In the construction and design of buildings and other structures, understanding the various loads acting upon them is crucial for ensuring both safety and economic efficiency. The design of a structure must take into account various forces that will affect its stability, durability, and performance over time. The classification of loads can be broadly divided into three categories: vertical loads, horizontal loads, and longitudinal loads. This article provides an overview of these types of loads, their importance, and how they are accounted for in structural design.
Classification of Loads
The loads acting on a structure can be classified as:
- Vertical Loads: Dead loads, live loads, and impact loads.
- Horizontal Loads: Wind loads and earthquake loads.
- Longitudinal Loads: Tractive and braking forces (mainly in bridges and gantry girders).
Each of these categories of loads plays a critical role in the design and safety of a structure, and understanding how to calculate and mitigate these forces is fundamental to successful building construction.
I. Dead Loads (DL)
Dead loads are permanent, stationary loads that are constantly acting on the structure throughout its lifespan. These loads are typically caused by the self-weight of the building materials, permanent partitions, fixed equipment, and the weight of structural elements such as roofs, walls, beams, and columns.
The calculation of dead loads involves determining the volume of each structural element and multiplying it by the unit weight of the material. For example, the weight of a concrete beam can be determined by calculating its volume and multiplying it by the unit weight of concrete (approximately 24 kN/m³).
Examples of Unit Weights for Common Construction Materials:
- Brick Masonry: 18.8 kN/m³
- Stone Masonry: 20.4-26.5 kN/m³
- Plain Cement Concrete: 24 kN/m³
- Reinforced Cement Concrete: 24 kN/m³
- Timber: 5-8 kN/m³
Dead loads are essential in the design process, as they help determine the overall weight and stability of the structure. These loads must be considered in every design phase, from foundations to the roof structure.
II. Imposed Loads or Live Loads (IL or LL)
Imposed loads, also known as live loads, are variable and temporary loads that occur due to the occupancy or use of a building. Unlike dead loads, which are constant, live loads change over time depending on how the space is used. These can include the weight of furniture, occupants, movable partitions, and other temporary elements.
The magnitude of live loads depends on the type of occupancy or use of the building. For instance, residential buildings, educational institutions, and industrial spaces each have different load requirements. The Indian Standard IS 875 (Part 2)-1987 provides guidelines for live load values based on building occupancy.
Common Building Classifications for Live Load Design:
- Residential Buildings: Dwelling houses, hotels, hostels, etc.
- Educational and Institutional Buildings.
- Assembly and Business Buildings.
- Industrial Buildings and Storage Rooms.
The design of floor slabs is based on either uniformly distributed loads or concentrated loads, depending on which one creates the higher stresses. In multi-story buildings, live loads are often reduced when designing columns, load-bearing walls, and foundations because it is unlikely that the maximum load will act on all floors simultaneously.
Reduction in Live Load for Multi-Story Buildings:
- For one floor: 0% reduction.
- For two floors: 10% reduction.
- For three floors: 20% reduction.
- For more than 10 floors: 50% reduction.
This reduction factor allows designers to optimize material use while still ensuring safety.
III. Wind Loads
Wind loads are horizontal forces exerted on structures due to the movement of air relative to the earth’s surface. Wind forces can significantly affect the stability of buildings, especially those that are taller or have large surface areas exposed to the wind. Wind load calculations are essential in structural design, particularly for buildings with a height greater than twice their dimension in the transverse direction.
For low-rise buildings (up to four or five stories), wind loads are generally not a major concern because the structure’s rigidity and continuity of floor systems provide sufficient resistance. However, as buildings grow taller, the effect of wind becomes more pronounced, requiring more detailed calculations.
Wind load calculations are based on several factors, including wind velocity, building size, and location. The Indian Standard IS 875 (Part 3)-1987 provides guidelines for calculating wind loads on structures. Designers can use a map of India to determine the basic wind pressure (Vb) for their specific location, and then calculate the design wind velocity (Vz) using the formula:
Vz = k1 × k2 × k3 × Vb
Where:
- k1 = Risk coefficient
- k2 = Coefficient based on terrain, height, and structure size
- k3 = Topography factor
The design wind pressure can be calculated using the formula:
pz = 0.6 × Vz²
For buildings taller than 30 meters, wind pressure increases with height and must be accounted for in the design.
IV. Snow Loads (SL)
Snow loads are vertical loads that are considered in areas prone to snowfall. Snow accumulation on roofs or exposed areas can add significant weight to a building, and this load must be considered in the design of structures in such regions.
The IS 875 (Part 4)-1987 provides guidelines for calculating snow loads on roofs and other areas. The design snow load on a roof or any area subjected to snow accumulation is determined by the formula:
S = Shape Coefficient × S0
Where:
- S = Design snow load
- S0 = Ground snow load
- Shape Coefficient = A factor that accounts for the geometry of the roof or surface
Snow loads are not a concern in all regions, but in areas where heavy snowfall occurs, they must be considered for the safety and integrity of the structure.
V. Earthquake Loads (EL)
Earthquake forces are unique in that they result in both vertical and horizontal loads on a building. The most significant forces come from the horizontal movement of the ground, which can cause vibrations and shaking of the structure.
The effects of earthquakes depend on factors such as the type of soil, the size of the building, and the intensity and duration of the seismic activity. Earthquake loads are especially critical in regions prone to seismic activity, and the IS 1893-2014 provides detailed guidelines for calculating these forces. The seismic coefficient is a key factor in determining earthquake-induced accelerations:
Seismic Coefficient = Acceleration due to Earthquake / Acceleration due to Gravity
The response of a structure to earthquake forces depends on its design, the type of foundation, and its location within seismic zones. In seismic zones 2 and 3, for instance, seismic forces may not be as critical for low-rise buildings.
VI. Other Loads and Effects on Structures
In addition to the primary loads discussed above, there are other forces and effects that may impact the safety and serviceability of a structure. These include:
- Foundation movement: Due to settlement or shifting of the ground.
- Elastic axial shortening: Due to axial loads on columns or beams.
- Soil and fluid pressure: Impacting underground structures.
- Vibration: From machinery or external sources.
- Fatigue: Due to repeated load cycles.
- Impact loads: Resulting from sudden forces or impacts on the structure.
- Erection loads: Temporary loads during construction.
The Indian Standard IS 456-2000 outlines these additional loads and stresses the need to account for them during the design process to ensure long-term stability and safety.
Conclusion
Understanding the various loads that act on structures is essential for designing safe and efficient buildings. Each type of load—whether vertical, horizontal, or longitudinal—presents unique challenges that require careful calculation and consideration. By following the relevant design codes, such as IS 875 and IS 1893, engineers can ensure that their structures are capable of withstanding the forces they will encounter throughout their lifespan. Proper load estimation is a critical factor in balancing safety and economy in construction, and it is key to building resilient, long-lasting structures.