Use AI tools to explore and evaluate structural framing strategies.
Example Prompt
Recommend several structural framing strategies for a 50,000 SF 10-story residential tower in San Francisco.
- Consider different material options -- concrete vs. steel vs. wood.
- Consider different stakeholder goals, including construction cost, construction speed, materials availability, carbon footprint, operational energy use, and aesthetics.
Create a table that contrasts the pros and cons of each strategy
Results from Claude
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Project assumptions
- Floor plate: 50,000 SF over 10 stories is about 5,000 SF per floor, roughly 50 × 100 ft. That's a small, slender tower, so the core takes up a large share of each floor, and overturning under seismic load needs early attention.
- Seismic: San Francisco is in Seismic Design Category D or E. Earthquake loads, not wind, will set the lateral system. Many sites (SoMa, Mission Bay, the waterfront) sit on fill or Bay Mud, so building weight directly drives foundation cost: deep piles or a mat, and liquefaction mitigation.
- Height limits: SF zoning sets height and bulk limits in feet (for example 85-X or 120-X). A system that adds 1.5 ft per floor adds about 15 ft to a 10-story building, which can cost you a story or push the project into a different review. Check the site's height district before choosing a system.
- Code limits on wood: Light-frame wood (Type III or V, including "5-over-2" podium buildings) is not viable at 10 stories. It tops out around 5 wood stories over a podium. Mass timber can reach 10 stories as Type IV-B, which California adopted in the 2022 CBC: up to 12 stories and 180 ft for residential.
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The five strategies
# | Strategy | Gravity system | Lateral system |
1 | Post-tensioned concrete flat plate (the SF baseline) | 8"–9" PT slabs on concrete columns | Concrete core and shear walls |
2 | Structural steel frame | Composite metal deck on steel beams and columns | Buckling-restrained braced frames (BRBF) or special moment frames |
3 | Mass timber, Type IV-B, over a concrete podium | CLT floors on glulam beams and columns, with a concrete topping | Concrete core (most practical in SF), with a concrete podium at levels 1–2 |
4 | Steel–timber hybrid | CLT floor panels on steel beams and columns | Steel BRBF or a concrete core |
5 | Volumetric modular (steel-framed modules) | Factory-built steel modules stacked on a concrete podium | Podium and core, or braced module frames |
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Pros and cons
Strategy | Pros | Cons |
1. PT concrete flat plate | • Lowest cost and least risk; SF contractors know it well<br>• Thinnest floors (about 9'-6" to 10' floor-to-floor), so the most stories fit under a height limit<br>• Flat soffits: no beams, easy ceilings<br>• Built-in 2-hr fire rating, good acoustics, low vibration<br>• Materials are local and plentiful | • Highest embodied carbon of the options (cement)<br>• Heaviest, so the largest seismic forces and the most expensive foundations on soft soil<br>• Slower: wet trades, curing, shoring, roughly 5–7 day cycles per floor<br>• Concrete balconies create cold bridges unless you add thermal breaks<br>• Hard to change later (PT tendons limit new openings) |
2. Steel frame | • Fast erection; fabrication happens off-site while the foundations go in<br>• Lighter than concrete, so smaller foundations and lower seismic mass<br>• Ductile, well-proven seismic systems (BRBF)<br>• Long spans and flexible future layouts<br>• High recycled content (US steel is mostly electric-arc furnace) | • Deepest floors (about 11'–12' floor-to-floor), which can cost a story under a height limit<br>• Usually more expensive than concrete for residential<br>• Needs spray fireproofing, plus extra detailing for acoustics and floor vibration<br>• Exposed to tariffs and steel price swings<br>• Beams have to be hidden behind ceilings or soffits |
3. Mass timber IV-B | • Lowest embodied carbon, and the wood stores carbon<br>• About 60–70% lighter than concrete, a big saving on seismic forces and foundations on SF soil<br>• Fast, quiet erection with small crews (about 3–5 days per floor) and less on-site waste<br>• Exposed wood is a strong visual and marketing feature<br>• Precise prefabrication fits well with BIM-driven fabrication | • Few suppliers (Pacific Northwest, British Columbia, Europe), long lead times, and supplier failure risk (for example Structurlam's 2023 bankruptcy)<br>• Often costs 0–10% more than concrete; builder's-risk insurance is higher<br>• Thicker floors (about 10'-6" to 11') because of the topping slab and acoustic layers<br>• Type IV-B limits how much wood can be left exposed; the rest must be covered with fire-rated board. The 2025 CBC loosens this; confirm with a code consultant<br>• Wood must be kept dry during construction<br>• SF building and fire officials are still getting familiar with it, so allow more time for plan review<br>• The concrete core and the timber frame move differently in an earthquake, which needs detailed design |
