Estimation of structural loads is a critical component of any engineering design. Structural loads used in design not only affect the structure’s safety but also its cost. As an engineer, if you overestimate the loads on a structure, then the economy of the project may be significantly affected. On other the hand, if you underestimate the loads, then you create a structural hazard.
The purpose of this article is to walk you through selecting loads for your engineering project. Please also note that as an engineer you are always liable for the decisions you make during your design process. In this article, we shall discuss the common loads typically subject to structures:
Dead loads, often called static or permanent loads, are the constant gravitational forces that structures must support throughout their lifespan. These loads primarily consist of a building’s own weight and the weight of fixed components within it.
A key factor in estimating dead loads in a building is density. Using density, the weight of different components in the building can be estimated. The following are formulas for estimating different types of dead loads:
- Dead loads (DL) on floors:
DL = height x density kN/m2
This can used to estimate the weight of concrete slab or finishes.
- Dead loads (DL) on beams:
DL = cross-sectional area x density kN/m
This can be used to estimate the self-weight of a beam.
- Concentrated dead loads (DL):
DL = volume x density kN
This can be used to estimate point loads due to fixed components in the building.
Since the density of different building materials is important in estimating the self-weight of a building or structure, here are the densities of some common materials. For ease of use, the densities are present in kN/m3.
|Stone Masonry||26 to 30|
|Brick Masonry||18 to 24|
|Timber||3.5 to 10.8|
|Steel||77.0 to 78.5|
Unlike dead loads, live loads are ever-changing and vary within the building from one instance to another. Examples of these loads include loads due to; people walking in a building, cars on a bridge, or even furniture being shifted around. In general, live loads depend on the function of the building or structure and the nature of the source. Below are some live loads for different categories of buildings.
|Category||UDL (kN/m2)||Concentrated load (kN)|
|Residential (rooms in residential, kitchens and toilets,bedrooms in hotels, hostels as well as bedrooms/wards in hospitals.||1.5 to 2.0||2.0 to 3.0|
|Balconies||2.5 to 4.0||2.0 to 3.0|
|Office areas||2.0 to 3.0||1.5 to 4.5|
|Areas with tables; Schools, Cafés, Restaurants, Dining||2.0 to 3.0||3.0 to 4.0|
|Areas with fixed seats; Churches,Conference rooms,Lecture halls and waiting rooms||3.0 to 4.0||2.5 to 7.0|
|Areas without obstacles for people moving; hotels and admin buildings.||3.0 to 5.0||4.0 to 7.0|
|Areas with physical activities; dance halls, gymnastic rooms, stages.||4.5 to 5.0||3.5 to 7.0|
|Areas with possible crowding; terraces and access||5.0 to 7.5||3.5 to 4.5|
|Shopping areas||4.0 to 5.0||3.5 to 7.0|
Staircases are subjected to live loads due to human traffic and these loads can be estimated in a range of 2 to 4.0 kN/m2.
Water tank loads
Water tanks in buildings exert loads on the floor. To estimate the pressure load from a water tank the following formula can be used:
Load (kN/m²) = ( Volume of Water × Density of Water × g ) / A
- Load (kN/m²) is the total load exerted by the water on the tank’s base.
- Volume of Water is the volume of water inside the tank (in cubic meters, m³).
- Density of Water is the density of water (approximately 1,000 kg/m³ or 9.81 kN/m³).
- g is the acceleration due to gravity (approximately 9.81 m/s²).
- A is the base area of the tank (in square meters, m²).
Structures below the ground level are subject to pressure loads as a result of the soil they retain. These loads need to be accounted for in the design of such structures. Here’s a simplified formula for computing earth pressure on a building wall or structure.
P = K ∗ γ ∗ H
- P is the lateral earth pressure (in Pascals, Pa).
- K is the coefficient of earth pressure (dimensionless) based on the soil properties and wall or structure type.
- γ is the unit weight of the soil (in Newtons per cubic meter, N/m³).
- H is the height of the soil or retaining wall (in meters, m).
The coefficient K depends on various factors, including the soil properties (e.g., friction angle, cohesion), the type of wall or structure (e.g., rigid or flexible), and the method of analysis (active or passive earth pressure). Different soil types and wall configurations may require different values of K.
Wind loads on buildings are among the common loads on any structure. To get an estimate of wind loads on any structure, you can use the formula below:
Wind Load (kN) = 0.5 x ρ x V2 x A x Cd
- ρ is the air density.
- V is the wind speed (in m/s).
- A is the effective wind-exposed area of the building or its components (in m2).
- Cd is the drag coefficient.
Seismic loads are dynamic forces generated by ground motion during an earthquake, and they have the potential to cause significant structural damage if not properly accounted for in the design process.
The process of estimating earth loads typically involves; establishing the seismic hazard at the site and considering factors such as historical earthquake data, geological conditions, and the region’s tectonic activity.
The structure’s dynamic properties, such as its natural frequency and mode shapes, can then be analyzed. These properties are used to calculate the modal response spectrum and spectral acceleration values. Finally, the seismic loads are calculated based on the structural response parameters and then applied to the building model during design.
The computation of earthquake loads is complex, and it is recommended to use design software to estimate these loads to minimize any errors.
Temperatures induced in a building or structure during its operation can affect its structural behaviour and hence they need to be accounted for during design.
The formulas for computing temperature loads are generally based on the principles of linear expansion and use coefficients of thermal expansion. The formula below shows how to estimate a load due to temperature change:
Temperature Load: PΔT=α∗ΔT∗A∗E
- PΔT is the temperature load (in Newtons, N).
- α is the coefficient of thermal expansion of the building material (in 1/°C or 1/°F).
- A is the cross-sectional area of the material subjected to temperature change in square meters (m²).
- ΔT is the temperature change in degrees Celsius (°C) or degrees Fahrenheit (°F).
- E is the modulus of elasticity of the material (in Pascals, Pa).
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