1. What Is Wind Load?
Wind load is the structural action produced by airflow around and through a building. Wind can create positive pressure, suction, and uplift on different surfaces at the same time.
For a typical steel building, relevant effects may include:
- Pressure and suction on windward, leeward, and side walls
- Uplift and suction on roof surfaces
- Concentrated local demand near roof edges and corners
- Lateral forces transferred through frames, bracing, and diaphragms
- Internal pressure caused by openings or enclosure classification
- Overturning, sliding, uplift, and anchor forces at the foundations
Wind design is not limited to a single horizontal force on the main frame. The global structural system and local cladding attachments may be governed by different pressure zones and tributary areas.
2. What Determines Design Wind Actions?
2.1 Site Wind Hazard
The governing standard establishes a basic or reference wind speed, pressure, or velocity parameter for the project location. Its definition can depend on return period, risk category, averaging time, height, terrain convention, and other code-specific rules.
For international projects, a wind speed should therefore never be transferred directly from one code framework into another without checking the definitions. Two values expressed in metres per second may represent different averaging periods or reliability levels and are not automatically equivalent.
2.2 Terrain, Exposure, and Topography
Open coastal terrain, suburban development, dense urban surroundings, and isolated hilltops do not produce the same wind profile.
Codes account for surrounding roughness or exposure and may also require topographic effects where hills, ridges, or escarpments accelerate the flow. The selected category must reflect the actual approach terrain in the relevant wind directions rather than a generic description of the city.
2.3 Building Height and Geometry
Wind pressure generally changes with height, while building proportions and roof geometry affect how pressure is distributed.
Relevant characteristics include:
- Mean roof height and overall building height
- Length-to-width proportions
- Roof slope and roof form
- Canopies, parapets, overhangs, and roof steps
- Large doors and wall openings
- Attached lower or higher building areas
Edges and corners often experience more severe local suction than central zones. This is particularly important for roof sheets, wall panels, fasteners, purlins, girts, and their connections.
2.4 Enclosure and Internal Pressure
Openings influence internal pressure. A building with a dominant opening, a partially enclosed façade, or planned large doors can behave differently from a fully enclosed building.
Door position alone does not determine the design assumption. The governing standard may consider opening size, distribution, permeability, and the possibility of openings occurring during the design event. The enclosure classification should therefore be confirmed before cladding and connection design is finalized.
2.5 Global and Local Wind Checks
Wind standards commonly distinguish between forces on the main wind-force-resisting system and pressures on components and cladding, although terminology varies by code.
The main system check addresses overall load transfer and stability. Local pressure checks address smaller tributary areas and zones such as roof corners, eaves, fasteners, and cladding panels. A pressure suitable for the global frame is not automatically suitable for every local component.
4. What Determines Roof Snow Actions?
4.1 Ground Snow Hazard and Site Conditions
Ground snow values depend on location, elevation, climate records, and the governing national or regional provisions. Mountainous sites may require more localized information than a broad municipal or regional value.
The ground value is not normally applied directly to the roof. It is transformed through code rules that reflect the building and design situation.
4.2 Roof Shape and Slope
Roof slope affects retention and possible sliding, while roof form influences distribution.
Gable, mono-slope, curved, multi-level, and sawtooth roofs may have different loading patterns. Even when a steeper surface sheds some snow, lower roofs or adjacent areas can receive the displaced accumulation.
4.3 Exposure and Thermal Conditions
Wind can scour snow from exposed roofs or deposit it in sheltered areas. Heated and unheated buildings may also be treated differently where the governing standard includes thermal effects.
These adjustments are code-specific. They should not be selected merely from a general statement that the building is “open,” “heated,” or “in a windy area.”
4.4 Unbalanced Snow and Drifting
Unbalanced loading can control rafters, trusses, purlins, and frame stability even when the total snow on the roof is lower than under a uniform case.
Drifts may form near:
- Parapets
- Roof steps
- Higher adjacent structures
- Projections and obstructions
- Changes in roof geometry
Because drift loads are localized, they can govern individual purlins or parts of the primary frame without governing the entire roof.
4.5 Other Roof Actions
Where relevant, the design basis may also need to consider sliding snow, rain-on-snow, ponding, ice, or blocked drainage. These are not interchangeable with ordinary roof snow load, and their treatment depends on the applicable standard and building configuration.
5. How Wind and Snow Affect the Steel Building System
Wind and snow follow different load paths and can govern different limit states.
| Element or check |
Wind influence |
Snow influence |
| Roof cladding and fasteners |
Pressure, local suction and uplift |
Downward roof load, including local accumulations |
| Wall cladding and girts |
External and internal pressure, suction and connection demand |
No ordinary direct roof-snow load; check special drift or sliding-snow interfaces only where applicable |
| Purlins and roof secondary steel |
Uplift, pressure reversal and restraint assumptions |
Uniform, unbalanced or drifted gravity loading |
| Main frames and trusses |
Lateral force, uplift, sway and stability |
Gravity force, deflection and asymmetric response |
| Bracing or engineered diaphragms |
Transfer of wind forces where included in the designed load path |
Indirect forces can arise from asymmetric roof loading; do not assume diaphragm action without a designed system |
| Anchors and foundations |
Uplift, overturning, sliding and lateral reaction |
Compression and unequal support reactions |
Wind can reverse the sign of force in members and connections. A roof connection that transfers downward gravity load may also need to resist uplift. Snow generally adds downward gravity demand, but unbalanced snow can also create frame sway, torsion, or unequal reactions.
Serviceability matters as well as strength. Excessive movement can affect cladding, doors, drainage, equipment alignment, and perceived building performance even when member resistance is adequate.
6. What Project Information Is Needed?
A reliable design basis normally identifies:
6.1 Wind Information
- Exact site location
- Governing standard and edition
- Basic or reference wind parameter and its definition
- Risk, consequence, or importance classification
- Terrain or exposure category
- Topographic conditions where relevant
- Building dimensions, roof geometry, and openings
- Enclosure or permeability assumptions
- Intended status of large doors during severe wind events
6.2 Snow Information
- Ground snow parameter and source
- Governing standard and edition
- Site elevation and local climate conditions
- Risk, consequence, or importance classification
- Roof slopes, levels, parapets, and adjacent structures
- Exposure and thermal assumptions where applicable
- Potential drift, unbalanced, sliding, and rain-on-snow conditions
Statements such as “wind speed: 40 m/s” or “snow load: 1.5 kN/m²” are incomplete unless the parameter definition and code basis are also known. The same numerical value can represent different stages of the calculation in different standards.
ASCE/SEI 7-22 addresses wind, snow, and load combinations in the U.S. framework. Within the Eurocodes, EN 1991 includes separate parts for snow loads and wind actions. Local adoption, national annexes, and the required edition must be confirmed for each project.
7. How These Actions Enter Design Combinations
Wind and snow are individual actions, not complete design conditions. Engineers evaluate different wind directions, pressure signs, snow distributions, and accompanying gravity loads through code-defined combinations.
The governing standard determines which actions accompany each wind or snow case and at what level. For the integration step, see load combination in steel structure design.
