Structural swap and carbon target, Alder Creek office building

Embodied carbon estimate built from the estimate export, with trucking, jobsite waste and generic values where EPDs are missing, run for every combination of structural system, concrete mix and cladding and compared with the owner’s target of 250 kg CO₂e/m² (23.2 kg CO₂e/SF). The proposed hollow-core plank on steel comes in at 258 and misses the target with the current proposals.

6-story office building, 90,400 GSF (8,398 m²)CM at-risk: Ironwood Builders, Inc. for Kettle Ridge Properties LLCUpfront embodied carbon, A1–A5Prepared Nov 6, 2026

Summary

Proposed swap · A1–A5

Hollow-core on steel comes in at 258 kg CO₂e/m², 8 over target

The plank and topping save 10 kg CO₂e/m² against the 8 in. post-tensioned flat plate and its columns, but the steel frame with SFRM adds 38. Lighter footings, less waste and less winter heat on site take back 9, and longer truck hauls add 6. The estimate’s PT baseline is 233 (Structural Alternates Memo Rev B, Cedarline proposal).

To confirm: plank spans, steel tonnage, lateral system and the 1-hour rating come from the preliminary memo and must be confirmed by the structural engineer of record before the structural system is locked.

Generic data · steel

No steel EPDs – with product-specific EPDs the swap comes in at 246

Cedarline’s proposal has no product-specific EPDs, so the 210 tons of steel use industry-wide values with the owner’s 1.2 factor: 1.30 kg CO₂e/kg for W-shapes and 2.39 for HSS. With EPDs at or below 0.92 for W-shapes and plates and 1.40 for HSS, the steel frame drops from 38 to 26 kg CO₂e/m² and the swap meets the target. At industry averages without the factor it is 239, so the outcome turns on the data rule. Require product-specific EPDs in the steel subcontract.

Combinations · 36

23 of 36 combinations meet the target; brick on steel only with the −40% mix

With the current data, every combination with the standard mix misses the target, and brick veneer on steel meets it only with the −40% mix. With product-specific steel EPDs, 24 of 36 meet it. The reduced-carbon alternate – plank on steel with steel EPDs and the −40% mix – keeps the brick and comes in at 230, 20 under the target.

Embodied carbon by combination

Pick a structural system, cladding and concrete mix, or one of the three alternatives. The result is recalculated against the owner’s target and the estimate baseline. Switch the data used for steel and for generic items, or edit quantities, haul distances and waste rates in the carbon estimate.

Alternative
Concrete mix
Applies to ready-mix. Plank and precast: −15% and −25%.
Steel data
Generic values
Report per
– –

–A1–A3 product stage
–A4 transport
–A5 waste and site energy
–Share from generic data

Values in the selected unit. Green meets the target, red is over. Click a cell to select that combination. “Est.” marks the estimate baseline.

Sources and method

Sources

  • AlderCreek_Office_Estimate_Export_2026-10-28.xlsxResource and carbon data from the estimating software, Resources and Carbon tabs. Product EPDs existed for concrete, plank, precast, glazing and two of the cladding options; other resources had no carbon data.1,212 lines
  • Structural_Alternates_Memo_RevB.pdfBirchline Structural Engineers: preliminary sizes for 10 in. hollow-core plank with 2 in. topping on W-shape beams and HSS columns, and for a precast frame16 pages
  • Basswood_Precast_Proposal_HC10.pdfUnit prices and product-specific EPDs for plank, columns and beams in three mixes11 pages
  • PrairieAsh_ReadyMix_Submittal_EPDs.pdfProduct-specific EPDs for 4000 and 5000 psi mixes: standard, −25% and −40% (slag cement, Type IL cement)8 pages
  • Cedarline_Steel_Proposal.pdfStructural steel and 1-hour SFRM, 210 tons, no product-specific EPDs6 pages
  • 01 81 13 Sustainable Design Requirements.pdfKettle Ridge Properties: upfront embodied carbon at or below 250 kg CO₂e/m² GFA, A1–A5; industry-wide values × 1.2 where no product-specific EPD is submitted5 pages
  • G-001 Code Analysis and Area Plan.pdf90,400 GSF; Type IIA construction, fully sprinklered2 sheets
  • A-201–A-204 Exterior Elevations.pdfWolden Architects: cladding materials, 35% window-to-wall ratio4 sheets
  • Site_Energy_Historical_2022-2025.xlsxIronwood Builders: electricity, fuel and temporary heat per m² for cast-in-place and precast or steel frames12 projects
  • Industry-wide EPDs: AISC, AISC/STI, CRSI and othersGeneric values for structural steel, HSS, rebar, gypsum, polyiso, EPS and brick; the LCA tool database for the restReference

