of floors
The ground floor assignment splits the floors above and below ground into 2 elevator trips (assuming everyone wants to get to the ground floor):
Using these two groups of floors, the Round Trip Time of each group is calculated and added together using this RTT model:
The outputs are computed in the Dynamo workspace as shown below:
Step 3 - Generative Design Study Results
The Dynamo study script is as follows, going left to right. Underneath is a zoom-in of the formation of the envelope cube, the shaft cube, and the floor surfaces that make up the building mass.
After running the optimization (10 generations of 20 samples), I found that the addition of basement floors definitively increased the elevator round trip time (because the passengers have to wait for the elevator to cycle both above and below ground). This was the opposite of what I expected. It is dependent, though, on the RTT modeling assumptions (no passengers on the return trip) and my choice to use all elevators in both directions.
So, the optimal choice was to place the bottommost floor at ground level. The optimization reduced down to a tradeoff between occupiable floor space and the elevator round trip time.
The floor area decreases linearly with the number of shafts. But the round trip time decreases nonlinearly. The largest time reduction was made going from 1 elevator/stair core to 2 cores (bolded in blue below). For this reason, I interpret the 2-core option as the optimal design.
