A structure is designed to resist the different types of loads acting on it, and these loads are among the most important parameters to consider during design. Different types of loads can cause stress, displacement, and deformation in structural members.
Determining the loads on a structure is important but complex, and manual analysis is time-consuming. Structural analysis software helps analyze these effects accurately and consistently.
Loads are classified on the basis of the following parameters:
- The direction of the load
- Duration of the load
- Spatial distribution of the load
- Source of the loading
Dead loads
The first vertical load considered in design is dead load. Dead loads are permanent or stationary loads that act on the structure throughout its life span. Dead load is primarily due to the self-weight of structural members, permanent partition walls, fixed permanent machinery, and the weight of various materials.
Imposed loads (live loads)
Imposed load is the load assumed to be produced by the intended occupancy or use of a structure, including distributed, concentrated, impact, dynamic, and inertia loads. Live loads are movable or moving loads without acceleration or impact.
Wind loads
Wind loads result from the movement of air relative to a structure, and their analysis draws on meteorology and aerodynamics as well as structural engineering. Wind load may not be significant for small, massive, low-rise buildings, but it gains importance with building height, the use of lighter materials, and shapes that affect airflow — typically roof forms.
Snow loads
Snow load is imposed by the accumulation of snow and is a greater concern in regions with heavy, frequent snowfall. Significant accumulation can add a sizable load to a structure.
Earthquake loads
Earthquake load arises from the inertia force generated in a building due to seismic excitation. Inertia force varies with mass — a structure with higher mass experiences higher earthquake loading. When earthquake load exceeds the moment of resistance offered by a structural element, the element can break or be damaged.
Analyzing loads on a portal frame
Evaluating dead, live, wind, and seismic loads by hand across every member of a portal frame is slow, and combining them into the governing load cases for each section leaves room for error. Pre-engineered building design software carries this further than a spreadsheet can: dead and live loads are applied automatically as the frame is modelled, and wind and seismic loads are generated to the applicable design code, so every member is checked against the actual combination that governs it — not a single conservative worst case.
Getting this translation right — from load type, to combination, to member check — is what determines whether a frame is safely designed or oversized out of excess caution.
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FAQs
What are the main types of loads acting on a structure?
Dead loads (permanent self-weight), live loads (occupancy and use), wind loads, snow loads, and earthquake (seismic) loads are the primary categories a structure is designed to resist.
What is the difference between dead load and live load?
Dead load is permanent and constant — the self-weight of the structure and fixed elements. Live load is variable and depends on occupancy or use, such as people, furniture, or movable equipment.
Why does wind load matter more for taller or lighter buildings?
Wind load becomes more significant as building height increases and as structures use lighter materials or roof shapes that affect airflow — for small, massive, low-rise buildings it's often a minor factor.
How is earthquake load different from other loads on a structure?
Earthquake load is an inertia force generated by ground motion, not an external force applied directly to the structure — it depends on the building's mass, so heavier structures experience higher seismic loading.
Does a structure need to resist all these loads at once?
Yes — design codes require checking combinations of these loads together (for example, dead plus snow plus wind), not each load type in isolation, since the governing case for any given member often comes from a combination rather than a single load acting alone.
