Steel Joist Load Capacity Chart and K-Series Span Tables Explained
Steel joist load capacity is determined using standardized load tables and charts that define the maximum uniform load a joist can safely support based on its span, series, and size.
These tables—such as the Steel Joist Institute (SJI) K-Series, LH-Series, and DLH-Series—provide essential data for engineers, builders, and contractors when selecting the correct joist for structural design.
Whether you’re calculating per-foot load ratings, checking span limits, or reviewing joist dimensions and weights, understanding the load capacity of steel joists is crucial for ensuring safety, strength, and code compliance in construction projects.
Key Takeaways
- Steel joist load capacity is measured in pounds per linear foot (plf) or pounds per square foot (psf).
- Capacity depends on joist size, span, spacing, and load type.
- K-Series load tables simplify design for standard uniform loads.
- Always check bending, shear, and deflection limits per SJI and IBC codes.
- For complex loads, use an engineer’s analysis or tools like a Steel Beam Calculator or Steel Deck Calculator.
Quick Reference Table: Typical Steel Joist Spans and Load Capacities
| Joist Type | Depth (in) | Typical Span (ft) | Max Uniform Load (plf) | Common Use |
|---|---|---|---|---|
| 8K1 | 8 | 10–20 | 450 | Small roof areas |
| 12K3 | 12 | 20–30 | 800 | Light commercial floors |
| 18K5 | 18 | 30–40 | 1200 | Roofs with moderate loads |
| 24K7 | 24 | 40–50 | 1600 | Long-span roof or mezzanine |
| 30LH03 | 30 | 50–70 | 2000 | Heavy floor or roof loads |
(Values shown are simplified examples; always check manufacturer load tables for actual ratings.)
What Determines Steel Joist Load Capacity
Steel joist load capacity is based on span, spacing, and the type of load applied. Each joist carries a share of the total building weight—its own dead load plus the live load from occupants, snow, or equipment.
Steel joists are designed to carry both dead loads (permanent structural weight) and live loads (temporary or variable loads like people, furniture, or snow). Load capacity is usually expressed in pounds per linear foot (plf) or pounds per square foot (psf) and depends on several key factors:
Many people think steel joists can handle “anything.” They can’t. The key is choosing a joist that matches both your load requirements and deflection limits.
If your floor feels springy or your ceiling sags slightly, the joist may be too light or the spacing too wide. That’s why we rely on Steel Joist Institute (SJI) load tables—they list safe uniform loads for standard joist sizes.
Main Factors
- Joist size and gauge: Larger, thicker sections handle more weight.
- Span: Longer spans need stronger joists or closer spacing.
- Spacing: Narrow spacing spreads loads evenly.
- Load type: Uniform vs concentrated.
- Deflection limit: How much the joist bends under load (per IBC).
When I first ran a design for a small warehouse, I underestimated how much deflection mattered. The joists were safe—but the ceiling tiles cracked. Lesson learned: safety isn’t just strength, it’s stiffness.
Understanding Steel Joist Load Tables
Steel joist load tables provide the simplest way to determine how much weight a joist can support. Tables usually list the maximum uniform load per linear foot for each joist size and span combination.
K-Series Joists
These are the most common open-web joists used in roofs and floors. They’re light, efficient, and follow standard depths (10 – 30 inches).
| K-Series Joist | Depth (in) | Typical Span (ft) | Total Load (plf) | Live Load (plf) | Dead Load (plf) |
|---|---|---|---|---|---|
| 10K1 | 10 | 15–25 | 600 | 400 | 200 |
| 16K3 | 16 | 25–35 | 1000 | 700 | 300 |
| 20K5 | 20 | 35–45 | 1400 | 900 | 500 |
| 24K7 | 24 | 40–50 | 1600 | 1000 | 600 |
Interpolation: If your exact span isn’t shown, find the two closest spans and linearly interpolate between load values.
Example: For a 22-ft span between the 20 ft and 24 ft columns, average the two load values.
These tables assume uniform gravity loads only. If you’ve got hanging equipment or a rooftop unit, that’s a different story.