Method

  1. Estimate resources were mapped to the owner’s scope in Section 01 81 13: foundations, structure, enclosure and interior partitions. MEP, sitework, hardscape and interior finishes are excluded.
  2. Each resource got an A1–A3 GWP factor: product-specific EPDs from the proposals where available, otherwise industry-wide EPDs or the LCA tool’s generic data, multiplied by 1.2 per Section 01 81 13. Generic items are flagged in the estimate. The EPDs are ISO 14025 Type III declarations under ISO 21930-based product category rules.
  3. A4 is shipping weight × haul distance to Alder Creek × 0.10 kg CO₂e per ton-mile for a tractor-trailer. A5 is jobsite waste, A1–A3 × waste %, plus site energy from the CM’s historical data, including winter heat and enclosures for cast-in-place work.
  4. Quantities for the plank-on-steel and precast alternates come from the structural alternates memo. Cores and shear walls are the same in all three systems.
  5. All 36 combinations were recalculated and compared with the owner’s target per m² of gross floor area. A combination meets the target when its rounded value is at or below it.

Assumptions and limitations

  • Target and reporting. The target is an owner requirement in Section 01 81 13, stated per m² of gross floor area; 250 kg CO₂e/m² equals 23.2 kg CO₂e/SF. It covers upfront carbon (A1–A5) only. Biogenic carbon is excluded.
  • LEED v5. The A1–A3 results for structure and enclosure can support the LEED v5 BD+C prerequisite Quantify and Assess Embodied Carbon. The whole-building LCA option of the Reduce Embodied Carbon credit compares a cradle-to-grave model with a baseline building and is not evaluated here.
  • Minnesota. The Minnesota Buy Clean Buy Fair Act (Minn. Stat. § 16B.312) sets GWP limits for ready-mix concrete and rebar on state-owned buildings over 50,000 GSF advertised on or after July 15, 2026, and the B3 Guidelines apply to projects funded with state general obligation bonds. Neither applies to this privately owned building.
  • Generic values come through the estimate’s LCA mapping, for example 1.08 kg CO₂e/kg for fabricated hot-rolled sections (AISC industry-wide EPD, 2025) and 1.99 for fabricated HSS (AISC/STI industry-wide EPD, 2022). Check them against current EPDs before the steel subcontract is written.
  • The steel EPD requirement is a scenario: 0.92 kg CO₂e/kg for W-shapes and plates and 1.40 for HSS are proposed procurement limits, not quoted products.
  • Reduced-carbon mixes follow the suppliers’ EPDs: −25% and −40% for ready-mix and −15% and −25% for plank and precast, against each supplier’s standard mix.
  • Site energy is the CM’s historical average, 14.0 kg CO₂e/m² for cast-in-place and 9.0–9.5 for precast or steel framing, not metered on this project.
  • Structure. The alternates use preliminary member sizes. The structural engineer of record should confirm plank spans, steel tonnage, lateral system and foundations, and the architect the 1-hour fire-resistance rating, before the structural system is locked.
  • Review. This is a preconstruction estimate, not a whole-building LCA. An LCA practitioner or sustainability consultant should check scope, mapping, generic values and EPD validity before the target is tracked through submittals.
  • Edits in the carbon estimate carry through the tool but not the summary at the top.