The carbon in a building is not spread evenly through it. Beyer Blinder Belle ran a cradle-to-gate life cycle assessment on a real job, a three-story vertical expansion of an existing steel-frame building now under construction, and found that steel accounted for 19 percent of the structure’s mass and 64 percent of its global warming potential. The firm published the results on 28 August.
Where the carbon actually sits
The as-designed baseline is a conventional steel frame with composite concrete slabs on a 20-by-20 grid, matching the building below it. BBB modeled that against three alternates using the Revit plugin Tally: cast-in-place concrete, light-gauge steel with load-bearing demising walls, and mass timber using glulam beams and columns with CLT floor panels.
Two findings travel beyond this project. In the concrete alternate, rebar came in at 5 percent of structural mass and 26 percent of global warming potential, so more than a quarter of the carbon was in the reinforcing rather than the mix. And a single additional inch of slab thickness raised GWP by 12 percent, which on this building works out to roughly 250,000 kilograms of CO2. Swapping a non-structural lightweight topping from a 6,000 PSI mix down to 4,000 PSI, using fly ash or other supplementary cementitious materials, cut carbon 8 percent.
None of those are exotic sustainability moves. They’re slab schedule decisions made in the first six weeks of a job, usually by a structural engineer working to a deflection criterion, and they’re worth more than most of what gets specified later.
The pricing claim
The timber alternate came out dramatically lighter, because CLT permits thinner slabs, and it scores carbon-negative under cradle-to-gate accounting because the wood sequesters carbon. That’s an accounting boundary, not a physical claim, and BBB says so: if the timber is burned at end of life the sequestered carbon comes back, while steel stays recyclable. The firm also flags that the climate benefit depends on sustainably managed forestry and short haul distances.
The line that will get quoted is the cost one. "When we’ve priced timber and steel schemes, they come back roughly equal. The assumption that mass timber carries a steep premium simply didn’t hold true," the firm writes. Treat that carefully. It’s a general statement about BBB’s pricing experience across projects, not a documented bid comparison on this one, and the three alternates were modeled rather than priced through subcontractors.
What to do with it
This is a self-published study by the design firm that ran it. There’s no peer review and no third-party verification, and the project isn’t named. What it does supply is a set of ratios a project team can test against its own numbers in an afternoon, without buying anything.
The broader point lands the same way as the embodied-carbon math behind large reclads, where keeping a frame in place does more than any material substitution can. Vornado’s over-clad approach at PENN 2 rests on exactly that arithmetic. BBB’s contribution is showing that on new structure, the same gain sits in the slab section. The firm’s full writeup is here.