LH-Series Joists
Long-span (LH) joists are used when spans exceed 50 ft. They’re heavier, deeper, and ideal for gymnasiums or warehouses.
| LH-Series Joist | Depth (in) | Typical Span (ft) | Max Load (plf) | Use |
|---|---|---|---|---|
| 24LH03 | 24 | 40–60 | 1800 | Roofs with a moderate snow load |
| 36LH06 | 36 | 60–80 | 2200 | Long-span roofs |
| 40LH10 | 40 | 70–90 | 2600 | Heavy industrial roofs |
| 48LH12 | 48 | 80–100 | 3200 | Large open structures |
Tip: Check both strength and deflection in the table columns. Even if a joist can carry the load, excessive deflection can cause floor vibration or wall cracking.
How to Use Steel Joist Load Tables
When you open a Vulcraft or SJI load table, it looks intimidating at first. But once you know what to look for, it’s quite simple.
Step-by-Step
- Identify your span (distance between supports).
- Find the series (K, LH, DLH) that covers your span range.
- Select your total load (dead + live load in plf).
- Cross-check with deflection limits and code requirements.
- If your span isn’t listed, use linear interpolation to estimate.
Let’s say you’re designing a 24 ft span with a total load of 1,200 plf. You’d look at the 20K5 or 24K7 joist series and confirm which one meets your load with acceptable deflection.
💡 Pro tip: Always check both strength (load capacity) and stiffness (deflection). A strong but flexible joist can still fail visually or cause discomfort.
Engineering Analysis for Non-Uniform Loads
Now, not every load is evenly distributed. Think about rooftop HVAC units, hung partitions, or pipe racks—these create concentrated loads.
Here’s where the simple load tables stop working. An engineer must calculate the bending moment and shear to ensure the joist can handle that uneven stress.
When I once checked a mezzanine supporting a small crane, the joist table said “fine.” But once I plotted the actual moment diagram, the mid-span stress spiked way above the table limit. Tables are for uniform loads only.
How Engineers Handle It
- Shear and bending checks: Using standard formulas or finite-element tools.
- Load paths: Tracking how roof or floor loads travel to the foundation.
- Collateral loads: Accounting for sprinklers, ducts, or ceiling systems.
If you’re not an engineer, tools like ClearCalcs Steel Joist Calculator make this easier. It lets you input loads and spans, then automatically shows bending, shear, and deflection checks.
Still, remember—accurate input equals accurate results. Garbage in, garbage out.
Design Standards and Code Compliance
Every joist system must follow Steel Joist Institute (SJI) specifications and International Building Code (IBC) deflection limits.
These standards ensure you’re not just building something that “stands up,” but something that performs safely for decades.
Deflection Limits
| Use Type | Typical Limit | Description |
|---|---|---|
| Floor | L/360 | Prevents visible floor sag |
| Roof (no ceiling) | L/240 | Accepts more deflection |
| Roof (with ceiling) | L/360 | Prevents ceiling cracking |
Example: If a joist spans 30 ft (360 in), a floor joist with a limit of L/360 can deflect no more than 1 inch.
Building inspectors often check this. Trust me—nothing’s worse than re-bracing an installed joist because of overlooked deflection limits.
Steel Joist Weight and Dimensions
Besides load, you’ll often see joist weights listed per linear foot. Knowing this helps plan lifting, transportation, and structural dead loads.
| Joist Type | Depth (in) | Weight (lb/ft) | Span Range (ft) |
|---|---|---|---|
| 10K1 | 10 | 6.0 | 10–20 |
| 16K3 | 16 | 8.2 | 20–30 |
| 20K5 | 20 | 10.0 | 30–40 |
| 24K7 | 24 | 11.8 | 40–50 |
| 30LH03 | 30 | 14.5 | 50–70 |
If you’re ever calculating total roof dead load, don’t forget to include joist self-weight. It’s small compared to concrete, but over hundreds of feet, it adds up fast.
Real-World Example: Estimating Load Capacity
Let’s walk through an example.
Goal: Support a 40-ft roof span using standard K-Series joists. Loads:
- Live load = 800 plf
- Dead load = 300 plf
Total = 1,100 plf
Check the K-Series table—a 24K7 joist handles about 1,600 plf over 45 ft.
