Load Combination Calculator
Plan your structural engineering project with our free load combination calculator. Get precise measurements, material lists, and budgets.
Formula
LRFD: 1.2D + 1.6L + 0.5(Lr or S) | ASD: D + L
ASCE 7 defines load combinations that account for the probability of multiple loads acting simultaneously. LRFD combinations apply load factors greater than 1.0 to amplify loads, while ASD combinations use service-level loads with reduced factors for coincident loading. The governing combination is the one that produces the highest demand on the structural element.
Worked Examples
Example 1: Typical Office Building Floor Beam
Problem: Dead load = 20 psf, Live load = 50 psf, Roof live = 20 psf, Snow = 30 psf, Wind = 25 psf, Seismic = 15 psf. Find the governing LRFD combination.
Solution: Combo 1: 1.4(20) = 28.0\nCombo 2: 1.2(20) + 1.6(50) + 0.5(30) = 119.0\nCombo 3: 1.2(20) + 1.6(30) + 50 = 122.0\nCombo 4: 1.2(20) + 25 + 50 + 15 = 114.0\nCombo 5: 1.2(20) + 15 + 50 + 6 = 95.0
Result: Combo 3 governs at 122.0 psf
Example 2: Uplift Check on Roof Connection
Problem: Same loads. Check if net uplift occurs with 0.9D + 1.0W.
Solution: Combo 6: 0.9(20) + 1.0(25) = 43.0 psf (downward)\nCombo 7: 0.9(20) + 1.0(15) = 33.0 psf\nNo net uplift in this case.
Result: No uplift — minimum gravity combination = 33.0 psf
Frequently Asked Questions
What are load combinations and why are they important?
Load combinations represent the simultaneous application of different types of loads that a structure might experience during its lifetime. Building codes require engineers to check multiple combinations because not all loads occur at their maximum values at the same time. For example, maximum live load and maximum wind load are unlikely to happen simultaneously. Load combinations use factors that account for the probability of coincident loading, ensuring the structure is designed for the most critical realistic scenario without being excessively conservative.
What is the difference between LRFD and ASD load combinations?
LRFD (Load and Resistance Factor Design) applies higher load factors to service-level loads and uses resistance factors less than 1.0 on the capacity side. The governing LRFD combination is typically 1.2D + 1.6L. ASD (Allowable Stress Design) uses service-level loads directly (or with modest factors like 0.75) and divides the nominal capacity by a safety factor. Both methods achieve roughly the same level of safety, but LRFD better accounts for the different uncertainties in dead vs live loads. LRFD is the preferred method in modern codes.
How do I determine which load combination governs?
The governing load combination is the one that produces the largest demand on the structural element being designed. You must evaluate all applicable combinations and identify the maximum. For gravity-only members like floor beams, combination 2 (1.2D + 1.6L in LRFD) often governs. For columns with lateral loads, combinations involving wind or seismic may control. For foundations and connections subject to uplift, the minimum gravity combinations (0.9D + W) may be critical. Always check all combinations since the controlling one varies by member and load direction.
How do I interpret the result?
Results are displayed with a label and unit to help you understand the output. Many calculators include a short explanation or classification below the result (for example, a BMI category or risk level). Refer to the worked examples section on this page for real-world context.
Does Load Combination Calculator work offline?
Once the page is loaded, the calculation logic runs entirely in your browser. If you have already opened the page, most calculators will continue to work even if your internet connection is lost, since no server requests are needed for computation.
How accurate are the results from Load Combination Calculator?
All calculations use established mathematical formulas and are performed with high-precision arithmetic. Results are accurate to the precision shown. For critical decisions in finance, medicine, or engineering, always verify results with a qualified professional.