4. Steel–timber hybrid | • Most of mass timber's carbon savings and visual appeal<br>• Steel connections are simpler and familiar to steel erectors<br>• Longer spans than all-timber; steel frames give strong, ductile seismic resistance<br>• Lighter than concrete | • Two supply chains to manage (timber and steel)<br>• Deep floors again (beam depth plus CLT), which hurts under a height limit<br>• Fire protection on both materials adds detailing and cost<br>• Few precedents, so less cost and schedule data |
5. Volumetric modular | • Fastest schedule: modules are built while site work proceeds, saving roughly 20–40% of the schedule<br>• Factory quality control gives tighter envelopes and better airtightness<br>• Much less on-site labor, which matters given SF labor costs<br>• Repeated unit layouts suit residential | • Thickest floors (separate floor and ceiling in each module), the worst option under a height limit<br>• Bay Area modular makers have failed before (for example Katerra), and few suppliers can build 10 stories<br>• Shipping limits module size, which limits unit layouts<br>• Hard to integrate with a seismic system at 10 stories<br>• Design must be frozen early, with little room for changes<br>• Repetitive, boxy exterior unless you invest in the facade |
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Scorecard by stakeholder goal
5 = best for that goal. Scores are relative and for early concept only; confirm them with your structural engineer and cost estimator.
Goal | 1. PT Concrete | 2. Steel | 3. Mass Timber | 4. Steel–Timber | 5. Modular |
Construction cost | 5 | 3 | 3 | 3 | 3 (wide range) |
Construction speed | 2 | 4 | 4 | 4 | 5 |
Materials availability (Bay Area) | 5 | 3 | 2 | 3 | 2 |
Embodied carbon | 2 (3 with low-carbon mix) | 3 | 5 | 4 | 3 |
Operational energy | 3 | 3 | 4 | 3 | 4 |
Aesthetics | 3 | 3 | 5 | 4 | 2 |
Height efficiency (floor-to-floor) | 5 | 2 | 3 | 2 | 1 |
Seismic mass and foundations | 2 | 4 | 5 | 4 | 4 |
On operational energy: the choice of structure has only a small effect. Envelope, glazing ratio, and all-electric mechanical systems (required for new SF construction) matter far more. Structure mainly affects thermal bridging: concrete slab edges and balconies, and steel penetrations through the envelope. Timber has an advantage because wood conducts heat poorly.
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Recommendations by priority
If the owner's main goal is… | Choose | Why |
Lowest cost and least risk | 1. PT concrete, specified with a low-carbon mix (30–50% cement replacement, performance-based specs, tendon layout optimized to save concrete) | Market default with the thinnest floors. A low-carbon mix can cut concrete's carbon by about 20–40% for little extra cost. |
Carbon leadership and design identity | 3. Mass timber IV-B over a 1–2 story concrete podium with a concrete core | Lowest embodied carbon, and the lighter weight saves on foundations, which can offset the cost premium on poor soils. |
Fastest time to occupancy | 2. Steel or 5. Modular | Off-site fabrication runs while foundations are built. Choose modular only if the height limit has room and the fabricator is well capitalized. |
Balanced | 4. Steel–timber hybrid, or 3. with a steel BRBF instead of a concrete core | Gets most of the carbon savings and visual appeal with familiar steel seismic systems. |
My overall pick for a typical SF site: carry Option 1 (low-carbon PT concrete) and Option 3 (mass timber over a podium) into schematic design. Two site facts usually decide between them:
- Soil: on poor soil, timber's weight savings can pay for most of its premium.
- Height limit: if the height district is tight, PT concrete's thinner floors win.
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Suggested next steps
- Geotechnical report: pile depth and liquefaction findings can change the cost ranking.
- Height district check: test 9'-6" against 11'-6" floor-to-floor for all 10 stories against the zoning limit.
- Concept-level whole-building carbon study on Options 1 and 3, using a tool such as One Click LCA, Tally, or EC3.
- Early meeting with SF DBI and the Fire Department if mass timber stays in the running.
- Revit Design Options for each structural scheme. A Dynamo material takeoff can then pull concrete, steel, and timber quantities for each option for the carbon and cost comparison. I can write that script if it helps.