Stage 1: Create Two New Evaluator Custom Nodes
For this stage, I created additional evaluation metrics to compare my tower form alternatives beyond the basic Revit outputs. The original table already includes gross floor area, gross surface area, and gross volume, but I wanted to add metrics that better explain the design tradeoffs from a developer and building performance perspective. In my study, I varied the Tower Top Height from 650 ft to 750 ft and the Tower Top Rotation from 90° to 150°, then compared the resulting geometry, efficiency, cost, and solar potential.
1st Evaluator Nodes: Embodied Carbon (Floor mass / Surface to volume ratio)
This parameter was created to evaluate how compact the building form is. The node divides the gross surface area by the gross volume. A lower ratio means the building has less envelope exposure per volume, which can be better for façade cost, heat transfer, and overall envelope efficiency.
2nd Evaluator Nodes: Cost Analysis
This parameter was created to estimate construction cost based on the prompt assumption that cost increases with height, from $500/SF at ground level to $1000/SF at 750 ft. This helps compare the alternatives from a developer’s perspective because a taller building may provide more floor area, but it can also become more expensive to construct.
3rd Evaluator Nodes: Solar Radiation Analysis / Daylight Potential
For the solar potential, I used the cumulative solar value to estimate how much solar exposure the envelope receives. This metric is useful for understanding daylight access, solar heat gain, and potential PV opportunities. Higher solar potential can be beneficial, but it also needs to be balanced with envelope efficiency and cost.
For Stage 1, the two parameters chosen were intended to compare the building alternatives beyond only gross floor area, gross surface area, and gross volume. The surface-to-volume ratio was included because the prompt asks us to minimize envelope surface area, and this metric helps show which massing is more compact. The estimated cost was included because the developer also needs to understand whether the additional floor area is worth the increased construction cost. I also included cumulative solar potential in the final table to connect the comparison to building performance. The test cases varied the Tower Top Height from 650 ft to 750 ft and the Tower Top Rotation from 90° to 150°.
Based on the completed table, the options with 750 ft tower height generally provide higher gross floor area and higher solar potential, but they also increase cost, surface area, and slenderness. The 650 ft and 700 ft options are more moderate and may be more efficient depending on the chosen weighting. This shows that the “best” alternative is not only the tallest or largest tower, but the one that balances floor area, cost, compactness, and solar performance.
Point to Ponder
Do the new evaluation metrics capture meaningful differences between the building form alternatives? Yes, the new metrics capture meaningful differences because they compare the tower alternatives beyond just basic size. The surface-to-volume ratio shows which form is more compact and efficient in terms of envelope exposure, while the estimated cost shows the financial impact of changing the tower height and geometry. The solar potential also adds a performance-based comparison by showing how much solar exposure each option receives.
What other metrics would be useful? Other useful metrics would include daylight access, embodied carbon, view quality, and rental value by floor level. These metrics would help make a stronger case for which alternative is truly better because they connect the form not only to cost and geometry, but also to sustainability, user experience, and developer value.
Stage 2: Develop a Single-Objective Optimization Scheme
For Stage 2, I created a single-objective optimization score to compare all tower alternatives using one combined value. The metrics included in the score were gross floor area, surface-to-volume ratio, estimated cost, and cumulative solar potential. Since these metrics have different units and scales, I normalized each value from 0 to 1 so they could be compared fairly. For gross floor area and solar potential, higher values were treated as better. For surface-to-volume ratio and estimated cost, lower values were treated as better, so the normalized values were inverted.
My overall strategy was to balance the developer’s goals with building performance. Gross floor area is important because the prompt requires 2.5M–3.0M SF of new floor area, while estimated cost reflects the financial feasibility of the option. Surface-to-volume ratio was included to reward more compact forms with less envelope exposure, and solar potential was included to represent environmental and daylight opportunity. After applying the combined score, the top three recommended alternatives were:
- Top Height = 750 ft, Top Rotation = 90° Score = 0.5689
- Top Height = 750 ft, Top Rotation = 120° Score = 0.5516
- Top Height = 750 ft, Top Rotation = 150° Score = 0.5328
The best option is the tower with 750 ft top height and 90° top rotation because it achieved the highest overall score. This option provides the largest gross floor area at 2,850,000 SF, which is still within the required range, while also maintaining the lowest cost among the 750 ft options. Even though the 150° rotation has the highest solar potential, it also has lower floor area and higher surface-to-volume ratio. Because of that, the 750 ft / 90° option gives the best balance between floor area, cost, compactness, and solar performance.
Point to Ponder: What overall strategy do you feel best captures the relationship between the evaluation metrics?
I think the best strategy is to normalize the metrics first and then combine them into one score, because each metric has a different unit and direction. This makes the comparison easier to understand and avoids one large-number metric dominating the result. It also helps show the tradeoffs between cost, floor area, envelope efficiency, and solar performance.
Point to Ponder: What propelled the recommended alternative to the top of the list?
The recommended alternative rose to the top because it performed well across the main categories instead of only excelling in one metric. It had the highest gross floor area, a reasonable surface-to-volume ratio, and the lowest estimated cost among the tallest options. Some tradeoffs are still simplified in this single score, especially because higher solar potential may not always be fully positive if it also increases cooling load. However, based on the selected metrics, this option gives the strongest overall balance.