At budget stage you usually have a floor area, a story count and maybe a column grid. Four published rules get you a defensible steel number from that: beam depth runs about half an inch per foot of span, a beam's weight in lb/ft is roughly 3.5 times its moment in foot-kips divided by its depth in inches (50 ksi steel), a braced steel frame weighs about (stories ÷ 3) + 7 pounds per square foot, and UK frames run 30 to 90 kg/m² depending on height and span. The same rules are just as useful after a takeoff, as a check that the total is in the right place.
This post is the companion to our steel per square foot rules of thumb, which covers psf ranges by building type and bay size. Here the focus is the published formulas behind those numbers and using them to check a finished takeoff.
These are estimating and budgeting references, not design values. Nothing here sizes a member. Loads, spans, deflection, vibration, fire and connections are the engineer of record's job. Use these rules to bracket a budget and to catch errors in a quantity, never to pick a section.
Rule 1: Structural depth from span
The most-cited source is "Rules of Thumb for Steel Design" by Socrates Ioannides and John Ruddy, published in Modern Steel Construction, February 2000. Their span-to-depth ratios (L/d):
| System | Span / depth (L/d) | Span range |
|---|---|---|
| Steel beam | 20 to 28 | 0 to 75 ft |
| Steel joist, floor | 20 | 8 to 144 ft |
| Steel joist, roof | 24 | 8 to 144 ft |
| Plate girder | 15 | 40 to 100 ft |
| Joist girder | 12 | 20 to 100 ft |
| Steel truss | 12 | 40 to 300 ft |
| Space frame | 12 to 20 | 80 to 300 ft |
The authors also give a shorthand with span L in feet and depth in inches: roof beams and joists about 0.5 × L, floor beams and joists about 0.6 × L, composite beams about 0.55 × L. Applied to common spans:
| Span (ft) | Roof beam or joist (in) | Composite floor beam (in) | Non-composite floor beam (in) |
|---|---|---|---|
| 20 | 10 | 11 | 12 |
| 25 | 12.5 | 13.8 | 15 |
| 30 | 15 | 16.5 | 18 |
| 40 | 20 | 22 | 24 |
| 50 | 25 | 27.5 | 30 |
What an estimator does with this: it tells you which W-shape depth family to expect. A 30 ft composite floor beam budgets as a W16 or W18. If the drawings arrive and every 30 ft beam is a W10, either the spans are not what you think or you are reading the wrong callouts. Our wide flange beam sizes guide lists the weights in each depth family.
Rule 2: Beam weight from moment
The same article gives a quick weight estimate for wide-flange beams. With M the bending moment in foot-kips and D the beam depth in inches:
- Fy = 36 ksi: weight (lb/ft) ≈ 5 × M ÷ D
- Fy = 50 ksi: weight (lb/ft) ≈ 3.5 × M ÷ D
Their worked example: a 30 ft beam carrying a 10 ft width of floor at 140 psf total load has a moment of 157.5 ft-kips. For an 18 in deep beam in 50 ksi steel, 3.5 × 157.5 ÷ 18 = 30.6 lb/ft, and the next economy section up is a W18x35. For composite floors the authors suggest using 70 to 75% of the simple-span moment.
For budgeting, this turns "a 30 by 30 bay of office floor" into a beam weight per foot you can multiply out, and it gives you a way to tell whether a beam size on a drawing is plausible for its span.
Rule 3: Steel weight per square foot from building height
For braced buildings in 50 ksi steel, Ioannides and Ruddy give:
Steel weight (psf) ≈ stories ÷ 3 + 7
| Stories | Approx. steel weight (psf) |
|---|---|
| 1 | 7.3 |
| 2 | 7.7 |
| 3 | 8.0 |
| 5 | 8.7 |
| 10 | 10.3 |
| 15 | 12.0 |
| 20 | 13.7 |
| 30 | 17.0 |
The authors call this an oversimplification, and it is. It covers braced frames only; moment frames, long spans, transfer girders and crane loads all push the number up. Its value is as a baseline. If your takeoff for a three-story braced office comes out at 14 psf, the formula is telling you to look for the reason before you send the number.