- Since your load (1,100 plf) is below 1,600 plf, this joist works.
- Verify deflection against L/240 (roof limit).
- Include joist weight (11.8 lb/ft) in the dead load if needed.
- Done—safe design, efficient span.
If your load was closer to 1,800 plf, you’d move up to an LH-Series joist instead.
How to Calculate Steel Joist Load Capacity
For uniform loads, load capacity can be determined directly from published tables or design software. For non-uniform or concentrated loads, a structural engineer must calculate:
- Maximum Bending Moment (M)
- Shear Forces (V)
- Bearing Loads (R)
Software such as ClearCalcs or manufacturer-provided joist calculators helps automate this process, dynamically tracking load paths from the roof to the foundation.
Example of Load Design
A 24CJ 1400/800/240 joist indicates:
- Depth: 24 inches
- Total Load: 1,400 plf
- Live Load: 800 plf
- Dead Load: 240 plf
These values come directly from SJI-compliant design tables and must align with IBC deflection limits for code compliance.
Using Calculators to Verify Design
You can cross-check your design with free or paid tools:
- 🧮 Steel Beam Calculator – Checks bending and shear for custom spans.
- 🧮 Steel Deck Calculator – Estimates deck capacity and diaphragm stiffness.
- 🧮 ClearCalcs Steel Joist Calculator – Evaluates complex load paths.
These tools are great for sanity checks or educational use. Still, they can’t replace a licensed engineer’s stamp when required by code.
When I mentor junior engineers, I tell them: “Always trust your math—but verify with a calculator.”
Benefits and Limitations of Load Tables
Pros:
- Simplifies design for standard gravity loads.
- Includes deflection information for safe structural design.
- Easy to apply using pre-calculated data.
Cons:
- Limited to uniform load cases.
- Requires interpolation for intermediate spans.
- May not apply to non-uniform or concentrated load conditions.
Additional Design Considerations
- Collateral Loads: Sprinkler systems, ducts, and suspended equipment must be included as uniform or concentrated loads.
- Multiple Joists: Loads are distributed among joists, but each must still meet its individual capacity requirements.
- Code Compliance: Always verify design against SJI and IBC standards to ensure safety and performance.
Common Mistakes When Reading Load Tables
Even experienced builders slip up. Here are frequent errors to avoid:
- Ignoring deflection: The joist may “work” structurally but fail visually.
- Mixing units: Don’t confuse psf with plf.
- Overlooking collateral loads: Ducts and sprinklers count.
- Assuming equal distribution: Concentrated loads require analysis.
- Skipping interpolation: Always calculate between given span values.
These small details often decide whether your project passes inspection or needs retrofitting.
Which Joists Are Load-Bearing?
A common question: “Are all joists load-bearing?”
No—only joists supported by beams, walls, or girders carry structural loads. Ceiling joists might simply support drywall, while floor or roof joists carry live and dead loads.
If you’re unsure, look for bearing points—ends of joists resting on steel beams or masonry walls.
A quick rule: If removing it could cause sagging, it’s load bearing.
How Much Weight Can You Hang from a Steel Joist?
For light fixtures, pipes, or fans, you can usually hang up to 50 lb per foot safely—if evenly distributed and approved by the engineer. Heavy loads like hoists or ductwork require custom brackets or extra joists.
Never drill or cut through joist chords without consulting the manufacturer. I’ve seen it happen—and it’s painful to fix.
Final Thoughts
Steel joists are efficient, strong, and versatile—but only when chosen correctly. The load capacity chart is your roadmap, not a guess.
If you take one thing away from this, it’s this: 👉 Always balance strength, deflection, and real-world use.
The charts, calculators, and tables are tools—but you, the designer or builder, make the judgment call.
When I help clients choose between joists, I remind them: “Overbuild a little for comfort, underbuild and you’ll regret it.”
Summary
Steel joist load capacity depends on span, spacing, series, and load type. K-Series and LH-Series load tables—based on SJI standards—offer quick guidance for safe design. For non-uniform loads or special cases, use engineering analysis or digital calculators. With proper planning and code checks, steel joists can safely carry heavy loads for decades.
