The main loads on a building are dead loads (the weight of the building itself), live loads (people, furniture, storage), snow, rain, wind and earthquake (seismic) loads. In the US, the building code (the IBC) points to ASCE 7 for how big each load is, and the engineer combines them to size the members.
As a steel detailer I don’t size beams; that’s the engineer’s job. But knowing where the loads on a building come from makes the engineer’s drawings a lot easier to read, and it helps you spot when something on a drawing looks off. This post covers the basics: how buildings are classified by risk, which codes set the loads, and how a floor load turns into the load on a single beam through tributary area.
Note: this is background for understanding drawings, not design guidance. Always check your local code and have a licensed engineer do the actual design.
What are the main types of loads on buildings?
| Load | What it is | ASCE 7 chapter |
|---|---|---|
| Dead | Weight of the structure and permanent parts | 3 |
| Live | People, furniture, equipment, storage | 4 |
| Snow | Snow on the roof, including drifts | 7 |
| Rain | Water ponding on the roof | 8 |
| Wind | Wind pressure on walls and roof | 26 to 31 |
| Seismic | Earthquake forces | 11 to 23 |
Chapter numbers are from ASCE 7-22. ASCE 7 is updated every few years and your local code may adopt an older edition, so check which one applies.
How are buildings classified by risk?
Buildings are put into risk categories from I to IV, based on how bad it would be if the building failed. Category I is something like a storage shed or a barn, where a failure wouldn’t put anyone in serious danger. Category IV is hospitals, fire stations and emergency centers: buildings that have to stay standing and keep working during a disaster.
A building goes into the highest category that fits how it’s used. The higher the category, the higher the design loads the engineer has to use.
Risk categories (ASCE 7, Table 1.5-1)
| Risk category | Typical buildings |
|---|---|
| I: Low risk | Farm buildings, temporary structures, minor storage. Little or no risk to people if they fail. |
| II: Standard | Most ordinary buildings: houses, offices, small shops. |
| III: Substantial risk | Buildings where more than 300 people gather in one area; daycares with more than 150 children; schools with more than 250 students, or colleges with more than 500; health care facilities with 50 or more residents; jails; utilities like power stations, water treatment plants and major communication hubs. |
| IV: Essential facilities | Hospitals, police and fire stations, ambulance facilities, emergency shelters and other buildings needed for disaster response. |
Which codes set design loads?
Building codes are the rules cities and states use to make sure buildings are designed and built safely. Before 2000, the US had three regional model codes:
- The BOCA National Building Code, used in the Northeast and Midwest
- The Uniform Building Code, used in the West
- The Standard Building Code, used mostly in the South
Having three different systems made life hard for anyone who worked in more than one region. In 1994 the International Code Council (ICC) was formed to combine them into one national model code, the International Building Code (IBC). The first IBC came out in 2000, and a new edition has come out every three years since. Today almost every state and city has adopted some edition of the IBC, often with local amendments. Chicago is a good example: it has its own building code. I cover that in Making a legal steel stair in Chicago.
Where do the load values come from?
For how much weight, wind, snow or earthquake force a building has to resist, the IBC points to ASCE 7, Minimum Design Loads for Buildings and Other Structures, published by the American Society of Civil Engineers. That’s the engineer’s reference for the loads used in design. The examples below follow ASCE 7-10. Newer editions exist, and the one that applies is whatever your local code adopted.
What is tributary area?
ASCE 7 mostly gives loads per unit area, in pounds per square foot (psf). But a beam doesn’t carry one square foot. It carries a strip of floor. To get the load on a beam, you multiply the unit load by the beam’s tributary area, the part of the floor that “drains” onto it.
The code also gives some concentrated loads. For those, what matters is where they’re placed, since you want the position that causes the worst effect.
In the parallel framing in Figure 1.1, beam CD picks up floor load halfway to the next beam on each side. Its tributary area is B × L, so the total load is:
W = w × B × L (w = unit load in psf)
An edge beam like AB only has floor on one side, so its tributary area is ½B × L.
An easy way to think about it: take a strip of floor 1 ft wide running across the beam. The load from that strip is w × B, and that’s the uniform load per foot along the beam.
Girders
Girders get point loads from the ends of the beams they support. If the beams are close together, you can treat the girder as carrying a uniform load from its tributary area instead.
A triangular framing layout
In Figure 1.2, beam AB carries two pieces of floor:
- A rectangle, A-B-2-1, with an area of BL/2
- A triangle, A-B-3, with an area of ½ × BL/2 = BL/4
So AB sees a uniform load plus a triangular load, as in Figure 1.3.
Beam AC carries the triangle A-C-3, which is also wBL/4. But AC runs on the diagonal, so its length is L₁ = √(L² + B²) instead of L, and that changes how the load is drawn along it (Figure 1.4).
Why the framing layout matters
How the floor is framed decides each member’s tributary area, and so its load. Change the layout and the loads change with it. That’s why engineers settle the framing scheme early.
How does a floor load become a beam load? (example)
Using the layout in Figure 1.2:
- Span L = 30 ft
- Beam spacing B = 10 ft
- Floor load w = 60 psf
For beam AB:
- Rectangular part: a 1-ft strip gives 1 × 5 = 5 sq ft per foot of beam, so the uniform load is 60 × 5 = 300 lb/ft.
- Triangular part: area = ½ × 5 × 30 = 75 sq ft, so the total load = 60 × 75 = 4,500 lb.
- The triangle goes from zero to a peak over the 30-ft span: ½ × w(peak) × 30 = 4,500, so w(peak) = 300 lb/ft.
So beam AB carries a uniform 300 lb/ft plus a triangular load that peaks at 300 lb/ft.
Once you see how the floor “collects” load and passes it down to the beams, girders and columns, the load notes on structural drawings make a lot more sense.
If you want to see how these loads turn into real steel, read what Advance Steel is and how detailers use it and my complete guide to stairs in Advance Steel.