Rule 4: Published UK frame weights
For UK work, the SCI and BCSA publish typical frame weights on steelconstruction.info, per square metre of gross internal floor area and including fittings. Converted at 1 kg/m² = 0.2048 psf:
| Building type (steelconstruction.info) | kg/m² | Approx. psf |
|---|---|---|
| Low eaves industrial shed (6 to 8 m eaves) | 30 to 40 | 6.1 to 8.2 |
| High eaves industrial (10 to 13 m eaves) | 40 to 50 | 8.2 to 10.2 |
| Low-rise, regular short spans | 50 to 60 | 10.2 to 12.3 |
| High-rise city-centre office (10 to 15 storeys) | 75 to 90 | 15.4 to 18.4 |
Two things to notice. These figures include fittings (connection material), so compare them with a takeoff total that includes connections, not a bare member list. And the UK figures run higher than the US formula for similar heights. Part of that is fittings, part is the long-span grids common in UK city offices. Know which basis a figure is on before you compare against it.
Sanity-checking a takeoff total
Once you have a real takeoff, turn the rules around and use them as checks. None of these tells you the right answer. Each one tells you when to go and look.
- Implied psf. Divide total pounds by gross floor area. Compare against Rule 3 (or Rule 4 for UK work) and the building-type ranges in our psf guide. A big gap in either direction needs a reason you can name, such as long spans, a transfer level or a moment frame.
- Depth check. Pick a typical bay and compare the beam depths on the drawings with Rule 1. Beams far shallower than the span suggests often mean you have picked up a secondary member or a detail callout as a main beam.
- Count check. Columns should roughly equal grid intersections times column tiers, and beams should roughly track bays times beams per bay. Big mismatches usually mean a sheet was counted twice (plan and enlarged plan both taken off) or a sheet was missed.
- Length check. Count times length should add up. A line with a count but no length, or every length set to a default, gives you a tonnage that is mostly guesswork. Our list of common steel takeoff mistakes covers this one in detail.
- Units check. On metric drawings priced in imperial, recompute one line by hand both ways. Mixing kg/m with lb/ft does not look wrong at a glance.
Worked example
A three-story braced office, 20,000 sf per floor, 60,000 sf gross.
- Rule 3 baseline: 3 ÷ 3 + 7 = 8 psf. 8 × 60,000 = 480,000 lb, or 240 tons.
- Takeoff comes back at 310 tons, which is 10.3 psf and 29% above the baseline. That is not automatically wrong. Look for 40 ft bays, a moment frame, a transfer girder, or duplicated sheets before you accept it.
- If the takeoff had come back at 150 tons (5 psf), start with missing sheets, especially columns, bracing and roof framing.
Then price the tonnage using current cost data, such as our structural steel cost per ton guide, and present a range rather than a single number. For taller buildings, where columns and the lateral system grow with height, see multi-story steel frame estimating.
Moving from a budget number to a real quantity is where the time goes. Steelflo reads the member callouts off your PDF set and boxes each one for review, so you get a count you can check against the rules above on the same day the drawings arrive.
Frequently Asked Questions
What is the rule of thumb for steel beam depth?
A common published rule is about half an inch of depth per foot of span (L/24), from Ioannides and Ruddy in Modern Steel Construction (2000). Floor beams run a bit deeper, around 0.6 inches per foot. It is a budgeting guide only; the engineer sizes the actual beam.
How many pounds of steel per square foot does a braced building have?
A published approximation for braced buildings in 50 ksi steel is stories ÷ 3 + 7 psf. A three-story building comes out around 8 psf and a 27-story building around 16 psf. Long spans, moment frames and heavy loads push the real number higher.
How do I check if my steel takeoff total is reasonable?
Divide the total weight by the gross floor area and compare the result with published psf figures for the building type and height. Then check beam depths against span, column counts against the grid, and that every line has a real length. A large gap is a reason to investigate, not proof of an error.
Can I use these rules of thumb to size steel members?
No. They are for estimating, budgeting and checking quantities. Member sizes depend on loads, deflection, vibration, connections and code requirements that only the engineer of record can account for.
Do published psf figures include connections?
It depends on the source. The UK figures on steelconstruction.info state that they include fittings. Many other published ranges do not say, so confirm the basis before comparing them with your takeoff.