Loads and load combinations

Why it matters

Every structural calculation starts with one question: what loads must this member carry, and in what combinations? Get the loads wrong and the most careful steel or concrete check is worthless. Load work is also where junior engineers spend much of their first years, so being fast and organised here is a real career skill.

Load types

US building design takes its minimum loads from ASCE/SEI 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures (the current edition is ASCE 7-22), adopted through the International Building Code and local amendments. The common load symbols are:

• D, dead load: the weight of the structure and permanent parts (slabs, beams, finishes, fixed equipment).
• L, live load: occupants, furniture and movable contents, set by occupancy (office, assembly, storage...). Lr is roof live load (workers and materials during maintenance).
• S, snow; R, rain; W, wind; E, earthquake; plus soil pressure, fluid pressure, flood, ice and others.

Minimum live, snow, wind and seismic values come from ASCE 7's chapters and maps, and many jurisdictions modify them. Look them up in the adopted edition (ASCE's hazard tool is at https://ascehazardtool.org); never copy numbers from a course or an old project. Checked on 2026-10-23.

From area load to line load

Floor and roof loads are given per unit area (kPa or psf). A beam carries the area halfway to the next beam on each side: its tributary width.

line load (kN/m) = area load (kPa) * tributary width (m)

Illustrative example. Beams at 3.0 m spacing carry a dead load of 4.0 kPa (slab, finishes, ceiling, services) and an office-type live load of 2.4 kPa. Then D = 4.0 × 3.0 = 12.0 kN/m and L = 2.4 × 3.0 = 7.2 kN/m. These area loads are illustrative; real ones come from a weight takeoff and the code.

Load combinations

Loads rarely peak at the same time, and each has a different uncertainty. Strength design (LRFD in steel, strength design in concrete) multiplies each load by a load factor and checks every combination in the code. The two you will meet first, written in the ASCE 7 style, are

1.4D
1.2D + 1.6L + 0.5(Lr or S or R)

Others combine roof, snow, wind and seismic loads, and one family uses 0.9D with wind or seismic to check uplift and overturning, where dead load helps. Allowable stress design (ASD) uses separate, unfactored-style combinations with the member's capacity divided by a safety factor. Take the full, current set from the adopted ASCE 7 edition; the factors and companion-load rules change between editions.

Worked example (continued)

For the floor beam:

• 1.4D = 1.4 × 12.0 = 16.8 kN/m
• 1.2D + 1.6L = 14.4 + 11.52 = 25.92 kN/m (no snow on an interior floor)

The second governs. For an 8.0 m simply supported span:

Mu = wu * L^2 / 8 = 25.92 * 8.0^2 / 8 = 207.4 kN*m
Vu = wu * L / 2   = 25.92 * 8.0 / 2   = 103.7 kN

These are the required strengths you compare with the member's design strength in the next two lessons.

Uplift: the other direction

On a light roof, wind can pull upward. With D = 3.0 kN/m and a wind uplift of W = -8.0 kN/m, the combination 0.9D + 1.0W = 2.7 - 8.0 = -5.3 kN/m: the beam and its connections must resist a net upward load, and the bottom flange may now be in compression. Always look for the minimum as well as the maximum.

Doing it in code

Spreadsheets and Python handle combinations well: store loads by type, store each combination as a dictionary of factors, compute every combination, and report the maximum and minimum along with its name. Keeping the name matters, because reviewers need to see which combination governed. Commercial analysis software does the same for frames with many load cases.

Common mistakes

• Using the wrong tributary width (the full spacing on an edge beam, or forgetting a cantilever).
• Mixing service and factored loads, for example checking deflection with factored loads.
• Stopping at 1.2D + 1.6L and missing wind, snow drift or uplift combinations.
• Old or unadopted editions: always confirm which ASCE 7 edition and local amendments apply.
• Unit slips between kN, kip, kPa and psf (1 kPa is about 20.9 psf).

Professional responsibility

This lesson teaches the method. Load determination for a real structure depends on code edition, site hazards, occupancy and engineering judgement. Real designs need review and sealing by a licensed professional engineer (PE) under the codes your jurisdiction adopts.

Recap

• Loads come from ASCE 7 (as adopted locally); area load times tributary width gives line load.
• Strength design factors and combines loads; the largest result governs, and the smallest checks uplift.
• Mu = wL^2/8 and Vu = wL/2 for a simple span with uniform load.

Try it

Open the Do tab to compute line loads, find the governing combination and the design moment in Python.