2021 IBC Chapter 16
Chapter 16 tells the structural engineer what forces a building must resist: its own weight, the people and contents in it, and the weather. A reviewer or inspector does not redo that math. The job is to read the design loads on the drawings, confirm they fit the use and risk category, and see that what gets built gives every load a continuous path to the ground. This lesson maps Chapter 16, gives the numbers you will meet most often, and covers Gulf Coast wind.
Section 1603 requires the construction documents to show the design loads and other structural information (1603.1). Section 1604 holds the general rules: a complete load path (1604.4), risk category (1604.5), and anchorage (1604.8). Load combinations are 1605, dead loads 1606, live loads 1607, snow 1608, wind 1609, soil loads 1610, rain 1611, flood 1612, and earthquake 1613. The IBC leans on the referenced standard ASCE/SEI 7-16 with Supplement 1 (Chapter 35) for the load math, so a design is only as current as the edition the jurisdiction has adopted.
Table 1604.5 sorts buildings into four risk categories by the hazard to people if they fail. The category drives the wind map, the opening protection, and other factors.
| Category | Examples from Table 1604.5 |
|---|---|
| I | Low hazard to human life: agricultural facilities, minor storage |
| II | Everything not in I, III, or IV: most offices, shops, warehouses, apartments |
| III | Public assembly over 300; Group E or I-4 over 250; I-3; any occupancy over 5,000 |
| IV | Essential facilities: fire, rescue, and police stations, designated shelters, emergency operations centers |
Where a building holds more than one occupancy, it takes the highest risk category, and a portion that shares egress or life safety components with a higher portion takes the higher category (1604.5.1). A church building whose primary occupancy is a 350-person sanctuary is Category III.
Dead load is the weight of the permanent construction (1606). Live loads are the people, furniture, and stored contents, and Table 1607.1 gives minimum uniform values in pounds per square foot (psf). Balconies and decks are designed for 1.5 times the live load of the area they serve, not required to exceed 100 psf.
| Use (Table 1607.1) | Uniform live load |
|---|---|
| Offices | 50 psf (2,000 lb concentrated) |
| Office lobbies and first-floor corridors | 100 psf |
| Classrooms | 40 psf |
| Assembly, fixed seats / movable seats | 60 psf / 100 psf |
| Retail, first floor / upper floors | 100 psf / 75 psf |
| Light storage and warehouses | 125 psf |
| Stairs and exits | 100 psf |
| Ordinary roofs | 20 psf |
Section 1607.5 adds a partition allowance of at least 15 psf in offices and other buildings where partitions can move, unless the specified live load is 80 psf or more.
Worked example: an office bay measures 24 feet by 30 feet, or 720 square feet. At 50 psf the live load is 720 times 50, or 36,000 pounds. If a tenant turns the bay into light storage at 125 psf, the live load becomes 720 times 125, or 90,000 pounds, two and a half times as much. That is the reason a change of use is a structural question, not only a code-classification one.
Section 1605.1 requires design to the ASCE 7 strength and allowable stress combinations, which add dead, live, wind, snow, and other loads with factors that are the engineer’s work. Snow follows ASCE 7 Chapter 7, and the design roof load may not be less than what 1607 requires (1608.1). Earthquake effects follow ASCE 7 (1613.1), with exceptions such as detached one- and two-family dwellings in low seismic design categories. Read these three as pointers and check the design criteria on the drawings.
Section 1609.1.1 requires wind loads on every building to be determined under ASCE 7 Chapters 26 to 30. The basic design wind speed V, the exposure category, and the opening protection are set under Section 1609 or ASCE 7. Wind is assumed to blow from any horizontal direction, and pressures act normal to surfaces, so a windward wall is pushed in, a leeward wall and much of the roof are pulled outward, and a roof can be lifted. The speed maps are Figures 1609.3(1) through 1609.3(12), and each risk category has its own set: Category II uses Figures 1609.3(1), (5), and (6), Category III uses (2), (7), and (8), Category IV uses (3), (9), and (10), and Category I uses (4), (11), and (12).
The Section 202 definition of hurricane-prone regions includes the Atlantic and Gulf coasts where V for Risk Category II buildings is greater than 115 mph. A windborne debris region is an area within a hurricane-prone region that is either within 1 mile of the mean high-water line where Exposure D exists upwind at the waterline and V is 130 mph or greater, or where V is 140 mph or greater. In that region glazing must be impact resistant or covered (1609.2). Openings within 30 feet of grade must meet the ASTM E1996 large missile test, and openings more than 30 feet above grade the small missile test. Exceptions include 7/16-inch wood structural panels on small Group R-3 and R-4 buildings, unoccupied Risk Category I greenhouses, and glazing over 60 feet above the ground and over 30 feet above certain roofs within 1,500 feet. Which wind speed applies to a particular site comes from the map or ASCE 7 and the local amendments. Texas Gulf Coast projects can also face Texas Department of Insurance windstorm requirements for coverage, so ask the jurisdiction and TDI which edition and rules apply.
Section 1604.4 requires a complete load path that transfers loads from their point of origin to the load-resisting elements. Section 1604.8.1 requires anchorage of the roof to the walls and columns, and of walls and columns to the foundations, to resist uplift and sliding, and 1604.8.2 anchors structural walls to the roof and to all floors and members that provide lateral support for the wall or are supported by it. In the field that means checking that the straps, clips, anchor bolts, and hold-downs on the structural drawings are actually installed, at the spacing and with the fasteners shown. A missing connector is a break in the load path.
In the field
A landlord wants to store paper records in a second-floor suite that was designed as offices. Before saying yes, look at the structural notes for the design floor live load, find out what the boxes and shelving will actually weigh per square foot, and ask the engineer of record to review it. The same discipline applies to a Gulf Coast reroof: the clips, straps, and fastener spacing on the drawings are the difference between a roof that stays and one that lifts.
1. Under Table 1604.5, into which risk category does an ordinary Group B office building with an occupant load of 120 fall?
2. A Group A-3 community hall is the primary occupancy of a building and has an occupant load of 350. What risk category does Table 1604.5 assign?
3. A new fire station with emergency vehicle bays is being designed. What risk category does Table 1604.5 assign?
4. A building holds a Risk Category II office suite and a structurally separated 400-person Group A-3 hall, classified Risk Category III, that share the same exit stairs. Under 1604.5.1, what category applies to the portions that share the egress?
5. What minimum uniform live load does Table 1607.1 give for the floor of an office area in a Group B building?
6. What minimum uniform live load does Table 1607.1 give for a first-floor corridor in a Group B office building?
7. What minimum uniform live load does Table 1607.1 give for an ordinary roof that is not used for assembly or occupancy?
8. A Group B second-floor bay of 400 square feet is converted from offices at 50 psf to light storage at 125 psf. How much greater is the total live load on the bay, in pounds?
9. During plan review of a change of use in a Group B building, an inspector notes the tenant will store heavy files where the design live load was for offices. Which action fits Chapter 16?
10. A Group B office building has a specified floor live load of 50 psf. Under 1607.5, what does the code require regarding partitions?
11. Which referenced standard does the 2021 IBC list in Chapter 35 for the design loads used with Chapter 16?
12. A site within 1 mile of the mean high-water line has Exposure D upwind at the waterline, and the basic design wind speed V is 135 mph. Is it in a windborne debris region, given it is also in a hurricane-prone region?
13. Glazing on the first floor of a Group M store in a windborne debris region is about 10 feet above grade. Under 1609.2, what missile test level applies?
14. At a framing inspection of a Group M store, wall-to-foundation anchor bolts shown on the structural drawings are missing along one wall. What is the most direct concern?