But how can you convert a drawing into an actual number of steel the fabricator must stick to? Estimating the structural steel consists of accounting for all of its elements and their dimensions, as well as multiplying these dimensions by the weight of each piece of steel in accordance with the AISC Manual. Everything else will be estimated additionally.
To estimate the structural steel tonnage, make a list of all the members from the drawings and find their shape and length. Then multiply the length of each member by the weight per foot in accordance with the AISC Manual. Add the plate and connection materials. Divide the total poundage by 2000 to get the tonnage. Miscellaneous metal is often estimated separately.
What Does a Structural Steel Takeoff Include?
“Steel” on a bid form can mean several different packages. Before you measure anything, sort your scope like the specification:
Construction Takeoff Services help organize these quantities by framing, connections, joists, deck, and miscellaneous metals before pricing begins.
- Structural framing (05 12 00). Columns, beams, girders, bracing, moment frames. The tons that most people talk about.
- Connection material. Shear tabs, clip angles, base plates, stiffeners, and bolts.
- Steel joists and joist girders (05 21 00). These are almost always measured on an FOB count/designation basis from the joist supplier, rather than being measured yourself.
- Steel deck (05 31 00). Priced based on area, profile, and gauge.
- Miscellaneous metals (05 50 00). Lintel, embed plate, stair, railing, ladder, and equipment support.
These should remain individual items. They tend to be quoted from different fabricators; labor costs vary; and combining them makes it difficult to see which element of the number is changing.
How Do You Read Member Sizes From the Drawings?
Wide flange members have their names telling you everything. For instance, W12x26 refers to a wide flange that is about 12 inches deep and 26 pounds per foot. The channels have a similar naming convention; therefore, C10x15.3 is 15.3 pounds per foot. However, in the case of angles and HSS, one needs to get the size of the member from the table in the AISC Steel Construction Manual.
| Designation | What the name tells you | Weight |
|---|---|---|
| W12x26 | Wide flange, about 12 in. deep | 26 lb/ft (in the name) |
| W10x33 | Wide flange, about 10 in. deep | 33 lb/ft (in the name) |
| C10x15.3 | Channel, 10 in. deep | 15.3 lb/ft (in the name) |
| L4x4x1/2 | Equal-leg angle, 4 in. legs, 1/2 in. thick | 12.8 lb/ft (from AISC tables) |
| HSS6x6x3/8 | Square tube, 6 in. sides, 3/8 in. wall | Look up in AISC tables |
| Plate | Thickness and plan dimensions | 40.8 lb/sq ft per inch of thickness |
Member sizes are spread across the set. Beams and girders come from the framing plans, columns from the column schedule, bracing from the bracing elevations, and a surprising amount of steel from sections and details that never appear in plan. Structural BIM Services can provide a coordinated model reference for checking beams, columns, bracing, and other structural elements across the complete project.
How Do You Calculate the Tonnage?
Go through the floors one-by-one, because now the takeoff can be verified against the plans and revised where there is a change of one floor:
- Build a member list. For each level, record mark, shape, count, and length.
- Measure lengths to the grid. Most estimators take beam lengths center to center of supports. That slightly overstates the actual cut length, which helps cover connection material.
- Convert to pounds. Multiply each length by its weight per foot.
- Add plates. Base plates, cap plates, and gussets are area x thickness x 40.8 lb per square foot per inch.
- Add connections. Either from a detailed takeoff of the connection details or from an allowance based on your own past jobs.
- Convert to tons. Total it all up and divide by 2,000.
Example calculation: a small building has 20 W12x26 beams at 30 feet and 12 W10x33 columns at 14 feet. The beams equal 600 ft x 26 = 15,600 pounds, and the columns equal 168 ft × 33 = 5,544 pounds. Total equals 21,144 pounds, or approximately 10.6 tons, excluding any plates and connections.
Note that the beams support nearly three-quarters of the load. This is usual for floor framing systems, and explains why the beam sizes and quantity should receive more scrutiny. A wrong column size moves the total a little; a typical bay counted on the wrong number of floors moves it a lot.
How Should Connections Be Handled?
Connections are where steel tonnage drifts most. If everything is shown in the drawings, remove them one by one – plate, angle, stiffener, and number of bolts by size. It’s painstaking, but it’s accurate.
Often the drawings don’t. On many projects, the engineer of record shows member sizes and design forces, and the connections are designed later by the fabricator’s engineer. In that case, an allowance is the only option at bid time, and it should be based on connection type. Bolted shear connections on a simple frame add far less than moment connections or heavy bracing. The final connection material isn’t known until the Structural Steel Shop Drawing Services develop the fabrication-level connection details. Which is one reason it pays to keep the allowance visible as its own line.
Either way, bolts warrant counting separately because they are not measured on a per-weight basis, but rather on their size, quality, and number. High-strength bolts used in slip-critical joints also cost substantially more to install than snug-tight bolts used in shear tabs.
Why Isn’t Tonnage Sufficient for Steel Pricing?
Per-ton pricing is a helpful tool for checking, but the cost of two projects with equal tonnage can vary greatly. Bid Estimating Services account for these differences by combining material quantities with fabrication, connection, coating, equipment, and erection costs. The things that move the number:
- Piece count. Twenty pounds of heavy columns are erected more quickly than twenty pounds of lighter beams and bracing.
- Connection type. Field-welded moment connections take far more labor than bolted shear tabs.
- Coatings. The cost of shop priming depends on the surface area, while galvanizing costs are calculated based on the weight. Fireproofing is typically a different specialty.
- Erection conditions. Cranes, building heights, and erection sequences all affect productivity.
This is why an accurate steel estimate always includes both tons and piece count, as well as connection type information. The tons give you the material to buy. The pieces give you the time to fabricate and erect.
What Gets Missed Most Often?
Most misses aren’t math errors. They’re members nobody counted:
- Steel that only appears in details. Kickers, lintels at openings, and roof frames for mechanical units often show up only in sections.
- Typical bays counted once. A typical framing plan that applies to six floors needs to be multiplied by six.
- Base and embed plates. Small individually, but they add up fast across a building.
- Old revisions. Steel changes late in design, so check the takeoff against the latest issued set and every addendum.
For a deeper look at the errors that cost steel bids the most, see our guide to structural steel estimating errors.
Need a steel estimate separated into members, levels, and connections before the bid day? Get your structural steel estimates prepared by the estimators at Optimar Precon, separated into framing, connections, joists, deck, and miscellaneous metals.
FAQs
The sum total pounds divided by 2,000 will yield the US short tons. In case the client uses metric tonnes, the sum total pounds is divided by 2,204.6.
It is a wide-flanged shape with an approximate depth of 12 inches and weighs 26 pounds per foot.
Approximately 40.8 pounds. The weight of steel is estimated to be 490 pounds per cubic foot, and hence the addition of an inch of thickness equals approximately 40.8 pounds per square foot.
Usually not. Joists are typically priced by count and designation from the supplier, and deck by area and gauge, so they’re carried as separate lines.
Take it off from the connection details if they’re drawn. If connections are delegated to the fabricator’s engineer, use an allowance based on connection type and your own past jobs, and show it as its own line.
