Budgets, not bids
Conceptual steel numbers are for feasibility studies, owner budgets, design-assist pricing and GC "rough order of magnitude" requests, when drawings are schematic or missing. AACE International's Recommended Practice 56R-08 (building and general construction) is the most useful way to explain to a customer what that number is worth:
| AACE class | Project definition | Typical method | Typical accuracy range (80% confidence) |
|---|---|---|---|
| Class 5 | 0% to 2% | Parametric, functional area | −20% to −30% low / +30% to +50% high |
| Class 4 | 1% to 15% | Assembly-driven, schematic design | −10% to −20% / +20% to +30% |
| Class 3 | 10% to 40% | Semi-detailed unit costs with assembly-level line items | −5% to −15% / +10% to +20% |
| Class 2–1 | 30% to 100% | Detailed unit cost with detailed takeoff | as narrow as −3% / +3%, at full definition |
AACE stresses that those ranges assume appropriate contingency is included and that a Class 5 estimate on a repeat building with good cost history can beat a Class 3 estimate on something novel. Put the class and the range on the budget letter. It is the single best protection against a conceptual number being treated as a quote.
Method 1: the typical bay
The typical-bay method is the closest you can get to a takeoff without framing plans. It works best on regular office, warehouse and retail frames.
- Define the bay. Column grid (say 30 x 30 ft), beam spacing, floor system (composite deck and slab, joists, or concrete on form deck), superimposed dead and live loads.
- Rough-size the members. From span tables, your own past jobs or, best, a preliminary call with the structural engineer. AISC notes that once bay size and loads are set, an engineer or fabricator can readily estimate preliminary beam and column sizes.
- Weigh the bay. Members x length x weight per foot, plus a connection and plate allowance from your history.
- Convert to lb/sf and multiply by floor area, keeping roof, typical floors and any transfer levels separate.
- Add the lateral system (braced frames or moment frames) as its own line. Moment frames in high-wind or seismic regions can add substantially to the gravity frame weight.
- List the extras: stairs, canopies, screen walls, RTU framing, lintels, embeds, as itemized allowances.
A 30 x 30 ft office bay with composite deck, beams at 10 ft on center.
The 8% connection allowance is an example only. Use the figure your completed jobs support, and use a different figure for braced-frame and moment-frame bays.
Method 2: ratios from your own jobs
Pounds (or tons) per square foot from completed jobs of similar type, height, bay size and lateral system is the fastest cross-check. AISC's estimating teaching module reproduces an older RSMeans table showing how widely those ratios move: roughly 8, 13 and 18 lb/sf for one-story steel-framed manufacturing buildings at 20 x 20, 30 x 30 and 40 x 40 ft bays, and roughly 10 lb/sf for low-rise office rising to 26 lb/sf for 20 to 30 stories. Those figures are illustrative and decades old; the lesson is the spread, not the numbers.
| Ratio | Good for | Weak spot |
|---|---|---|
| lb/sf by building type | Budget cross-check | Hides span, load and lateral differences |
| Pieces per ton | Labor sanity check | Varies with framing layout and infill spacing |
| Connection allowance % | Early material allowance | Moment and braced frames vary widely |
| $/ton installed | Order of magnitude | Mixes material, shop, field and purchased items with today's market |
What moves a conceptual number most
- Bay size and span direction. Longer spans mean heavier members but fewer pieces and connections. AISC notes this tradeoff can lower total frame cost even as tonnage rises.
- Lateral system. Braced frames are generally lighter and cheaper than moment frames; seismic detailing adds cost beyond weight.
- Floor system and vibration criteria. Labs, hospitals and long-span floors are often sized for vibration, not strength.
- Transfer levels and irregular geometry. Carry them as separate items, not blended into lb/sf.
- Market. Mill price, service-center premiums, shop backlog and erector availability. Date every conceptual number.
Rules for using rules of thumb
- Use ratios as checks. A number far from your history needs an explanation, not a fudge.
- Write every assumption down: bay sizes, loads, floor and lateral system, design criteria, exclusions.
- Carry contingency as a visible line. Do not hide it in rates.
- Price delegated connection design and engineering separately if the delivery method may assign it to you.
- Re-run the typical bay when the engineer's preliminary sizes arrive.
When to switch to a takeoff
Once framing plans show member sizes for most of the building, the conceptual method is no longer the right tool. Even a quick member list is more accurate and far easier to defend. This is where teams lose days, because the takeoff starts from zero under a design-development deadline. RenderDraw's AI takeoff is built for that moment: it proposes the member list from framing plans with source sheets and flags so your estimator reviews rather than starts from scratch. Try it on a schematic or DD set with a free sample takeoff.
Document the basis
Every conceptual number should carry its basis: the drawings or narrative used and their dates, assumed bays and loads, which completed jobs each ratio came from, the AACE class you believe it represents, contingency and exclusions. When the design progresses, compare the takeoff with the conceptual estimate and record the difference by system. That comparison is how your ratios improve. For a quick weight check from a member list, use the steel tonnage calculator; for the full estimate, see the structural steel estimating guide.
